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In search of life / Planet hunters scan heavens for warm, Earthlike worlds
"Curiouser and curiouser," said Alice in Wonderland. "Now I'm opening out like the largest telescope that ever was!"
In something like a celestial land rush that began only 10 years ago, more than 50 teams of astronomers around the world are aiming their telescopes from mountaintops and aboard an orbiting spacecraft to scan the Milky Way in a race to detect more planets circling far-off stars.
They are hoping to discover Earthlike planets throughout the galaxy -- even ones that harbor life -- but the more than 210 objects they have spotted so far are as curious, even bizarre, as anyone can imagine.
Known as "exoplanets," or "hot Jupiters," or "gas giants," they are all huge and they all move in orbits so close to their suns that a trip around -- which takes an Earth year -- takes only a few days. . One gas giant, detected by the Hubble space telescope, whirls around its sun in only 10 hours, compared with 12 years for the Jupiter in our own solar system.
"I think there are definitely water-rich, habitable planets out there," says astronomer Sean Raymond at the University of Colorado, "and my feeling is that single-celled microbial life should be really common on some of them."
But any life on those planets would be very different from our own given the many evolutionary steps that would be required between planet formation and a point at which "advanced life forms (are) looking back at us," Raymond says.
In an interview, he estimated that at least one-third of the planets discovered outside the solar system could lie within so-called habitable zones -- regions around their parent stars where temperatures are benign enough to sustain liquid water and support life.
And in a report in the journal Science, Raymond and his colleagues, Avi Mandell and Steinn Sigurdsson of Penn State University, proposed that many of the giant gas planets that orbit close to their suns could have migrated inward from far more distant orbits as they were forming. And, in the process, they might have shed rocky debris that coalesced outside their orbits to form planets, including some rich in water and very much like Earth.
"I think Sean Raymond's estimate is quite defensible and plausible," said Geoffrey Marcy, the UC Berkeley astronomer whose planet-hunting team has found more of the gas giants than any other group.
"Those traveling, close-in Jupiters provide evidence that planet-building material is spread out in wide orbits where Earthlike planets can form," he said.
But Marcy's team -- using the telescopes at Lick Observatory on Mount Hamilton near San Jose and the Keck Observatory on the Big Island of Hawaii -- has also found cold gas giants orbiting many millions of miles away from their stars, rather than close in. Marcy reasoned in a recent interview that warm rocky planets could just as well form inside the orbits of distant Jupiters -- "as in our own solar system."
"Rocky planets such as Earth, Mars and Venus are so small that they can form from very little material almost anywhere in a solar system," he said -- that is, if some giant planet like Jupiter isn't so close that it would destroy them.
The astronomers who first detected extrasolar planets in the galaxy were Michel Mayor and Didier Queloz of the Geneva Observatory in Switzerland, in 1995, and Marcy's team found two more that same year at Lick Observatory. Their discoveries started a worldwide rush, and scientists in France, Australia, Canada and the European Space Agency are all preparing for the hunt. Details are on the Web at exoplanet.eu, the Extrasolar Planet Encyclopedia, maintained by astronomer Jean Schneider at the Paris Observatory.
NASA spacecraft carrying some of the most sensitive instruments ever made have been designed to hunt far beyond our solar system to find these rocky, warm planets and to scan their surfaces for telltale evidence that their environments could make them habitable.
Astronomers at the Space Telescope Institute in Baltimore, who direct the Hubble's mission, reported Wednesday that their powerful orbiting instrument has detected 16 "candidate" planets; at least seven are most probably true planets, they said after analyzing the data, and two have been confirmed by astronomers at a ground-based observatory in Chile. For these observations, the Hubble peered far, far out into a section of the galaxy, 26,000 light-years away in the constellation Sagittarius the Archer, much farther away than the ground-based telescopes can possibly detect any planets.
NASA now has one new planet-hunting spacecraft scheduled for launch in 2008. Two others, called the Terrestrial Planet Finder, or TPF, and the Space Interferometry Mission Planet Quest (SIM), had been well along in their schedules to fly within the next few years, but construction has been put on hold indefinitely. Changes in NASA's budget priorities and cuts for space science now commit the agency to focus on the Bush administration's decision to send astronauts back to the moon by 2020, and on to Mars in the more distant future.
"This change goes against the wishes of the vast majority of scientists," says Marcy. "A manned mission to the moon comes with a poorly articulated, and perhaps nonexistent, scientific rationale."
Astronomers haven't given up though, and NASA has not canceled one particular planet-seeking spacecraft already under construction by the Ball Aerospace Corp. in Boulder, Colo. It is overseen by scientists and engineers at the Ames Research Center in Mountain View, where the space agency's Astrobiology Institute has its headquarters and the independent SETI Institute is nearby.
The spacecraft is named after Johannes Kepler, the 17th century German astronomer who discovered the laws of planetary motion.
Kepler is scheduled for launch in November 2008, and from its flight path trailing Earth's orbit around the sun, the spacecraft's telescope will scan at least 100,000 stars in the region of the constellation Cygnus the Swan during its four-year mission. Its extraordinarily sensitive telescope is designed to detect planets at least the size of Earth by measuring the almost-imperceptible dimming of the star's light whenever a planet's orbit carries it directly across its parent sun's face.
"Earth-size planets will be really hard to spot," says Janice Voss, a former astronaut and veteran of five space shuttle missions, who is director of the Kepler science office at Ames. "But by watching each planet as it transits its star at least three times, we should be able to determine that planet's orbit and its mass."
When Kepler does detect any extrasolar planets roughly the size of Earth, Voss said, their locations will be sent to major observatories around the world and the Hubble space telescope for more detailed observations. They hope to learn whether they are orbiting in a habitable zone and whether their atmospheres contain gases, like nitrogen and oxygen, that support life on Earth.
At the Lick Observatory atop Mount Hamilton, a team led by Marcy and astronomer Paul Butler of the Carnegie Institution in Washington is well into building a powerful new ground-based telescope informally named the Rocky Planet Finder. It will be programmed like a sensitive robot to search for planets in habitable zones around some 200 nearby stars. It will operate continuously every night of the year, seeking planets with rocky surfaces and lukewarm temperatures.
Instead of seeking evidence of planets by noting how their transits dim the light of their suns, the instruments of Marcy's team will measure the faint wobble of the stars caused by the gravity of planets orbiting them.
"We expect to discover nearby Earthlike planets," Marcy said, "and then point the world's largest radio telescopes toward them to hunt for radio and television signals from intelligent civilizations." | <urn:uuid:9e8c5c5e-43c2-4ba6-aceb-8b97a7e70f08> | 3.09375 | 1,604 | Truncated | Science & Tech. | 32.67013 | 95,504,494 |
Reporting in the journal Nature Physics, William Irvine and Dustin Kleckner, physicists at the University of Chicago, have created a knotted fluid vortex in the lab ? a scientific first, they say. The knots resemble smoke rings ? except these are made of water, and they're shaped like pretzels, not donuts. Understanding knottiness has extra-large applications, like understanding dynamics of the sun.
IRA FLATOW, HOST:
Flora Lichtman, our correspondent and managing editor for video is here with our...
FLORA LICHTMAN, BYLINE: Video Pick of the Week.
FLATOW: You've got it.
LICHTMAN: I got it.
FLATOW: Let me just remind everybody that I'm Ira Flatow with Flora. This is SCIENCE FRIDAY from NPR. And our Video Pick...
LICHTMAN: This week, Ira, it's out there. This one's loopy. That's a joke, and you're going to find out why.
FLATOW: It's literally loopy.
LICHTMAN: It's literally loopy.
FLATOW: You're not - you were looking at me saying loopy. So...
LICHTMAN: The Video Pick. The Video Pick this week is a study that appeared in Nature Physics from William Irvine and Dustin Kleckner at the University of Chicago. They're physicists, and they did a scientific first, they believe, which is they tied water into a knot.
FLATOW: They tied water into a knot.
LICHTMAN: What could that even mean, you might be thinking.
FLATOW: You have to see it.
LICHTMAN: You have to see it.
FLATOW: You have to see it. It's on our website up there at sciencefriday.com. But...
LICHTMAN: It's really neat. OK. Here's how you can understand or here's how I did. If you take a smoke ring, people are familiar with these, these vortex rings, right?
LICHTMAN: So the kind of donuts of smoke. And then you were able to twist that flow, the fluid flow into a pretzel, you would have what these researchers have created.
FLATOW: Like a knot, a pretzel - tied up into a pretzel.
LICHTMAN: A pretzel of bubbles and water.
FLATOW: So you can see the bubbles - so it starts out like a smoke ring and then gets twisted into a pretzel of bubbles?
LICHTMAN: No. But...
LICHTMAN: ...here's out here's how it works. They - and this is part of the innovation of this study.
LICHTMAN: They printed these three - using a 3-D printer. 3-D printer saves the day again, right?
FLATOW: Of course.
LICHTMAN: These wings, they look sort of like twisted - it looks a lot like a pretzel, but in a wing shape and it traces the outline of the knot. And then they put that in water and little bubbles attached to it and then they accelerate it really fast and the bubbles fly off and the water flows in this knotted fluid flow. You have to see it. It's...
FLATOW: You have to see it. You know, and one of the fun parts about the video, which is up there on our Video Pick of the Week at sciencefriday.com, is also watching them blow smoke rings out of a cannon. When you talk about...
LICHTMAN: There's an added bonus. That was from Dan (Unintelligible) in University of Maryland, only semi-related. But, you know, the same idea. The idea is to understand how fluids flow, and this is an idea, this idea of knottiness of fluid, so air or water, had been suggested over a hundred a years ago by Lord Kelvin. So it's an old idea, but no one had managed to actually make one of these in the lab until now. And the reason, you know, even - it's kind of out there and interesting on its own, but there are also applications. For instance, the sun's corona, which you may have seen from NASA, these beautiful images of the sun...
FLATOW: Oh, yeah.
LICHTMAN: ...with these projectiles of plasma, those are thought to be knotty too. And so part of this is trying to understand what happens to these knotted liquids. Do the knots untie? If they untie, how do they untie? Do you conserve knottiness? That's the idea. Maybe knottiness doesn't quite go away. Maybe it has to be translated into twisting. So there's really some interesting questions raised by this - interesting physical questions.
FLATOW: You can't make this at home, though, right? You can't make these bubble knots on your own at home, I don't think yet.
LICHTMAN: You've got to have a very - I was trying to think if you could do like a smoke ring with your tongue - I don't think so.
FLATOW: No, no. Blow a smoke ring...
LICHTMAN: It's way too complicated.
FLATOW: By the way, what's also gorgeous about the video up there is that there are these three dimensional - you go around them, right?
LICHTMAN: Yeah. Thanks for bringing that up, because I think one of the most amazing parts of this study is how they image to this. OK. So first of all we're talking very high-speed video. But then to see the knot of liquid - they use bubbles to see it - but they have to do it very fast and with a laser. So the laser scans across the bubble knot, you know, many times over the course of a second. I think it's like a hundred times, making these sheets of images and then they are stacked together digitally so that you have a 3D picture of what the knot looks like. And then you can fly around the knot.
FLATOW: It's like a CAT scan of - a CAT scan without the X-rays.
FLATOW: It's like a CAT scan of a 3D - and you fly around there and it's all up there on our Video Pick of the Week. If you want to - beside the cannon that shoots these great smokes.
LICHTMAN: Ira, you really love the cannon.
FLATOW: I love the cannon part. The three dimensional bubble images. How you take bubbles and they twist them into a pretzel is quite fascinating, Flora. Thank you.
LICHTMAN: Thanks, Ira.
FLATOW: So our Video Pick of the Week up there on our website at sciencefriday.com and also you can get them downloaded onto your app. We have a video app that you can get up there. We have a new SCIENCE FRIDAY app up there on iTunes, brand new. It's got great new features on it. I want to make sure you download it.
NPR transcripts are created on a rush deadline by a contractor for NPR, and accuracy and availability may vary. This text may not be in its final form and may be updated or revised in the future. Please be aware that the authoritative record of NPR's programming is the audio. | <urn:uuid:3e1d8627-451d-47eb-bd45-b72e3651e5d2> | 3.109375 | 1,593 | Audio Transcript | Science & Tech. | 76.014651 | 95,504,498 |
EELS study on BST thin film under electron beam irradiation
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It was found that BST thin film was damaged by the irradiation of high density electron beam (the current density was about 2 nA/cm2). In-situ and real time EELS showed that the intensity ratio of Ti to O edge and the distance between Ti and O edge changed. It indicated that the film lost oxygen and thus the oxidation states of positive ions lowered. EELS study with high spatial resolution proved that compared with the inner of columnar grains, the grain boundaries with special structure and chemical environment were the main passageway of oxygen loss.
KeywordsBST thin film irradiation damage EELS
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- 1.Kingon, A. I., Streiffer, S. K., Basceri, C. et al., High-permittivity perovskite thin films for dynamic ran-dom-access memories, MRS Bull., 1996, 21: 46–52.Google Scholar
- 12.Egerton, R. F., Electron Energy-Loss Spectroscopy in the Electron Microscope, New York: Plenum Press, 1996, 245–300.Google Scholar | <urn:uuid:d36a36ee-8b07-490c-afcd-54abf9f29aa0> | 2.71875 | 259 | Academic Writing | Science & Tech. | 63.602406 | 95,504,532 |
Nicknamed the Southern Pinwheel, M83 is undergoing more rapid star formation than our own Milky Way galaxy, especially in its nucleus. The sharp "eye" of the Wide Field Camera 3 (WFC3) has captured hundreds of young star clusters, ancient swarms of globular star clusters, and hundreds of thousands of individual stars, mostly blue supergiants and red supergiants.
The image at right is Hubble's close-up view of the myriad stars near the galaxy's core, the bright whitish region at far right. An image of the entire galaxy, taken by the European Southern Observatory's Wide Field Imager on the ESO/MPG 2.2-meter telescope at La Silla, Chile, is shown at left. The white box outlines Hubble's view.
WFC3's broad wavelength range, from ultraviolet to near-infrared, reveals stars at different stages of evolution, allowing astronomers to dissect the galaxy's star-formation history.
The image reveals in unprecedented detail the current rapid rate of star birth in this famous "grand design" spiral galaxy. The newest generations of stars are forming largely in clusters on the edges of the dark dust lanes, the backbone of the spiral arms. These fledgling stars, only a few million years old, are bursting out of their dusty cocoons and producing bubbles of reddish glowing hydrogen gas.
The excavated regions give a colorful "Swiss cheese" appearance to the spiral arm. Gradually, the young stars' fierce winds (streams of charged particles) blow away the gas, revealing bright blue star clusters. These stars are about 1 million to 10 million years old. The older populations of stars are not as blue.
A bar of stars, gas, and dust slicing across the core of the galaxy may be instigating most of the star birth in the galaxy's core. The bar funnels material to the galaxy's center, where the most active star formation is taking place. The brightest star clusters reside along an arc near the core.
The remains of about 60 supernova blasts, the deaths of massive stars, can be seen in the image, five times more than known previously in this region. WFC3 identified the remnants of exploded stars. By studying these remnants, astronomers can better understand the nature of the progenitor stars, which are responsible for the creation and dispersal of most of the galaxy's heavy elements.
M83, located in the Southern Hemisphere, is often compared to M51, dubbed the Whirlpool galaxy, in the Northern Hemisphere. Located 15 million light-years away in the constellation Hydra, M83 is two times closer to Earth than M51.
Credit for Hubble image: NASA, ESA, R. O'Connell (University of Virginia), B. Whitmore (Space Telescope Science Institute), M. Dopita (Australian National University), and the Wide Field Camera 3 Science Oversight Committee
The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. Goddard manages the telescope. The Space Telescope Science Institute conducts Hubble science operations. The institute is operated for NASA by the Association of Universities for Research in Astronomy, Inc. in Washington, and is an International Year of Astronomy 2009 program partner.
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ATLASGAL is a survey of the Galactic Plane at a wavelength of 0.87 mm. It has revealed an unprecedented number of cold dense clumps of gas and dust as the cradles of massive stars, thus providing a complete view of their birthplaces in the Milky Way.
Based on this census, an international team of scientists led by Timea Csengeri from the Max Planck Institute for Radio Astronomy in Bonn has estimated the time scale for these nurseries to grow stars. This has been found to be a very fast process: with only 75,000 years on average it is much shorter than the corresponding time scales typically found for nurseries of lower mass stars.
The ATLASGAL survey covers two thirds of the surface area of the Galaxy within 50,000 light years of the Galactic center. Thus it includes practically all (97%) of the star-formation within the Solar Circle, i.e. the inner Galaxy. The image displays a part of ATLASGAL, a region located between the giant molecular complexes called W33 and M17 in the Sagittarius constellation. Zooms in color scale show the 3-color emission from the mid-infrared GLIMPSE survey, and sub-millimeter dust emission from ATLASGAL is shown in red and traced with contours. One region corresponds to a cold, pristine massive clump (upper left inset), and another one to a young massive star (upper right inset). Both objects have sizes of only a few light-years across. In the lower right inset we present a schematic of the Milky Way and show the position of the Solar Circle (green) and region of the Galaxy covered by ATLASGAL (shaded region).
Stars significantly more massive than the Sun end their fast and furious lives in violent supernova explosions producing the heavy elements in the Universe. Throughout their lives, their powerful stellar winds and high-energy radiation shape their local environments and have a significant impact on the appearance and future evolution of their host galaxies.
These stars form at the densest and coldest places in the Milky Way deeply embedded in dust cocoons, which are so dense that they absorb most of the radiation from the young stars within. It is in these dense cocoons of gas and dust, hidden from visible and infrared wavelengths, where the next generation of stars are being born.
An international team of astronomers used the APEX telescope with its sub-millimeter camera, LABOCA, built at the Max Planck Institute for Radio Astronomy (MPIfR), to survey the inner Galaxy to search for the birthplaces of the most massive stars currently forming in the Milky Way. The APEX telescope is located on the Chajnantor Plateau in Chile at 5100 m altitude, which is one of the few places on Earth where observations at sub-millimeter wavelengths are possible.
The ATLASGAL survey covers more than 420 square degrees of the Galactic plane, which corresponds to 97% of the inner Galaxy within the Solar circle. Thus it includes large sections of all four spiral arms, and approximately two thirds of the entire molecular disc of the Milky Way (see lower right inset of Fig. 1). This data set therefore includes the majority of all massive star forming nurseries in the Galaxy and is being used to construct a 3D map of the Milky Way.
The APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) provides an unprecedented census of the cold and dense environments where the most massive stars in our Galaxy begin their lives. The material in these stellar nurseries is so dense that optical and infrared light emitted from the embedded young high-mass stars cannot escape.
Therefore, the earliest stages of star formation are effectively hidden at these short wavelengths and longer wavelengths are required to probe these regions. The ATLASGAL survey detects emission at sub-millimeter wavelengths, which is dominated by emission from cold dust. It provides a detailed view of the birthplaces where the next generation of massive stars is being formed.
As these dusty corners of our Galaxy are very difficult to access, such surveys provide the large scale coverage to search for the stellar nurseries forming the most massive stars in our Galaxy. “Our team has now analyzed this survey revealing the largest sample of the so-far hidden places of massive star-formation”, states Timea Csengeri from MPIfR, the lead author of the study. "We have identified many new potential sites where the most massive stars currently form in our Galaxy."
Providing an unprecedented statistics, scientists reveal that the processes to build up the cold, dense sites where the most massive stars in our Galaxy form, occur rapidly, taking place within only 75,000 years, which is much shorter than the corresponding timescales in nurseries of lower mass –stars like our Sun. This is the first global indication that star-formation is a fast process in our Galaxy.
"We characterized these places to search for signatures revealing how massive stars form within them," continues James Urquhart, also from MPIfR. “The fast and furious life of the most massive stars was already known. And now we could also show that it is initiated by a pretty short infancy within their stellar cocoons.” The lifetime of massive stars is about 1000 times shorter than the lifetime of stars like the Sun, and the new results reveal that they also form on short timescales and in a much more dynamic star formation process.
“Only telescopes at exceptional locations, such as the high and dry Chajnantor Plateau in Chile at 5100 m are capable to observe in this frequency range”, adds Frederic Schuller from ESO, co-author of the study. “This is the largest area in the sky surveyed from a ground-based telescope in the sub-millimeter wavelength regime".
“ATLASGAL also provides a “finding chart” for the most extreme dust cocoons, where the innermost processes of stellar birth can be studied at much higher angular-resolution with the new ALMA interferometer, located just next to the APEX telescope” concludes Friedrich Wyrowski, the APEX project scientist at MPIfR.
The ATLASGAL survey: a catalog of dust condensations in the Galactic plane,
T. Csengeri, J. S. Urquhart, F. Schuller, F. Motte, S. Bontemps, F. Wyrowski, K. M. Menten, L. Bronfman, H. Beuther, Th. Henning, L. Testi, A. Zavagno, M. Walmsley, Astronomy & Astrophysics, Vol. 565, A75 (May 2014).
See also: astro-ph.GA: arXiv:1312.0937.
Dr. Timea Csengeri,
Max-Planck-Institut für Radioastronomie.
Dr. James Urquhart,
Max-Planck-Institut für Radioastronomie.
Dr. Norbert Junkes,
Press and Public Outreach,
Max-Planck-Institut für Radioastronomie.
Norbert Junkes | Max-Planck-Institut
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For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.
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Image credit: SLAC National Accelerator Laboratory
MENLO PARK, CALIF. — A number of important biological processes, such as photosynthesis and vision, depend on light. But it’s hard to capture responses of biomolecules to light because they happen almost instantaneously.
Now, researchers have made a giant leap forward in taking snapshots of these ultrafast reactions in a bacterial light sensor. Using the world’s most powerful X-ray laser at the Department of Energy’s SLAC National Accelerator Laboratory, they were able to see atomic motions as fast as 100 quadrillionths of a second—1,000 times faster than ever before.
Further, “We’re the first to succeed in taking real-time snapshots of an ultrafast structure transition in a protein, in which a molecule excited by light relaxes by rearranging its structure in what is known as trans-to-cisisomerization,” says the study’s principal investigator, Marius Schmidt from the University of Wisconsin-Milwaukee.
The technique could widely benefit studies of light-driven, ultrafast atomic motions. For example, it could reveal:
- How visual pigments in the human eye respond to light, and how absorbing too much of it damages them.
- How photosynthetic organisms turn light into chemical energy—a process that could serve as a model for the development of new energy technologies.
- How atomic structures respond to light pulses of different shape and duration—an important first step toward controlling chemical reactions with light.
“The new data show for the first time how the bacterial sensor reacts immediately after it absorbs light,” says Andy Aquila, a researcher at SLAC’s Linac Coherent Light Source (LCLS), a DOE Office of Science User Facility. “The initial response, which is almost instantaneous, is absolutely crucial because it creates a ripple effect in the protein, setting the stage for its biological function. Only LCLS’s X-ray pulses are bright enough and short enough to capture biological processes on this ultrafast timescale.” The results were published May 5 in Science.
High-speed X-ray Camera Reveals Extremely Fast Biology
The team looked at the light-sensitive part of a protein called “photoactive yellow protein,” or PYP. It functions as an “eye” in purple bacteria, helping them sense blue light and stay away from light that is too energetic and potentially harmful.
The researchers had already studied light-induced structural changes in PYP at LCLS, revealing atomic motions as fast as 10 billionths of a second. By tweaking their experiment, they were now able to improve their speed limit 100,000 times and capture reactions in the protein that are 1,000 times faster than any seen in an X-ray experiment before.
Both studies followed a very similar approach: At LCLS, the team sent a stream of tiny PYP crystals into a sample chamber. There, each crystal was struck by a flash of optical laser light and then an X-ray pulse, which took an image of the protein’s structural response to the light. By varying the time between the two pulses, scientists were able to see how the protein morphed over time.
Since LCLS’s X-ray pulses are extremely short, lasting only a few quadrillionths of a second, they can in principle probe processes on that very timescale—but only if the optical laser also matches the tremendous speed. For the new experiment, the team replaced the old optical laser with a new one whose pulses were 100 quadrillionths of a second long—100,000 times shorter than before and much closer to the X-ray pulse length.
The researchers also applied better timing tools to measure the relative arrival time between the optical and X-ray laser pulses, enhancing the ability to precisely track ultrafast events.
“These improvements allowed us to see what no one has ever directly seen before,” Schmidt says.
Other institutions involved in the study were: Center for Free-Electron Laser Science/Deutsches Elektronen-Synchrotron, Germany; Imperial College, UK; University of Jyväskylä, Finland; Arizona State University; Max Planck Institute for Structure and Dynamics of Matter, Germany; State University of New York at Buffalo; University of Chicago; Lawrence Livermore National Laboratory; and University of Hamburg, Germany. Funding sources included: National Science Foundation; National Institutes of Health; Helmholtz Association; German Federal Ministry of Education and Research; Engineering and Physical Sciences Research Council; Academy of Finland; and the European Union.
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Methods of detecting exoplanets
Any planet is an extremely faint light source compared to its parent star. For example, a star like the Sun is about a billion times as bright as the reflected light from any of the planets orbiting it. In addition to the intrinsic difficulty of detecting such a faint light source, the light from the parent star causes a glare that washes it out. For those reasons, very few of the extrasolar planets reported as of April 2014[update] have been observed directly, with even fewer being resolved from their host star.
Instead, astronomers have generally had to resort to indirect methods to detect extrasolar planets. As of 2016, several different indirect methods have yielded success.
- 1 Established detection methods
- 1.1 Radial velocity
- 1.2 Transit photometry
- 1.3 Reflection/Emission modulations
- 1.4 Relativistic beaming
- 1.5 Ellipsoidal variations
- 1.6 Pulsar timing
- 1.7 Variable star timing
- 1.8 Transit timing
- 1.9 Transit duration variation
- 1.10 Eclipsing binary minima timing
- 1.11 Gravitational microlensing
- 1.12 Direct imaging
- 1.13 Polarimetry
- 1.14 Astrometry
- 2 Other possible methods
- 3 Detection of extrasolar asteroids and debris disks
- 4 Space telescopes
- 5 Primary and secondary detection
- 6 Verification and falsification methods
- 7 Characterization methods
- 8 See also
- 9 References
- 10 External links
Established detection methods
The following methods have at least once proved successful for discovering a new planet or detecting an already discovered planet:
A star with a planet will move in its own small orbit in response to the planet's gravity. This leads to variations in the speed with which the star moves toward or away from Earth, i.e. the variations are in the radial velocity of the star with respect to Earth. The radial velocity can be deduced from the displacement in the parent star's spectral lines due to the Doppler effect. The radial-velocity method measures these variations in order to confirm the presence of the planet using the binary mass function.
The speed of the star around the system's center of mass is much smaller than that of the planet, because the radius of its orbit around the center of mass is so small. (For example, the Sun moves by about 13 m/s due to Jupiter, but only about 9 cm/s due to Earth). However, velocity variations down to 1 m/s or even somewhat less can be detected with modern spectrometers, such as the HARPS (High Accuracy Radial Velocity Planet Searcher) spectrometer at the ESO 3.6 meter telescope in La Silla Observatory, Chile, or the HIRES spectrometer at the Keck telescopes. An especially simple and inexpensive method for measuring radial velocity is "externally dispersed interferometry".
Until around 2012, the radial-velocity method (also known as Doppler spectroscopy) was by far the most productive technique used by planet hunters (after 2012, the transit method from the Kepler spacecraft overtook it in number). The radial velocity signal is distance independent, but requires high signal-to-noise ratio spectra to achieve high precision, and so is generally used only for relatively nearby stars, out to about 160 light-years from Earth, to find lower-mass planets. It is also not possible to simultaneously observe many target stars at a time with a single telescope. Planets of Jovian mass can be detectable around stars up to a few thousand light years away. This method easily finds massive planets that are close to stars. Modern spectrographs can also easily detect Jupiter-mass planets orbiting 10 astronomical units away from the parent star, but detection of those planets requires many years of observation. Earth-mass planets are currently detectable only in very small orbits around low-mass stars, e.g. Proxima b.
It is easier to detect planets around low-mass stars, for two reasons: First, these stars are more affected by gravitational tug from planets. The second reason is that low-mass main-sequence stars generally rotate relatively slowly. Fast rotation makes spectral-line data less clear because half of the star quickly rotates away from observer's viewpoint while the other half approaches. Detecting planets around more massive stars is easier if the star has left the main sequence, because leaving the main sequence slows down the star's rotation.
Sometimes Doppler spectrography produces false signals, especially in multi-planet and multi-star systems. Magnetic fields and certain types of stellar activity can also give false signals. When the host star has multiple planets, false signals can also arise from having insufficient data, so that multiple solutions can fit the data, as stars are not generally observed continuously. Some of the false signals can be eliminated by analyzing the stability of the planetary system, conducting photometry analysis on the host star and knowing its rotation period and stellar activity cycle periods.
Planets with orbits highly inclined to the line of sight from Earth produce smaller visible wobbles, and are thus more difficult to detect. One of the advantages of the radial velocity method is that eccentricity of the planet's orbit can be measured directly. One of the main disadvantages of the radial-velocity method is that it can only estimate a planet's minimum mass (). The posterior distribution of the inclination angle i depends on the true mass distribution of the planets. However, when there are multiple planets in the system that orbit relatively close to each other and have sufficient mass, orbital stability analysis allows one to constrain the maximum mass of these planets. The radial-velocity method can be used to confirm findings made by the transit method. When both methods are used in combination, then the planet's true mass can be estimated.
Although radial velocity of the star only gives a planet's minimum mass, if the planet's spectral lines can be distinguished from the star's spectral lines then the radial velocity of the planet itself can be found, and this gives the inclination of the planet's orbit. This enables measurement of the planet's actual mass. This also rules out false positives, and also provides data about the composition of the planet. The main issue is that such detection is possible only if the planet orbits around a relatively bright star and if the planet reflects or emits a lot of light.
Technique, advantages, and disadvantages
While the radial velocity method provides information about a planet's mass, the photometric method can determine the planet's radius. If a planet crosses (transits) in front of its parent star's disk, then the observed visual brightness of the star drops by a small amount, depending on the relative sizes of the star and the planet. For example, in the case of HD 209458, the star dims by 1.7%. However, most transit signals are considerably smaller; for example, an Earth-size planet transiting a Sun-like star produces a dimming of only 80 parts per million (0.008 percent).
This method has two major disadvantages. First, planetary transits are observable only when the planet's orbit happens to be perfectly aligned from the astronomers' vantage point. The probability of a planetary orbital plane being directly on the line-of-sight to a star is the ratio of the diameter of the star to the diameter of the orbit (in small stars, the radius of the planet is also an important factor). About 10% of planets with small orbits have such an alignment, and the fraction decreases for planets with larger orbits. For a planet orbiting a Sun-sized star at 1 AU, the probability of a random alignment producing a transit is 0.47%. Therefore, the method cannot guarantee that any particular star is not a host to planets. However, by scanning large areas of the sky containing thousands or even hundreds of thousands of stars at once, transit surveys can find more extrasolar planets than the radial-velocity method. Several surveys have taken that approach, such as the ground-based MEarth Project, SuperWASP, KELT, and HATNet, as well as the space-based COROT and Kepler missions. The transit method has also the advantage of detecting planets around stars that are located a few thousand light years away. The most distant planets detected by Sagittarius Window Eclipsing Extrasolar Planet Search are located near the galactic center. However, reliable follow-up observations of these stars are nearly impossible with current technology.
The second disadvantage of this method is a high rate of false detections. A 2012 study found that the rate of false positives for transits observed by the Kepler mission could be as high as 40% in single-planet systems. For this reason, a star with a single transit detection requires additional confirmation, typically from the radial-velocity method or orbital brightness modulation method. The radial velocity method is especially necessary for Jupiter-sized or larger planets, as objects of that size encompass not only planets, but also brown dwarfs and even small stars. As the false positive rate is very low in stars with two or more planet candidates, such detections often can be validated without extensive follow-up observations. Some can also be confirmed through the transit timing variation method.
Red giant branch stars have another issue for detecting planets around them: while planets around these stars are much more likely to transit due to the larger star size, these transit signals are hard to separate from the main star's brightness light curve as red giants have frequent pulsations in brightness with a period of a few hours to days. This is especially notable with subgiants. In addition, these stars are much more luminous, and transiting planets block a much smaller percentage of light coming from these stars. In contrast, planets can completely occult a very small star such as a neutron star or white dwarf, an event which would be easily detectable from Earth. However, due to the small star sizes, the chance of a planet aligning with such a stellar remnant is extremely small.
The main advantage of the transit method is that the size of the planet can be determined from the lightcurve. When combined with the radial-velocity method (which determines the planet's mass), one can determine the density of the planet, and hence learn something about the planet's physical structure. The planets that have been studied by both methods are by far the best-characterized of all known exoplanets.
The transit method also makes it possible to study the atmosphere of the transiting planet. When the planet transits the star, light from the star passes through the upper atmosphere of the planet. By studying the high-resolution stellar spectrum carefully, one can detect elements present in the planet's atmosphere. A planetary atmosphere, and planet for that matter, could also be detected by measuring the polarisation of the starlight as it passed through or is reflected off the planet's atmosphere.
Additionally, the secondary eclipse (when the planet is blocked by its star) allows direct measurement of the planet's radiation and helps to constrain the planet's orbital eccentricity without needing the presence of other planets. If the star's photometric intensity during the secondary eclipse is subtracted from its intensity before or after, only the signal caused by the planet remains. It is then possible to measure the planet's temperature and even to detect possible signs of cloud formations on it. In March 2005, two groups of scientists carried out measurements using this technique with the Spitzer Space Telescope. The two teams, from the Harvard-Smithsonian Center for Astrophysics, led by David Charbonneau, and the Goddard Space Flight Center, led by L. D. Deming, studied the planets TrES-1 and HD 209458b respectively. The measurements revealed the planets' temperatures: 1,060 K (790°C) for TrES-1 and about 1,130 K (860 °C) for HD 209458b. In addition, the hot Neptune Gliese 436 b is known to enter secondary eclipse. However, some transiting planets orbit such that they do not enter secondary eclipse relative to Earth; HD 17156 b is over 90% likely to be one of the latter.
A French Space Agency mission, CoRoT, began in 2006 to search for planetary transits from orbit, where the absence of atmospheric scintillation allows improved accuracy. This mission was designed to be able to detect planets "a few times to several times larger than Earth" and performed "better than expected", with two exoplanet discoveries (both of the "hot Jupiter" type) as of early 2008. In June 2013, CoRoT's exoplanet count was 32 with several still to be confirmed. The satellite unexpectedly stopped transmitting data in November 2012 (after its mission had twice been extended), and was retired in June 2013.
In March 2009, NASA mission Kepler was launched to scan a large number of stars in the constellation Cygnus with a measurement precision expected to detect and characterize Earth-sized planets. The NASA Kepler Mission uses the transit method to scan a hundred thousand stars for planets. It was hoped that by the end of its mission of 3.5 years, the satellite would have collected enough data to reveal planets even smaller than Earth. By scanning a hundred thousand stars simultaneously, it was not only able to detect Earth-sized planets, it was able to collect statistics on the numbers of such planets around Sun-like stars.
On 2 February 2011, the Kepler team released a list of 1,235 extrasolar planet candidates, including 54 that may be in the habitable zone. On 5 December 2011, the Kepler team announced that they had discovered 2,326 planetary candidates, of which 207 are similar in size to Earth, 680 are super-Earth-size, 1,181 are Neptune-size, 203 are Jupiter-size and 55 are larger than Jupiter. Compared to the February 2011 figures, the number of Earth-size and super-Earth-size planets increased by 200% and 140% respectively. Moreover, 48 planet candidates were found in the habitable zones of surveyed stars, marking a decrease from the February figure; this was due to the more stringent criteria in use in the December data. By June 2013, the number of planet candidates was increased to 3,278 and some confirmed planets were smaller than Earth, some even Mars-sized (such as Kepler-62c) and one even smaller than Mercury (Kepler-37b).
The Transiting Exoplanet Survey Satellite launched in April, 2018.
Short-period planets in close orbits around their stars will undergo reflected light variations because, like the Moon, they will go through phases from full to new and back again. In addition, as these planets receive a lot of starlight, it heats them, making thermal emissions potentially detectable. Since telescopes cannot resolve the planet from the star, they see only the combined light, and the brightness of the host star seems to change over each orbit in a periodic manner. Although the effect is small — the photometric precision required is about the same as to detect an Earth-sized planet in transit across a solar-type star – such Jupiter-sized planets with an orbital period of a few days are detectable by space telescopes such as the Kepler Space Observatory. Like with the transit method, it is easier to detect large planets orbiting close to their parent star than other planets as these planets catch more light from their parent star. When a planet has a high albedo and is situated around a relatively luminous star, its light variations are easier to detect in visible light while darker planets or planets around low-temperature stars are more easily detectable with infrared light with this method. In the long run, this method may find the most planets that will be discovered by that mission because the reflected light variation with orbital phase is largely independent of orbital inclination and does not require the planet to pass in front of the disk of the star. It still cannot detect planets with circular face-on orbits from Earth's viewpoint as the amount of reflected light does not change during its orbit.
The phase function of the giant planet is also a function of its thermal properties and atmosphere, if any. Therefore, the phase curve may constrain other planet properties, such as the size distribution of atmospheric particles. When a planet is found transiting and its size is known, the phase variations curve helps calculate or constrain the planet's albedo. It is more difficult with very hot planets as the glow of the planet can interfere when trying to calculate albedo. In theory, albedo can also be found in non-transiting planets when observing the light variations with multiple wavelengths. This allows scientists to find the size of the planet even if the planet is not transiting the star.
The first-ever direct detection of the spectrum of visible light reflected from an exoplanet was made in 2015 by an international team of astronomers. The astronomers studied light from 51 Pegasi b – the first exoplanet discovered orbiting a main-sequence star (a Sunlike star), using the High Accuracy Radial velocity Planet Searcher (HARPS) instrument at the European Southern Observatory's La Silla Observatory in Chile.
Both Corot and Kepler have measured the reflected light from planets. However, these planets were already known since they transit their host star. The first planets discovered by this method are Kepler-70b and Kepler-70c, found by Kepler.
A separate novel method to detect exoplanets from light variations uses relativistic beaming of the observed flux from the star due to its motion. It is also known as Doppler beaming or Doppler boosting. The method was first proposed by Abraham Loeb and Scott Gaudi in 2003 . As the planet tugs the star with its gravitation, the density of photons and therefore the apparent brightness of the star changes from observer's viewpoint. Like the radial velocity method, it can be used to determine the orbital eccentricity and the minimum mass of the planet. With this method, it is easier to detect massive planets close to their stars as these factors increase the star's motion. Unlike the radial velocity method, it does not require an accurate spectrum of a star, and therefore can be used more easily to find planets around fast-rotating stars and more distant stars.
One of the biggest disadvantages of this method is that the light variation effect is very small. A Jovian-mass planet orbiting 0.025 AU away from a Sun-like star is barely detectable even when the orbit is edge-on. This is not an ideal method for discovering new planets, as the amount of emitted and reflected starlight from the planet is usually much larger than light variations due to relativistic beaming. This method is still useful, however, as it allows for measurement of the planet's mass without the need for follow-up data collection from radial velocity observations.
Massive planets can cause slight tidal distortions to their host stars. When a star has a slightly ellipsoidal shape, its apparent brightness varies, depending if the oblate part of the star is facing the observer's viewpoint. Like with the relativistic beaming method, it helps to determine the minimum mass of the planet, and its sensitivity depends on the planet's orbital inclination. The extent of the effect on a star's apparent brightness can be much larger than with the relativistic beaming method, but the brightness changing cycle is twice as fast. In addition, the planet distorts the shape of the star more if it has a low semi-major axis to stellar radius ratio and the density of the star is low. This makes this method suitable for finding planets around stars that have left the main sequence.
A pulsar is a neutron star: the small, ultradense remnant of a star that has exploded as a supernova. Pulsars emit radio waves extremely regularly as they rotate. Because the intrinsic rotation of a pulsar is so regular, slight anomalies in the timing of its observed radio pulses can be used to track the pulsar's motion. Like an ordinary star, a pulsar will move in its own small orbit if it has a planet. Calculations based on pulse-timing observations can then reveal the parameters of that orbit.
This method was not originally designed for the detection of planets, but is so sensitive that it is capable of detecting planets far smaller than any other method can, down to less than a tenth the mass of Earth. It is also capable of detecting mutual gravitational perturbations between the various members of a planetary system, thereby revealing further information about those planets and their orbital parameters. In addition, it can easily detect planets which are relatively far away from the pulsar.
There are two main drawbacks to the pulsar timing method: pulsars are relatively rare, and special circumstances are required for a planet to form around a pulsar. Therefore, it is unlikely that a large number of planets will be found this way. Also, life as we know it could not survive on planets orbiting pulsars due to the intensity of high-energy radiation there.
In 1992, Aleksander Wolszczan and Dale Frail used this method to discover planets around the pulsar PSR 1257+12. Their discovery was quickly confirmed, making it the first confirmation of planets outside our Solar System.
Variable star timing
Like pulsars, some other types of pulsating variable stars are regular enough that radial velocity could be determined purely photometrically from the Doppler shift of the pulsation frequency, without needing spectroscopy. This method is not as sensitive as the pulsar timing variation method, due to the periodic activity being longer and less regular. The ease of detecting planets around a variable star depends on the pulsation period of the star, the regularity of pulsations, the mass of the planet, and its distance from the host star.
The transit timing variation method considers whether transits occur with strict periodicity, or if there is a variation. When multiple transiting planets are detected, they can often be confirmed with the transit timing variation method. This is useful in planetary systems far from the Sun, where radial velocity methods cannot detect them due to the low signal-to-noise ratio. If a planet has been detected by the transit method, then variations in the timing of the transit provide an extremely sensitive method of detecting additional non-transiting planets in the system with masses comparable to Earth's. It is easier to detect transit-timing variations if planets have relatively close orbits, and when at least one of the planets is more massive, causing the orbital period of a less massive planet to be more perturbed.
The main drawback of the transit timing method is that usually not much can be learned about the planet itself. Transit timing variation can help to determine the maximum mass of a planet. In most cases, it can confirm if an object has a planetary mass, but it does not put narrow constraints on its mass. There are exceptions though, as planets in the Kepler-36 and Kepler-88 systems orbit close enough to accurately determine their masses.
The first significant detection of a non-transiting planet using TTV was carried out with NASA's Kepler spacecraft. The transiting planet Kepler-19b shows TTV with an amplitude of five minutes and a period of about 300 days, indicating the presence of a second planet, Kepler-19c, which has a period which is a near-rational multiple of the period of the transiting planet.
In circumbinary planets, variations of transit timing are mainly caused by the orbital motion of the stars, instead of gravitational perturbations by other planets. These variations make it harder to detect these planets through automated methods. However, it makes these planets easy to confirm once they are detected.
Transit duration variation
"Duration variation" refers to changes in how long the transit takes. Duration variations may be caused by an exomoon, apsidal precession for eccentric planets due to another planet in the same system, or general relativity.
When a circumbinary planet is found through the transit method, it can be easily confirmed with the transit duration variation method. In close binary systems, the stars significantly alter the motion of the companion, meaning that any transiting planet has significant variation in transit duration. The first such confirmation came from Kepler-16b.
Eclipsing binary minima timing
When a binary star system is aligned such that – from the Earth's point of view – the stars pass in front of each other in their orbits, the system is called an "eclipsing binary" star system. The time of minimum light, when the star with the brighter surface is at least partially obscured by the disc of the other star, is called the primary eclipse, and approximately half an orbit later, the secondary eclipse occurs when the brighter surface area star obscures some portion of the other star. These times of minimum light, or central eclipses, constitute a time stamp on the system, much like the pulses from a pulsar (except that rather than a flash, they are a dip in brightness). If there is a planet in circumbinary orbit around the binary stars, the stars will be offset around a binary-planet center of mass. As the stars in the binary are displaced back and forth by the planet, the times of the eclipse minima will vary. The periodicity of this offset may be the most reliable way to detect extrasolar planets around close binary systems. With this method, planets are more easily detectable if they are more massive, orbit relatively closely around the system, and if the stars have low masses.
The eclipsing timing method allows the detection of planets further away from the host star than the transit method. However, signals around cataclysmic variable stars hinting for planets tend to match with unstable orbits.[clarification needed] In 2011, Kepler-16b became the first planet to be definitely characterized via eclipsing binary timing variations.
Gravitational microlensing occurs when the gravitational field of a star acts like a lens, magnifying the light of a distant background star. This effect occurs only when the two stars are almost exactly aligned. Lensing events are brief, lasting for weeks or days, as the two stars and Earth are all moving relative to each other. More than a thousand such events have been observed over the past ten years.
If the foreground lensing star has a planet, then that planet's own gravitational field can make a detectable contribution to the lensing effect. Since that requires a highly improbable alignment, a very large number of distant stars must be continuously monitored in order to detect planetary microlensing contributions at a reasonable rate. This method is most fruitful for planets between Earth and the center of the galaxy, as the galactic center provides a large number of background stars.
In 1991, astronomers Shude Mao and Bohdan Paczyński proposed using gravitational microlensing to look for binary companions to stars, and their proposal was refined by Andy Gould and Abraham Loeb in 1992 as a method to detect exoplanets. Successes with the method date back to 2002, when a group of Polish astronomers (Andrzej Udalski, Marcin Kubiak and Michał Szymański from Warsaw, and Bohdan Paczyński) during project OGLE (the Optical Gravitational Lensing Experiment) developed a workable technique. During one month, they found several possible planets, though limitations in the observations prevented clear confirmation. Since then, several confirmed extrasolar planets have been detected using microlensing. This was the first method capable of detecting planets of Earth-like mass around ordinary main-sequence stars.
Unlike most other methods, which have detection bias towards planets with small (or for resolved imaging, large) orbits, the microlensing method is most sensitive to detecting planets around 1-10 astronomical units away from Sun-like stars.
A notable disadvantage of the method is that the lensing cannot be repeated, because the chance alignment never occurs again. Also, the detected planets will tend to be several kiloparsecs away, so follow-up observations with other methods are usually impossible. In addition, the only physical characteristic that can be determined by microlensing is the mass of the planet, within loose constraints. Orbital properties also tend to be unclear, as the only orbital characteristic that can be directly determined is its current semi-major axis from the parent star, which can be misleading if the planet follows an eccentric orbit. When the planet is far away from its star, it spends only a tiny portion of its orbit in a state where it is detectable with this method, so the orbital period of the planet cannot be easily determined. It is also easier to detect planets around low-mass stars, as the gravitational microlensing effect increases with the planet-to-star mass ratio.
The main advantages of the gravitational microlensing method are that it can detect low-mass planets (in principle down to Mars mass with future space projects such as WFIRST); it can detect planets in wide orbits comparable to Saturn and Uranus, which have orbital periods too long for the radial velocity or transit methods; and it can detect planets around very distant stars. When enough background stars can be observed with enough accuracy, then the method should eventually reveal how common Earth-like planets are in the galaxy.
Observations are usually performed using networks of robotic telescopes. In addition to the European Research Council-funded OGLE, the Microlensing Observations in Astrophysics (MOA) group is working to perfect this approach.
The PLANET (Probing Lensing Anomalies NETwork)/RoboNet project is even more ambitious. It allows nearly continuous round-the-clock coverage by a world-spanning telescope network, providing the opportunity to pick up microlensing contributions from planets with masses as low as Earth's. This strategy was successful in detecting the first low-mass planet on a wide orbit, designated OGLE-2005-BLG-390Lb.
Planets are extremely faint light sources compared to stars, and what little light comes from them tends to be lost in the glare from their parent star. So in general, it is very difficult to detect and resolve them directly from their host star. Planets orbiting far enough from stars to be resolved reflect very little starlight, so planets are detected through their thermal emission instead. It is easier to obtain images when the star system is relatively near to the Sun, and when the planet is especially large (considerably larger than Jupiter), widely separated from its parent star, and hot so that it emits intense infrared radiation; images have then been made in the infrared, where the planet is brighter than it is at visible wavelengths. Coronagraphs are used to block light from the star, while leaving the planet visible. Direct imaging of an Earth-like exoplanet requires extreme optothermal stability. During the accretion phase of planetary formation, the star-planet contrast may be even better in H alpha than it is in infrared – an H alpha survey is currently underway.
Direct imaging can give only loose constraints of the planet's mass, which is derived from the age of the star and the temperature of the planet. Mass can vary considerably, as planets can form several million years after the star has formed. The cooler the planet is, the less the planet's mass needs to be. In some cases it is possible to give reasonable constraints to the radius of a planet based on planet's temperature, its apparent brightness, and its distance from Earth. The spectra emitted from planets do not have to be separated from the star, which eases determining the chemical composition of planets.
Sometimes observations at multiple wavelengths are needed to rule out the planet being a brown dwarf. Direct imaging can be used to accurately measure the planet's orbit around the star. Unlike the majority of other methods, direct imaging works better with planets with face-on orbits rather than edge-on orbits, as a planet in a face-on orbit is observable during the entirety of the planet's orbit, while planets with edge-on orbits are most easily observable during their period of largest apparent separation from the parent star.
The planets detected through direct imaging currently fall into two categories. First, planets are found around stars more massive than the Sun which are young enough to have protoplanetary disks. The second category consists of possible sub-brown dwarfs found around very dim stars, or brown dwarfs which are at least 100 AU away from their parent stars.
Planetary-mass objects not gravitationally bound to a star are found through direct imaging as well.
In 2004, a group of astronomers used the European Southern Observatory's Very Large Telescope array in Chile to produce an image of 2M1207b, a companion to the brown dwarf 2M1207. In the following year, the planetary status of the companion was confirmed. The planet is estimated to be several times more massive than Jupiter, and to have an orbital radius greater than 40 AU.
In September 2008, an object was imaged at a separation of 330 AU from the star 1RXS J160929.1−210524, but it was not until 2010, that it was confirmed to be a companion planet to the star and not just a chance alignment.
The first multiplanet system, announced on 13 November 2008, was imaged in 2007, using telescopes at both the Keck Observatory and Gemini Observatory. Three planets were directly observed orbiting HR 8799, whose masses are approximately ten, ten, and seven times that of Jupiter. On the same day, 13 November 2008, it was announced that the Hubble Space Telescope directly observed an exoplanet orbiting Fomalhaut, with a mass no more than 3 MJ. Both systems are surrounded by disks not unlike the Kuiper belt.
In 2012, it was announced that a "Super-Jupiter" planet with a mass about 12.8 MJ orbiting Kappa Andromedae was directly imaged using the Subaru Telescope in Hawaii. It orbits its parent star at a distance of about 55 AU, or nearly twice the distance of Neptune from the sun.
Other possible exoplanets to have been directly imaged include GQ Lupi b, AB Pictoris b, and SCR 1845 b. As of March 2006, none have been confirmed as planets; instead, they might themselves be small brown dwarfs.
Some projects to equip telescopes with planet-imaging-capable instruments include the ground-based telescopes Gemini Planet Imager, VLT-SPHERE, Subaru-HiCIAO, Palomar Project 1640, and the space telescope WFIRST-AFTA. The New Worlds Mission proposes a large occulter in space designed to block the light of nearby stars in order to observe their orbiting planets. This could be used with existing, already planned or new, purpose-built, telescopes.
In 2010, a team from NASAs Jet Propulsion Laboratory demonstrated that a vortex coronagraph could enable small scopes to directly image planets. They did this by imaging the previously imaged HR 8799 planets, using just a 1.5 meter-wide portion of the Hale Telescope.
It has also been proposed that space-telescopes that focus light using zone plates instead of mirrors would provide higher-contrast imaging, and be cheaper to launch into space due to being able to fold up the lightweight foil zone plate.
Light given off by a star is un-polarized, i.e. the direction of oscillation of the light wave is random. However, when the light is reflected off the atmosphere of a planet, the light waves interact with the molecules in the atmosphere and become polarized.
By analyzing the polarization in the combined light of the planet and star (about one part in a million), these measurements can in principle be made with very high sensitivity, as polarimetry is not limited by the stability of the Earth's atmosphere. Another main advantage is that polarimetry allows for determination of the composition of the planet's atmosphere. The main disadvantage is that it will not be able to detect planets without atmospheres. Larger planets and planets with higher albedo are easier to detect through polarimetry, as they reflect more light.
Astronomical devices used for polarimetry, called polarimeters, are capable of detecting polarized light and rejecting unpolarized beams. Groups such as ZIMPOL/CHEOPS and PlanetPol are currently using polarimeters to search for extrasolar planets. The first successful detection of an extrasolar planet using this method came in 2008, when HD 189733 b, a planet discovered three years earlier, was detected using polarimetry. However, no new planets have yet been discovered using this method.
This method consists of precisely measuring a star's position in the sky, and observing how that position changes over time. Originally, this was done visually, with hand-written records. By the end of the 19th century, this method used photographic plates, greatly improving the accuracy of the measurements as well as creating a data archive. If a star has a planet, then the gravitational influence of the planet will cause the star itself to move in a tiny circular or elliptical orbit. Effectively, star and planet each orbit around their mutual centre of mass (barycenter), as explained by solutions to the two-body problem. Since the star is much more massive, its orbit will be much smaller. Frequently, the mutual centre of mass will lie within the radius of the larger body. Consequently, it is easier to find planets around low-mass stars, especially brown dwarfs.
Astrometry is the oldest search method for extrasolar planets, and was originally popular because of its success in characterizing astrometric binary star systems. It dates back at least to statements made by William Herschel in the late 18th century. He claimed that an unseen companion was affecting the position of the star he cataloged as 70 Ophiuchi. The first known formal astrometric calculation for an extrasolar planet was made by William Stephen Jacob in 1855 for this star. Similar calculations were repeated by others for another half-century until finally refuted in the early 20th century. For two centuries claims circulated of the discovery of unseen companions in orbit around nearby star systems that all were reportedly found using this method, culminating in the prominent 1996 announcement, of multiple planets orbiting the nearby star Lalande 21185 by George Gatewood. None of these claims survived scrutiny by other astronomers, and the technique fell into disrepute. Unfortunately, changes in stellar position are so small—and atmospheric and systematic distortions so large—that even the best ground-based telescopes cannot produce precise enough measurements. All claims of a planetary companion of less than 0.1 solar mass, as the mass of the planet, made before 1996 using this method are likely spurious. In 2002, the Hubble Space Telescope did succeed in using astrometry to characterize a previously discovered planet around the star Gliese 876.
The space-based observatory Gaia, launched in 2013, is expected to find thousands of planets via astrometry, but prior to the launch of Gaia, no planet detected by astrometry had been confirmed.
One potential advantage of the astrometric method is that it is most sensitive to planets with large orbits. This makes it complementary to other methods that are most sensitive to planets with small orbits. However, very long observation times will be required — years, and possibly decades, as planets far enough from their star to allow detection via astrometry also take a long time to complete an orbit.
Planets orbiting around one of the stars in binary systems are more easily detectable, as they cause perturbations in the orbits of stars themselves. However, with this method, follow-up observations are needed to determine which star the planet orbits around.
In 2009, the discovery of VB 10b by astrometry was announced. This planetary object, orbiting the low mass red dwarf star VB 10, was reported to have a mass seven times that of Jupiter. If confirmed, this would be the first exoplanet discovered by astrometry, of the many that have been claimed through the years. However recent radial velocity independent studies rule out the existence of the claimed planet.
Other possible methods
An optical/infrared interferometer array doesn't collect as much light as a single telescope of equivalent size, but has the resolution of a single telescope the size of the array. For bright stars, this resolving power could be used to image a star's surface during a transit event and see the shadow of the planet transiting. This could provide a direct measurement of the planet's angular radius and, via parallax, its actual radius. This is more accurate than radius estimates based on transit photometry, which are dependent on stellar radius estimates which depend on models of star characteristics. Imaging also provides more accurate determination of the inclination than photometry does.
Magnetospheric radio emissions
Radio emissions from magnetospheres could be detected with future radio telescopes. This could enable determination of the rotation rate of a planet, which is difficult to detect otherwise.
Auroral radio emissions
By looking at the wiggles of an interferogram using a Fourier-Transform-Spectrometer, enhanced sensitivity could be obtained in order to detect faint signals from Earth-like planets.
Detection of extrasolar asteroids and debris disks
Disks of space dust (debris disks) surround many stars. The dust can be detected because it absorbs ordinary starlight and re-emits it as infrared radiation. Even if the dust particles have a total mass well less than that of Earth, they can still have a large enough total surface area that they outshine their parent star in infrared wavelengths.
The Hubble Space Telescope is capable of observing dust disks with its NICMOS (Near Infrared Camera and Multi-Object Spectrometer) instrument. Even better images have now been taken by its sister instrument, the Spitzer Space Telescope, and by the European Space Agency's Herschel Space Observatory, which can see far deeper into infrared wavelengths than the Hubble can. Dust disks have now been found around more than 15% of nearby sunlike stars.
The dust is thought to be generated by collisions among comets and asteroids. Radiation pressure from the star will push the dust particles away into interstellar space over a relatively short timescale. Therefore, the detection of dust indicates continual replenishment by new collisions, and provides strong indirect evidence of the presence of small bodies like comets and asteroids that orbit the parent star. For example, the dust disk around the star tau Ceti indicates that that star has a population of objects analogous to our own Solar System's Kuiper Belt, but at least ten times thicker.
More speculatively, features in dust disks sometimes suggest the presence of full-sized planets. Some disks have a central cavity, meaning that they are really ring-shaped. The central cavity may be caused by a planet "clearing out" the dust inside its orbit. Other disks contain clumps that may be caused by the gravitational influence of a planet. Both these kinds of features are present in the dust disk around epsilon Eridani, hinting at the presence of a planet with an orbital radius of around 40 AU (in addition to the inner planet detected through the radial-velocity method). These kinds of planet-disk interactions can be modeled numerically using collisional grooming techniques.
Contamination of stellar atmospheres
Spectral analysis of white dwarfs' atmospheres often finds contamination of heavier elements like magnesium and calcium. These elements cannot originate from the stars' core, and it is probable that the contamination comes from asteroids that got too close (within the Roche limit) to these stars by gravitational interaction with larger planets and were torn apart by star's tidal forces. Up to 50% of young white dwarfs may be contaminated in this manner.
Additionally, the dust responsible for the atmospheric pollution may be detected by infrared radiation if it exists in sufficient quantity, similar to the detection of debris discs around main sequence stars. Data from the Spitzer Space Telescope suggests that 1-3% of white dwarfs possess detectable circumstellar dust.
In 2015, minor planets were discovered transiting the white dwarf WD 1145+017. This material orbits with a period of around 4.5 hours, and the shapes of the transit light curves suggest that the larger bodies are disintegrating, contributing to the contamination in the white dwarf's atmosphere.
Most confirmed extrasolar planets have been found using space-based telescopes (as of 01/2015). Many of the detection methods can work more effectively with space-based telescopes that avoid atmospheric haze and turbulence. COROT (2007-2012) and Kepler were space missions dedicated to searching for extrasolar planets using transits. COROT discovered about 30 new exoplanets. Kepler (2009-2013) and K2 (2013- ) have discovered over 2000 verified exoplanets. Hubble Space Telescope and MOST have also found or confirmed a few planets. The infrared Spitzer Space Telescope has been used to detect transits of extrasolar planets, as well as occultations of the planets by their host star and phase curves.
The Gaia mission, launched in December 2013, will use astrometry to determine the true masses of 1000 nearby exoplanets. CHEOPS and TESS, to be launched in 2018, and PLATO in 2024 will use the transit method.
Primary and secondary detection
|Transit||Primary eclipse. Planet passes in front of star.||Secondary eclipse. Star passes in front of planet.|
|Radial velocity||Radial velocity of star||Radial velocity of planet. This has been done for Tau Boötis b.|
|Astrometry||Astrometry of star. Position of star moves more for large planets with large orbits.||Astrometry of planet. Color-differential astrometry. Position of planet moves quicker for planets with small orbits. Theoretical method—has been proposed for use for the SPICA spacecraft.|
Verification and falsification methods
- Verification by multiplicity
- Transit color signature
- Doppler tomography
- Dynamical stability testing
- Distinguishing between planets and stellar activity
- Transit offset
- Transmission spectroscopy
- Emission spectroscopy, phase-resolved
- Speckle imaging / Lucky imaging to detect companion stars that the planets could be orbiting instead of the primary star, which would alter planet parameters that are derived from stellar parameters.
- Photoeccentric Effect
- Rossiter–McLaughlin effect
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Climate is changing and evidence suggests that the impact of climate change would influence our everyday lives, including agriculture, built environment, energy management, food security and water resources. Brunei Darussalam located within the heart of Borneo will be affected both in terms of precipitation and temperature. Therefore, it is crucial to comprehend and assess how important climate indicators like temperature and precipitation are expected to vary in the future in order to minimise its impact. This study assesses the application of a statistical downscaling model (SDSM) for downscaling General Circulation Model (GCM) results for maximum and minimum temperatures along with precipitation in Brunei Darussalam. It investigates future climate changes based on numerous scenarios using Hadley Centre Coupled Model, version 3 (HadCM3), Canadian Earth System Model (CanESM2) and third-generation Coupled Global Climate Model (CGCM3) outputs. The SDSM outputs were improved with the implementation of bias correction and also using a monthly sub-model instead of an annual sub-model. The outcomes of this assessment show that monthly sub-model performed better than the annual sub-model. This study indicates a satisfactory applicability for generation of maximum temperatures, minimum temperatures and precipitation for future periods of 2017–2046 and 2047–2076. All considered models and the scenarios were consistent in predicting increasing trend of maximum temperature, increasing trend of minimum temperature and decreasing trend of precipitations. Maximum overall trend of Tmax was also observed for CanESM2 with Representative Concentration Pathways (RCP) 8.5 scenario. The increasing trend is 0.014 °C per year. Accordingly, by 2076, the highest prediction of average maximum temperatures is that it will increase by 1.4 °C. The same model predicts an increasing trend of Tmin of 0.004 °C per year, while the highest trend is seen under CGCM3-A2 scenario which is 0.009 °C per year. The highest change predicted for the Tmin is therefore 0.9 °C by 2076. The precipitation showed a maximum trend of decrease of 12.7 mm year. It is also seen in the output using CanESM2 data that precipitation will be more chaotic with some reaching 4800 mm per year and also producing low rainfall about 1800 mm per year. All GCMs considered are consistent in predicting it is very likely that Brunei is expected to experience more warming as well as less frequent precipitation events but with a possibility of intensified and drastically high rainfalls in the future.
Brunei Darussalam Statistical downscaling model Temperature Precipitation
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We acknowledge that this study comprises of part of the research work of the first author conducted at Universiti Teknologi Brunei (UTB). We would like to thank the Department of Brunei Darussalam Meteorological Services for giving access to long-term temperature and precipitation data to conduct the research. Additionally, we would like to express our gratitude and thanks to the developers of SDSM program for providing this model. We would also like to express our deepest gratitude to Brunei Research Council (BRC), Department of Economic Planning and Development (DEPD) of Brunei for funding this project and UTB for giving permission to conduct this research.
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X-ray crystallography is a technique used for determining the atomic and molecular structure of a crystal, in which the crystalline structure cause a beam of incident X-rays to diffract into many specific directions. By measuring the angles and intensities of these diffracted beams, a crystallographer can produce a three-dimensional picture of the density of electrons within the crystal. From this electron density, the mean positions of the atoms in the crystal can be determined, as well as their chemical bonds, their crystallographic disorder, and various other information.
Since many materials can form crystals—such as salts, metals, minerals, semiconductors, as well as various inorganic, organic, and biological molecules—X-ray crystallography has been fundamental in the development of many scientific fields. In its first decades of use, this method determined the size of atoms, the lengths and types of chemical bonds, and the atomic-scale differences among various materials, especially minerals and alloys. The method also revealed the structure and function of many biological molecules, including vitamins, drugs, proteins and nucleic acids such as DNA. X-ray crystallography is still the chief method for characterizing the atomic structure of new materials and in discerning materials that appear similar by other experiments. X-ray crystal structures can also account for unusual electronic or elastic properties of a material, shed light on chemical interactions and processes, or serve as the basis for designing pharmaceuticals against diseases.
In a single-crystal X-ray diffraction measurement, a crystal is mounted on a goniometer. The goniometer is used to position the crystal at selected orientations. The crystal is illuminated with a finely focused monochromatic beam of X-rays, producing a diffraction pattern of regularly spaced spots known as reflections. The two-dimensional images taken at different orientations are converted into a three-dimensional model of the density of electrons within the crystal using the mathematical method of Fourier transforms, combined with chemical data known for the sample. Poor resolution (fuzziness) or even errors may result if the crystals are too small, or not uniform enough in their internal makeup.
X-ray crystallography is related to several other methods for determining atomic structures. Similar diffraction patterns can be produced by scattering electrons or neutrons, which are likewise interpreted by Fourier transformation. If single crystals of sufficient size cannot be obtained, various other X-ray methods can be applied to obtain less detailed information; such methods include fiber diffraction, powder diffraction and (if the sample is not crystallized) small-angle X-ray scattering (SAXS). If the material under investigation is only available in the form of nanocrystalline powders or suffers from poor crystallinity, the methods of electron crystallography can be applied for determining the atomic structure.
For all above mentioned X-ray diffraction methods, the scattering is elastic; the scattered X-rays have the same wavelength as the incoming X-ray. By contrast, inelastic X-ray scattering methods are useful in studying excitations of the sample such as plasmons, crystal-field and orbital excitations, magnons, and phonons, rather than the distribution of its atoms.
- 1 History
- 2 Contributions to chemistry and material science
- 3 Relationship to other scattering techniques
- 4 Methods
- 4.1 Overview of single-crystal X-ray diffraction
- 4.2 Crystallization
- 4.3 Data collection
- 4.4 Data analysis
- 4.5 Deposition of the structure
- 5 Diffraction theory
- 6 Nobel Prizes involving X-ray crystallography
- 7 Applications of X-ray diffraction
- 8 See also
- 9 References
- 10 Further reading
- 11 External links
Early scientific history of crystals and X-rays
Crystals, though long admired for their regularity and symmetry, were not investigated scientifically until the 17th century. Johannes Kepler hypothesized in his work Strena seu de Nive Sexangula (A New Year's Gift of Hexagonal Snow) (1611) that the hexagonal symmetry of snowflake crystals was due to a regular packing of spherical water particles.
The Danish scientist Nicolas Steno (1669) pioneered experimental investigations of crystal symmetry. Steno showed that the angles between the faces are the same in every exemplar of a particular type of crystal, and René Just Haüy (1784) discovered that every face of a crystal can be described by simple stacking patterns of blocks of the same shape and size. Hence, William Hallowes Miller in 1839 was able to give each face a unique label of three small integers, the Miller indices which remain in use today for identifying crystal faces. Haüy's study led to the correct idea that crystals are a regular three-dimensional array (a Bravais lattice) of atoms and molecules; a single unit cell is repeated indefinitely along three principal directions that are not necessarily perpendicular. In the 19th century, a complete catalog of the possible symmetries of a crystal was worked out by Johan Hessel, Auguste Bravais, Evgraf Fedorov, Arthur Schönflies and (belatedly) William Barlow (1894). From the available data and physical reasoning, Barlow proposed several crystal structures in the 1880s that were validated later by X-ray crystallography; however, the available data were too scarce in the 1880s to accept his models as conclusive.
Wilhelm Röntgen discovered X-rays in 1895, just as the studies of crystal symmetry were being concluded. Physicists were initially uncertain of the nature of X-rays, but soon suspected (correctly) that they were waves of electromagnetic radiation, in other words, another form of light. At that time, the wave model of light—specifically, the Maxwell theory of electromagnetic radiation—was well accepted among scientists, and experiments by Charles Glover Barkla showed that X-rays exhibited phenomena associated with electromagnetic waves, including transverse polarization and spectral lines akin to those observed in the visible wavelengths. Single-slit experiments in the laboratory of Arnold Sommerfeld suggested that X-rays had a wavelength of about 1 angstrom. However, X-rays are composed of photons, and thus are not only waves of electromagnetic radiation but also exhibit particle-like properties. Albert Einstein introduced the photon concept in 1905, but it was not broadly accepted until 1922, when Arthur Compton confirmed it by the scattering of X-rays from electrons. Therefore, these particle-like properties of X-rays, such as their ionization of gases, caused William Henry Bragg to argue in 1907 that X-rays were not electromagnetic radiation. Nevertheless, Bragg's view was not broadly accepted and the observation of X-ray diffraction by Max von Laue in 1912 confirmed for most scientists that X-rays were a form of electromagnetic radiation.
Crystals are regular arrays of atoms, and X-rays can be considered waves of electromagnetic radiation. Atoms scatter X-ray waves, primarily through the atoms' electrons. Just as an ocean wave striking a lighthouse produces secondary circular waves emanating from the lighthouse, so an X-ray striking an electron produces secondary spherical waves emanating from the electron. This phenomenon is known as elastic scattering, and the electron (or lighthouse) is known as the scatterer. A regular array of scatterers produces a regular array of spherical waves. Although these waves cancel one another out in most directions through destructive interference, they add constructively in a few specific directions, determined by Bragg's law:
Here d is the spacing between diffracting planes, is the incident angle, n is any integer, and λ is the wavelength of the beam. These specific directions appear as spots on the diffraction pattern called reflections. Thus, X-ray diffraction results from an electromagnetic wave (the X-ray) impinging on a regular array of scatterers (the repeating arrangement of atoms within the crystal).
X-rays are used to produce the diffraction pattern because their wavelength λ is typically the same order of magnitude (1–100 angstroms) as the spacing d between planes in the crystal. In principle, any wave impinging on a regular array of scatterers produces diffraction, as predicted first by Francesco Maria Grimaldi in 1665. To produce significant diffraction, the spacing between the scatterers and the wavelength of the impinging wave should be similar in size. For illustration, the diffraction of sunlight through a bird's feather was first reported by James Gregory in the later 17th century. The first artificial diffraction gratings for visible light were constructed by David Rittenhouse in 1787, and Joseph von Fraunhofer in 1821. However, visible light has too long a wavelength (typically, 5500 angstroms) to observe diffraction from crystals. Prior to the first X-ray diffraction experiments, the spacings between lattice planes in a crystal were not known with certainty.
The idea that crystals could be used as a diffraction grating for X-rays arose in 1912 in a conversation between Paul Peter Ewald and Max von Laue in the English Garden in Munich. Ewald had proposed a resonator model of crystals for his thesis, but this model could not be validated using visible light, since the wavelength was much larger than the spacing between the resonators. Von Laue realized that electromagnetic radiation of a shorter wavelength was needed to observe such small spacings, and suggested that X-rays might have a wavelength comparable to the unit-cell spacing in crystals. Von Laue worked with two technicians, Walter Friedrich and his assistant Paul Knipping, to shine a beam of X-rays through a copper sulfate crystal and record its diffraction on a photographic plate. After being developed, the plate showed a large number of well-defined spots arranged in a pattern of intersecting circles around the spot produced by the central beam. Von Laue developed a law that connects the scattering angles and the size and orientation of the unit-cell spacings in the crystal, for which he was awarded the Nobel Prize in Physics in 1914.
As described in the mathematical derivation below, the X-ray scattering is determined by the density of electrons within the crystal. Since the energy of an X-ray is much greater than that of a valence electron, the scattering may be modeled as Thomson scattering, the interaction of an electromagnetic ray with a free electron. This model is generally adopted to describe the polarization of the scattered radiation.
Hence the atomic nuclei, which are much heavier than an electron, contribute negligibly to the scattered X-rays.
Development from 1912 to 1920
After Von Laue's pioneering research, the field developed rapidly, most notably by physicists William Lawrence Bragg and his father William Henry Bragg. In 1912–1913, the younger Bragg developed Bragg's law, which connects the observed scattering with reflections from evenly spaced planes within the crystal. The Braggs, father and son, shared the 1915 Nobel Prize in Physics for their work in crystallography. The earliest structures were generally simple and marked by one-dimensional symmetry. However, as computational and experimental methods improved over the next decades, it became feasible to deduce reliable atomic positions for more complicated two- and three-dimensional arrangements of atoms in the unit-cell.
The potential of X-ray crystallography for determining the structure of molecules and minerals—then only known vaguely from chemical and hydrodynamic experiments—was realized immediately. The earliest structures were simple inorganic crystals and minerals, but even these revealed fundamental laws of physics and chemistry. The first atomic-resolution structure to be "solved" (i.e., determined) in 1914 was that of table salt. The distribution of electrons in the table-salt structure showed that crystals are not necessarily composed of covalently bonded molecules, and proved the existence of ionic compounds. The structure of diamond was solved in the same year, proving the tetrahedral arrangement of its chemical bonds and showing that the length of C–C single bond was 1.52 angstroms. Other early structures included copper, calcium fluoride (CaF2, also known as fluorite), calcite (CaCO3) and pyrite (FeS2) in 1914; spinel (MgAl2O4) in 1915; the rutile and anatase forms of titanium dioxide (TiO2) in 1916; pyrochroite Mn(OH)2 and, by extension, brucite Mg(OH)2 in 1919;. Also in 1919 sodium nitrate (NaNO3) and caesium dichloroiodide (CsICl2) were determined by Ralph Walter Graystone Wyckoff, and the wurtzite (hexagonal ZnS) structure became known in 1920.
The structure of graphite was solved in 1916 by the related method of powder diffraction, which was developed by Peter Debye and Paul Scherrer and, independently, by Albert Hull in 1917. The structure of graphite was determined from single-crystal diffraction in 1924 by two groups independently. Hull also used the powder method to determine the structures of various metals, such as iron and magnesium.
Cultural and aesthetic importance
In what has been called his scientific autobiography, The Development of X-ray Analysis, Sir William Lawrence Bragg mentioned that he believed the field of crystallography was particularly welcoming to women because the techno-aesthetics of the molecular structures resembled textiles and household objects. Bragg was known to compare crystal formation to "curtains, wallpapers, mosaics, and roses".
In 1951, the Festival Pattern Group at the Festival of Britain hosted a collaborative group of textile manufacturers and experienced crystallographers to design lace and prints based on the X-ray crystallography of insulin, china clay, and hemoglobin. One of the leading scientists of the project was Dr. Helen Megaw (1907–2002), the Assistant Director of Research at the Cavendish Laboratory in Cambridge at the time. Megaw is credited as one of the central figures who took inspiration from crystal diagrams and saw their potential in design. In 2008, the Wellcome Collection in London curated an exhibition on the Festival Pattern Group called "From Atom to Patterns".
Contributions to chemistry and material science
X-ray crystallography has led to a better understanding of chemical bonds and non-covalent interactions. The initial studies revealed the typical radii of atoms, and confirmed many theoretical models of chemical bonding, such as the tetrahedral bonding of carbon in the diamond structure, the octahedral bonding of metals observed in ammonium hexachloroplatinate (IV), and the resonance observed in the planar carbonate group and in aromatic molecules. Kathleen Lonsdale's 1928 structure of hexamethylbenzene established the hexagonal symmetry of benzene and showed a clear difference in bond length between the aliphatic C–C bonds and aromatic C–C bonds; this finding led to the idea of resonance between chemical bonds, which had profound consequences for the development of chemistry. Her conclusions were anticipated by William Henry Bragg, who published models of naphthalene and anthracene in 1921 based on other molecules, an early form of molecular replacement.
Also in the 1920s, Victor Moritz Goldschmidt and later Linus Pauling developed rules for eliminating chemically unlikely structures and for determining the relative sizes of atoms. These rules led to the structure of brookite (1928) and an understanding of the relative stability of the rutile, brookite and anatase forms of titanium dioxide.
The distance between two bonded atoms is a sensitive measure of the bond strength and its bond order; thus, X-ray crystallographic studies have led to the discovery of even more exotic types of bonding in inorganic chemistry, such as metal-metal double bonds, metal-metal quadruple bonds, and three-center, two-electron bonds. X-ray crystallography—or, strictly speaking, an inelastic Compton scattering experiment—has also provided evidence for the partly covalent character of hydrogen bonds. In the field of organometallic chemistry, the X-ray structure of ferrocene initiated scientific studies of sandwich compounds, while that of Zeise's salt stimulated research into "back bonding" and metal-pi complexes. Finally, X-ray crystallography had a pioneering role in the development of supramolecular chemistry, particularly in clarifying the structures of the crown ethers and the principles of host-guest chemistry.
In material sciences, many complicated inorganic and organometallic systems have been analyzed using single-crystal methods, such as fullerenes, metalloporphyrins, and other complicated compounds. Single-crystal diffraction is also used in the pharmaceutical industry, due to recent problems with polymorphs. The major factors affecting the quality of single-crystal structures are the crystal's size and regularity; recrystallization is a commonly used technique to improve these factors in small-molecule crystals. The Cambridge Structural Database contains over 800,000 structures as of September 2016; over 99% of these structures were determined by X-ray diffraction.
Mineralogy and metallurgy
Since the 1920s, X-ray diffraction has been the principal method for determining the arrangement of atoms in minerals and metals. The application of X-ray crystallography to mineralogy began with the structure of garnet, which was determined in 1924 by Menzer. A systematic X-ray crystallographic study of the silicates was undertaken in the 1920s. This study showed that, as the Si/O ratio is altered, the silicate crystals exhibit significant changes in their atomic arrangements. Machatschki extended these insights to minerals in which aluminium substitutes for the silicon atoms of the silicates. The first application of X-ray crystallography to metallurgy likewise occurred in the mid-1920s. Most notably, Linus Pauling's structure of the alloy Mg2Sn led to his theory of the stability and structure of complex ionic crystals.
On October 17, 2012, the Curiosity rover on the planet Mars at "Rocknest" performed the first X-ray diffraction analysis of Martian soil. The results from the rover's CheMin analyzer revealed the presence of several minerals, including feldspar, pyroxenes and olivine, and suggested that the Martian soil in the sample was similar to the "weathered basaltic soils" of Hawaiian volcanoes.
Early organic and small biological molecules
The first structure of an organic compound, hexamethylenetetramine, was solved in 1923. This was followed by several studies of long-chain fatty acids, which are an important component of biological membranes. In the 1930s, the structures of much larger molecules with two-dimensional complexity began to be solved. A significant advance was the structure of phthalocyanine, a large planar molecule that is closely related to porphyrin molecules important in biology, such as heme, corrin and chlorophyll.
X-ray crystallography of biological molecules took off with Dorothy Crowfoot Hodgkin, who solved the structures of cholesterol (1937), penicillin (1946) and vitamin B12 (1956), for which she was awarded the Nobel Prize in Chemistry in 1964. In 1969, she succeeded in solving the structure of insulin, on which she worked for over thirty years.
Biological macromolecular crystallography
Crystal structures of proteins (which are irregular and hundreds of times larger than cholesterol) began to be solved in the late 1950s, beginning with the structure of sperm whale myoglobin by Sir John Cowdery Kendrew, for which he shared the Nobel Prize in Chemistry with Max Perutz in 1962. Since that success, 132055 X-ray crystal structures of proteins, nucleic acids and other biological molecules have been determined. For comparison, the nearest competing method in terms of structures analyzed is nuclear magnetic resonance (NMR) spectroscopy, which has resolved 11904 chemical structures. Moreover, crystallography can solve structures of arbitrarily large molecules, whereas solution-state NMR is restricted to relatively small ones (less than 70 kDa). X-ray crystallography is now used routinely by scientists to determine how a pharmaceutical drug interacts with its protein target and what changes might improve it. However, intrinsic membrane proteins remain challenging to crystallize because they require detergents or other means to solubilize them in isolation, and such detergents often interfere with crystallization. Such membrane proteins are a large component of the genome, and include many proteins of great physiological importance, such as ion channels and receptors. Helium cryogenics are used to prevent radiation damage in protein crystals.
On the other end of the size scale, even relatively small molecules may pose challenges for the resolving power of X-ray crystallography. The structure assigned in 1991 to the antibiotic isolated from a marine organism, diazonamide A (C40H34Cl2N6O6, molar mass 765.65 g/mol), proved to be incorrect by the classical proof of structure: a synthetic sample was not identical to the natural product. The mistake was attributed to the inability of X-ray crystallography to distinguish between the correct -OH / >NH and the interchanged -NH2 / -O- groups in the incorrect structure. With advances in instrumentation, however, it is now routinely possible to distinguish between such similar groups using modern single-crystal X-ray diffractometers.
Relationship to other scattering techniques
Elastic vs. inelastic scattering
X-ray crystallography is a form of elastic scattering; the outgoing X-rays have the same energy, and thus same wavelength, as the incoming X-rays, only with altered direction. By contrast, inelastic scattering occurs when energy is transferred from the incoming X-ray to the crystal, e.g., by exciting an inner-shell electron to a higher energy level. Such inelastic scattering reduces the energy (or increases the wavelength) of the outgoing beam. Inelastic scattering is useful for probing such excitations of matter, but not in determining the distribution of scatterers within the matter, which is the goal of X-ray crystallography.
X-rays range in wavelength from 10 to 0.01 nanometers; a typical wavelength used for crystallography is 1 Å (0.1 nm), which is on the scale of covalent chemical bonds and the radius of a single atom. Longer-wavelength photons (such as ultraviolet radiation) would not have sufficient resolution to determine the atomic positions. At the other extreme, shorter-wavelength photons such as gamma rays are difficult to produce in large numbers, difficult to focus, and interact too strongly with matter, producing particle-antiparticle pairs. Therefore, X-rays are the "sweetspot" for wavelength when determining atomic-resolution structures from the scattering of electromagnetic radiation.
Other X-ray techniques
Other forms of elastic X-ray scattering include powder diffraction, Small-Angle X-ray Scattering (SAXS) and several types of X-ray fiber diffraction, which was used by Rosalind Franklin in determining the double-helix structure of DNA. In general, single-crystal X-ray diffraction offers more structural information than these other techniques; however, it requires a sufficiently large and regular crystal, which is not always available.
These scattering methods generally use monochromatic X-rays, which are restricted to a single wavelength with minor deviations. A broad spectrum of X-rays (that is, a blend of X-rays with different wavelengths) can also be used to carry out X-ray diffraction, a technique known as the Laue method. This is the method used in the original discovery of X-ray diffraction. Laue scattering provides much structural information with only a short exposure to the X-ray beam, and is therefore used in structural studies of very rapid events (Time resolved crystallography). However, it is not as well-suited as monochromatic scattering for determining the full atomic structure of a crystal and therefore works better with crystals with relatively simple atomic arrangements.
The Laue back reflection mode records X-rays scattered backwards from a broad spectrum source. This is useful if the sample is too thick for X-rays to transmit through it. The diffracting planes in the crystal are determined by knowing that the normal to the diffracting plane bisects the angle between the incident beam and the diffracted beam. A Greninger chart can be used to interpret the back reflection Laue photograph.
Electron and neutron diffraction
Other particles, such as electrons and neutrons, may be used to produce a diffraction pattern. Although electron, neutron, and X-ray scattering are based on different physical processes, the resulting diffraction patterns are analyzed using the same coherent diffraction imaging techniques.
As derived below, the electron density within the crystal and the diffraction patterns are related by a simple mathematical method, the Fourier transform, which allows the density to be calculated relatively easily from the patterns. However, this works only if the scattering is weak, i.e., if the scattered beams are much less intense than the incoming beam. Weakly scattered beams pass through the remainder of the crystal without undergoing a second scattering event. Such re-scattered waves are called "secondary scattering" and hinder the analysis. Any sufficiently thick crystal will produce secondary scattering, but since X-rays interact relatively weakly with the electrons, this is generally not a significant concern. By contrast, electron beams may produce strong secondary scattering even for relatively thin crystals (>100 nm). Since this thickness corresponds to the diameter of many viruses, a promising direction is the electron diffraction of isolated macromolecular assemblies, such as viral capsids and molecular machines, which may be carried out with a cryo-electron microscope. Moreover, the strong interaction of electrons with matter (about 1000 times stronger than for X-rays) allows determination of the atomic structure of extremely small volumes. The field of applications for electron crystallography ranges from bio molecules like membrane proteins over organic thin films to the complex structures of (nanocrystalline) intermetallic compounds and zeolites.
Neutron diffraction is an excellent method for structure determination, although it has been difficult to obtain intense, monochromatic beams of neutrons in sufficient quantities. Traditionally, nuclear reactors have been used, although sources producing neutrons by spallation are becoming increasingly available. Being uncharged, neutrons scatter much more readily from the atomic nuclei rather than from the electrons. Therefore, neutron scattering is very useful for observing the positions of light atoms with few electrons, especially hydrogen, which is essentially invisible in the X-ray diffraction. Neutron scattering also has the remarkable property that the solvent can be made invisible by adjusting the ratio of normal water, H2O, and heavy water, D2O.
Overview of single-crystal X-ray diffraction
The oldest and most precise method of X-ray crystallography is single-crystal X-ray diffraction, in which a beam of X-rays strikes a single crystal, producing scattered beams. When they land on a piece of film or other detector, these beams make a diffraction pattern of spots; the strengths and angles of these beams are recorded as the crystal is gradually rotated. Each spot is called a reflection, since it corresponds to the reflection of the X-rays from one set of evenly spaced planes within the crystal. For single crystals of sufficient purity and regularity, X-ray diffraction data can determine the mean chemical bond lengths and angles to within a few thousandths of an angstrom and to within a few tenths of a degree, respectively. The atoms in a crystal are not static, but oscillate about their mean positions, usually by less than a few tenths of an angstrom. X-ray crystallography allows measuring the size of these oscillations.
The technique of single-crystal X-ray crystallography has three basic steps. The first—and often most difficult—step is to obtain an adequate crystal of the material under study. The crystal should be sufficiently large (typically larger than 0.1 mm in all dimensions), pure in composition and regular in structure, with no significant internal imperfections such as cracks or twinning.
In the second step, the crystal is placed in an intense beam of X-rays, usually of a single wavelength (monochromatic X-rays), producing the regular pattern of reflections. The angles and intensities of diffracted X-rays are measured, with each compound having a unique diffraction pattern. As the crystal is gradually rotated, previous reflections disappear and new ones appear; the intensity of every spot is recorded at every orientation of the crystal. Multiple data sets may have to be collected, with each set covering slightly more than half a full rotation of the crystal and typically containing tens of thousands of reflections.
In the third step, these data are combined computationally with complementary chemical information to produce and refine a model of the arrangement of atoms within the crystal. The final, refined model of the atomic arrangement—now called a crystal structure—is usually stored in a public database.
As the crystal's repeating unit, its unit cell, becomes larger and more complex, the atomic-level picture provided by X-ray crystallography becomes less well-resolved (more "fuzzy") for a given number of observed reflections. Two limiting cases of X-ray crystallography—"small-molecule" (which includes continuous inorganic solids) and "macromolecular" crystallography—are often discerned. Small-molecule crystallography typically involves crystals with fewer than 100 atoms in their asymmetric unit; such crystal structures are usually so well resolved that the atoms can be discerned as isolated "blobs" of electron density. By contrast, macromolecular crystallography often involves tens of thousands of atoms in the unit cell. Such crystal structures are generally less well-resolved (more "smeared out"); the atoms and chemical bonds appear as tubes of electron density, rather than as isolated atoms. In general, small molecules are also easier to crystallize than macromolecules; however, X-ray crystallography has proven possible even for viruses and proteins with hundreds of thousands of atoms, through improved crystallographic imaging and technology. Though normally X-ray crystallography can only be performed if the sample is in crystal form, new research has been done into sampling non-crystalline forms of samples.
Although crystallography can be used to characterize the disorder in an impure or irregular crystal, crystallography generally requires a pure crystal of high regularity to solve the structure of a complicated arrangement of atoms. Pure, regular crystals can sometimes be obtained from natural or synthetic materials, such as samples of metals, minerals or other macroscopic materials. The regularity of such crystals can sometimes be improved with macromolecular crystal annealing and other methods. However, in many cases, obtaining a diffraction-quality crystal is the chief barrier to solving its atomic-resolution structure.
Small-molecule and macromolecular crystallography differ in the range of possible techniques used to produce diffraction-quality crystals. Small molecules generally have few degrees of conformational freedom, and may be crystallized by a wide range of methods, such as chemical vapor deposition and recrystallization. By contrast, macromolecules generally have many degrees of freedom and their crystallization must be carried out so as to maintain a stable structure. For example, proteins and larger RNA molecules cannot be crystallized if their tertiary structure has been unfolded; therefore, the range of crystallization conditions is restricted to solution conditions in which such molecules remain folded.
Protein crystals are almost always grown in solution. The most common approach is to lower the solubility of its component molecules very gradually; if this is done too quickly, the molecules will precipitate from solution, forming a useless dust or amorphous gel on the bottom of the container. Crystal growth in solution is characterized by two steps: nucleation of a microscopic crystallite (possibly having only 100 molecules), followed by growth of that crystallite, ideally to a diffraction-quality crystal. The solution conditions that favor the first step (nucleation) are not always the same conditions that favor the second step (subsequent growth). The crystallographer's goal is to identify solution conditions that favor the development of a single, large crystal, since larger crystals offer improved resolution of the molecule. Consequently, the solution conditions should disfavor the first step (nucleation) but favor the second (growth), so that only one large crystal forms per droplet. If nucleation is favored too much, a shower of small crystallites will form in the droplet, rather than one large crystal; if favored too little, no crystal will form whatsoever. Other approaches involves, crystallizing proteins under oil, where aqueous protein solutions are dispensed under liquid oil, and water evaporates through the layer of oil. Different oils have different evaporation permeabilities, therefore yielding changes in concentration rates from different percipient/protein mixture. The technique relies on bringing the protein directly into the nucleation zone by mixing protein with the appropriate amount of percipient[clarification needed] to prevent the diffusion of water out of the drop.
It is extremely difficult to predict good conditions for nucleation or growth of well-ordered crystals. In practice, favorable conditions are identified by screening; a very large batch of the molecules is prepared, and a wide variety of crystallization solutions are tested. Hundreds, even thousands, of solution conditions are generally tried before finding the successful one. The various conditions can use one or more physical mechanisms to lower the solubility of the molecule; for example, some may change the pH, some contain salts of the Hofmeister series or chemicals that lower the dielectric constant of the solution, and still others contain large polymers such as polyethylene glycol that drive the molecule out of solution by entropic effects. It is also common to try several temperatures for encouraging crystallization, or to gradually lower the temperature so that the solution becomes supersaturated. These methods require large amounts of the target molecule, as they use high concentration of the molecule(s) to be crystallized. Due to the difficulty in obtaining such large quantities (milligrams) of crystallization-grade protein, robots have been developed that are capable of accurately dispensing crystallization trial drops that are in the order of 100 nanoliters in volume. This means that 10-fold less protein is used per experiment when compared to crystallization trials set up by hand (in the order of 1 microliter).
Several factors are known to inhibit or mar crystallization. The growing crystals are generally held at a constant temperature and protected from shocks or vibrations that might disturb their crystallization. Impurities in the molecules or in the crystallization solutions are often inimical to crystallization. Conformational flexibility in the molecule also tends to make crystallization less likely, due to entropy. Molecules that tend to self-assemble into regular helices are often unwilling to assemble into crystals. Crystals can be marred by twinning, which can occur when a unit cell can pack equally favorably in multiple orientations; although recent advances in computational methods may allow solving the structure of some twinned crystals. Having failed to crystallize a target molecule, a crystallographer may try again with a slightly modified version of the molecule; even small changes in molecular properties can lead to large differences in crystallization behavior.
Mounting the crystal
The crystal is mounted for measurements so that it may be held in the X-ray beam and rotated. There are several methods of mounting. In the past, crystals were loaded into glass capillaries with the crystallization solution (the mother liquor). Nowadays, crystals of small molecules are typically attached with oil or glue to a glass fiber or a loop, which is made of nylon or plastic and attached to a solid rod. Protein crystals are scooped up by a loop, then flash-frozen with liquid nitrogen. This freezing reduces the radiation damage of the X-rays, as well as the noise in the Bragg peaks due to thermal motion (the Debye-Waller effect). However, untreated protein crystals often crack if flash-frozen; therefore, they are generally pre-soaked in a cryoprotectant solution before freezing. Unfortunately, this pre-soak may itself cause the crystal to crack, ruining it for crystallography. Generally, successful cryo-conditions are identified by trial and error.
The capillary or loop is mounted on a goniometer, which allows it to be positioned accurately within the X-ray beam and rotated. Since both the crystal and the beam are often very small, the crystal must be centered within the beam to within ~25 micrometers accuracy, which is aided by a camera focused on the crystal. The most common type of goniometer is the "kappa goniometer", which offers three angles of rotation: the ω angle, which rotates about an axis perpendicular to the beam; the κ angle, about an axis at ~50° to the ω axis; and, finally, the φ angle about the loop/capillary axis. When the κ angle is zero, the ω and φ axes are aligned. The κ rotation allows for convenient mounting of the crystal, since the arm in which the crystal is mounted may be swung out towards the crystallographer. The oscillations carried out during data collection (mentioned below) involve the ω axis only. An older type of goniometer is the four-circle goniometer, and its relatives such as the six-circle goniometer.
Small scale can be done on a local X-ray tube source, typically coupled with an image plate detector. These have the advantage of being (relatively) inexpensive and easy to maintain, and allow for quick screening and collection of samples. However, the wavelength light produced is limited by anode material, typically copper. Further, intensity is limited by the power applied and cooling capacity available to avoid melting the anode. In such systems, electrons are boiled off of a cathode and accelerated through a strong electric potential of ~50 kV; having reached a high speed, the electrons collide with a metal plate, emitting bremsstrahlung and some strong spectral lines corresponding to the excitation of inner-shell electrons of the metal. The most common metal used is copper, which can be kept cool easily, due to its high thermal conductivity, and which produces strong Kα and Kβ lines. The Kβ line is sometimes suppressed with a thin (~10 µm) nickel foil. The simplest and cheapest variety of sealed X-ray tube has a stationary anode (the Crookes tube) and run with ~2 kW of electron beam power. The more expensive variety has a rotating-anode type source that run with ~14 kW of e-beam power.
X-rays are generally filtered (by use of X-ray filters) to a single wavelength (made monochromatic) and collimated to a single direction before they are allowed to strike the crystal. The filtering not only simplifies the data analysis, but also removes radiation that degrades the crystal without contributing useful information. Collimation is done either with a collimator (basically, a long tube) or with a clever arrangement of gently curved mirrors. Mirror systems are preferred for small crystals (under 0.3 mm) or with large unit cells (over 150 Å).
Rotating anodes were used by Joanna (Joka) Maria Vandenberg in the first experiments that demonstrated the power of X rays for quick (in real time production) screening of large InGaAsP thin film wafers for quality control of quantum well lasers.
Synchrotron radiation are some of the brightest lights on earth. It is the single most powerful tool available to X-ray crystallographers. It is made of X-ray beams generated in large machines called synchrotrons. These machines accelerate electrically charged particles, often electrons, to nearly the speed of light and confine them in a (roughly) circular loop using magnetic fields.
Synchrotrons are generally national facilities, each with several dedicated beamlines where data is collected without interruption. Synchrotrons were originally designed for use by high-energy physicists studying subatomic particles and cosmic phenomena. The largest component of each synchrotron is its electron storage ring. This ring is actually not a perfect circle, but a many-sided polygon. At each corner of the polygon, or sector, precisely aligned magnets bend the electron stream. As the electrons’ path is bent, they emit bursts of energy in the form of X-rays.
Using synchrotron radiation frequently has specific requirements for X-ray crystallography. The intense ionizing radiation can cause radiation damage to samples, particularly macromolecular crystals. Cryo crystallography protects the sample from radiation damage, by freezing the crystal at liquid nitrogen temperatures (~100 K). However, synchrotron radiation frequently has the advantage of user selectable wavelengths, allowing for anomalous scattering experiments which maximizes anomalous signal. This is critical in experiments such as SAD and MAD.
Free electron laser
Recently, free-electron lasers have been developed for use in X-ray crystallography. These are the brightest X-ray sources currently available; with the X-rays coming in femtosecond bursts. The intensity of the source is such that atomic resolution diffraction patterns can be resolved for crystals otherwise too small for collection. However, the intense light source also destroys the sample, requiring multiple crystals to be shot. As each crystal is randomly oriented in the beam, hundreds of thousands of individual diffraction images must be collected in order to get a complete data-set. This method, serial femtosecond crystallography, has been used in solving the structure of a number of protein crystal structures, sometimes noting differences with equivalent structures collected from synchrotron sources.
Recording the reflections
When a crystal is mounted and exposed to an intense beam of X-rays, it scatters the X-rays into a pattern of spots or reflections that can be observed on a screen behind the crystal. A similar pattern may be seen by shining a laser pointer at a compact disc. The relative intensities of these spots provide the information to determine the arrangement of molecules within the crystal in atomic detail. The intensities of these reflections may be recorded with photographic film, an area detector or with a charge-coupled device (CCD) image sensor. The peaks at small angles correspond to low-resolution data, whereas those at high angles represent high-resolution data; thus, an upper limit on the eventual resolution of the structure can be determined from the first few images. Some measures of diffraction quality can be determined at this point, such as the mosaicity of the crystal and its overall disorder, as observed in the peak widths. Some pathologies of the crystal that would render it unfit for solving the structure can also be diagnosed quickly at this point.
One image of spots is insufficient to reconstruct the whole crystal; it represents only a small slice of the full Fourier transform. To collect all the necessary information, the crystal must be rotated step-by-step through 180°, with an image recorded at every step; actually, slightly more than 180° is required to cover reciprocal space, due to the curvature of the Ewald sphere. However, if the crystal has a higher symmetry, a smaller angular range such as 90° or 45° may be recorded. The rotation axis should be changed at least once, to avoid developing a "blind spot" in reciprocal space close to the rotation axis. It is customary to rock the crystal slightly (by 0.5–2°) to catch a broader region of reciprocal space.
Multiple data sets may be necessary for certain phasing methods. For example, MAD phasing requires that the scattering be recorded at least three (and usually four, for redundancy) wavelengths of the incoming X-ray radiation. A single crystal may degrade too much during the collection of one data set, owing to radiation damage; in such cases, data sets on multiple crystals must be taken.
Crystal symmetry, unit cell, and image scaling
The recorded series of two-dimensional diffraction patterns, each corresponding to a different crystal orientation, is converted into a three-dimensional model of the electron density; the conversion uses the mathematical technique of Fourier transforms, which is explained below. Each spot corresponds to a different type of variation in the electron density; the crystallographer must determine which variation corresponds to which spot (indexing), the relative strengths of the spots in different images (merging and scaling) and how the variations should be combined to yield the total electron density (phasing).
Data processing begins with indexing the reflections. This means identifying the dimensions of the unit cell and which image peak corresponds to which position in reciprocal space. A byproduct of indexing is to determine the symmetry of the crystal, i.e., its space group. Some space groups can be eliminated from the beginning. For example, reflection symmetries cannot be observed in chiral molecules; thus, only 65 space groups of 230 possible are allowed for protein molecules which are almost always chiral. Indexing is generally accomplished using an autoindexing routine. Having assigned symmetry, the data is then integrated. This converts the hundreds of images containing the thousands of reflections into a single file, consisting of (at the very least) records of the Miller index of each reflection, and an intensity for each reflection (at this state the file often also includes error estimates and measures of partiality (what part of a given reflection was recorded on that image)).
A full data set may consist of hundreds of separate images taken at different orientations of the crystal. The first step is to merge and scale these various images, that is, to identify which peaks appear in two or more images (merging) and to scale the relative images so that they have a consistent intensity scale. Optimizing the intensity scale is critical because the relative intensity of the peaks is the key information from which the structure is determined. The repetitive technique of crystallographic data collection and the often high symmetry of crystalline materials cause the diffractometer to record many symmetry-equivalent reflections multiple times. This allows calculating the symmetry-related R-factor, a reliability index based upon how similar are the measured intensities of symmetry-equivalent reflections,[clarification needed] thus assessing the quality of the data.
The data collected from a diffraction experiment is a reciprocal space representation of the crystal lattice. The position of each diffraction 'spot' is governed by the size and shape of the unit cell, and the inherent symmetry within the crystal. The intensity of each diffraction 'spot' is recorded, and this intensity is proportional to the square of the structure factor amplitude. The structure factor is a complex number containing information relating to both the amplitude and phase of a wave. In order to obtain an interpretable electron density map, both amplitude and phase must be known (an electron density map allows a crystallographer to build a starting model of the molecule). The phase cannot be directly recorded during a diffraction experiment: this is known as the phase problem. Initial phase estimates can be obtained in a variety of ways:
- Ab initio phasing or direct methods – This is usually the method of choice for small molecules (<1000 non-hydrogen atoms), and has been used successfully to solve the phase problems for small proteins. If the resolution of the data is better than 1.4 Å (140 pm), direct methods can be used to obtain phase information, by exploiting known phase relationships between certain groups of reflections.
- Molecular replacement – if a related structure is known, it can be used as a search model in molecular replacement to determine the orientation and position of the molecules within the unit cell. The phases obtained this way can be used to generate electron density maps.
- Anomalous X-ray scattering (MAD or SAD phasing) – the X-ray wavelength may be scanned past an absorption edge[when defined as?] of an atom, which changes the scattering in a known way. By recording full sets of reflections at three different wavelengths (far below, far above and in the middle of the absorption edge) one can solve for the substructure of the anomalously diffracting atoms and hence the structure of the whole molecule. The most popular method of incorporating anomalous scattering atoms into proteins is to express the protein in a methionine auxotroph (a host incapable of synthesizing methionine) in a media rich in seleno-methionine, which contains selenium atoms. A MAD experiment can then be conducted around the absorption edge, which should then yield the position of any methionine residues within the protein, providing initial phases.
- Heavy atom methods (multiple isomorphous replacement) – If electron-dense metal atoms can be introduced into the crystal, direct methods or Patterson-space methods can be used to determine their location and to obtain initial phases. Such heavy atoms can be introduced either by soaking the crystal in a heavy atom-containing solution, or by co-crystallization (growing the crystals in the presence of a heavy atom). As in MAD phasing, the changes in the scattering amplitudes can be interpreted to yield the phases. Although this is the original method by which protein crystal structures were solved, it has largely been superseded by MAD phasing with selenomethionine.
Model building and phase refinement
Having obtained initial phases, an initial model can be built. This model can be used to refine the phases, leading to an improved model, and so on. Given a model of some atomic positions, these positions and their respective Debye-Waller factors (or B-factors, accounting for the thermal motion of the atom) can be refined to fit the observed diffraction data, ideally yielding a better set of phases. A new model can then be fit to the new electron density map and a further round of refinement is carried out. This continues until the correlation between the diffraction data and the model is maximized. The agreement is measured by an R-factor defined as
where F is the structure factor. A similar quality criterion is Rfree, which is calculated from a subset (~10%) of reflections that were not included in the structure refinement. Both R factors depend on the resolution of the data. As a rule of thumb, Rfree should be approximately the resolution in angstroms divided by 10; thus, a data-set with 2 Å resolution should yield a final Rfree ~ 0.2. Chemical bonding features such as stereochemistry, hydrogen bonding and distribution of bond lengths and angles are complementary measures of the model quality. Phase bias is a serious problem in such iterative model building. Omit maps are a common technique used to check for this.[clarification needed]
It may not be possible to observe every atom in the asymmetric unit. In many cases, Crystallographic disorder smears the electron density map. Weakly scattering atoms such as hydrogen are routinely invisible. It is also possible for a single atom to appear multiple times in an electron density map, e.g., if a protein sidechain has multiple (<4) allowed conformations. In still other cases, the crystallographer may detect that the covalent structure deduced for the molecule was incorrect, or changed. For example, proteins may be cleaved or undergo post-translational modifications that were not detected prior to the crystallization.
A common challenge in refinement of crystal structures results from crystallographic disorder. Disorder can take many forms but in general involves the coexistence of two or more species or conformations. Failure to recognize disorder results in flawed interpretation. Pitfalls from improper modeling of disorder are illustrated by the discounted hypothesis of bond stretch isomerism. Disorder is modelled with respect to the relative population of the components, often only two, and their identity. In structures of large molecules and ions, solvent and counterions are often disordered.
Deposition of the structure
Once the model of a molecule's structure has been finalized, it is often deposited in a crystallographic database such as the Cambridge Structural Database (for small molecules), the Inorganic Crystal Structure Database (ICSD) (for inorganic compounds) or the Protein Data Bank (for protein structures). Many structures obtained in private commercial ventures to crystallize medicinally relevant proteins are not deposited in public crystallographic databases.
The main goal of X-ray crystallography is to determine the density of electrons f(r) throughout the crystal, where r represents the three-dimensional position vector within the crystal. To do this, X-ray scattering is used to collect data about its Fourier transform F(q), which is inverted mathematically to obtain the density defined in real space, using the formula
where the integral is taken over all values of q. The three-dimensional real vector q represents a point in reciprocal space, that is, to a particular oscillation in the electron density as one moves in the direction in which q points. The length of q corresponds to 2 divided by the wavelength of the oscillation. The corresponding formula for a Fourier transform will be used below
where the integral is summed over all possible values of the position vector r within the crystal.
The intensities of the reflections observed in X-ray diffraction give us the magnitudes |F(q)| but not the phases φ(q). To obtain the phases, full sets of reflections are collected with known alterations to the scattering, either by modulating the wavelength past a certain absorption edge or by adding strongly scattering (i.e., electron-dense) metal atoms such as mercury. Combining the magnitudes and phases yields the full Fourier transform F(q), which may be inverted to obtain the electron density f(r).
Crystals are often idealized as being perfectly periodic. In that ideal case, the atoms are positioned on a perfect lattice, the electron density is perfectly periodic, and the Fourier transform F(q) is zero except when q belongs to the reciprocal lattice (the so-called Bragg peaks). In reality, however, crystals are not perfectly periodic; atoms vibrate about their mean position, and there may be disorder of various types, such as mosaicity, dislocations, various point defects, and heterogeneity in the conformation of crystallized molecules. Therefore, the Bragg peaks have a finite width and there may be significant diffuse scattering, a continuum of scattered X-rays that fall between the Bragg peaks.
Intuitive understanding by Bragg's law
An intuitive understanding of X-ray diffraction can be obtained from the Bragg model of diffraction. In this model, a given reflection is associated with a set of evenly spaced sheets running through the crystal, usually passing through the centers of the atoms of the crystal lattice. The orientation of a particular set of sheets is identified by its three Miller indices (h, k, l), and let their spacing be noted by d. William Lawrence Bragg proposed a model in which the incoming X-rays are scattered specularly (mirror-like) from each plane; from that assumption, X-rays scattered from adjacent planes will combine constructively (constructive interference) when the angle θ between the plane and the X-ray results in a path-length difference that is an integer multiple n of the X-ray wavelength λ.
A reflection is said to be indexed when its Miller indices (or, more correctly, its reciprocal lattice vector components) have been identified from the known wavelength and the scattering angle 2θ. Such indexing gives the unit-cell parameters, the lengths and angles of the unit-cell, as well as its space group. Since Bragg's law does not interpret the relative intensities of the reflections, however, it is generally inadequate to solve for the arrangement of atoms within the unit-cell; for that, a Fourier transform method must be carried out.
Scattering as a Fourier transform
The incoming X-ray beam has a polarization and should be represented as a vector wave; however, for simplicity, let it be represented here as a scalar wave. We also ignore the complication of the time dependence of the wave and just concentrate on the wave's spatial dependence. Plane waves can be represented by a wave vector kin, and so the strength of the incoming wave at time t=0 is given by
At position r within the sample, let there be a density of scatterers f(r); these scatterers should produce a scattered spherical wave of amplitude proportional to the local amplitude of the incoming wave times the number of scatterers in a small volume dV about r
where S is the proportionality constant.
Let's consider the fraction of scattered waves that leave with an outgoing wave-vector of kout and strike the screen at rscreen. Since no energy is lost (elastic, not inelastic scattering), the wavelengths are the same as are the magnitudes of the wave-vectors |kin|=|kout|. From the time that the photon is scattered at r until it is absorbed at rscreen, the photon undergoes a change in phase
The net radiation arriving at rscreen is the sum of all the scattered waves throughout the crystal
which may be written as a Fourier transform
where q = kout – kin. The measured intensity of the reflection will be square of this amplitude
Friedel and Bijvoet mates
For every reflection corresponding to a point q in the reciprocal space, there is another reflection of the same intensity at the opposite point -q. This opposite reflection is known as the Friedel mate of the original reflection. This symmetry results from the mathematical fact that the density of electrons f(r) at a position r is always a real number. As noted above, f(r) is the inverse transform of its Fourier transform F(q); however, such an inverse transform is a complex number in general. To ensure that f(r) is real, the Fourier transform F(q) must be such that the Friedel mates F(−q) and F(q) are complex conjugates of one another. Thus, F(−q) has the same magnitude as F(q) but they have the opposite phase, i.e., φ(q) = −φ(q)
The equality of their magnitudes ensures that the Friedel mates have the same intensity |F|2. This symmetry allows one to measure the full Fourier transform from only half the reciprocal space, e.g., by rotating the crystal slightly more than 180° instead of a full 360° revolution. In crystals with significant symmetry, even more reflections may have the same intensity (Bijvoet mates); in such cases, even less of the reciprocal space may need to be measured. In favorable cases of high symmetry, sometimes only 90° or even only 45° of data are required to completely explore the reciprocal space.
The Friedel-mate constraint can be derived from the definition of the inverse Fourier transform
Since Euler's formula states that eix = cos(x) + i sin(x), the inverse Fourier transform can be separated into a sum of a purely real part and a purely imaginary part
The function f(r) is real if and only if the second integral Isin is zero for all values of r. In turn, this is true if and only if the above constraint is satisfied
since Isin = −Isin implies that Isin=0.
Each X-ray diffraction image represents only a slice, a spherical slice of reciprocal space, as may be seen by the Ewald sphere construction. Both kout and kin have the same length, due to the elastic scattering, since the wavelength has not changed. Therefore, they may be represented as two radial vectors in a sphere in reciprocal space, which shows the values of q that are sampled in a given diffraction image. Since there is a slight spread in the incoming wavelengths of the incoming X-ray beam, the values of|F(q)|can be measured only for q vectors located between the two spheres corresponding to those radii. Therefore, to obtain a full set of Fourier transform data, it is necessary to rotate the crystal through slightly more than 180°, or sometimes less if sufficient symmetry is present. A full 360° rotation is not needed because of a symmetry intrinsic to the Fourier transforms of real functions (such as the electron density), but "slightly more" than 180° is needed to cover all of reciprocal space within a given resolution because of the curvature of the Ewald sphere. In practice, the crystal is rocked by a small amount (0.25-1°) to incorporate reflections near the boundaries of the spherical Ewald's shells.
A well-known result of Fourier transforms is the autocorrelation theorem, which states that the autocorrelation c(r) of a function f(r)
has a Fourier transform C(q) that is the squared magnitude of F(q)
Therefore, the autocorrelation function c(r) of the electron density (also known as the Patterson function) can be computed directly from the reflection intensities, without computing the phases. In principle, this could be used to determine the crystal structure directly; however, it is difficult to realize in practice. The autocorrelation function corresponds to the distribution of vectors between atoms in the crystal; thus, a crystal of N atoms in its unit cell may have N(N-1) peaks in its Patterson function. Given the inevitable errors in measuring the intensities, and the mathematical difficulties of reconstructing atomic positions from the interatomic vectors, this technique is rarely used to solve structures, except for the simplest crystals.
Advantages of a crystal
In principle, an atomic structure could be determined from applying X-ray scattering to non-crystalline samples, even to a single molecule. However, crystals offer a much stronger signal due to their periodicity. A crystalline sample is by definition periodic; a crystal is composed of many unit cells repeated indefinitely in three independent directions. Such periodic systems have a Fourier transform that is concentrated at periodically repeating points in reciprocal space known as Bragg peaks; the Bragg peaks correspond to the reflection spots observed in the diffraction image. Since the amplitude at these reflections grows linearly with the number N of scatterers, the observed intensity of these spots should grow quadratically, like N2. In other words, using a crystal concentrates the weak scattering of the individual unit cells into a much more powerful, coherent reflection that can be observed above the noise. This is an example of constructive interference.
In a liquid, powder or amorphous sample, molecules within that sample are in random orientations. Such samples have a continuous Fourier spectrum that uniformly spreads its amplitude thereby reducing the measured signal intensity, as is observed in SAXS. More importantly, the orientational information is lost. Although theoretically possible, it is experimentally difficult to obtain atomic-resolution structures of complicated, asymmetric molecules from such rotationally averaged data. An intermediate case is fiber diffraction in which the subunits are arranged periodically in at least one dimension.
Nobel Prizes involving X-ray crystallography
|1914||Max von Laue||Physics||"For his discovery of the diffraction of X-rays by crystals", an important step in the development of X-ray spectroscopy.|
|1915||William Henry Bragg||Physics||"For their services in the analysis of crystal structure by means of X-rays",|
|1915||William Lawrence Bragg||Physics||"For their services in the analysis of crystal structure by means of X-rays",|
|1962||Max F. Perutz||Chemistry||"for their studies of the structures of globular proteins"|
|1962||John C. Kendrew||Chemistry||"for their studies of the structures of globular proteins"|
|1962||James Dewey Watson||Medicine||"For their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material"|
|1962||Francis Harry Compton Crick||Medicine||"For their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material"|
|1962||Maurice Hugh Frederick Wilkins||Medicine||"For their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material"|
|1964||Dorothy Hodgkin||Chemistry||"For her determinations by X-ray techniques of the structures of important biochemical substances"|
|1972||Stanford Moore||Chemistry||"For their contribution to the understanding of the connection between chemical structure and catalytic activity of the active centre of the ribonuclease molecule"|
|1972||William H. Stein||Chemistry||"For their contribution to the understanding of the connection between chemical structure and catalytic activity of the active centre of the ribonuclease molecule"|
|1976||William N. Lipscomb||Chemistry||"For his studies on the structure of boranes illuminating problems of chemical bonding"|
|1985||Jerome Karle||Chemistry||"For their outstanding achievements in developing direct methods for the determination of crystal structures"|
|1985||Herbert A. Hauptman||Chemistry||"For their outstanding achievements in developing direct methods for the determination of crystal structures"|
|1988||Johann Deisenhofer||Chemistry||"For their determination of the three-dimensional structure of a photosynthetic reaction centre"|
|1988||Hartmut Michel||Chemistry||"For their determination of the three-dimensional structure of a photosynthetic reaction centre"|
|1988||Robert Huber||Chemistry||"For their determination of the three-dimensional structure of a photosynthetic reaction centre"|
|1997||John E. Walker||Chemistry||"For their elucidation of the enzymatic mechanism underlying the synthesis of adenosine triphosphate (ATP)"|
|2003||Roderick MacKinnon||Chemistry||"For discoveries concerning channels in cell membranes [...] for structural and mechanistic studies of ion channels"|
|2003||Peter Agre||Chemistry||"For discoveries concerning channels in cell membranes [...] for the discovery of water channels"|
|2006||Roger D. Kornberg||Chemistry||"For his studies of the molecular basis of eukaryotic transcription"|
|2009||Ada E. Yonath||Chemistry||"For studies of the structure and function of the ribosome"|
|2009||Thomas A. Steitz||Chemistry||"For studies of the structure and function of the ribosome"|
|2009||Venkatraman Ramakrishnan||Chemistry||"For studies of the structure and function of the ribosome"|
|2012||Brian Kobilka||Chemistry||"For studies of G-protein-coupled receptors"|
Applications of X-ray diffraction
X-ray diffraction has wide and various applications in the chemical, biochemical, physical, material and mineralogical sciences. Laue claimed in 1937 that the technique "has extended the power of observing minute structure ten thousand times beyond that given us by the microscope". X-ray diffraction is analogous to a microscope with atomic-level resolution which shows the atoms and their electron distribution.
X-ray diffraction, electron diffraction, and neutron diffraction give information about the structure of matter, crystalline and non-crystalline, at the atomic and molecular level. In addition, these methods may be applied in the study of properties of all materials, inorganic, organic or biological. Due to the importance and variety of applications of diffraction studies of crystals, many Nobel Prizes have been awarded for such studies.
X-ray method for investigation of drugs
X-ray diffraction has been used for the identification of antibiotic drugs such as: eight β-lactam (ampicillin sodium, penicillin G procaine, cefalexin, ampicillin trihydrate, benzathine penicillin, benzylpenicillin sodium, cefotaxime sodium, Ceftriaxone sodium), three tetracycline (doxycycline hydrochloride, oxytetracycline dehydrate, tetracycline hydrochloride) and two macrolide (azithromycin, erythromycin estolate) antibiotic drugs. Each of these drugs has a unique XRD pattern that makes their identification possible.
X-ray method for investigation of textile fibers and polymers
Forensic examination of any trace evidence is based upon Locard's exchange principle. This states that "every contact leaves a trace". In practice, even though a transfer of material has taken place, it may be impossible to detect, because the amount transferred is very small.
Textile fibers are a mixture of crystalline and amorphous substances. Therefore, the measurement of the degree of crystalline gives useful data in the characterization of fibers using X-ray diffractometry. It has been reported that X-ray diffraction was used to identify of a "crystalline" deposit which was found on a chair. The deposit was found to be amorphous, but the diffraction pattern present matched that of polymethylmethacrylate. Pyrolysis mass spectrometry later identified the deposit as polymethylcyanoacrylaon of Boin crystal parameters.
X-ray method for investigation of bones
Hiller investigated the effects of heating and burning on bone mineral using X-ray diffraction (XRD) techniques. The bone samples were heated in temperature of 500, 700, and 900 C° for 15 and 45 min. The results show bone crystals began to change during the first 15 min of heating at 500 C0 and above. At higher temperatures, thickness and shape of crystals of bones appear stabilized, but when the samples were heated at lower temperature or for shorter period, XRD traces showed extreme changes in crystal parameters.
- Beevers–Lipson strip
- Bragg diffraction
- Crystallographic database
- Crystallographic point groups
- Difference density map
- Electron diffraction
- Energy Dispersive X-Ray Diffraction
- Henderson limit
- International Year of Crystallography
- John Desmond Bernal
- Multipole density formalism
- Neutron diffraction
- Powder diffraction
- Scherrer equation
- Small angle X-ray scattering (SAXS)
- Structure determination
- Ultrafast x-ray
- Wide angle X-ray scattering (WAXS)
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- "The Nobel Prize in Chemistry 1962". Nobelprize.org. Retrieved 2008-10-06.
- "The Nobel Prize in Physiology or Medicine 1962". Nobel Foundation. Retrieved 2007-07-28.
- "The Nobel Prize in Chemistry 1964". Nobelprize.org. Retrieved 2008-10-06.
- "The Nobel Prize in Chemistry 1972". Nobelprize.org. Retrieved 2008-10-06.
- "The Nobel Prize in Chemistry 1976". Nobelprize.org. Retrieved 2008-10-06.
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- "The Nobel Prize in Chemistry 1988". Nobelprize.org. Retrieved 2008-10-06.
- "The Nobel Prize in Chemistry 1997". Nobelprize.org. Retrieved 2008-10-06.
- "The Nobel Prize in Chemistry 2003". Nobelprize.org. Retrieved 2008-10-06.
- "The Nobel Prize in Chemistry 2006". Nobelprize.org. Retrieved 2008-10-06.
- "The Nobel Prize in Chemistry 2009". Nobelprize.org. Retrieved 2009-10-07.
- "The Nobel Prize in Chemistry 2012". Nobelprize.org. Retrieved 2012-10-13.
- Max von Laue (1937). Laue Diagrams. Bangalore Press. p. 9.
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International Tables for Crystallography
- Theo Hahn, ed. (2002). International Tables for Crystallography. Volume A, Space-group Symmetry (5th ed.). Dordrecht: Kluwer Academic Publishers, for the International Union of Crystallography. ISBN 0-7923-6590-9.
- Michael G. Rossmann; Eddy Arnold, eds. (2001). International Tables for Crystallography. Volume F, Crystallography of biological molecules. Dordrecht: Kluwer Academic Publishers, for the International Union of Crystallography. ISBN 0-7923-6857-6.
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Bound collections of articles
- Charles W. Carter; Robert M. Sweet., eds. (1997). Macromolecular Crystallography, Part A (Methods in Enzymology, v. 276). San Diego: Academic Press. ISBN 0-12-182177-3.
- Charles W. Carter Jr.; Robert M. Sweet., eds. (1997). Macromolecular Crystallography, Part B (Methods in Enzymology, v. 277). San Diego: Academic Press. ISBN 0-12-182178-1.
- A. Ducruix; R. Giegé, eds. (1999). Crystallization of Nucleic Acids and Proteins: A Practical Approach (2nd ed.). Oxford: Oxford University Press. ISBN 0-19-963678-8.
- B.E. Warren (1969). X-ray Diffraction. New York. ISBN 0-486-66317-5.
- Blow D (2002). Outline of Crystallography for Biologists. Oxford: Oxford University Press. ISBN 0-19-851051-9.
- Burns G.; Glazer A M (1990). Space Groups for Scientists and Engineers (2nd ed.). Boston: Academic Press, Inc. ISBN 0-12-145761-3.
- Clegg W (1998). Crystal Structure Determination (Oxford Chemistry Primer). Oxford: Oxford University Press. ISBN 0-19-855901-1.
- Cullity B.D. (1978). Elements of X-Ray Diffraction (2nd ed.). Reading, Massachusetts: Addison-Wesley Publishing Company. ISBN 0-534-55396-6.
- Drenth J (1999). Principles of Protein X-Ray Crystallography. New York: Springer-Verlag. ISBN 0-387-98587-5.
- Giacovazzo C (1992). Fundamentals of Crystallography. Oxford: Oxford University Press. ISBN 0-19-855578-4.
- Glusker JP; Lewis M; Rossi M (1994). Crystal Structure Analysis for Chemists and Biologists. New York: VCH Publishers. ISBN 0-471-18543-4.
- Massa W (2004). Crystal Structure Determination. Berlin: Springer. ISBN 3-540-20644-2.
- McPherson A (1999). Crystallization of Biological Macromolecules. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press. ISBN 0-87969-617-6.
- McPherson A (2003). Introduction to Macromolecular Crystallography. John Wiley & Sons. ISBN 0-471-25122-4.
- McRee DE (1993). Practical Protein Crystallography. San Diego: Academic Press. ISBN 0-12-486050-8.
- O'Keeffe M; Hyde B G (1996). Crystal Structures; I. Patterns and Symmetry. Washington, DC: Mineralogical Society of America, Monograph Series. ISBN 0-939950-40-5.
- Rhodes G (2000). Crystallography Made Crystal Clear. San Diego: Academic Press. ISBN 0-12-587072-8., PDF copy of select chapters
- Rupp B (2009). Biomolecular Crystallography: Principles, Practice and Application to Structural Biology. New York: Garland Science. ISBN 0-8153-4081-8.
- Zachariasen WH (1945). Theory of X-ray Diffraction in Crystals. New York: Dover Publications. LCCN 67026967.
Applied computational data analysis
- Young, R.A., ed. (1993). The Rietveld Method. Oxford: Oxford University Press & International Union of Crystallography. ISBN 0-19-855577-6.
- Bijvoet JM, Burgers WG, Hägg G, eds. (1969). Early Papers on Diffraction of X-rays by Crystals. I. Utrecht: published for the International Union of Crystallography by A. Oosthoek's Uitgeversmaatschappij N.V.
- Bijvoet JM; Burgers WG; Hägg G, eds. (1972). Early Papers on Diffraction of X-rays by Crystals. II. Utrecht: published for the International Union of Crystallography by A. Oosthoek's Uitgeversmaatschappij N.V.
- Bragg W L; Phillips D C & Lipson H (1992). The Development of X-ray Analysis. New York: Dover. ISBN 0-486-67316-2.
- Ewald, PP, and numerous crystallographers, eds. (1962). Fifty Years of X-ray Diffraction. Utrecht: published for the International Union of Crystallography by A. Oosthoek's Uitgeversmaatschappij N.V. doi:10.1007/978-1-4615-9961-6. ISBN 978-1-4615-9963-0.
- Ewald, P. P., editor 50 Years of X-Ray Diffraction (Reprinted in pdf format for the IUCr XVIII Congress, Glasgow, Scotland, International Union of Crystallography).
- Friedrich W (1922). "Die Geschichte der Auffindung der Röntgenstrahlinterferenzen". Die Naturwissenschaften. 10 (16): 363. Bibcode:1922NW.....10..363F. doi:10.1007/BF01565289.
- Lonsdale, K (1949). Crystals and X-rays. New York: D. van Nostrand.
- "The Structures of Life". U.S. Department of Health and Human Services. 2007.
|Library resources about |
|Wikibooks has a book on the topic of: Xray Crystallography|
- Learning Crystallography
- Simple, non technical introduction
- The Crystallography Collection, video series from the Royal Institution
- "Small Molecule Crystalization" (PDF) at Illinois Institute of Technology website
- International Union of Crystallography
- Crystallography 101
- Interactive structure factor tutorial, demonstrating properties of the diffraction pattern of a 2D crystal.
- Picturebook of Fourier Transforms, illustrating the relationship between crystal and diffraction pattern in 2D.
- Lecture notes on X-ray crystallography and structure determination
- Online lecture on Modern X-ray Scattering Methods for Nanoscale Materials Analysis by Richard J. Matyi
- Interactive Crystallography Timeline from the Royal Institution
- Crystallography Open Database (COD)
- Protein Data Bank (PDB)
- Nucleic Acid Databank (NDB)
- Cambridge Structural Database (CSD)
- Inorganic Crystal Structure Database (ICSD)
- Biological Macromolecule Crystallization Database (BMCD)
- Proteopedia – the collaborative, 3D encyclopedia of proteins and other molecules
- HIC-Up database of PDB ligands
- Structural Classification of Proteins database
- CATH Protein Structure Classification
- List of transmembrane proteins with known 3D structure
- Orientations of Proteins in Membranes database
- MolProbity structural validation suite
- NQ-Flipper (check for unfavorable rotamers of Asn and Gln residues)
- DALI server (identifies proteins similar to a given protein) | <urn:uuid:37c30d56-d9ab-485e-a5db-24f3c6beb050> | 4.375 | 24,920 | Knowledge Article | Science & Tech. | 54.729205 | 95,504,616 |
- Generate new knowledge by directly supporting original, high-quality research to identify indoor chemical sources, characterize the chemical and physical transformations taking place indoors, and determine how indoor chemistry is shaped by building attributes and occupancy.
- Develop a modeling consortium to improve the cohesiveness of the community and its ability to integrate findings.
- Build a thriving, multidisciplinary research community of chemists; environmental, civil, and mechanical engineers; architects; atmospheric scientists; microbiologists; and environmental health experts that will endure beyond the program’s timeline.
- Train the next generation of scholars and practitioners. An important component of this program is introducing new voices into the field and training the next generation of researchers.
- Develop community-wide research protocols, and norms.
- Advance capacity for discovery through development of new tools for data collection, sampling, analysis, and visualization.
Nation’s Largest, Most Comprehensive Indoor Chemistry Study Now Underway
(VIDEO) HOMEChem: Meet the Test House
(Video) Meet HOMEChem: The Frontier of Indoor Chemistry
Los Angeles Times
Mad about L.A.'s air quality? Blame common products like hairspray and paint, not just cars
In a surprising study, scientists say everyday chemicals now rival cars as a source of air pollution
Science Magazine Podcast
LISTEN: Understanding the Chemistry of the Indoor Environment
- Environment and Occupancy: How does the built environment and its human and microbial inhabitants affect indoor chemistry? How does indoor chemistry affect the built environment and its inhabitants?
- Sources: What are the primary sources of reactive compounds indoors? What role does outdoor air play in affecting the abundance and distribution of chemicals in indoor air?
- Chemical and Physical Transformations: What is the nature of indoor gas, aerosol, and surface chemistry? What indoor processes drive transitions between gas, aerosol, and surface chemistries?
Chemistry of Indoor Environments
Program Goal To grow a new field of scientific inquiry focused on understanding the fundamental chemistry taking place in indoor environments and how that chemistry is shaped by building attributes and human occupancy. Strategy The program makes grants to achieve a series of interrelated goals. Generate new knowledge by directly supporting original, high-quality research to identify indoor chem… | <urn:uuid:c14b5b43-77a5-444e-a39c-4e4ed3ce9eb9> | 3.203125 | 468 | Product Page | Science & Tech. | 11.652699 | 95,504,619 |
WASHINGTON -- A peculiar cosmic explosion first detected by NASA's Swift observatory on Christmas Day 2010 was caused either by a novel type of supernova located billions of light-years away or an unusual collision much closer to home, within our own galaxy. Papers describing both interpretations appear in the Dec. 1 issue of the journal Nature.
Gamma-ray bursts (GRBs) are the universe's most luminous explosions, emitting more energy in a few seconds than our sun will during its entire energy-producing lifetime. What astronomers are calling the "Christmas burst" is so unusual that it can be modeled in such radically different ways.
"What the Christmas burst seems to be telling us is that the family of gamma-ray bursts is more diverse than we fully appreciate,” said Christina Thoene, the supernova study's lead author, at the Institute of Astrophysics of Andalusia in Granada, Spain. It's only by rapidly detecting hundreds of them, as Swift is doing, that we can catch some of the more eccentric siblings."
Common to both scenarios is the presence of a neutron star, the crushed core that forms when a star many times the sun's mass explodes. When the star's fuel is exhausted, it collapses under its own weight, compressing its core so much that about a half-million times Earth's mass is squeezed into a sphere no larger than a city.
The Christmas burst, also known as GRB 101225A, was discovered in the constellation Andromeda by Swift's Burst Alert Telescope at 1:38 p.m. EST on Dec. 25, 2010. The gamma-ray emission lasted at least 28 minutes, which is unusually long. Follow-up observations of the burst's afterglow by the Hubble Space Telescope and ground-based observatories were unable to determine the object's distance.
Thoene's team proposes that the burst occurred in an exotic binary system where a neutron star orbited a normal star that had just entered its red giant phase, enormously expanding its outer atmosphere. This expansion engulfed the neutron star, resulting in both the ejection of the giant's atmosphere and rapid tightening of the neutron star's orbit.
Once the two stars became wrapped in a common envelope of gas, the neutron star may have merged with the giant's core after just five orbits, or about 18 months. The end result of the merger was the birth of a black hole and the production of oppositely directed jets of particles moving at nearly the speed of light, followed by a weak supernova.
The particle jets produced gamma rays. Jet interactions with gas ejected before the merger explain many of the burst's signature oddities. Based on this interpretation, the event took place about 5.5 billion light-years away, and the team has detected what may be a faint galaxy at the right location.
"Deep exposures using Hubble may settle the nature of this object," said Sergio Campana, who led the collision study at Brera Observatory in Merate, Italy.
If it is indeed a galaxy, that would be evidence for the binary model. On the other hand, if NASA's Chandra X-ray Observatory finds an X-ray point source or if radio telescopes detect a pulsar, that goes against it.
Campana's team supports an alternative model that involves the tidal disruption of a large comet-like object and the ensuing crash of debris onto a neutron star located only about 10,000 light-years away. The scenario requires the break-up of an object with about half the mass of the dwarf planet Ceres. While rare in the asteroid belt, such objects are thought to be common in the icy Kuiper belt beyond Neptune. Similar objects located far away from the neutron star may have survived the supernova that formed it.
Gamma-ray emission occurred when debris fell onto the neutron star. Clumps of cometary material likely made a few orbits, with different clumps following different paths before settling into a disk around the neutron star. X-ray variations detected by Swift's X-Ray Telescope that lasted several hours may have resulted from late-arriving clumps that struck the neutron star as the disk formed.
In the early years of studying GRBs, astronomers had very few events to study in detail and dozens of theories to explain them. In the Swift era, astronomers have settled into two basic scenarios, either the collapse of a massive star or the merger of a compact binary system.
"The beauty of the Christmas burst is that we must invoke two exotic scenarios to explain it, but such rare oddballs will help us advance the field,” said Chryssa Kouveliotou, a co-author of the supernova study at NASA's Marshall Space Flight Center in Huntsville, Ala.
NASA's Swift was launched in November 2004 and is managed by Goddard. It is operated in collaboration with several U.S. institutions and partners in the United Kingdom, Italy, Germany and Japan.
For more information and video associated with this release, visit http://www.nasa.gov/swift.
- end - | <urn:uuid:cd367e3f-b7c8-46bf-a863-ef492570f34d> | 3.703125 | 1,037 | News Article | Science & Tech. | 49.10467 | 95,504,625 |
How Ribosomes Shape the Proteome
Right panel: interaction of positively charged proteins (dark blue) with the ribosome complex (light blue/yellow). Negatively charged proteins do not interact. At high ionic strength (left panel) the positive proteins hardly interact with the ribosome. Credit: Poolman lab, University of Groningen.
Cells are crowded with macromolecules, which limits the diffusion of proteins, especially in prokaryotic cells without active transport in the cytoplasm. While investigating the relationship between crowding, ionic strength and protein diffusion, University of Groningen biochemists made a fascinating discovery: positively charged proteins stick to the surface of ribosome complexes. This explains why most water-soluble proteins carry an overall negative charge. The discovery will appear soon in the journal eLife.
The speed of movement of proteins inside cells is important: many processes in biological cells depend on interactions between macromolecules (proteins and nucleic acids) and thus on their ability to find each other. ‘But the cell cytoplasm is a bustling place and this will affect protein and RNA diffusion’, remarks Professor of Biochemistry Bert Poolman.
His group studied the effects of crowding on diffusion, and found a correlation between protein size and diffusion speed. ‘But for some proteins we did not find this correlation, so we set out to investigate why.’ The team used three different prokaryotes with increasing ionic strength: the Gram-negative bacterium Escherichia coli, the Gram-positive Lactococcus lactis and the extremophile Haloferax volcanii, which lives at very high salt concentrations.
For this study, the researchers constructed different variants of Green Fluorescent Protein (GFP), with surface charges ranging from -30 to +25. They then studied the movement of these GFP variants in the three cell types. ‘We saw that positively charged proteins would diffuse very slowly. They got stuck in the cell’, explains Poolman. Further analysis showed that the positive proteins didn’t bind to the DNA or the cell membrane but to the ribosome complex.
A bioinformatics analysis of the proteomes of microorganisms and eukaryotic cells showed that in most cases roughly 70 percent of the proteins are negatively charged. ‘Interestingly, the remaining 30 percent are either membrane proteins or proteins involved in the functioning or folding of the ribosome or mRNA.’
The membrane proteins are shielded by chaperones during biogenesis, so they won’t stick to the ribosomes. There are therefore no ‘free’ cytoplasmic proteins with a high enough positive charge to make them settle onto ribosomes. The negative charge of the ribosome complex and the ambient ionic strength of the cytoplasm appear to have shaped the evolution of charges in the cellular proteome.
The new and unexpected insight that protein mobility is a function of protein charge may explain why it is hard to express some proteins in bacterial systems with low ionic strength. ‘We observed that a higher ionic strength reduces the stickiness of positively charged proteins. That could be a valuable insight for the construction of protein expression platforms.’
A final observation in the eLife paper is that the genomes of several endosymbionts show an abundance of positively charged proteins. ‘This finding really baffles us’, admits Poolman. ‘You would expect all these proteins to be attracted to the endosymbionts ribosomes. So far, we have no explanation of how these organisms are able to deal with slow diffusion and ribosomes being engulfed with positive proteins.’
This article has been republished from materials provided by the University of Groningen. Note: material may have been edited for length and content. For further information, please contact the cited source.
Paul E Schavemaker Wojciech M Śmigiel Bert Poolman: Ribosome surface properties may impose limits on the nature of the cytoplasmic proteome. eLife, online 'as accepted' 20 november. DOI: 10.7554/eLife.30084.
What Makes Good Brain Proteins Turn Bad?News
The protein FUS is implicated in two neurodegenerative diseases: amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). Using a newly developed fruit fly model, researchers have zoomed in on the protein structure of FUS to gain more insight into how it causes neuronal toxicity and disease. | <urn:uuid:082708b1-7a09-4df2-a24b-2916bb164646> | 3.109375 | 964 | Truncated | Science & Tech. | 30.21649 | 95,504,632 |
A principle that is emphasized throughout this book is that the physics of any multicomponent seismic technology cannot be understood unless that technology is viewed in terms of the particle-displacement vectors associated with the various modes of a seismic wavefield. This material therefore begins with a discussion of seismic vector-wavefield behavior to set the stage for subsequent chapters.
Several approaches can be used to explain why each wave mode of nine-component (9C) and three-component (3C) seismic data that propagates through subsurface geology provides a different amount and type of rock/fluid information about the geology that the wave modes illuminate. Some approaches appeal to people who have limited interest in mathematics. Other options need to be structured for people who have an appreciation of the mathematics of wavefield reflectivity. Another argument that can be used focuses on the fundamental differences in P-wave and S-wave radiation patterns and the distinctions in target illuminations associated with 9C and 3C seismic sources. We will consider all of those paths of logic.
A principle that will be stressed is that each mode of a multicomponent seismic wavefield senses a different earth fabric along its propagation path because its particle-displacement vector is oriented in a different direction than are the particle-displacement vectors of its companion modes. Although estimations of earth fabric obtained from various modes of a multicomponent seismic wavefield can differ, each estimate still can be correct because each wave mode deforms a unit volume of rock in a different direction, depending on the orientation of its | <urn:uuid:d63055eb-95fa-4e81-88b3-e27321c9705a> | 3.859375 | 318 | Academic Writing | Science & Tech. | 12.299935 | 95,504,653 |
Supported tags and respective
Where to file issues:
the Docker Community
Supported Docker versions:
the latest release (down to 1.6 on a best-effort basis)
What is Ruby?
Ruby is a dynamic, reflective, object-oriented, general-purpose, open-source programming language. According to its authors, Ruby was influenced by Perl, Smalltalk, Eiffel, Ada, and Lisp. It supports multiple programming paradigms, including functional, object-oriented, and imperative. It also has a dynamic type system and automatic memory management.
How to use this image
Dockerfile in your Ruby app project
FROM ruby:2.5 # throw errors if Gemfile has been modified since Gemfile.lock RUN bundle config --global frozen 1 WORKDIR /usr/src/app COPY Gemfile Gemfile.lock ./ RUN bundle install COPY . . CMD ["./your-daemon-or-script.rb"]
Put this file in the root of your app, next to the
You can then build and run the Ruby image:
$ docker build -t my-ruby-app . $ docker run -it --name my-running-script my-ruby-app
The above example
Dockerfile expects a
Gemfile.lock in your app directory. This
docker run will help you generate one. Run it in the root of your app, next to the
$ docker run --rm -v "$PWD":/usr/src/app -w /usr/src/app ruby:2.5 bundle install
Run a single Ruby script
For many simple, single file projects, you may find it inconvenient to write a complete
Dockerfile. In such cases, you can run a Ruby script by using the Ruby Docker image directly:
$ docker run -it --rm --name my-running-script -v "$PWD":/usr/src/myapp -w /usr/src/myapp ruby:2.5 ruby your-daemon-or-script.rb
By default, Ruby inherits the locale of the environment in which it is run. For most users running Ruby on their desktop systems, that means it's likely using some variation of
en_US.UTF-8, etc). In Docker however, the default locale is
C, which can have unexpected results. If your application needs to interact with UTF-8, it is recommended that you explicitly adjust the locale of your image/container via
-e LANG=C.UTF-8 or
ENV LANG C.UTF-8.
This image sets several environment variables which change the behavior of Bundler and Gem for running a single application within a container (especially in such a way that the development sources of the application can be bind-mounted inside a container and not have
.bundle from the host interfere with the proper functionality of the container).
The environment variables we set are canonically listed in the above-linked
Dockerfiles, but some of them include
If these cause issues for your use case (running multiple Ruby applications in a single container, for example), setting them to the empty string should be sufficient for undoing their behavior.
ruby images come in many flavors, each designed for a specific use case.
This is the defacto image. If you are unsure about what your needs are, you probably want to use this one. It is designed to be used both as a throw away container (mount your source code and start the container to start your app), as well as the base to build other images off of. This tag is based off of
buildpack-deps is designed for the average user of docker who has many images on their system. It, by design, has a large number of extremely common Debian packages. This reduces the number of packages that images that derive from it need to install, thus reducing the overall size of all images on your system.
This image does not contain the common packages contained in the default tag and only contains the minimal packages needed to run
ruby. Unless you are working in an environment where only the
ruby image will be deployed and you have space constraints, we highly recommend using the default image of this repository.
This image is based on the popular Alpine Linux project, available in the
alpine official image. Alpine Linux is much smaller than most distribution base images (~5MB), and thus leads to much slimmer images in general.
This variant is highly recommended when final image size being as small as possible is desired. The main caveat to note is that it does use musl libc instead of glibc and friends, so certain software might run into issues depending on the depth of their libc requirements. However, most software doesn't have an issue with this, so this variant is usually a very safe choice. See this Hacker News comment thread for more discussion of the issues that might arise and some pro/con comparisons of using Alpine-based images.
To minimize image size, it's uncommon for additional related tools (such as
bash) to be included in Alpine-based images. Using this image as a base, add the things you need in your own Dockerfile (see the
alpine image description for examples of how to install packages if you are unfamiliar).
ONBUILD image variants are deprecated, and their usage is discouraged. For more details, see docker-library/official-images#2076.
onbuild variant is really useful for "getting off the ground running" (zero to Dockerized in a short period of time), it's not recommended for long-term usage within a project due to the lack of control over when the
ONBUILD triggers fire (see also
Once you've got a handle on how your project functions within Docker, you'll probably want to adjust your
Dockerfile to inherit from a non-
onbuild variant and copy the commands from the
Dockerfile (moving the
ONBUILD lines to the end and removing the
ONBUILD keywords) into your own file so that you have tighter control over them and more transparency for yourself and others looking at your
Dockerfile as to what it does. This also makes it easier to add additional requirements as time goes on (such as installing more packages before performing the previously-
View license information for the software contained in this image.
As with all Docker images, these likely also contain other software which may be under other licenses (such as Bash, etc from the base distribution, along with any direct or indirect dependencies of the primary software being contained).
Some additional license information which was able to be auto-detected might be found in the
As for any pre-built image usage, it is the image user's responsibility to ensure that any use of this image complies with any relevant licenses for all software contained within. | <urn:uuid:841464e6-086b-4a2d-9bfb-14f6dfde3617> | 2.671875 | 1,422 | Documentation | Software Dev. | 45.14568 | 95,504,666 |
These are the questions the TEEB – The Economics of Ecosystems and Biodiversity – project is seeking to answer. The pilot study led by Pavan Sukhdev, who is head of Deutsche Bank’s Global Market Centre in London, was commissioned by the German Federal Ministry for the Environment (BMU) and the European Union. The BMU asked the Helmholtz Centre for Environmental Research (UFZ) to co-manage the scientific contributions to the study.
Preliminary results will be presented at a press conference during the 9th UN Conference on Biological Diversity (COP9) in Bonn, Germany on 29 May, which will be followed by an event open to the general public the same evening.
"Biological diversity not only maintains the equilibrium of ecosystems, it is also an inexhaustible source of potential new drugs. It helps sustain a healthy food chain and promotes water and soil quality," says Prof. Jürgen Mlynek, President of the Helmholtz Association. "Its value goes far beyond anything we can describe using economic indices, yet the material benefits it offers humankind are also tremendous."Five thousand United Nations delegates from 190 countries will gather in Bonn from 19 to 30 May 2008. During the conference, they will focus primarily on discussing potential ways to halt the steady decline of biological diversity. "We are currently experiencing the sixth wave of extinctions in the history of our planet; this one has primarily been caused by hu-mans encroaching on the habitats of other species. And we are only now beginning to understand the economic value of biological diversity," explains Mlynek.
Financial expert Pavan Sukhdev estimates that the "value" of the services offered in the nature reserves on the world’s five continents (not counting marine parks and reserves) –adds up to around $5 billion per year. Yet establishing the global value of biodiversity is not the main focus of the study. As is the case with global warming, it is the poor, particularly those in developing and emerging economies, who stand to suffer the most from the loss of so-called ecosystemic services. Preserving biodiversity is thus necessary if we are to fight global poverty and attain the Millennium Development Goals.
The Helmholtz Centre for Environmental Research acted as co-coordinator of the scientific contributions to the study. Researchers at the UFZ are currently preparing to continue collaborating on the report, which will move on to the next phase after COP9. Dr. Heidi Wittmer, a senior researcher at UFZ who helped compile the report, spoke of her hopes for the project: "The Stern Review changed the way we look at the economic consequences of climate change. It is our hope that the TEEB Report will do the same for biodiversity. It is becoming clear that stopping the extinction of species is not merely a romantic notion, but is actually crucial for human survival."
Exhibition: "Millionen Arten zu leben" (Millions of Ways of Life), Plaza of Diversity, near Robert-Schumann-Platz, Stand 30; open Monday to Friday from 1 p.m. to 3 p.m. and from 6 p.m. to 8 p.m. from 19 to 26 May, and daily between 10 a.m. to 8 p.m. from 27 to 30 May.
Tilo Arnhold | alfa
Upcycling of PET Bottles: New Ideas for Resource Cycles in Germany
25.06.2018 | Fraunhofer-Institut für Betriebsfestigkeit und Systemzuverlässigkeit LBF
Dry landscapes can increase disease transmission
20.06.2018 | Forschungsverbund Berlin e.V.
A new manufacturing technique uses a process similar to newspaper printing to form smoother and more flexible metals for making ultrafast electronic devices.
The low-cost process, developed by Purdue University researchers, combines tools already used in industry for manufacturing metals on a large scale, but uses...
For the first time ever, scientists have determined the cosmic origin of highest-energy neutrinos. A research group led by IceCube scientist Elisa Resconi, spokesperson of the Collaborative Research Center SFB1258 at the Technical University of Munich (TUM), provides an important piece of evidence that the particles detected by the IceCube neutrino telescope at the South Pole originate from a galaxy four billion light-years away from Earth.
To rule out other origins with certainty, the team led by neutrino physicist Elisa Resconi from the Technical University of Munich and multi-wavelength...
For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.
Whirlpools are an everyday experience in a bath tub: When the water is drained a circular vortex is formed. Typically, such whirls are rather stable. Similar...
Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.
A frequently used reaction in organic chemistry is nucleophilic substitution. It plays, for example, an important role in in the synthesis of new chemical...
Optical spectroscopy allows investigating the energy structure and dynamic properties of complex quantum systems. Researchers from the University of Würzburg present two new approaches of coherent two-dimensional spectroscopy.
"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy....
13.07.2018 | Event News
12.07.2018 | Event News
03.07.2018 | Event News
23.07.2018 | Science Education
23.07.2018 | Health and Medicine
23.07.2018 | Life Sciences | <urn:uuid:6438534f-4920-4e50-a7f9-4363d54298ea> | 3.046875 | 1,292 | Content Listing | Science & Tech. | 44.296774 | 95,504,675 |
In this age of the pseudo-factual, its more important than ever to acquaint ourselves with the foundations of the scientific tradition, such as Darwin's Origin of Species.
Why was one gene mutation that affects hair, teeth, sweat glands and breasts ubiquitous among ice age Arctic people? New research points to the advantage it provided for ancestors of Native Americans.
New research shows the Bajau Laut people of Southeast Asia have evolved bigger spleens to store more oxygen-rich blood.
A core idea in molecular biology is that one gene codes for one protein. Now biologists have found an example of a gene that yields two forms of a protein – enabling it to evolve new functionality.
New research uses pathology in dinosaur bones to look at predator-prey interactions in the fossil record.
Aliens are highly likely to undergo natural selection, shows new research.
Did we evolve the capacity to produce sublime music simply to get more sex? Or are we driven by higher ideals related to living together in harmony? Maybe it's a bit of both.
Comparing genomes of more than 200,000 people, researchers identified genetic variants that are less common in older people, suggesting natural selection continues to weed out disadvantageous traits.
A group of lizards in Brazil have evolved bigger heads in just 15 years thanks to their new environment.
Many scientists say there's no purpose to life – but a theoretical study suggests there could be.
We don't know much about the origins of most human achievements – scientific and otherwise. Like evolution, does progress occur as random insights are selected for or against?
While there is currently interest interest in artificial intelligence, it offers limited achievements, such as the autonomous car. Tomorrow, machines will learn alone and forge solutions.
In science, the word 'theory' has a very specific meaning that's easy for nonscientists to misunderstand or misconstrue. Here's what a theory must withstand to be accepted by the scientific community.
It's a watery battle of the sexes.
Animals that group together to fight off evolutionary pressure may actually evolve faster.
Scientists of all kinds turn to computer models to investigate questions they can't get at any other way. Here's how models work and why we can trust them.
Robots that can reproduce could improve their design in ways we wouldn't think of but still within our control.
Is a woman's longing for a child evolution at work, or social conditioning? And what about those who don't want kids? Are they defying nature? Probably not, as almost everyone wants sex.
Evolution also does not claim humans evolved from primates. Neither does it say non-human primates, including monkeys, baboons, chimpanzees and gorillas, will evolve into humans with time.
Darwin's insights into life on Earth may have revolutionised biology, but scientists sometimes get carried away. It's not the only game in town. | <urn:uuid:e6c2ef75-89cf-405b-8a99-2beae7561046> | 3.09375 | 594 | Content Listing | Science & Tech. | 44.642106 | 95,504,681 |
|Scientific Name:||Atelopus orcesi Coloma, Duellman, Almendáriz, Ron, Terán-Valdez & Guayasamin, 2010|
|Taxonomic Source(s):||Frost, D.R. 2013. Amphibian Species of the World: an Online Reference. Version 5.6 (9 January 2013). Electronic Database. American Museum of Natural History, New York, USA. Available at: http://research.amnh.org/herpetology/amphibia/index.html.|
|Red List Category & Criteria:||Critically Endangered (Possibly Extinct) B2ab(iii); D ver 3.1|
|Assessor(s):||IUCN SSC Amphibian Specialist Group|
|Facilitator/Compiler(s):||Jarvis, L., NatureServe|
Listed as Critically Endangered (Possibly Extinct) as it is known only from the type locality and has not been recorded since 1988 despite recent surveys. Its area of occupancy (AOO) is estimated at 4 km2, it is only known from a single location, and there is a continuing decline in its montane forest habitat in northern Ecuador due to encroaching agriculture, agricultural pollution, livestock farming and human habitation. If a population still exists, it is suspected to be less than 50 mature individuals.
|Date last seen:||1988|
|Previously published Red List assessments:|
|Range Description:||This species is known only from the type locality, La Alegría-Sibundoy, at 2,400 m asl, in the eastern versant of the Cordillera Oriental in Provincia Sucumbíos, Ecuador (Coloma et al. 2010). Searches in the surroundings of the type locality at suitable elevations have not been able to locate this species (Coloma et al. 2010). The area of occupancy (AOO) is estimated at 4 km2 (using a grid cell of 2 x 2 km) and occurs in one location, based on the main threats affecting the species.|
|Range Map:||Click here to open the map viewer and explore range.|
|Population:||It is known only from the type series, collected in 1988 and comprised of two specimens. The population probably declined dramatically in the past because of climate change and the impact of pathogens (Coloma et al. 2010). One intensive 40 person/hour survey conducted in 2009 at the type locality and nearby areas with suitable elevations yielded no new records (Coloma et al. 2010). Although it is difficult to infer this species' historical relative abundance based on the type series, given patterns observed in other congeners in montane areas and the results from the recent survey it is possible that this species may have experienced a population decline. If a population still exists it is thought to have less than 50 individuals. Due to ongoing decline in the extent and quality of habitat, the population is suspected to be decreasing.|
|Current Population Trend:||Decreasing|
|Habitat and Ecology:||It occurs in montane cloud forest (Coloma et al. 2010). There is no information on this species' ecology, although based on congener ecology it is expected to breed by larval development. There is continuing decline in the quality of this species' habitat due to encroaching agriculture, agricultural pollution, livestock farming and human habitation.|
|Continuing decline in area, extent and/or quality of habitat:||Yes|
|Movement patterns:||Not a Migrant|
|Use and Trade:||
There are no records of this species being utilized.
|Major Threat(s):||The habitat occupied by this species has been extensively deforested and replaced by intensive crop agriculture and livestock. The indiscriminate use of pesticides and urban growth are also other threats (Coloma and Duellman 2012). In addition, chytrid fungus has been found in a neighbouring area and climate change and other pathogens are suspected as possible threat factors (Coloma et al. 2010, Coloma and Duellman 2012).|
It is not known to occur in any protected areas.
In situ conservation is urgently needed to preserve the last species' habitat remnants.
Further surveys of its type locality and surrounding areas in northern Ecuador are urgently required to determine if this species is still extant.
|Citation:||IUCN SSC Amphibian Specialist Group. 2018. Atelopus orcesi. The IUCN Red List of Threatened Species 2018: e.T18435530A56602182.Downloaded on 16 July 2018.|
|Feedback:||If you see any errors or have any questions or suggestions on what is shown on this page, please provide us with feedback so that we can correct or extend the information provided| | <urn:uuid:642e973c-4af3-44d8-95af-f2b0186706e8> | 2.640625 | 1,016 | Knowledge Article | Science & Tech. | 40.196857 | 95,504,682 |
Atomic nuclei are normally compact structures defined by a sharp border. About twenty-five years ago, it was discovered at the University of California in Berkeley that there are exceptions to this picture: Certain exotic atomic nuclei contain particles that shear off from the central core and create a cloud, which surrounds the central core like a 'heiligenschein' or halo.
The \'halo\' nucleus 11Be consists of a core of 10Be and loosely bound neutron. The neutron orbits at a mean distance of 7 fm from the center-of-mass. illustration: Dirk Tiedemann, Institute of Nuclear Chemistry
An example of such a halo occurs in beryllium-11, a specific isotope of the metal beryllium. Here, the halo is made up of a single neutron. For the first time ever, scientists at the Institute of Nuclear Chemistry of the Johannes Gutenberg University Mainz in cooperation with colleagues from other institutes have succeeded in precisely measuring this one-neutron halo by means of a laser, and in evaluating the dimensions of the cloud. By studying neutron halos, scientists hope to gain further understanding of the forces within the atomic nucleus that bind atoms together, taking into account the fact that the degree of displacement of halo neutrons from the atomic nuclear core is incompatible with the concepts of classical nuclear physics.
"We intuitively imagine the atomic nucleus as a compact sphere consisting of positively charged protons and uncharged neutrons," explains Dr Wilfried Nörtershäuser of the Institute of Nuclear Chemistry. "In fact, we have known since the 1980s that atomic nuclei of certain neutron-rich isotopes of the lightest elements - lithium, helium and beryllium - completely contradict this conception." These isotopes consist of a compact nuclear core and a cloud made of diluted nuclear material - called 'heiligenschein' or 'halo'. A halo consists mostly of neutrons that are very weakly bound to the nuclear core, "normally with only one-tenth of the usual binding energy of a neutron inside the core," explains Nörtershäuser.
The discovery of these exotic atomic nuclei created a new area of research, which Nörtershäuser as the head of a young investigators group funded by the German Helmholtz Association has pursued since 2005 at the University in Mainz and at the GSI Helmholtz Center for Heavy Ion Research in Darmstadt. Measuring halo nuclei is extremely difficult, since they can only be artificially created in minute amounts. In addition, these synthesized nuclei decay within seconds, mostly even in milliseconds.
Nörtershäuser's team has now succeeded for the first time in measuring the nuclear charge radius in beryllium-11. This nucleus consists of a dense core with 4 protons and 6 neutrons and a single weakly bound neutron that forms the halo. In order to accomplish this ultra-precise laser spectroscopic measurement, the scientists used a method developed 30 years ago at the University of Mainz, but combined it now for the first time with the most modern techniques for precise laser frequency measurement, i.e., by employing an optical frequency comb. This combination alone was not sufficient, though. Only by further expanding the method using an additional laser system it was possible to achieve the right level of precision. The technique was then applied to beryllium isotopes at the Isotope Separator On Line (ISOLDE) facility for radioactive ion beams at the European Organization for Nuclear Research (CERN) in Geneva. The professional journal Physical Review Letters published this work in its latest February 13 issue.
The measurements revealed that the average distance between the halo neutrons and the dense core of the nucleus is 7 femtometers. Thus, the halo neutron is about three times as far from the dense core as is the outermost proton, since the core itself has a radius of only 2.5 femtometers. "This is an impressive direct demonstration of the halo character of this isotope. It is interesting that the halo neutron is thus much farther from the other nucleons than would be permissible according to the effective range of strong nuclear forces in the classical model," explains Nörtershäuser. The strong interaction that holds atoms together can only extend to a distance of between 2 to 3 femtometers. The riddle as to how the halo neutron can exist at such a great distance from the core nucleus can only be resolved by means of the principles of quantum mechanics: In this model, the neutron must be characterized in terms of a so-called wave function. Because of the low binding energy, the wave function only falls off very slowly with increasing distance from the core. Thus, it is highly likely that the neutron can expand into classically forbidden distances, thereby inducing the expansive 'heiligenschein'.
This work was supported by the Helmholtz Association, the GSI Darmstadt and the Federal Ministry of Education and Research (BMBF).Original publication:
Petra Giegerich | idw
Further reports about: > Atomic > Atomic Nucleus > Atomic nucleus of beryllium > Beryllium > GSI > HALO > Heiligenschein > Helium > Lithium > Nuclear > One-Neutron Halo > Physical > atomic nuclei > beryllium isotopes > classical nuclear physics > ion beam > isotopes > laser system > lasers > precise laser frequency measurement > ultra-precise laser spectroscopic measurement
Nano-kirigami: 'Paper-cut' provides model for 3D intelligent nanofabrication
16.07.2018 | Chinese Academy of Sciences Headquarters
Theorists publish highest-precision prediction of muon magnetic anomaly
16.07.2018 | DOE/Brookhaven National Laboratory
For the first time ever, scientists have determined the cosmic origin of highest-energy neutrinos. A research group led by IceCube scientist Elisa Resconi, spokesperson of the Collaborative Research Center SFB1258 at the Technical University of Munich (TUM), provides an important piece of evidence that the particles detected by the IceCube neutrino telescope at the South Pole originate from a galaxy four billion light-years away from Earth.
To rule out other origins with certainty, the team led by neutrino physicist Elisa Resconi from the Technical University of Munich and multi-wavelength...
For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.
Whirlpools are an everyday experience in a bath tub: When the water is drained a circular vortex is formed. Typically, such whirls are rather stable. Similar...
Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.
A frequently used reaction in organic chemistry is nucleophilic substitution. It plays, for example, an important role in in the synthesis of new chemical...
Optical spectroscopy allows investigating the energy structure and dynamic properties of complex quantum systems. Researchers from the University of Würzburg present two new approaches of coherent two-dimensional spectroscopy.
"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy....
Ultra-short, high-intensity X-ray flashes open the door to the foundations of chemical reactions. Free-electron lasers generate these kinds of pulses, but there is a catch: the pulses vary in duration and energy. An international research team has now presented a solution: Using a ring of 16 detectors and a circularly polarized laser beam, they can determine both factors with attosecond accuracy.
Free-electron lasers (FELs) generate extremely short and intense X-ray flashes. Researchers can use these flashes to resolve structures with diameters on the...
13.07.2018 | Event News
12.07.2018 | Event News
03.07.2018 | Event News
16.07.2018 | Physics and Astronomy
16.07.2018 | Transportation and Logistics
16.07.2018 | Agricultural and Forestry Science | <urn:uuid:291b5783-c627-4d6b-94a4-b1bc9cab0cf9> | 4.15625 | 1,803 | Content Listing | Science & Tech. | 33.539186 | 95,504,692 |
U.S. Army Dugway Proving Ground (DPG) is a major defense test range located in the remote west desert of Utah, USA. DPG is made up of various testing facilities, extensive test grids, and impact areas. DPG’s mission is testing for chemical and biological defense. Recently, a series of large-scale chlorine releases were held at DPG, known as the Jack Rabbit II test program. The purpose of the testing was to better define public safety parameters in the event of a large-scale chlorine release. DPG deployed 100s of point sensors to quantify the test events. Three single-wavelength UV lidar systems were also developed and deployed with the goal of providing a more overall picture of these events. This was an experimental effort using principles similar to Differential Absorption Lidar (DIAL) to estimate chlorine concentration and track clouds downrange. Lidar systems are typically configured with two wavelengths for DIAL measurements. As our effort was experimental and had very limited funds, we used on hand ND:YAG lasers at the 355 nm wavelength only. The second wavelength was later simulated from portions of the data in which no chlorine was present. The main assumption made in using only a single wavelength was that very limited aerosols and other types of chemicals would be mixed with the chlorine cloud. This single-wavelength approach was found to be an effective method for tracking absorbing chemical vapors. We obtained an overall picture of the test event and were able to estimate concentrations in post processing.
James T. Pearson, George W. Lemire, William L. Brown, R. James Berry, and Joshua P. Herron, "Remote sensing of chlorine using UV lidar at U.S. Army Dugway Proving Ground," Proc. SPIE 10629, Chemical, Biological, Radiological, Nuclear, and Explosives (CBRNE) Sensing XIX, 106290S (Presented at SPIE Defense + Security: April 17, 2018; Published: 16 May 2018); https://doi.org/10.1117/12.2300263.
Conference Presentations are recordings of oral presentations given at SPIE conferences and published as part of the conference proceedings. They include the speaker's narration along with a video recording of the presentation slides and animations. Many conference presentations also include full-text papers. Search and browse our growing collection of more than 12,000 conference presentations, including many plenary and keynote presentations. | <urn:uuid:958c68b4-97c7-4803-a676-0ff4f03d1f97> | 3 | 506 | Truncated | Science & Tech. | 45.256866 | 95,504,693 |
A View from Martin LaMonica
Nuclear generator powers Curiosity Mars mission
Solar panels, used in the past Mars missions, were passed up in favor of a space battery for powering the car-size Curiosity robot.
When the Curiosity rover touched down on Mars yesterday, a specially designed nuclear generator kicked into action.
Previous Mars missions have relied on solar panels to power the rovers, but exploration was slowed down by dust build-up on the solar panels or short winters days with little sunlight. The Curiosity Rover, which is as big as a large car, is also significantly larger and ten times heavier than previous Martian rovers.
Enter the Multi-Mission Radioisotope Thermoelectric Generator, or MMRTP, an energy source that relies on the heat generated by decaying plutonium dioxide to run Curiosity. It’s designed to run at least one Martian year, which is almost two Earth years.
The Curiosity is essentially a robotic science lab, equipped with sophisticated instruments for taking ground samples and analyzing their chemical make-up in the search for signs of life. This testing and communications equipment needs a lot of power to operate and needs to maintain a certain temperature to effectively operate on Mars where temperatures can go far below freezing.
The nuclear generator delivers both heat and 110 watts of steady electric power from an array of iridium capsules holding a ceramic form of plutonium dioxide. The heat is piped through the Curiosity carried by liquid Freon. Thermoelectric devices on the generator convert the heat into electricity with no moving parts. Idaho National Laboratory, which designed and tested the energy system, says it can operate for years.
Nuclear power has been used in 26 previous space missions over the past 50 years. The Idaho National Lab team began assembling the power source in the summer of 2008, which included tests for vibrations to simulate rocket launch conditions and making sure the generator’s electric field won’t affect on-board scientific instruments.
Couldn't make it to EmTech Next to meet experts in AI, Robotics and the Economy?Go behind the scenes and check out our video | <urn:uuid:ff0b95e7-72ca-401b-bf25-33df03ed24fc> | 4.0625 | 427 | Truncated | Science & Tech. | 33.393052 | 95,504,694 |
Gravitational waves, as “ripples” of the space-time, were predicted by Einstein more than a century ago. As a new way of observing the universe, the detection of gravitational waves may unveil a lot of mysteries, such as the nature of spacetime and the properties of black holes. In 2003, when young Caltech Ph.D. Chen Yanbei went back to China to give a talk on gravitational wave detection, people thought it was an impossible undertaking — the detectors were too expensive, and the sensitivities required to detect gravitational waves were too high, said critics of the program.
However, popular opinion has changed dramatically in the past 15 years, particularly after the signal of gravitational waves was first detected in 2015 at the U.S.-based Laser Interferometer Gravitational-Wave Observatory (LIGO).
“Recently if you talked to people about gravitational wave detection, they would say LIGO is so cheap, why can’t we do something like that,” said Chen, now a Professor of Physics at Caltech, and a key member in the LIGO group.
In recent years, as gravitational wave detection became a hot topic, the Chinese government put a lot of effort in building detectors and telescopes, hoping to grasp a leadership position in gravitational wave detection. By 2018, China had launched its first Hard X-ray Modulation Telescope, Insight, to find electromagnetic counterparts to gravitational wave sources, and FAST, the world’s largest radio telescope, to look for pulsars, which can be used to detect ultra-low frequency gravitational waves.
Ongoing projects include building a gravitational wave observatory in Ngari Prefecture, a domestic gravitational wave project “Ali” that aims at detecting primordial gravitational waves, and two space exploration projects: “Taiji” and “Tianqin.” Both groups of scientists are aiming at building space-based gravitational wave detectors, to differentiate them from LIGO, which consists of ground-based detectors.
Professor Chen pointed out that China used to lag behind Europeans and Americans in space-based gravitational wave detection, but is now trying to catch up by sending scientists to Europe. He added that the reason Chinese scientists didn’t play a role in the early stage of research on the subject is that the importance of gravitational wave detection was not recognized at the very beginning, so the government didn’t fund much in this direction in the past.
The Chinese economy has grown quickly in the past 15 years. Now China has enough funding to support large-scale science projects, and to enhance international collaborations. In the meantime, more and more people have started to realize the importance of cutting-edge scientific research. As a result, China is catching up with the scientific development in America and Europe, and has become a key player in multiple fields, such as high energy physics and plasma physics. Now the issue is, in the future, how can China become a leader, instead of a follower?
“In the longer term, the government should invest in a field before it becomes important,” said Professor Chen. “But how do they do that? I don’t know.”
True, government support is crucial for scientific development, but it is the scientists, not the government, that discover the importance of a field. The question then boils down to who is taking the initiative in directing scientific research. In the U.S., funding is distributed by the National Science Foundation, where the decision-makers are scientists from research institutes. However, in China, the system of scientific research is strongly influenced by the government. While such a top-down intervention to science development allows China to quickly catch up with leading foreign research groups, it impedes the development of new ideas.
“Groundbreaking science ideas are fueled in a liberal environment that encourages curiosity and different opinions, like trees grow from the soil,” commented a Chinese scientist from Caltech who prefers to remain anonymous. “Now the scientific development in China is like planting potted flowers, with designated sizes and types.” He also pointed out that political factors had been playing a huge role in scientific development for a long time, and such intervention is likely to continue in the future.
Western scholars had been discussing Chinese scientific development for a long time. John K. Fairbank, the renowned scholar of Chinese history, argued that the Chinese political system is hostile to scientific progress. Yasheng Huang, a professor at the MIT Sloan School of Management, pointed out that government intervention leads to bureaucracies, and the dissemination of research findings (research results that impact policy and stability may not be widely published), which weaken the impact of China’s massive investments in science and technology.
Aside from government intervention, another problem facing Chinese scientists is that they need to have the courage to start something completely new.
“If you want to be the person that follows the leader, you just follow the articles in top journals and do some follow-up work. That way you are safe, and you can still get a lot of citations. That’s why it is easy to catch up and to be the second. But to be the first, it takes a lot more courage and confidence,” said Professor Chen.
Do we have scientists who are willing to take the risk to be the first in a field? Or, a better question might be, do we have an environment that welcomes challenges and tolerates failures? Unarguably, in a result-oriented climate of the past ten years, the influence and status of the research of Chinese scientists had increased tremendously. However, to push innovation forward, China needs a more liberal academic environment for scientists to explore their own ideas.
It has taken decades for China to be at the forefront of scientific development, and it is now at an interesting stage, on the precipice of transforming itself from a successful follower to a bold leader. China’s rapid economic growth provides solid financial support for large-scale scientific projects, yet an ideological revolution is even more crucial to the role transformation. Luckily, the Chinese scientific community is becoming more diverse, and internationalized.
“Chinese scientists are now better connected with the international scientific community,” remarked Professor Chen. “It used to be people come to the U.S. and stay here, now more people are willing to return to China.” The returning scientists bring not only new technologies and opportunities for collaboration, but also a spirit of innovation. Their efforts may revolutionize the Chinese science system, and bring China one step closer to a leading position. | <urn:uuid:73a02b5e-0af3-4904-a8a4-3386aaab4499> | 3.78125 | 1,361 | Truncated | Science & Tech. | 34.209392 | 95,504,716 |
Niche (protein structural motif)
In the area of protein structural motifs, niches are three or four amino acid residue features in which main-chain CO groups are bridged by positively charged or δ+ groups. The δ+ groups include groups with two hydrogen bond donor atoms such as NH2 groups and water molecules. In typical proteins, 7% of amino acid residues belong to niches bound to a δ+ group, while another 7% have the conformation but no single cationic bridging group is detected.
Niches are of two kinds, distinguished as niche3 (3 residues, i to i+2) and niche4 (4 residues, i to i+3). In a niche3 motif the δ+-binding carbonyl groups are from residues i and i+2 while in a niche4 motif they are from residues i and i+3.
A niche3 has the α conformation for residue i+1 and the β conformation for residue i+2; a niche4 has the α conformation for residues i+1 and i+2 and the β conformation for residue i+3.
Metal ions that occur bound to niches in proteins are Na+, K+, Ca++ and Mg++. Proteins with regulatory cations often employ niches for metal binding (thrombin, Na+; annexin, Ca++; pyruvate dehydrogenase, K+).
A major cation transporter in cells is calcium ATPase. In the Ca++-bound crystal structures the two calcium ions side-by-side within the transmembrane domain are thought to be at the halfway stage of being transported. As well as being bound by various side chain carbonyl groups, one of these calcium ions is bound by a niche3/niche4 (both in the one motif) at residues 304–307 at the C-terminus of an α-helix.
A lysine side chain in the nuclear export receptor CRM1 is recognised specifically by a niche conformation that has to be adopted as a key part of the nuclear export signal of proteins exiting the nucleus.
A sodium ion in the Fluc fluoride channel is situated at the dyad axis of the dimer, bound tetrahedrally by two niche4s, one from each subunit.
The Hsp70 interdomain linker region of 10 residues enables allosteric communication between two folded domains. The N-terminal part of the linker has a niche4 structure that is water-bound .
Another small tripeptide motif that binds cations or δ+ groups via main-chain CO groups is called the catgrip.
- Torrance, GM; Leader DP (2009). "A Novel Main Chain Motif in Proteins Bridged by Cationic Groups: The Niche". Journal of Molecular Biology. 385 (4): 1076–1086. doi:10.1016/j.jmb.2008.11.007. PMID 19038265.
- Regad, L; Martin J (2011). "Dissecting protein loops with a statistical scalpel suggests a functional implication of some structural motifs". BMC Bioinformatics. 12 (1): 247. doi:10.1186/1471-2105-12-247. PMC . PMID 21689388.
- Cianci, M; Tomaszewski (2010). "Crystallographic Analysis of Counterion Effects on Subtilisin Enzymatic Action in Acetonitrile". Journal of the American Chemical Society. 132 (7): 2293–2300. doi:10.1021/ja908703c. PMID 20099851.
- Toyoshima, C; Mizutani (2004). "Crystal structure of the calcium pump with a bound ATP analogue". Nature. 430 (6999): 529–535. doi:10.1038/nature02680.
- Fung, HYJ; Fu S-C; Chook YM (2017). "Nuclear export receptor CRM1 recognizes diverse conformations in nuclear export signals". eLIFE. 6: e23961. doi:10.7554/eLife.23961.
- Stockbridge, RB; Kolmakova-Partensky L; Shane T (2015). "Crystal structures of a double-barrelled fluoride ion channel". Nature. 525: 548–551.
- English, CA; Sherman W; Meng W (2017). "The Hsp70 interdomain linker is a dynamic switch that enables allosteric communication between two structured domains". J Biol Chem. 292: 14765–14774. doi:10.1074/jbc.M117.789313.
|This protein-related article is a stub. You can help Wikipedia by expanding it.|
- Leader, DP; Milner-White (2009). "Motivated Proteins: A web application for studying small three-dimensional protein motifs". BMC Bioinformatics. 10 (1): 60. doi:10.1186/1471-2105-10-60. PMC . PMID 19210785.
- Golovin, A; Henrick (2008). "MSDmotif: exploring protein sites and motifs". BMC Bioinformatics. 9 (1): 312. doi:10.1186/1471-2105-9-312. PMC . PMID 18637174. | <urn:uuid:7e7ca487-bf49-4d18-a104-bff15eae8aa9> | 2.96875 | 1,155 | Knowledge Article | Science & Tech. | 76.146167 | 95,504,741 |
What is the weight of the wood cuboid 15 cm, 20 cm, 3 m if 1 m3 wood weighs 800 kg?
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Young stars in an old bulge: A natural outcome of internal evolution in the Milky Way
The center of our disk galaxy, the Milky Way, is dominated by a boxy/peanut-shaped bulge. Numerous studies of the bulge based on stellar photometry have concluded that the bulge stars are exclusively old. The perceived lack of young stars in the bulge strongly constrains its likely formation scenarios, providing evidence that the bulge is a unique population that formed early and separately from the disk. However, recent studies of individual bulge stars using the microlensing technique have...[Show more]
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Slime molds have such an unappealing name that it may seem hard to imagine why anyone would study them. But Dictyostelium discoideum is an unusual organism, one that straddles the boundary between the unicellular and the multicellular.Its life cycle includes individual amoeba-like cells, a multicellular migrating “slug,” and a spore-producing structure.
Dictyostelium discoideum is useful as a model because, like other model organisms, it is easy to grow in the laboratory and has a short generation time. In addition, its cells are readily accessible to microscopy and genetic studies. As a result, D. discoideum (affectionately called “Dicty” by its researchers) remains a fascinating organism.
Researchers have made several discoveries in this species:
A “Dicty” cell eats by producing extensions that engulf and absorb food particles by phagocytosis. Scientists have discovered that this movement is possible because proteins such as actin and myosin move rapidly within the cell. These same proteins produce muscle movement in animals.
Researchers observing cell division in D. discoideum have discovered that the protein myosin is also required for cytokinesis (the physical division of one cell into two).
Starving Dicty cells move toward one another and form a multicellular “slug.” This movement toward a chemical stimulus, called chemotaxis, requires membrane proteins that not only detect the signals from other Dicty cells but also transmit the information to the inside of the cell. Similar signal transduction systems occur in many organisms.
When individual Dicty cells come together, chemical signals presumably determine which cells will become stalk cells (and die) and which will become spore cells (and survive). Such research may help answer questions about the origin of multicellularity. | <urn:uuid:8733f5ec-0ae8-4c38-929f-7217fbcd6951> | 3.625 | 401 | Knowledge Article | Science & Tech. | 21.955074 | 95,504,818 |
|Illustration by John Gerrard Keulemans, 1905|
The Auckland rail (Lewinia muelleri) is a small nearly flightless rail endemic to the Auckland Islands 460 km south of New Zealand. It is somewhat of a biogeographical anomaly, being the only species in the genus Lewinia to have reached the islands of New Zealand, skipping over the main islands to reach the remote Auckland group. Its closest relative is Lewin's rail of Australia. The species is currently restricted to two islands in the Auckland group, Adams Island and Disappointment Island.
The Auckland rail is a small rail with chestnut back plumage and a grey breast. The flanks are barred black and white and the head is red-brown, with a red bill. It is smaller than the Australian Lewins' rail. There are conflicting reports about its ability to fly. Early accounts suggested it could; recent researchers have found little evidence for this. If the species is able to fly, it does so very infrequently. Auckland rails have a variety of calls, the most common being a crex call made at one second intervals 10 or more times in a row. The function of the calls is unknown.
Little is known about the reproductive biology of the Auckland rail. The few nests that have been found contained clutches of two eggs, probably laid in early November. The eggs are cream coloured with red, brown and grey spots.
The Auckland rail is highly secretive and was considered to be extinct for many years before its rediscovery. The population is currently stable on the two islands it survives on. It is thought to have become extinct on the main Auckland Islands due to the presence of introduced feral cats and pigs; it is hoped the eventual removal of these from the islands will allow for reintroductions to other islands in the group. The species is currently considered vulnerable by the IUCN and BirdLife International due to the possibility of rats or other predators reaching the two islands it survives on.
- BirdLife International (2016) Species factsheet: Lewinia muelleri. Downloaded from on 8/Jul/2016
- Elliott, G. P.; Walker, K. J.; Buckingham, R. (1991). "The Auckland Islands Rail". Notornis. 38 (3): 199–209. | <urn:uuid:552787d1-a217-47c3-82c0-bd871e36b07d> | 3.03125 | 465 | Knowledge Article | Science & Tech. | 59.728279 | 95,504,832 |
Genetic Circuit Engineering with Gene Editing
The human body is an amazing machine. Think about all of the cellular programming that went into making you, starting with a single cell. Also, notice how when we talk about biology these days we draw the inevitable analogy with computers and machines. Artificial intelligence is one obvious example, as artificial neural networks attempt to replicate the complex computational power of the human brain. But we can find other places where biology and machines intersect. Specifically, we’re talking about computer circuits, as in those that help computers perform specific tasks, and genetic circuits, which have similar “components” as their inorganic counterparts but drive complex biological processes.
And, like computer circuits, biological circuits can be engineered and programmed. Genetic circuit engineering falls into a broad scientific discipline called synthetic biology that we’ve covered before. For example, there are companies that can genetically modify organisms to create milk and meat proteins using techniques similar to brewing beer with yeast. AI tools such as machine learning and other automation technologies, combined with the low costs of gene sequencing and advances in gene editing, have enabled companies like Ginkgo Bioworks to rapidly design, build and test organisms for specific applications such as organic pesticides or artificial fragrances.
How It Works
Genetic circuit engineering goes even further. Scientists can design genetic circuits and insert them into cells, giving them new functions such as producing drugs when they detect disease in the body. Of course, it’s much harder than it sounds—and it sounds really, really hard. Until recently, only a select few could pull it off, requiring years of design and trial-and-error to get these synthetic circuits to perform as intended, despite the fact we’ve been working on these problems since the 1960s.
That was before researchers, particularly some big brains out of MIT and Boston University, figured out a way to automate the process much as chip designers today use sophisticated software to design, test and manufacture hardware. In fact, they use a computing language called Verilog, which is used to design silicon circuits, to create specialized genetic circuits. The user types in commands to design the gene circuit with specific functions. The code is then turned into a DNA sequence using a software program called Cello that sanity checks the design. Below is a little demo that will probably only appeal to the computer scientist geeks among you. You can also get a more in-depth but readable explanation of gene circuit engineering here.
As Christopher Voigt, one of the key minds at MIT behind gene circuit engineering, said: “It’s literally a programming language for bacteria”.
What’s It Good For
Before we do a brief dive into possible applications, let’s get the big caveat out of the way: Gene circuit engineering is cutting edge technology and very few companies (two of which we’ll talk about shortly) are even trying to commercialize it yet. So, at this point, there’s no direct exposure to this technology for the retail investor, which is probably a good thing. As we’ve recently discussed, investing in synthetic biology stocks is still a pretty risky venture, but we’d bet you a million bitcoins that healthcare, agriculture and other industries that rely on understanding and manipulating biological systems will one day be dominated by synbio. That’s why we want you to understand this emerging technology.
Now for the fun stuff: What can we do with gene circuit engineering? One possible application would be to design cells that when they detect a tumor can produce a drug to attack the cancer. It would be like turning the cells in our bodies into nanobots, able to detect, diagnose and treat disease without the hassles or costs of insurance and deductibles.
Maybe we’ll make smarter plants that can respond to droughts or produce their own insecticide. Remember those meat-protein-fermenting yeasts? Those yeast cells, as more than one expert has noted, could be engineered to halt their own fermentation process if too many toxic byproducts build up. Or perhaps we could reprogram cells so people are no longer lactose intolerant or gluten intolerant by helping them digest what was once indigestible.
It’s Not Science Fiction Despite the Name
Those are some of the possibilities that inspired the founding of Asimov last year. The Cambridge, Massachusetts startup raised $4.7 million in Seed funding last December, led by the powerhouse VC firm Andreessen Horowitz. Most of the company’s co-founders, all of whom hail from MIT or Boston University, published a seminal paper on gene circuit automation in the journal Science in 2016. Their research had been funded by the Navy and the shadowy government agency known as DARPA. In other words, these guys are pretty smart, and their business plan sounds like it comes out of the Ginkgo Bioworks playbook: They want to design and sell engineered genetic circuits to other biotech companies for various applications. (By the way, Ginkgo Bioworks became the first synbio unicorn last year.)
Asimov’s platform can reportedly predict whether or not a biological circuit will work with up to 90 percent accuracy.
It’s a Small World After All
Not surprisingly, the other startup that we found attempting to commercialize gene circuit engineering also has roots in MIT, though Senti Bio is headquartered on the other side of the country in San Francisco. The company, founded in 2016, raised a $53 million Series A last month from about a dozen investors including names like Amgen. It’s also not surprising to find one of the pioneers of synthetic biology, MIT professor Timothy Lu, at the helm. Lu has co-founded a number of biotech companies including Synlogic (NASDAQ:SYBX). Sitting on the company’s advisory board is Voigt, a colleague at MIT and a co-founder at Asimov. No doubt synthetic biology is a small world in more ways than one.
Senti Bio is specifically concerned with disease, which in the context of our biology-machine analogy can be thought of as an error in computer code. The company’s technology platform enables it to rapidly design, build, and test various genetic circuits to enhance human cell and gene therapies to fix those errors. The company has pulled together experts in not just gene circuit engineering but therapeutic synthetic biology, immune cell engineering, and engineered cell therapies. It sounds like Senti Bio will target cancer and autoimmune diseases to start.
We only learned about gene circuit engineering a few months ago when Asimov had its high-profile coming out party late last year. And then Senti Bio came out of stealth mode last month. It probably won’t be long before we have to revisit this topic to cover the latest top 10 gene circuit engineering startups on the planet. That’s just how quickly (well, 60 years after gene circuits were first recognized) these technologies are emerging and being commercialized. As long as venture capitalists remain fearless when it comes to investing in synbio—up to $1 billion per year now—we wouldn’t be surprised if another biotech unicorn is born soon. Could it be Senti Bio or even Asimov? It’s just one SoftBank-sized deal away from happening.
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Why are the units of work and energy same?
Work and Energy have the same unit because they are intimately related by the Work-Energy Theorem. This states that the Work done on an object moving from point A to point B is the difference in the Kinetic Energy it has at the two points. In simplified form it says that in order to do Work, you must use Energy; you can never do Work without using Energy, and so for convenience we give them the same unit. You didn't ask about Heat, but it really ought to be mentioned here, too; when you use Energy to do Work, some of the Energy usually gets converted to Heat by friction, so we can and do also use the same unit for Heat. Work and Energy and Heat are not the same thing, but they are inextricably related. By extension, the Theorem lets us define forms of Energy other than Kinetic Energy which enable us to do Work: Chemical Energy, Electrical Energy, Nuclear Energy, Gravitational Energy, etc. The SI unit of work or energy is Newton-meter (Nm). Another name for it is Joule (J). 1 joule is defined as the work done when a force of one newton acts over one metre of distance. 1 joule is also the amount of heat dissipated when a current of 1 ampere passes through a resistor of 1 ohm for 1 second (or 1 watt-second). Please use the following link to confirm the accuracy of the information presented here, or for more information.
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Work and Energy are analogous to Debt and Payment. If you owe me $1, you should pay me $1 to cancel your debt.. If work requires 1 J, then you must have the energy of 1 J to… satisfy the work requirment, or the job is NOT DONE.
Because energy and work have the same dimensions: energy is the ability to do work and work is the expenditure of energy. The SI unit is the joule.
Joule- a metric unit of work and energy
Because energy and work have the same 'dimensions', which is the scientist's way of saying they both have the same make-up of the basic units, which are length, mass, and time…. In fact energy can be defined as the capacity for doing work, which is potential energy, and when work is done energy is expended. When a power plant generates say 1000 MegaWatts, this is called Energy, and it is sent out to the grid. At the same time many different loads are using this energy, some for electric motors, some for heating, others in various industrial processes, some to run computers, and so on. These loads are all doing work, and consuming energy in doing so. 1000 MW is 10 9 Watts, and this means 10 9 Joules per second, so that is the work being done every second by all these various loads combined.
Volts and watts.
Energy and work have the same units because......we say energy is the ability to do work....work is the energy transferred from a body to another.....the donor of energy is sa…id to have done work upon the acceptor...
Heat is a energy form.So unit of heat is the same as energy
because work is a transformation of energy. If there is some loss of energy in a process then same amount of work will be created. For this reason the unit of work and energy …is same.
The Joule is the SI unit of energy or work. 1 Joule = (1 Newton)*(1 meter) or 1 kg*m 2 /s 2
Consider "work" as a "change in energy" or "transfer of energy". It makes sense to use energy units for work.
The derived SI unit of energy, and work is the joule. One joule is the energy used, or the work done, in applying a force of one newton through a distance of one metre; als…o defined as the energy to pass a current of one ampere through a resistance of one ohm for one second.
They are closely related. You can think of work as the transfer of energy. | <urn:uuid:f32f83d2-9dfd-4216-9da2-0c2313a1f9cb> | 3.484375 | 869 | Q&A Forum | Science & Tech. | 63.788894 | 95,504,864 |
Conditional and Closure Properties
The union theorem, introduced in section 8.2.3, is the main tool for the study of asynchronous compositions of programs. The major virtue of this theorem is that it provides a simple rule for deducing the co-properties and transient predicates of a system from those of its component boxes. The major shortcoming is that it does not provide a simple rule for deducing the leads-to properties of a system from those of its components. The only way we can use a progress property of a component, p→ q in F, to deduce a similar property of the system is to either (1) apply the union theorem for progress (see section 8.2.6), which requires introduction of a well-founded order over the values of the shared variables, or (2) exhibit the proof of p→ q in F and show that the other components do not falsify the proof steps. The first strategy is cumbersome, and the second defeats the whole purpose of modular program construction.
KeywordsUnion Theorem Global Variable Closure Property Conditional Property Type Declaration
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Which equations are relevent to pendulum physics problems?
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Pendulum Question watch
- Thread Starter
Last edited by dodgewc25; 08-03-2018 at 20:47.
- 08-03-2018 16:03
(Original post by dodgewc25)
- 08-03-2018 16:07
The simple pendulum shown below in Figure Q3 is released from rest in the horizontal position. After it reaches the bottom position, the cord wraps around the smooth fixed pin at B and the pendulum continues in its motion. Calculate the magnitude of the force, FR supported by the pin at B when the pendulum passes the position, θ = 37.
I can find the speed of the ball when the string hits the pin, but have no idea what to do about finding the force supported by the pin..
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Tidal locking (also called gravitational locking or captured rotation) occurs when the long-term interaction between a pair of co-orbiting astronomical bodies drives the rotation rate of at least one of them into the state where there is no more net transfer of angular momentum between this body (e.g. a planet) and its orbit around the second body (e.g. a star); this condition of "no net transfer" must be satisfied over the course of one orbit around the second body. This does not mean that the rotation and spin rates are always perfectly synchronized throughout an orbit, as there can be some back and forth transfer over the course of an orbit.
This effect arises from the gravitational gradient (tidal force) between the co-orbiting bodies, acting over a sufficiently long period of time.
In the special case where the orbital eccentricity and obliquity are nearly zero, tidal locking results in one hemisphere of the revolving object constantly facing its partner, an effect known as synchronous rotation. For example, the same side of the Moon always faces the Earth, although there is some libration because the Moon's orbit is not perfectly circular. A tidally locked body in synchronous rotation takes just as long to rotate around its own axis as it does to revolve around its partner.
Usually, only the satellite is tidally locked to the larger body. However, if both the mass difference between the two bodies and the distance between them are relatively small, each may be tidally locked to the other; this is the case for Pluto and Charon.
Consider a pair of co-orbiting objects, A and B. The change in rotation rate necessary to tidally lock body B to the larger body A is caused by the torque applied by A's gravity on bulges it has induced on B by tidal forces.
The gravitational force from object A upon B will vary with distance, being greatest at the nearest surface to A and least at the most distant. This creates a gravitational gradient across object B that will distort its equilibrium shape slightly. The body of object B will become elongated along the axis oriented toward A, and conversely, slightly reduced in dimension in directions orthogonal to this axis. The elongated distortions are known as tidal bulges. (For the solid Earth, these bulges can reach displacements of up to around 0.4 metres (1.3 ft).) When B is not yet tidally locked, the bulges travel over its surface due to orbital motions, with one of the two "high" tidal bulges traveling close to the point where body A is overhead. For large astronomical bodies that are nearly spherical due to self-gravitation, the tidal distortion produces a slightly prolate spheroid, i.e. an axially symmetric ellipsoid that is elongated along its major axis. Smaller bodies also experience distortion, but this distortion is less regular.
The material of B exerts resistance to this periodic reshaping caused by the tidal force. In effect, some time is required to reshape B to the gravitational equilibrium shape, by which time the forming bulges have already been carried some distance away from the A–B axis by B's rotation. Seen from a vantage point in space, the points of maximum bulge extension are displaced from the axis oriented toward A. If B's rotation period is shorter than its orbital period, the bulges are carried forward of the axis oriented toward A in the direction of rotation, whereas if B's rotation period is longer, the bulges instead lag behind.
Because the bulges are now displaced from the A–B axis, A's gravitational pull on the mass in them exerts a torque on B. The torque on the A-facing bulge acts to bring B's rotation in line with its orbital period, whereas the "back" bulge, which faces away from A, acts in the opposite sense. However, the bulge on the A-facing side is closer to A than the back bulge by a distance of approximately B's diameter, and so experiences a slightly stronger gravitational force and torque. The net resulting torque from both bulges, then, is always in the direction that acts to synchronize B's rotation with its orbital period, leading eventually to tidal locking.
The angular momentum of the whole A–B system is conserved in this process, so that when B slows down and loses rotational angular momentum, its orbital angular momentum is boosted by a similar amount (there are also some smaller effects on A's rotation). This results in a raising of B's orbit about A in tandem with its rotational slowdown. For the other case where B starts off rotating too slowly, tidal locking both speeds up its rotation, and lowers its orbit.
Locking of the larger bodyEdit
The tidal locking effect is also experienced by the larger body A, but at a slower rate because B's gravitational effect is weaker due to B's smaller mass. For example, Earth's rotation is gradually being slowed by the Moon, by an amount that becomes noticeable over geological time as revealed in the fossil record. Current estimations are that this (together with the tidal influence of the Sun) has helped lengthen the Earth day from about 6 hours to the current 24 hours. Currently, atomic clocks show that Earth's day lengthens by about 15 microseconds every year. Given enough time, this would create a mutual tidal locking between Earth and the Moon, where the length of a day has increased and the length of a lunar month has shortened until the two are the same. However, Earth is not expected to become tidally locked to the Moon before the Sun becomes a red giant and engulfs Earth and the Moon.
For bodies of similar size the effect may be of comparable size for both, and both may become tidally locked to each other on a much shorter timescale. An example is the dwarf planet Pluto and its satellite Charon. They have already reached a state where Charon is only visible from one hemisphere of Pluto and vice versa.
For orbits that do not have an eccentricity close to zero, the rotation rate tends to become locked with the orbital speed when the body is at periapsis, which is the point of strongest tidal interaction between the two objects. If the orbiting object has a companion, this third body can cause the rotation rate of the parent object to vary in an oscillatory manner. This interaction can also drive an increase in orbital eccentricity of the orbiting object around the primary – an effect known as eccentricity pumping.
In some cases where the orbit is eccentric and the tidal effect is relatively weak, the smaller body may end up in a so-called spin–orbit resonance, rather than being tidally locked. Here, the ratio of the rotation period of a body to its own orbital period is some simple fraction different from 1:1. A well known case is the rotation of Mercury, which is locked to its own orbit around the Sun in a 3:2 resonance.
Many exoplanets (especially the close-in ones) are expected to be in spin–orbit resonances higher than 1:1. A Mercury-like terrestrial planet can, for example, become captured in a 3:2, 2:1, or 5:2 spin–orbit resonance, with the probability of each being dependent on the orbital eccentricity.
Most major moons in the Solar System − the gravitationally rounded satellites − are tidally locked with their primaries, because they orbit very closely and tidal force increases rapidly (as a cubic function) with decreasing distance. Notable exceptions are the irregular outer satellites of the gas giants, which orbit much farther away than the large well-known moons.
Pluto and Charon are an extreme example of a tidal lock. Charon is a relatively large moon in comparison to its primary and also has a very close orbit. This results in Pluto and Charon being mutually tidally locked. Pluto's other moons are not tidally locked; Styx, Nix, Kerberos, and Hydra all rotate chaotically due to the influence of Charon.
The Moon's rotation and orbital periods are tidally locked with each other, so no matter when the Moon is observed from Earth the same hemisphere of the Moon is always seen. The far side of the Moon was not seen until 1959, when photographs of most of the far side were transmitted from the Soviet spacecraft Luna 3.
When the Earth is observed from the moon, the Earth does not appear to translate across the sky but appears to remain in the same place, rotating on its own axis.
Despite the Moon's rotational and orbital periods being exactly locked, about 59% of the Moon's total surface may be seen with repeated observations from Earth due to the phenomena of libration and parallax. Librations are primarily caused by the Moon's varying orbital speed due to the eccentricity of its orbit: this allows up to about 6° more along its perimeter to be seen from Earth. Parallax is a geometric effect: at the surface of Earth we are offset from the line through the centers of Earth and Moon, and because of this we can observe a bit (about 1°) more around the side of the Moon when it is on our local horizon.
It was thought for some time that Mercury was in synchronous rotation with the Sun. This was because whenever Mercury was best placed for observation, the same side faced inward. Radar observations in 1965 demonstrated instead that Mercury has a 3:2 spin–orbit resonance, rotating three times for every two revolutions around the Sun, which results in the same positioning at those observation points. Modeling has demonstrated that Mercury was captured into the 3:2 spin–orbit state very early in its history, within 20 (and more likely even 10) million years after its formation.
Venus's 583.92-day interval between successive close approaches to Earth is equal to 5.001444 Venusian solar days, making approximately the same face visible from Earth at each close approach. Whether this relationship arose by chance or is the result of some kind of tidal locking with Earth is unknown.
Proxima Centauri b, the "Earth-like planet" discovered in 2016 that orbits around the star Proxima Centauri is tidally locked, either in synchronized rotation, or otherwise expresses a 3:2 spin–orbit resonance like that of Mercury.
Close binary stars throughout the universe are expected to be tidally locked with each other, and extrasolar planets that have been found to orbit their primaries extremely closely are also thought to be tidally locked to them. An unusual example, confirmed by MOST, may be Tau Boötis, a star that is probably tidally locked by its planet Tau Boötis b. If so, the tidal locking is almost certainly mutual. However, since stars are gaseous bodies that can rotate with a different rate at different latitudes, the tidal lock is with Tau Boötis's magnetic field.
An estimate of the time for a body to become tidally locked can be obtained using the following formula:
- is the initial spin rate expressed in radians per second,
- is the semi-major axis of the motion of the satellite around the planet (given by the average of the periapsis and apoapsis distances),
- is the moment of inertia of the satellite, where is the mass of the satellite and is the mean radius of the satellite,
- is the dissipation function of the satellite,
- is the gravitational constant,
- is the mass of the planet, and
- is the tidal Love number of the satellite.
and are generally very poorly known except for the Moon, which has . For a really rough estimate it is common to take (perhaps conservatively, giving overestimated locking times), and
- is the density of the satellite
- is the surface gravity of the satellite
- is the rigidity of the satellite. This can be roughly taken as 3×1010 N·m−2 for rocky objects and 4×109 N·m−2 for icy ones.
Even knowing the size and density of the satellite leaves many parameters that must be estimated (especially ω, Q, and μ), so that any calculated locking times obtained are expected to be inaccurate, even to factors of ten. Further, during the tidal locking phase the semi-major axis may have been significantly different from that observed nowadays due to subsequent tidal acceleration, and the locking time is extremely sensitive to this value.
Because the uncertainty is so high, the above formulas can be simplified to give a somewhat less cumbersome one. By assuming that the satellite is spherical, , and it is sensible to guess one revolution every 12 hours in the initial non-locked state (most asteroids have rotational periods between about 2 hours and about 2 days)
with masses in kilograms, distances in meters, and in newtons per meter squared; can be roughly taken as 3×1010 N·m−2 for rocky objects and 4×109 N·m−2 for icy ones.
There is an extremely strong dependence on semi-major axis .
For the locking of a primary body to its satellite as in the case of Pluto, the satellite and primary body parameters can be swapped.
One conclusion is that, other things being equal (such as and ), a large moon will lock faster than a smaller moon at the same orbital distance from the planet because grows as the cube of the satellite radius . A possible example of this is in the Saturn system, where Hyperion is not tidally locked, whereas the larger Iapetus, which orbits at a greater distance, is. However, this is not clear cut because Hyperion also experiences strong driving from the nearby Titan, which forces its rotation to be chaotic.
The above formulae for the timescale of locking may be off by orders of magnitude, because they ignore the frequency dependence of . More importantly, they may be inapplicable to viscous binaries (double stars, or double asteroids that are rubble), because the spin–orbit dynamics of such bodies is defined mainly by their viscosity, not rigidity.
List of known tidally locked bodiesEdit
|Parent body||Tidally-locked satellites|
|Sun||Mercury (3:2 spin–orbit resonance)|
|Mars||Phobos · Deimos|
|Jupiter||Metis · Adrastea · Amalthea · Thebe · Io · Europa · Ganymede · Callisto|
|Saturn||Pan · Atlas · Prometheus · Pandora · Epimetheus · Janus · Mimas · Enceladus · Telesto · Tethys · Calypso · Dione · Rhea · Titan · Iapetus|
|Uranus||Miranda · Ariel · Umbriel · Titania · Oberon|
|Neptune||Proteus · Triton|
|Pluto||Charon (Pluto is itself locked to Charon)|
Bodies likely to be lockedEdit
Based on comparison between the likely time needed to lock a body to its primary, and the time it has been in its present orbit (comparable with the age of the Solar System for most planetary moons), a number of moons are thought to be locked. However their rotations are not known or not known enough. These are:
Probably locked to Saturn
Probably locked to Uranus
Probably locked to Neptune
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- All planets in the TRAPPIST-1 system are likely to be tidally locked.
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Building LiteBSD Kernels with Config
The document is based on article "Building 4.4BSD Kernels with Config" by Samuel J. Leffler and Michael J. Karels, with modifications relevant to LiteBSD operating system and target PIC32 platform.
This document describes the use of config(8) to configure and create LiteBSD kernel images. It discusses the structure of kernel configuration files and how to configure systems with non-standard hardware configurations. Sections describing the preferred way to add new code to the kernel and how the system’s autoconfiguration process operates are included. An appendix contains a summary of the rules used by the kernel in calculating the size of system data structures, and also indicates some of the standard kernel size limitations (and how to change them). Other configuration options are also listed.
Revised June 4, 2015
Config is a tool used in building LiteBSD system images (the UNIX kernel). It takes a file describing a kernel’s tunable parameters and hardware support, and generates a collection of files which are then used to build a copy of UNIX appropriate to that configuration. Config simplifies kernel maintenance by isolating system dependencies in a single, easy to understand, file.
This document describes the content and format of kernel configuration files and the rules which must be followed when creating these files. Example configuration files are constructed and discussed.
Later sections suggest guidelines to be used in modifying kernel source and explain some of the inner workings of the autoconfiguration process. Appendix C summarizes the rules used in calculating the most important kernel data structures and indicates some inherent kernel data structure size limitations (and how to go about modifying them).
2. Configuration File Contents
A kernel configuration must include at least the following pieces of information:
- machine type
- cpu type
- system identification
- maximum number of users
- location of the root file system
- available hardware
Config allows multiple kernel images to be generated from a single configuration description. Each kernel image is configured for identical hardware, but may have different locations for the root file system and, possibly, other system devices.
2.1. Machine type
The machine type indicates if the system is going to operate on a PIC32 microcontroller, or some other platform on which LiteBSD operates. The machine type is used to locate certain data files which are platform specific, and also to select rules used in constructing the resultant configuration files.
2.2. Cpu type
The cpu type indicates which, of possibly many, cpu’s the kernel is to operate on. For example, if the system is being configured as a PIC32, it could be running on a PIC32MZ, or other variants (not yet). Specifying more than one cpu type implies that the system should be configured to run on any of the cpu’s specified. For some types of machines this is not possible and config will print a diagnostic indicating such.
2.3. System identification
The system identification is a moniker attached to the system, and often identifies the board type, on which the system is to run. For example, for PIC32MZ we have boards SDZL, MEBII, WIFIRE, and so on. The system identifier selected is used to create a global C "#define" which may be used to isolate board dependent pieces of code in the kernel.
The system identifier "GENERIC" is given to a system which will run on any cpu of a particular machine type; it should not otherwise be used for a system identifier.
The timezone in which the system is to run is used to define the information returned by the gettimeofday(2) system call. This value is specified as the number of hours east or west of GMT. Negative numbers indicate a value east of GMT. The timezone specification may also indicate the type of daylight savings time rules to be applied.
2.5. Maximum number of users
The kernel allocates many system data structures at boot time based on the maximum number of users the system will support. This number is normally between 8 and 40, depending on the hardware and expected job mix. The rules used to calculate kernel data structures are discussed in Appendix C.
2.6. Root file system location
When the kernel boots it must know the location of the root of the file system tree. This location and the part(s) of the disk(s) to be used for paging and swapping must be specified in order to create a complete configuration description. Config uses many rules to calculate default locations for these items; these are described in Appendix B.
When a generic system is configured, the root file system is left undefined until the system is booted. In this case, the root file system need not be specified, only that the system is a generic system.
2.7. Hardware devices
When the kernel boots it goes through an autoconfiguration phase. During this period, the kernel searches for all those hardware devices which the system builder has indicated might be present. This probing sequence requires certain pieces of information such as register addresses, bus interconnects, etc. A system’s hardware may be configured in a very flexible manner or be specified without any flexibility whatsoever. Most people do not configure hardware devices into the kernel unless they are currently present on the machine, expect them to be present in the near future, or are simply guarding against a hardware failure somewhere else at the site (it is often wise to configure in extra disks in case an emergency requires moving one off a machine which has hardware problems).
The specification of hardware devices usually occupies the majority of the configuration file. As such, a large portion of this document will be spent understanding it. Section 6.3 contains a description of the autoconfiguration process, as it applies to those planning to write, or modify existing, device drivers.
2.8. Pseudo devices
Several kernel facilities are configured in a manner like that used for hardware devices although they are not associated with specific hardware. These system options are configured as pseudo-devices. Some pseudo devices allow an optional parameter that sets the limit on the number of instances of the device that are active simultaneously.
2.9. System options
Other than the mandatory pieces of information described above, it is also possible to include various optional kernel facilities or to modify system behavior and/or limits. Optional support is provided for disk quotas and tracing the performance of the virtual memory subsystem. Any optional facilities to be configured into the kernel are specified in the configuration file. The resultant files generated by config will automatically include the necessary pieces of the system.
3. System Building Process
In this section we consider the steps necessary to build a bootable kernel image. We assume the kernel source is located in the "$BSDSRC/sys" directory and that, initially, the system is being configured from source code.
Under normal circumstances there are 5 steps in building a kernel.
- Create a configuration file for the kernel.
- Make a directory for the kernel to be constructed in.
- Run config on the configuration file to generate the files required to compile and load the kernel image.
- Construct the source code interdependency rules for the configured kernel with makedepend using make(1).
- Compile and load the kernel with make.
Steps 1 and 2 are usually done only once. When a kernel configuration changes it usually suffices to just run config on the modified configuration file, rebuild the source code dependencies, and remake the kernel. Sometimes, however, configuration dependencies may not be noticed in which case it is necessary to clean out the relocatable object files saved in the kernel’s directory; this will be discussed later.
3.1. Creating a configuration file
Configuration files normally reside in the machine-dependent directory like "$BSDSRC/sys/mips/conf". A configuration file is most easily constructed by copying an existing configuration file and modifying it. The LiteBSD distribution contains a number of configuration files for well known boards; one may be suitable for you, or a copy of the generic configuration file may be edited.
The configuration file must have the same name as the directory in which the configured kernel is to be built. Further, config assumes this directory is located in the parent directory of the directory in which it is run. For example, the generic system has a configuration file "sys/mips/conf/GENERIC" and an accompanying directory named "sys/compile/GENERIC". Although it is not required that the kernel sources and configuration files reside in "sys", the configuration and compilation procedure depends on the relative locations of directories within that hierarchy, as most of the system code and the files created by config use pathnames of the form "../". If the system files are not located in "$BSDSRC/sys", it is desirable to make a symbolic link there for use in installation of other parts of the system that share files with the kernel.
When building the configuration file, be sure to include the items described in section 2. In particular, the machine type, cpu type, timezone, system identifier, maximum users, and root device must be specified. The specification of the hardware present may take a bit of work; particularly if your hardware is configured at non-standard places (e.g. device registers located at funny places or devices not supported by the system). Section 4 of this document gives a detailed description of the configuration file syntax, section 5 explains some sample configuration files, and section 6 discusses how to add new devices to the kernel. If the devices to be configured are not already described in one of the existing configuration files you should check the manual pages in section 4 of the UNIX Programmers Manual. For each supported device, the manual page synopsis entry gives a sample configuration line.
Once the configuration file is complete, run it through config and look for any errors. Never try and use a kernel which config has complained about; the results are unpredictable. For the most part, config’s error diagnostics are self explanatory. It may be the case that the line numbers given with the error messages are off by one.
A successful run of config on your configuration file will generate a number of files in the configuration directory. These files are:
- A file to be used by make(1) in compiling and loading the kernel, Makefile.
- One file for each possible kernel image for this machine, swapxxx.c, where xxx is the name of the kernel image, which describes where swapping, the root file system, and other miscellaneous system devices are located.
- A collection of header files, one per possible device the system supports, which define the hardware configured.
- A file containing the I/O configuration tables used by the kernel during its autoconfiguration phase, ioconf.c.
- An assembly language file of interrupt vectors which connect interrupts from the machine’s external buses to the main system path for handling interrupts, and a file that contains counters and names for the interrupt vectors.
Unless you have reason to doubt config, or are curious how the kernel’s autoconfiguration scheme works, you should never have to look at any of these files.
3.2. Constructing source code dependencies
When config is done generating the files needed to compile and link your kernel it will terminate with a message of the form "Don’t forget to run make depend". This is a reminder that you should change over to the configuration directory for the kernel just configured and type "make depend" to build the rules used by make to recognize interdependencies in the system source code. This will insure that any changes to a piece of the system source code will result in the proper modules being recompiled the next time make is run.
This step is particularly important if your site makes changes to the system include files. The rules generated specify which source code files are dependent on which include files. Without these rules, make will not recognize when it must rebuild modules due to the modification of a system header file. The dependency rules are generated by a pass of the C preprocessor and reflect the global system options. This step must be repeated when the configuration file is changed and config is used to regenerate the system makefile.
3.3. Building the kernel
The makefile constructed by config should allow a new kernel to be rebuilt by simply typing "make image-name". For example, if you have named your bootable kernel image "vmunix", then "make vmunix" will generate a bootable image named "vmunix". Alternate kernel image names are used when the root file system location and/or swapping configuration is done in more than one way. The makefile which config creates has entry points for each kernel image defined in the configuration file. Thus, if you have configured "vmunix" to be a kernel with the root file system on an "sd" device and "hkvmunix" to be a kernel with the root file system on an "hk" device, then "make vmunix hkvmunix" will generate binary images for each.
Note that the name of a bootable image is different from the system identifier. All bootable images are configured for the same system; only the information about the root file system and paging devices differ. (This is described in more detail in section 4.)
4. Configuration File Syntax
In this section we consider the specific rules used in writing a configuration file. A complete grammar for the input language can be found in Appendix A and may be of use if you should have problems with syntax errors.
A configuration file is broken up into three logical pieces:
- configuration parameters global to all kernel images specified in the configuration file,
- parameters specific to each kernel image to be generated, and
- device specifications.
4.1. Global configuration parameters
The global configuration parameters are the type of machine, cpu types, options, timezone, system identifier, and maximum users. Each is specified with a separate line in the configuration file.
- machine "type"
The kernel is to run on the machine type specified. No more than one machine type can appear in the configuration file. Currently the only legal value is pic32. Other variants are outdated (like vax, tahoe, hp300, i386, mips, pmax, luna68k and news3400).
- cpu "mtype"
This kernel is to run on the cpu type specified. More than one cpu type specification can appear in a configuration file. Legal type for a pic32 machine is PIC32MZ.
- options optionlist
Compile the listed optional code into the kernel. Options in this list are separated by commas. Possible options are listed at the top of the generic makefile. A line of the form "options FUNNY,HAHA" generates global "#define"s −DFUNNY −DHAHA in the resultant makefile. An option may be given a value by following its name with "=", then the value enclosed in (double) quotes. The following are major options are currently in use: INET (Internet communication protocols), and QUOTA (enable disk quotas). Other kernel options controlling system sizes and limits are listed in Appendix C; options for the network are found in Appendix D. There are additional options which are associated with certain peripheral devices; those are listed in the Synopsis section of the manual page for the device.
- makeoptions optionlist
Options that are used within the kernel makefile and evaluated by make are listed as makeoptions. Options are listed with their values with the form "makeoptions name=value,name2=value2". The values must be enclosed in double quotes if they include numerals or begin with a dash.
- timezone number [ dst [ number ] ]
Specifies the timezone used by the system. This is measured in the number of hours your timezone is west of GMT. EST is 5 hours west of GMT, PST is 8. Negative numbers indicate hours east of GMT. If you specify dst, the system will operate under daylight savings time. An optional integer or floating point number may be included to specify a particular daylight saving time correction algorithm; the default value is 1, indicating the United States. Other values are: 2 (Australian style), 3 (Western European), 4 (Middle European), and 5 (Eastern European). See gettimeofday(2) and ctime(3) for more information.
- ident name
This system is to be known as name. This is usually a short name like WIFIRE (for chipKIT Wi-Fire board) or MEBII (for Microchip Multimedia Expansion Board II). This value is defined for use in conditional compilation, and is also used to locate an optional list of source files specific to this system.
- maxusers number
The maximum expected number of simultaneously active users on this system is number. This number is used to size several kernel data structures.
4.2. Kernel image parameters
Multiple bootable images may be specified in a single configuration file. The kernels will have the same global configuration parameters and devices, but the location of the root file system and other system specific devices may be different. A kernel image is specified with a "config" line:
config sysname config-clauses
The sysname field is the name given to the loaded kernel image; almost everyone names their standard kernel image "vmunix". The configuration clauses are one or more specifications indicating where the root file system is located and the number and location of paging devices.
A configuration clause is one of the following
root [ on ] root-device swap [ on ] swap-device [ and swap-device ] ... dumps [ on ] dump-device
(the "on" is optional.) Multiple configuration clauses are separated by white space; config allows specifications to be continued across multiple lines by beginning the continuation line with a tab character. The "root" clause specifies where the root file system is located, the "swap" clause indicates swapping and paging area(s), and the "dumps" clause can be used to force kernel dumps to be taken on a particular device.
The device names supplied in the clauses may be fully specified as a device, unit, and file system partition; or underspecified in which case config will use builtin rules to select default unit numbers and file system partitions.
The defaulting rules are a bit complicated as they are dependent on the overall kernel configuration. For example, the swap area need not be specified at all if the root device is specified; in this case the swap area is placed in the "b" partition of the same disk where the root file system is located. Appendix B contains a complete list of the defaulting rules used in selecting kernel configuration devices.
The device names are translated to the appropriate major and minor device numbers on a per-machine basis. A file like "$BSDSRC/sys/mips/conf/devices.machine" (where "machine" is the machine type specified in the configuration file) is used to map a device name to its major block device number. The minor device number is calculated using the standard disk partitioning rules: on unit 0, while drive is minor device 0, partition "a" is minor device 1, partition "b" is minor device 2, and so on; for units other than 0, add 8 times the unit number to get the minor device.
If the default mapping of device name to major/minor device number is incorrect for your configuration, it can be replaced by an explicit specification of the major/minor device. This is done by substituting
major x minor y
where the device name would normally be found. For example,
config vmunix root on major 99 minor 1
Normally, the areas configured for swap space are sized by the kernel at boot time. If a non-standard size is to be used for one or more swap areas (less than the full partition), this can also be specified. To do this, the device name specified for a swap area should have a "size" specification appended. For example,
config vmunix root on sd0 swap on sd0b size 1200
would force swapping to be done in partition "b" of "sd0" and the swap partition size would be set to 1200 sectors. A swap area sized larger than the associated disk partition is trimmed to the partition size.
To create a generic configuration, only the clause "swap generic" should be specified; any extra clauses will cause an error.
4.3. Device specifications
Each device attached to a machine must be specified to config so that the kernel generated will know to probe for it during the autoconfiguration process carried out at boot time. Hardware specified in the configuration need not actually be present on the machine where the generated kernel is to be run. Only the hardware actually found at boot time will be used by the system.
A device specification takes one of the following forms:
controller device-name device-info [ interrupt-spec ] device device-name device-info interrupt-spec disk device-name device-info
A "controller" is typically an SPI or I2C bus port. A "device" is an autonomous device which connects directly to the processor (as opposed to something like SPI devices which connects through an SPI controller). "Disk" identify disk drives like SD cards connected to SPI "controller".
The device-name is one of the standard device names, as indicated in section 4 of the UNIX Programmers Manual, concatenated with the logical unit number to be assigned the device (the logical unit number may be different than the physical unit number indicated on the front of something like a disk; the logical unit number is used to refer to the UNIX device, not the physical unit number). For example, "sd0" is logical unit 0 of a storage device.
The device-info clause specifies how the hardware is connected in the interconnection hierarchy. On the PIC32, SD cards and Wi-Fi controllers are connected through SPI ports. Thus, one of the following specifications would be used:
disk sd0 at spi2 drive 0 flags 0x79 device mrf0 at spi3 flags 0x71641f49
Certain device drivers require extra information passed to them at boot time to tailor their operation to the actual hardware present. The line printer driver, for example, needs to know how many columns are present on each non-standard line printer (i.e. a line printer with other than 80 columns). The drivers for the terminal multiplexors need to know which lines are attached to modem lines so that no one will be allowed to use them unless a connection is present. For this reason, one last parameter may be specified to a device, a flags field. It has the syntax
and is usually placed after other specifications. The number is passed directly to the associated driver. The manual pages in section 4 should be consulted to determine how each driver uses this value (if at all). Communications interface drivers commonly use the flags to indicate whether modem control signals are in use.
The exact syntax for each specific device is given in the Synopsis section of its manual page in section 4 of the manual.
A number of drivers and software subsystems are treated like device drivers without any associated hardware. To include any of these pieces, a "pseudo-device" specification must be used. A specification for a pseudo device takes the form
pseudo-device device-name [ howmany ]
Examples of pseudo devices are pty, the pseudo terminal driver (where the optional howmany value indicates the number of pseudo terminals to configure, 32 default), and loop, the software loopback network pseudo-interface. Other pseudo devices for the network include imp (required when a CSS or ACC imp is configured) and ether (used by the Address Resolution Protocol on 10 Mb/sec Ethernets). More information on configuring each of these can also be found in section 4 of the manual.
5. Sample Configuration Files
In this section we will consider how to configure a sample PIC32MZ system for SDZL board. We then study the rules needed to configure a PIC32MZ to run in a networking environment.
5.1. PIC32MZ System
Our PIC32MZ kernel is configured with hardware peripherals avalable on SDZL board. Table 1 lists the pertinent hardware to be configured.
Table 1. PIC32MZ Hardware support.
|UART port 1||cpu||uart1|
|UART port 2||cpu||uart2|
|SPI port 1||cpu||spi1|
|SPI port 2||cpu||spi2|
|SPI port 3||cpu||spi3|
|SPI port 4||cpu||spi4|
We will call this machine SDZL and construct a configuration file one step at a time.
The first step is to fill in the global configuration parameters. The target is a PIC32MZ microcontroller, so the machine type is "pic32". We will assume this kernel will run only on this one processor, so the cpu type is "PIC32MZ". The options are empty since this is going to be a "vanilla" PIC32. The system identifier, as mentioned before, is "SDZL", and the maximum number of users we plan to support is about 40. Thus the beginning of the configuration file looks like this:
# # SDZL board # machine "pic32" cpu "PIC32MZ" timezone 8 dst ident SDZL maxusers 2
To this we must then add the specification for the kernel image. It will be our standard kernel with the root on "sd0" and swapping on the same drive as the root.
config vmunix root on sd0
Finally, the hardware must be specified. Let us first just try transcribing the information from Table 1.
device uart1 flags 0x4243 # pins rx=RD2, tx=RD3 device uart2 flags 0x2726 # pins rx=RB7, tx=RB6 controller spi1 flags 0x6160 # pins sdi=RF1, sdo=RF0 controller spi2 flags 0x7778 # pins sdi=RG7, sdo=RG8 controller spi3 flags 0x2923 # pins sdi=RB9, sdo=RB3 controller spi4 flags 0x4b40 # pins sdi=RD11, sdo=RD0 disk sd0 at spi2 drive 0 flags 0x79 # select pin RG9
The completed configuration file for SDZL is available here: SDZL.pic32.
5.2. PIC32MZ with network support
Our PIC32MZ system will work on MEB-II board connected to 100Mb/s Ethernet local area network. First the global parameters:
# # Microchip Multimedia Expansion Board II # machine "pic32" cpu "PIC32MZ" ident MEBII timezone 8 dst maxusers 2 options INET
The value of 2 given for the maximum number of users is done to keep the kernel data structures compact. The "INET" indicates that we plan to use the standard Internet protocols on this machine.
The kernel image, serial ports and disks are configured next.
config vmunix root on sd0 device uart1 flags 0x1e1f # pins rx=RA14, tx=RA15 controller spi1 flags 0x4e2a # pins sdi=RD14, sdo=RB10 controller spi2 flags 0x4778 # pins sdi=RD7, sdo=RG8 controller spi4 flags 0x7723 # pins sdi=RG7, sdo=RB3 disk sd0 at spi2 drive 0 flags 0x2e # select pin RB14
Finally, we add in the network devices. Pseudo terminals are needed to allow users to log in across the network (remember the only hardwired terminal is the console). The software loopback device is used for on-machine communications. And, finally, there are the two Ethernet devices. These use a special protocol, the Address Resolution Protocol (ARP), to map between Internet and Ethernet addresses. Thus, yet another pseudo-device is needed. The additional device specifications are show below.
pseudo-device pty 4 pseudo-device loop pseudo-device ether controller en0
The completed configuration file for MEB-II is available here: MEBII.pic32.
6. Adding New System Software
This section is not for the novice, it describes some of the inner workings of the configuration process as well as the pertinent parts of the system autoconfiguration process. It is intended to give those people who intend to install new device drivers and/or other kernel facilities sufficient information to do so in the manner which will allow others to easily share the changes.
This section is broken into four parts:
- general guidelines to be followed in modifying kernel code,
- how to add non-standard kernel facilities to LiteBSD,
- how to add a device driver to LiteBSD, and
6.1. Modifying kernel code
If you wish to make site-specific modifications to the system it is best to bracket them with
#ifdef SITENAME ... #endif
to allow your source to be easily distributed to others, and also to simplify diff(1) listings. If you choose not to use a source code control system (e.g. SCCS, RCS), and perhaps even if you do, it is recommended that you save the old code with something of the form:
#ifndef SITENAME ... #endif
We try to isolate our site-dependent code in individual files which may be configured with pseudo-device specifications.
Indicate machine-specific code with "#ifdef PIC32MZ" (or other machine, as appropriate). LiteBSD underwent extensive work to make it extremely portable to machines with similar architectures − you may someday find yourself trying to use a single copy of the source code on multiple machines.
6.2. Adding non-standard kernel facilities
This section considers the work needed to augment config’s data base files for non-standard kernel facilities. Config uses a set of files that list the source modules that may be required when building a system. The data bases are taken from the directory in which config is run, like $BSDSRC/sys/mips/conf. Three such files may be used: files, files.machine, and files.ident. The first is common to all systems, the second contains files unique to a single machine type, and the third is an optional list of modules for use on a specific machine. This last file may override specifications in the first two. The format of the files file has grown somewhat complex over time. Entries are normally of the form
dir/source.c type option-list modifiers
mips/dev/spi.c optional spi device-driver
The type is one of standard or optional. Files marked as standard are included in all system configurations. Optional file specifications include a list of one or more system options that together require the inclusion of this module. The options in the list may be either names of devices that may be in the configuration file, or the names of system options that may be defined. An optional file may be listed multiple times with different options; if all of the options for any of the entries are satisfied, the module is included.
If a file is specified as a device-driver, any special compilation options for device drivers will be invoked. On the PIC32 it make no difference, but on other architectures it can result in the use of some special options for the C compiler.
Two other optional keywords modify the usage of the file. Config understands that certain files are used especially for kernel profiling. These files are indicated in the files files with a profiling-routine keyword. For example, the current profiling subroutines are sequestered off in a separate file with the following entry:
sys/subr_mcount.c optional profiling-routine
The profiling-routine keyword forces config not to compile the source file with the −pg option.
The second keyword which can be of use is the config-dependent keyword. This causes config to compile the indicated module with the global configuration parameters. This allows certain modules, such as machdep.c to size kernel data structures based on the maximum number of users configured for the system.
6.3. Adding device drivers to LiteBSD
The I/O system and config have been designed to easily allow new device support to be added. The kernel source directories are organized as follows:
sys/sys machine independent include files sys/kern machine-independent kernel source files sys/libkern kernel library routines sys/conf site configuration files and basic templates sys/dev machine-independent device drivers sys/mips MIPS PIC32-specific code and drivers sys/miscfs code for optional filesystems sys/net network-related, protocol-independent code sys/netinet Internet protocol code sys/nfs network filesystem code sys/ufs Unix filesystem code sys/vm virtual memory code
Existing device drivers for the PIC32 reside in "$BSDSRC/sys/mips/dev". Any new device drivers should be placed in the appropriate source code directory and named so as not to conflict with existing devices. Normally, definitions for things like device registers are placed in a separate file in the same directory. For example, the "dh" device driver is named "dh.c" and its associated include file is named "dhreg.h".
Once the source for the device driver has been placed in a directory, the file "../files.machine", and possibly "../devices.machine" should be modified. The files files in the conf directory contain a line for each C source or binary-only file in the system. Those files which are machine independent are located in "$BSDSRC/sys/conf/files", while machine specific files are in "$BSDSRC/sys/ARCH/conf/files.machine". The "devices.machine" file is used to map device names to major block device numbers. If the device driver being added provides support for a new disk you will want to modify this file (the format is obvious).
In addition to including the driver in the files file, it must also be added to the device configuration tables. These are located in "$BSDSRC/sys/mips/pic32/conf.c", or similar for machines other than the PIC32. If you don’t understand what to add to this file, you should study an entry for an existing driver. Remember that the position in the device table specifies the major device number. The block major number is needed in the "devices.machine" file if the device is a disk.
With the configuration information in place, your configuration file appropriately modified, and a system reconfigured and rebooted you should incorporate the shell commands needed to install the special files in the file system to the file "/dev/MAKEDEV" or "/dev/MAKEDEV.local". This is discussed in the document "Installing and Operating LiteBSD".
Appendix A. Configuration File Grammar
The following grammar is a compressed form of the actual yacc(1) grammar used by config to parse configuration files. Terminal symbols are shown all in upper case, literals are emboldened; optional clauses are enclosed in brackets, "[" and "]"; zero or more instantiations are denoted with "*".
Configuration ::= [ Spec ; ]* Spec ::= Config_spec | Device_spec | trace | /* lambda */ /* configuration specifications */ Config_spec ::= machine ID | cpu ID | options Opt_list | ident ID | System_spec | timezone [ − ] NUMBER [ dst [ NUMBER ] ] | timezone [ − ] FPNUMBER [ dst [ NUMBER ] ] | maxusers NUMBER /* system configuration specifications */ System_spec ::= config ID System_parameter [ System_parameter ]* System_parameter ::= swap_spec | root_spec | dump_spec | arg_spec swap_spec ::= swap [ on ] swap_dev [ and swap_dev ]* swap_dev ::= dev_spec [ size NUMBER ] root_spec ::= root [ on ] dev_spec dump_spec ::= dumps [ on ] dev_spec arg_spec ::= args [ on ] dev_spec dev_spec ::= dev_name | major_minor major_minor ::= major NUMBER minor NUMBER dev_name ::= ID [ NUMBER [ ID ] ] /* option specifications */ Opt_list ::= Option [ , Option ]* Option ::= ID [ = Opt_value ] Opt_value ::= ID | NUMBER Mkopt_list ::= Mkoption [ , Mkoption ]* Mkoption ::= ID = Opt_value /* device specifications */ Device_spec ::= device Dev_name Dev_info Int_spec | disk Dev_name Dev_info | controller Dev_name Dev_info [ Int_spec ] | pseudo-device Dev [ NUMBER ] Dev_name ::= Dev NUMBER Dev ::= ID Dev_info ::= Con_info [ Info ]* Con_info ::= at Dev NUMBER Info ::= drive NUMBER | flags NUMBER Int_spec ::= priority NUMBER
The terminal symbols are loosely defined as:
One or more alphabetics, either upper or lower case, and underscore, "_".
Approximately the C language specification for an integer number. That is, a leading "0x" indicates a hexadecimal value, a leading "0" indicates an octal value, otherwise the number is expected to be a decimal value. Hexadecimal numbers may use either upper or lower case alphabetics.
A floating point number without exponent. That is a number of the form "nnn.ddd", where the fractional component is optional.
In special instances a question mark, "?", can be substituted for a "NUMBER" token. This is used to effect wildcarding in device interconnection specifications.
Comments in configuration files are indicated by a "#" character at the beginning of the line; the remainder of the line is discarded.
A specification is interpreted as a continuation of the previous line if the first character of the line is tab.
Appendix B. Rules for Defaulting System Devices
When config processes a "config" rule which does not fully specify the location of the root file system, paging area(s), and device for kernel dumps, it applies a set of rules to define those values left unspecified. The following list of rules are used in defaulting system devices.
- If a root device is not specified, the swap specification must indicate a "generic" system is to be built.
- If the root device does not specify a unit number, it defaults to unit 0.
- If the root device does not include a partition specification, it defaults to the "a" partition.
- If no swap area is specified, it defaults to the "b" partition of the root device.
- If no device is specified for processing argument lists, the first swap partition is selected.
- If no device is chosen for kernel dumps, the first swap partition is selected (see below to find out where dumps are placed within the partition).
The following table summarizes the default partitions selected when a device specification is incomplete, e.g. "sd0".
Multiple swap/paging areas
When multiple swap partitions are specified, the kernel treats the first specified as a "primary" swap area which is always used. The remaining partitions are then interleaved into the paging system at the time a swapon(2) system call is made. This is normally done at boot time with a call to swapon(8) from the /etc/rc file.
Kernel dumps are automatically taken after a system crash, provided the device driver for the "dumps" device supports this. The dump contains the contents of memory, but not the swap areas. Normally the dump device is a disk in which case the information is copied to a location at the back of the partition. The dump is placed in the back of the partition because the primary swap and dump device are commonly the same device and this allows the system to be rebooted without immediately overwriting the saved information. When a dump has occurred, the system variable dumpsize is set to a non-zero value indicating the size (in bytes) of the dump. The savecore(8) program then copies the information from the dump partition to a file in a "crash" directory and also makes a copy of the kernel which was running at the time of the crash (usually "/vmunix"). The offset to the kernel dump is defined in the system variable dumplo (a sector offset from the front of the dump partition). The savecore program operates by reading the contents of dumplo, dumpdev, and dumpmagic from /dev/kmem, then comparing the value of dumpmagic read from /dev/kmem to that located in corresponding location in the dump area of the dump partition. If a match is found, savecore assumes a crash occurred and reads dumpsize from the dump area of the dump partition. This value is then used in copying the kernel dump. Refer to savecore(8) for more information about its operation.
The value dumplo is calculated to be
dumpdev-size - memsize
where dumpdev-size is the size of the disk partition where kernel dumps are to be placed, and memsize is the size of physical memory. If the disk partition is not large enough to hold a full dump, dumplo is set to 0 (the start of the partition).
Appendix C. Kernel Data Structure Sizing Rules
Certain kernel data structures are sized at compile time according to the maximum number of simultaneous users expected, while others are calculated at boot time based on the physical resources present, e.g. memory. This appendix lists both sets of rules and also includes some hints on changing built-in limitations on certain data structures.
Compile time rules
The file $BSDSRC/sys/conf/param.c contains the definitions of almost all data structures sized at compile time. This file is copied into the directory of each configured kernel to allow configuration-dependent rules and values to be maintained. (Each copy normally depends on the copy in $BSDSRC/sys/conf, and global modifications cause the file to be recopied unless the makefile is modified.) The rules implied by its contents are summarized below (here MAXUSERS refers to the value defined in the configuration file in the "maxusers" rule). Most limits are computed at compile time and stored in global variables for use by other modules; they may generally be patched in the kernel binary image before rebooting to test new values.
The maximum number of processes which may be running at any time. It is referred to in other calculations as NPROC and is defined to be
8 + 8 * MAXUSERS
The maximum number of files in the file system which may be active at any time. This includes files in use by users, as well as directory files being read or written by the system and files associated with bound sockets in the UNIX IPC domain. It is defined as
NPROC + 2 * MAXUSERS + 24
The number of "file table" structures. One file table structure is used for each open, unshared, file descriptor. Multiple file descriptors may reference a single file table entry when they are created through a dup call, or as the result of a fork. This is defined to be
2 * NPROC + 16
The number of "callout" structures. One callout structure is used per internal system event handled with a timeout. Timeouts are used for terminal delays, watchdog routines in device drivers, protocol timeout processing, etc. This is defined as
16 + NPROC
The maximum number of pages which may be allocated by the network. This is machine-dependent and defined as 16 (64 kbytes of memory) for PIC32 in $BSDSRC/sys/mips/include/param.h. In practice, network starts off by allocating 8 kilobytes of memory, then requesting more as required. This value represents an upper bound.
The most important data structures sized at run-time are those used in the buffer cache. Allocation is done by allocating physical memory (and system virtual memory) immediately after the kernel has been started up; look in the file $BSDSRC/sys/mips/pic32/machdep.c. The amount of physical memory which may be allocated to the buffer cache is constrained by the size of the kernel page tables, among other things. While the system may calculate a large amount of memory to be allocated to the buffer cache, if the kernel page table is too small to map this physical memory into the virtual address space of the system, only as much as can be mapped will be used.
The buffer cache is comprised of a number of "buffer headers" and a pool of pages attached to these headers. Buffer headers are divided into two categories: those used for swapping and paging, and those used for normal file I/O. The kernel tries to allocate 10% of the first two megabytes and 5% of the remaining available physical memory for the buffer cache (where available does not count that space occupied by the kernel’s text and data segments). If this results in fewer than 16 pages of memory allocated, then 16 pages are allocated. This value is kept in the initialized variable bufpages so that it may be patched in the binary image (to allow tuning without recompiling the kernel), or the default may be overridden with a configuration-file option. For example, the option options BUFPAGES="3200" causes 3200 pages (3.2M bytes) to be used by the buffer cache. A sufficient number of file I/O buffer headers are then allocated to allow each to hold 2 pages each. Each buffer maps 8K bytes. If the number of buffer pages is larger than can be mapped by the buffer headers, the number of pages is reduced. The number of buffer headers allocated is stored in the global variable nbuf, which may be patched before the kernel is booted. The system option options NBUF="1000" forces the allocation of 1000 buffer headers. Half as many swap I/O buffer headers as file I/O buffers are allocated, but no more than 256.
Kernel size limitations
As distributed, the sum of the virtual sizes of the core-resident processes is limited to 256M bytes. The size of the text segment of a single process is currently limited to 6M bytes. It may be increased to no greater than the data segment size limit (see below) by redefining MAXTSIZ. This may be done with a configuration file option, e.g. options MAXTSIZ="(10*1024*1024)" to set the limit to 10 million bytes. Other per-process limits discussed here may be changed with similar options with names given in parentheses. Soft, user-changeable limits are set to 512K bytes for stack (DFLSSIZ) and 6M bytes for the data segment (DFLDSIZ) by default; these may be increased up to the hard limit with the setrlimit(2) system call. The data and stack segment size hard limits are set by a kernel configuration option to one of 17M, 33M or 64M bytes. One of these sizes is chosen based on the definition of MAXDSIZ; with no option, the limit is 17M bytes; with an option options MAXDSIZ="(32*1024*1024)" (or any value between 17M and 33M), the limit is increased to 33M bytes, and values larger than 33M result in a limit of 64M bytes. You must be careful in doing this that you have adequate paging space. As normally configured, the kernel has 16M or 32M bytes per paging area, depending on disk size. The best way to get more space is to provide multiple, thereby interleaved, paging areas. Increasing the virtual memory limits results in interleaving of swap space in larger sections (from 500K bytes to 1M or 2M bytes).
Because the file system block numbers are stored in page table pg_blkno entries, the maximum size of a file system is limited to 2^24 1024 byte blocks. Thus no file system can be larger than 8 gigabytes.
Appendix D. Network Configuration Options
The network support in the kernel is self-configuring according to the INET protocol support option and the network hardware discovered during autoconfiguration. There are several changes that may be made to customize network behavior due to local restrictions. Within the Internet protocol routines, the following options set in the kernel configuration file are supported:
The machine is to be used as a gateway. This option currently makes only minor changes. First, the size of the network routing hash table is increased. Secondly, machines that have only a single hardware network interface will not forward IP packets; without this option, they will also refrain from sending any error indication to the source of unforwardable packets. Gateways with only a single interface are assumed to have missing or broken interfaces, and will return ICMP unreachable errors to hosts sending them packets to be forwarded.
Normally, LiteBSD machines with multiple network interfaces will forward IP packets received that should be resent to another host. If the line `options IPFORWARDING="0"' is in the kernel configuration file, IP packet forwarding will be disabled.
When forwarding IP packets, LiteBSD IP will note when a packet is forwarded using the same interface on which it arrived. When this is noted, if the source machine is on the directly-attached network, an ICMP redirect is sent to the source host. If the packet was forwarded using a route to a host or to a subnet, a host redirect is sent, otherwise a network redirect is sent. The generation of redirects may be inhibited with the configuration option `options IPSENDREDIRECTS="0"'.
TCP calculates a maximum segment size to use for each connection, and sends no datagrams larger than that size. This size will be no larger than that supported on the outgoing interface. Furthermore, if the destination is not on the local network, the size will be no larger than 576 bytes. For this test, other subnets of a directly-connected subnetted network are considered to be local unless the line `options SUBNETSARELOCAL="0"' is used in the kernel configuration file. | <urn:uuid:15e4ddac-af27-409f-8c54-49fa3ad64c25> | 2.9375 | 10,655 | Documentation | Software Dev. | 40.352251 | 95,504,953 |
Importance sampling is a way to predict the probability of a rare event. Along with Markov Chain Monte Carlo, it is the primary simulation tool for generating models of hard-to-define probability distributions.
Rare events can usually be found on the tails of probability distributions. For example, on a bell curve for IQ, the Albert Einsteins of the world are found above three standard deviations from the mean. The rarity of finding results like this makes it extremely difficult to sample large enough numbers for any meaningful statistical analysis. In addition, the probability distribution for these rare events are going to look markedly different from the bell curve. Although predicting when another Einstein might be born is probably not that critical, predicting other rare events — like fatigue in engineering structures or landfall for category 5 hurricanes — can be a matter of life and death.
As well as finding probabilities in tails, importance sampling can also be used to find expectations of random functions.
One way to produce large enough samples is to change the probability density function to generate more rare events. This alternate density function is derived from the original function of interest (in the above example, the bell curve) and is usually called the biasing density. The end goal is to reduce the variance of your estimates. The basic steps are:
- Choose a model for the process you want to study (i.e. derive the biasing density function and define the model’s parameters (e.g. the mean and variance)),
- Draw random samples from the parameterized model,
- Run your statistical analysis on the biasing density function,
- Modify those results to reflect the changes you made to the probability distribution.
- Analyze the output.
Importance Sampling and Monte Carlo Procedures
Importance sampling speeds up Monte Carlo procedures for rare events (a “Monte Carlo procedure” is sampling based on random walks). As it speeds up the process, it’s sometimes referred to as “fast simulation using importance sampling.” It’s also called a “forced Monte Carlo procedure” because it’s forcing the Monte Carlo procedure to behave somewhat abnormally.
If you’re using Monte Carlo procedures, you’re more than likely using software because of the large number of computations involved. Many statistical software packages include Monte Carlo algorithms, including Minitab, R and SPSS.
The formulas behind Importance Sampling are somewhat esoteric, mainly because of the calculus involved. As a (relatively) simple example, let’s say you wanted to create an expectation for some function, f:
μf = ℰp[f(X)], with
Then for any probability density function q(x) that satisfies q(x) > 0 when f(x)p(x)≠ 0, you have:
- w(x) = p(x)/q(x)
- ℰq = expectation with respect to q(x).
Neal, R. M. (2001). Annealed Importance Sampling. Statistics and
Computing (11) 125-139.
Oh, M.-S. and Berger, J. O. (1992). Adaptive Importance Samplin in
Monte Carlo Integration. Journal of Statistical Computation and Simulation
Srinivasan, R. (2013). Importance Sampling: Applications in Communications and Detection. Springer Science & Business Media.
Tokdar, S. & Kass, R. (2009). Importance Sampling: A Review. Retrieved 8/18/2017 from: http://www2.stat.duke.edu/~st118/Publication/impsamp.pdf
If you prefer an online interactive environment to learn R and statistics, this free R Tutorial by Datacamp is a great way to get started. If you're are somewhat comfortable with R and are interested in going deeper into Statistics, try this Statistics with R track.Comments? Need to post a correction? Please post on our Facebook page. | <urn:uuid:9f1cc0e3-6db9-4ed3-86a1-9c64253dee7b> | 3.859375 | 833 | Tutorial | Science & Tech. | 45.01625 | 95,504,982 |
here is a link that sheds a bit of light on it. In a stable orbit, velocity is perfectly balanced with the force of gravity at a right angle to the velocity. I think the vector arrows in the diagram make it a whole lot easier to visualize. Too little velocity, and the orbit decays. Too much velocity and you escape the gravity well.
Here is another link from that page with a funny little MSpaint diagram.
You can prove to yourself that the amount of gravity in low earth orbit is very similar to gravity on the surface of the planet by using Newton's Universal Gravitation Equation: F = GMm/R2
F = force of attraction in Newtons
G = the factor relating force to mass and distance in Newton's law of gravitation. It is a universal constant with the value 6.673 ✕ 10-11 N m2 kg-2. (convoluted)
M = mass of first object
m = mass of second object
R = distance, in meters, between objects center of mass (that's important)
The mean radius of the Earth is 6,371km. So when you are on the surface, you are 6,371km from the center of mass.
For example, the ISS, is, on average, around 330km above the surface of the earth. So, even without doing any calculations, you know that your unsquared denominator in the universal gravitation equation is only increasing from 6,371 km to around 6701 km, about a 5% increase. Therefore, you are still experiencing most of the gravity you would if you were on the surface of the earth.
CAVEAT - I am not nearly the among the most sciency people on reddit. I welcome corrections, I know my general concept is right, but my explanations may not be correct...if anybody wants to make this assertion more correct, I'd be happy to edit this post
EDIT: as IConrad has pointed out, I got the headline a bit wrong...my bad...I don't think it ruins the point overall, though.
6371 * 6371 == 40,589,641
6701 * 6701 == 44,903,401
40,589,641 / 44,903,401 == 0.903932444
The ISS is experiencing ~90% of the gravity it would if it were at sea level. | <urn:uuid:c7cedcb9-fde3-47d0-ae35-fa930240f17b> | 3.203125 | 496 | Comment Section | Science & Tech. | 74.257092 | 95,505,021 |
Appleby said that even though the pest has been in the news since it was first identified in ash trees near Detroit in 2002, there are still people who are unaware of the problem and inadvertently transport the pest to uninfested areas. "I talked with a homeowner in Michigan who had a number of dead ash trees on her property. She said that she had never heard of the ash borer and didn't know it was a problem." Appleby described an innocent scenario that may occur in which her friend or grandson from Illinois visits and she says, "I have plenty of firewood. Take as much as you want." The wood filled with ash borer larvae, gets thrown into the trunk of the car, destined for a new area. Adult beetles would emerge from the wood during May and June and infest a new neighborhood of unsuspecting ash trees.
The natural spread of an infestation is probably no more than a half mile per year. But over the years, it is the transportation of infested firewood into new locations that continues to bring new infestations. Today the insect is commonly found in the southern half of Michigan with scattered infestations in northern Michigan, an area in Canada just east of Detroit, northwestern and several other locations in Ohio, northeastern Indiana and just north of Indianapolis, and in 2006 Illinois infestations were found in Cook and Kane counties.
"Nurseries and businesses that sell trees are monitored and controlled," said Appleby. "But all it takes is one uninformed person to transport infested firewood to bring the pests into a new area."
The beetle may have been in the Detroit area for 10 to 12 years before it was even discovered there. Shortly afterward, federal and state agencies imposed a quarantine in the infested areas, which prohibited the movement of any ash trees, logs, and firewood out of the areas. Unfortunately, prior to the quarantine, infested firewood and some infested trees were already moved to other areas. Households having fireplaces use firewood and a common practice for campers and people who own cabins in outlying areas is to take firewood to their camps and cabins. So, unknowingly infested firewood was distributed to many areas and even to adjoining states.
Appleby says that the emerald ash borer joins a long list of exotic species that have become invasive --most have been accidentally transported. "The emerald ash borer beetle is native to China, Korea, Japan, Taiwan, and parts of Russia. We can only speculate that the beetle probably arrived in the larval stage inside wood crating material from China. Shortly after arriving, adult beetles emerged from the wood and flew to local ash trees infesting them."
Although Appleby says that woodpeckers are natural predators, they feed on larger larvae but by that time the larger larvae have already done extensive damage.
From studies in the state of Michigan it is known that the insect overwinters in the larval stage under the bark of ash trees. In April the larva changes into the inactive pupal stage and then beginning in early to mid May it changes into the adult stage. The adult beetle then chews a distinctive D-shaped exit hole, about an eighth of an inch in diameter, in the bark. The adult beetle flies to ash foliage where it feeds on the edges of the leaf. After feeding for about a week, the beetles will mate. Seven to 10 days after mating the female beetle is attracted to the upper branches of a living ash tree where she deposits eggs in bark crevices. The eggs hatch in about 10 days. The larva bores into the bark and feeds just under the bark where it makes serpentine tunnels.
It is the feeding of the larval stage that is so destructive to the tree. The feeding causes a disruption in the tree’s ability to transport nutrients and so as the number of tunnels continues to increase each year, the upper tree branches begin to die and kill the tree in three to five years.
To date, white, green, and black ash trees all appear to be susceptible to attack by the emerald ash borer. In some urban areas ash may comprise as much as 20 percent of their ornamental trees. "In some Michigan communities where ash lined the streets on both sides, now only stumps remain and the residents have no shade," said Appleby. "We should learn never to plant all of one tree species in a given area but plant a diversity of tree species."
Appleby said that some communities are no longer planting ash trees. "This might be a good course to take, particularly in northeastern Illinois, but in other areas it might be too drastic not to include the planting of at least some ash. Time will tell whether or not we will be successful in limiting the spread of this very destructive ash insect."
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For the first time ever, scientists have determined the cosmic origin of highest-energy neutrinos. A research group led by IceCube scientist Elisa Resconi, spokesperson of the Collaborative Research Center SFB1258 at the Technical University of Munich (TUM), provides an important piece of evidence that the particles detected by the IceCube neutrino telescope at the South Pole originate from a galaxy four billion light-years away from Earth.
To rule out other origins with certainty, the team led by neutrino physicist Elisa Resconi from the Technical University of Munich and multi-wavelength...
For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.
Whirlpools are an everyday experience in a bath tub: When the water is drained a circular vortex is formed. Typically, such whirls are rather stable. Similar...
Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.
A frequently used reaction in organic chemistry is nucleophilic substitution. It plays, for example, an important role in in the synthesis of new chemical...
Optical spectroscopy allows investigating the energy structure and dynamic properties of complex quantum systems. Researchers from the University of Würzburg present two new approaches of coherent two-dimensional spectroscopy.
"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy....
Ultra-short, high-intensity X-ray flashes open the door to the foundations of chemical reactions. Free-electron lasers generate these kinds of pulses, but there is a catch: the pulses vary in duration and energy. An international research team has now presented a solution: Using a ring of 16 detectors and a circularly polarized laser beam, they can determine both factors with attosecond accuracy.
Free-electron lasers (FELs) generate extremely short and intense X-ray flashes. Researchers can use these flashes to resolve structures with diameters on the...
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Abstract / Notes
Most plant families are distinguished by characteristic secondary metabolites, which can function as putative defence against herbivores. However, many herbivorous insects of different orders can make use of these plant-synthesised compounds by ingesting and storing them in their body tissue or integument. Such sequestration of putatively unpalatable or toxic metabolites can enhance the insects' own defence against enemies and may also be involved in reproductive behaviour. This review gives a comprehensive overview of all groups of secondary plant metabolites for which sequestration by insect herbivores belonging to different orders has been demonstrated. Sequestered compounds include various aromatic compounds, nitrogen-containing metabolites such as alkaloids, cyanogenic glycosides, glucosinolates and other sulphur-containing metabolites, and isoprenoids such as cardiac glycosides, cucurbitacins, iridoid glycosides and others. Sequestration of plant compounds has been investigated most in insects feeding or gathering on Apocynaceae s.l. (Apocynoideae, Asclepiaoideae), Aristolochiaceae, Asteraceae, Boraginaceae, Fabaceae and Plantaginaceae, but it also occurs for some gymnosperms and even lichens. In total, more than 250 insect species have been shown to sequester plant metabolites from at least 40 plant families. Sequestration predominates in the Coleoptera and Lepidoptera, but also occurs frequently in the orders Heteroptera, Hymenoptera, Orthoptera and Sternorrhyncha. Patterns of sequestration mechanisms for various compound classes and common or individual features occurring in different insect orders are highlighted. More research is needed to elucidate the specific transport mechanisms and the physiological processes of sequestration in various insect species.
Opitz S, Müller C. Plant chemistry and insect sequestration. CHEMOECOLOGY. 2009;19(3):117-154.
Opitz, S., & Müller, C. (2009). Plant chemistry and insect sequestration. CHEMOECOLOGY, 19(3), 117-154. doi:10.1007/s00049-009-0018-6
Opitz, S., and Müller, C. (2009). Plant chemistry and insect sequestration. CHEMOECOLOGY 19, 117-154.
Opitz, S., & Müller, C., 2009. Plant chemistry and insect sequestration. CHEMOECOLOGY, 19(3), p 117-154.
S. Opitz and C. Müller, “Plant chemistry and insect sequestration”, CHEMOECOLOGY, vol. 19, 2009, pp. 117-154.
Opitz, S., Müller, C.: Plant chemistry and insect sequestration. CHEMOECOLOGY. 19, 117-154 (2009).
Opitz, Sebastian, and Müller, Caroline. “Plant chemistry and insect sequestration”. CHEMOECOLOGY 19.3 (2009): 117-154.
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By: Thomas Pochapsky and Susan Pochapsky
Nuclear Magnetic Resonance spectroscopy is a dynamic way for scientists of all kinds to investigate the physical, chemical, and biological properties of matter. Its many applications make it a versatile tool previously subject to monolithic treatment in reference-style texts. Based on a course taught for over ten years at Brandeis University, this is the first textbook on NMR spectroscopy for a one-semester course or self-instruction. In keeping with the authors' efforts to make it a useful textbook, they have included problems at the end of each chapter. The book not only covers the latest developments in the field, such as GOESY (Gradient Enhanced Overhauser Spectroscopy) and multidimensional NMR, but includes practical examples using real spectra and associated problem sets.
Assuming the reader has a background of chemistry, physics and calculus, this textbook will be ideal for graduate students in chemistry and biochemistry, as well as biology, physics, and biophysics. "NMR for Physical and Biological Scientists" will also be useful to medical schools, research facilities, and the many chemical, pharmaceutical, and biotech firms that offer in-house instruction on NMR spectroscopy.
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In order to produce a porous material it is necessary to have multiple components. When the minor component is removed, small pores are left in its place. Until now, creating nanoporous materials was limiting as it was believed the minor component had to be connected throughout the structure as well as to the outside in order for it to be removed.
However, new research published today (Sunday, 27 November) in the journal Nature Materials has demonstrated a much more effective, flexible method called collective osmotic shock (COS) for creating porous structures. The research, by scientists at the University of Cambridge, has shown how by using osmotic forces even structures with minor components entirely encapsulated in a matrix can be made porous (or nanoporous).
The lead author, Dr Easan Sivaniah from the University of Cambridge's Cavendish Laboratory, explains how the process works: "The experiment is rather similar to the classroom demonstration using a balloon containing salty water. How does one release the salt from the balloon? The answer is to put the balloon in a bath of fresh water. The salt can't leave the balloon but the water can enter, and it does so to reduce the saltiness in the balloon. As more water enters, the balloon swells, and eventually bursts, releasing the salt completely.
"In our experiments, we essentially show this works in materials with these trapped minor components, leading to a series of bursts that connect together and to the outside, releasing the trapped components and leaving an open porous material."
The researchers have also demonstrated how the nanoporous materials created by the unique process can be used to develop filters capable of removing very small dyes from water.
Dr Sivaniah added: "It is currently an efficient filter system that could be used in countries with poor access to fresh potable water, or to remove heavy metals and industrial waste products from ground water sources. Though, with development, we hope it can also be used in making sea-water drinkable using low-tech and low-power routes."
Other applications were explored in collaboration with groups having expertise in photonics (Dr Hernan Miguez, University of Sevilla) and optoelectronics (Professor Sir Richard Friend, Cavendish Laboratory). Light-emitting devices were demonstrated using titania electrodes templated from COS materials whilst the novel stack-like arrangement of materials provide uniquely efficient photonic multilayers with potential applications as sensors that change colour in response to absorbing trace amounts of chemicals, or for use in optical components.
Dr Sivaniah added, "We are currently exploring a number of applications, to include use in light-emitting devices, solar cells, electrodes for supercapacitors as well as fuels cells."
More information: The paper 'Collective osmotic shock in ordered materials' will be published in the 27 November 2011 edition of Nature Materials. DOI: 10.1038/nmat3179
Wednesday, November 30, 2011
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The discovery of a millisecond pulsar in a triple system challenges current consensus. Thomas Tauris (Bonn) and Ed van den Heuvel (Amsterdam) have developed a semi-analytical model, which can resolve the puzzling formation of this exotic triple system.
Triple millisecond pulsar with its two white dwarf companions. According to the new model, the system survived three phases of mass transfer and a supernova explosion, remaining dynamically stable.
Through theoretical calculations on the base of stellar evolution, they have demonstrated a plausible model which brings new insight to our knowledge of stellar interactions in multiple star systems. Their study can also help explain an increasing number of observed binary millisecond pulsars which seem to require a triple system origin.
Pulsars are among the most extreme celestial bodies known. They have radii of only 10 kilometres, but at the same time a mass exceeding that of our Sun. Pulsars are formed as the remnants of violent supernova explosions of massive stars. The fastest rotating neutron stars are known as millisecond pulsars. They are thought to be strongly magnetized, old neutron stars which have been spun up to high rotational frequencies by accumulation of mass and angular momentum from a companion star in a binary system. Today we know of about 200 such pulsars with spin periods between 1.4 and 10 milliseconds. These are located in both the Galactic Disk and in Globular Clusters.
Since the first binary pulsar was detected in 1974, theoretical astrophysicists have investigated mass transfer between stars and other binary interactions in order to explain their origin. A surprising new discovery has now revealed a millisecond pulsar in a triple system with two white dwarf companions, posing a unique challenge to stellar physicists to explain its formation.
"This is a truly amazing system with three degenerate objects. It has survived three phases of mass transfer and a supernova explosion, and yet it remained dynamically stable", says Thomas Tauris, theoretical astrophysicist and first author of the present study. "Pulsars have previously been found with planets and in recent years my observational colleagues have discovered a number of peculiar binary pulsars which seem to require a triple system origin. But this new millisecond pulsar is the first to be detected with two white dwarfs".
The new triple millisecond pulsar J0337+1715 was discovered recently by a joint American-European collaboration led by Scott Ransom from National Radio Astronomy Observatory (USA). The system is located in the constellation of Taurus at a distance of about 4000 light-years. It is in the Galactic disk, and not inside a globular cluster. Therefore, its existence cannot be explained simply as a result of dynamical encounter events in a dense stellar environment. During the last 6 months, Thomas Tauris and Ed van den Heuvel have developed a semi-analytical model to explain its existence. One of the key results obtained from their investigation is that the observed parameters reflect that both white dwarfs were indeed produced in the present system.
Triple systems often become dynamically unstable during their evolution leading to expulsion of one of the three stars. A major challenge was to find a solution that remained dynamically stable throughout the entire evolution, including the stage of the supernova explosion. "An interesting result of our new investigation is that the system evolved through a common envelope stage where both white dwarf progenitor stars were dragged into the envelope of the massive star due to frictional forces, causing their orbits to shrink by a large factor, thereby enabling survival of its subsequent explosion", says Ed van den Heuvel.
"Actually, we can apply several tests of stellar evolution with this new system and also make predictions about its 3-dimensional velocity which can be measured within a few years", concludes Thomas Tauris. "This will allow us to constrain the mass of the exploding star."
This work has profited from a recent effort to bridge the Fundamental Physics in Radio Astronomy group at the Max-Planck-Institut für Radioastronomie (MPIfR), led by Michael Kramer, with the Stellar Physics group at the Argelander-Institut für Astronomie (AIfA) at University of Bonn, led by Norbert Langer. Michael Kramer and his colleagues are using the 100-m Effelsberg Radio Telescope to participate in several ongoing searches and discoveries of millisecond pulsars, while the stellar physicists at AIfA are modelling their formation and evolution.
Thomas Tauris has been working at the AIfA and MPIfR as a visiting research professor since 2010. Some of his recent work on the recycling of millisecond pulsars has been published jointly with Norbert Langer, Michael Kramer and other colleagues in Bonn. Together they host twice per year an international one-day workshop in Bonn, called Formation and Evolution of Neutron stars.
Formation of the Galactic Millisecond Pulsar Triple System PSR J0337+1715 - a Neutron Star with Two Orbiting White Dwarfs , T. M. Tauris & E. P. J. van den Heuvel, 2014, Astrophysical Journal Letters, scheduled for online publication on January 06, 2014.
Pulsar Discovery Paper:A millisecond pulsar in a stellar triple system, S.M. Ransom et al., 2014, Nature Online Publishing, doi:10.1038/nature12917.
Local Contact:Dr. Thomas M. Tauris
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In geometry, a torus (plural tori) is a surface of revolution generated by revolving a circle in three-dimensional space about an axis coplanar with the circle. If the axis of revolution does not touch the circle, the surface has a ring shape and is called a torus of revolution.
Real-world examples of toroidal objects include inner tubes.
A torus should not be confused with a solid torus, which is formed by rotating a disc, rather than a circle, around an axis. A solid torus is a torus plus the volume inside the torus. Real-world approximations include doughnuts, many lifebuoys, and O-rings.
In topology, a ring torus is homeomorphic to the Cartesian product of two circles: S1 × S1, and the latter is taken to be the definition in that context. It is a compact 2-manifold of genus 1. The ring torus is one way to embed this space into three-dimensional Euclidean space, but another way to do this is the Cartesian product of the embedding of S1 in the plane with itself. This produces a geometric object called the Clifford torus, a surface in 4-space.
- θ, φ are angles which make a full circle, so that their values start and end at the same point,
- R is the distance from the center of the tube to the center of the torus,
- r is the radius of the tube.
R is known as the "major radius" and r is known as the "minor radius". The ratio R divided by r is known as the "aspect ratio". The typical doughnut confectionery has an aspect ratio of about 3 to 2.
or the solution of f(x, y, z) = 0, where
The three different classes of standard tori correspond to the three possible aspect ratios between R and r:
- When R > r, the surface will be the familiar ring torus or anchor ring.
- R = r corresponds to the horn torus, which in effect is a torus with no "hole".
- R < r describes the self-intersecting spindle torus.
- When R = 0, the torus degenerates to the sphere.
When R ≥ r, the interior
of this torus is diffeomorphic (and, hence, homeomorphic) to a product of an Euclidean open disc and a circle. The volume of this solid torus and the surface area of its torus are easily computed using Pappus's centroid theorem, giving
These formulas are the same as for a cylinder of length 2πR and radius r, obtained from cutting the tube along the plane of a small circle, and unrolling it by straightening out (rectifying) the line running around the center of the tube. The losses in surface area and volume on the inner side of the tube exactly cancel out the gains on the outer side.
Expressing the surface area and the volume by the distance p of an outermost point on the surface of the torus to the center, and the distance q of an innermost point (so that R = p + q/ and r = p − q/), yields
As a torus is the product of two circles, a modified version of the spherical coordinate system is sometimes used. In traditional spherical coordinates there are three measures, R, the distance from the center of the coordinate system, and θ and φ, angles measured from the center point.
As a torus has, effectively, two center points, the centerpoints of the angles are moved; φ measures the same angle as it does in the spherical system, but is known as the "toroidal" direction. The center point of θ is moved to the center of r, and is known as the "poloidal" direction. These terms were first used in a discussion of the Earth's magnetic field, where "poloidal" was used to denote "the direction toward the poles".
In modern use these terms are more commonly used to discuss magnetic confinement fusion devices.
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Topologically, a torus is a closed surface defined as the product of two circles: S1 × S1. This can be viewed as lying in C2 and is a subset of the 3-sphere S3 of radius √. This topological torus is also often called the Clifford torus. In fact, S3 is filled out by a family of nested tori in this manner (with two degenerate circles), a fact which is important in the study of S3 as a fiber bundle over S2 (the Hopf bundle).
The surface described above, given the relative topology from R3, is homeomorphic to a topological torus as long as it does not intersect its own axis. A particular homeomorphism is given by stereographically projecting the topological torus into R3 from the north pole of S3.
Intuitively speaking, this means that a closed path that circles the torus' "hole" (say, a circle that traces out a particular latitude) and then circles the torus' "body" (say, a circle that traces out a particular longitude) can be deformed to a path that circles the body and then the hole. So, strictly 'latitudinal' and strictly 'longitudinal' paths commute. An equivalent statement may be imagined as two shoelaces passing through each other, then unwinding, then rewinding.
If a torus is punctured and turned inside out then another torus results, with lines of latitude and longitude interchanged. This is equivalent to building a torus from a cylinder, by joining the circular ends together, in two different ways: around the outside like joining two ends of a garden hose, or through the inside like rolling a sock (with the toe cut off). Additionally, if the cylinder was made by gluing two opposite sides of a rectangle together, choosing the other two sides instead will cause the same reversal of orientation.
The 2-torus double-covers the 2-sphere, with four ramification points. Every conformal structure on the 2-torus can be represented as a two-sheeted cover of the 2-sphere. The points on the torus corresponding to the ramification points are the Weierstrass points. In fact, the conformal type of the torus is determined by the cross-ratio of the four points.
The torus has a generalization to higher dimensions, the n-dimensional torus, often called the n-torus or hypertorus for short. (This is one of two different meanings of the term "n-torus".) Recalling that the torus is the product space of two circles, the n-dimensional torus is the product of n circles. That is:
The 1-torus is just the circle: T1 = S1. The torus discussed above is the 2-torus, T2. And similar to the 2-torus, the n-torus, Tn can be described as a quotient of Rn under integral shifts in any coordinate. That is, the n-torus is Rn modulo the action of the integer lattice Zn (with the action being taken as vector addition). Equivalently, the n-torus is obtained from the n-dimensional hypercube by gluing the opposite faces together.
An n-torus in this sense is an example of an n-dimensional compact manifold. It is also an example of a compact abelian Lie group. This follows from the fact that the unit circle is a compact abelian Lie group (when identified with the unit complex numbers with multiplication). Group multiplication on the torus is then defined by coordinate-wise multiplication.
Toroidal groups play an important part in the theory of compact Lie groups. This is due in part to the fact that in any compact Lie group G one can always find a maximal torus; that is, a closed subgroup which is a torus of the largest possible dimension. Such maximal tori T have a controlling role to play in theory of connected G. Toroidal groups are examples of protori, which (like tori) are compact connected abelian groups, which are not required to be manifolds.
Automorphisms of T are easily constructed from automorphisms of the lattice Zn, which are classified by invertible integral matrices of size n with an integral inverse; these are just the integral matrices with determinant ±1. Making them act on Rn in the usual way, one has the typical toral automorphism on the quotient.
The fundamental group of an n-torus is a free abelian group of rank n. The k-th homology group of an n-torus is a free abelian group of rank n choose k. It follows that the Euler characteristic of the n-torus is 0 for all n. The cohomology ring H•(Tn, Z) can be identified with the exterior algebra over the Z-module Zn whose generators are the duals of the n nontrivial cycles.
As the n-torus is the n-fold product of the circle, the n-torus is the configuration space of n ordered, not necessarily distinct points on the circle. Symbolically, Tn = (S1)n. The configuration space of unordered, not necessarily distinct points is accordingly the orbifold Tn/Sn, which is the quotient of the torus by the symmetric group on n letters (by permuting the coordinates).
For n = 2, the quotient is the Möbius strip, the edge corresponding to the orbifold points where the two coordinates coincide. For n = 3 this quotient may be described as a solid torus with cross-section an equilateral triangle, with a twist; equivalently, as a triangular prism whose top and bottom faces are connected with a 1/3 twist (120°): the 3-dimensional interior corresponds to the points on the 3-torus where all 3 coordinates are distinct, the 2-dimensional face corresponds to points with 2 coordinates equal and the 3rd different, while the 1-dimensional edge corresponds to points with all 3 coordinates identical.
These orbifolds have found significant applications to music theory in the work of Dmitri Tymoczko and collaborators (Felipe Posada and Michael Kolinas, et al.), being used to model musical triads.
In three dimensions, one can bend a rectangle into a torus, but doing this typically stretches the surface, as seen by the distortion of the checkered pattern.
Seen in stereographic projection, a 4D flat torus can be projected into 3-dimensions and rotated on a fixed axis.
The flat torus is a torus with the metric inherited from its representation as the quotient, R2/L, where L is a discrete subgroup of R2 isomorphic to Z2. This gives the quotient the structure of a Riemannian manifold. Perhaps the simplest example of this is when L = Z2: R2/Z2, which can also be described as the Cartesian plane under the identifications (x, y) ~ (x + 1, y) ~ (x, y + 1). This particular flat torus (and any uniformly scaled version of it) is known as the "square" flat torus.
This metric of the square flat torus can also be realised by specific embeddings of the familiar 2-torus into Euclidean 4-space or higher dimensions. Its surface has zero Gaussian curvature everywhere. Its surface is "flat" in the same sense that the surface of a cylinder is "flat". In 3 dimensions one can bend a flat sheet of paper into a cylinder without stretching the paper, but you cannot then bend this cylinder into a torus without stretching the paper (unless you give up some regularity and differentiability conditions, see below).
A simple 4-dimensional Euclidean embedding of a rectangular flat torus (more general than the square one) is as follows:
where R and P are constants determining the aspect ratio. It is diffeomorphic to a regular torus but not isometric. It can not be analytically embedded (smooth of class Ck, 2 ≤ k ≤ ∞) into Euclidean 3-space. Mapping it into 3-space requires you to stretch it, in which case it looks like a regular torus, for example, the following map
If R and P in the above flat torus form a unit vector (R, P) = (cos(η), sin(η)) then u, v, and η can be used to parameterize the unit 3-sphere in a parameterization associated with the Hopf map. In particular, for certain very specific choices of a square flat torus in the 3-sphere S3, where η = π/4 above, the torus will partition the 3-sphere into two congruent solid tori subsets with the aforesaid flat torus surface as their common boundary. One example is the torus T defined by
Other tori in S3 having this partitioning property include the square tori of the form Q⋅T, where Q is a rotation of 4-dimensional space R4, or in other words Q is a member of the Lie group SO(4).
It is known that there exists no C2 (twice continuously differentiable) embedding of a flat torus into 3-space. (The idea of the proof is to take a large sphere containing such a flat torus in its interior, and shrink the radius of the sphere until it just touches the torus for the first time. Such a point of contact must be a tangency. But that would imply that part of the torus, since it has zero curvature everywhere, must lie strictly outside the sphere, which is a contradiction.) On the other hand, according to the Nash-Kuiper theorem, proven in the 1950s, an isometric C1 embedding exists. This is solely an existence proof, and does not provide explicit equations for such an embedding.
In April 2012, an explicit C1 (continuously differentiable) embedding of a flat torus into 3-dimensional Euclidean space R3 was found. It is similar in structure to a fractal as it is constructed by repeatedly corrugating a normal torus. Like fractals, it has no defined Gaussian curvature. However, unlike fractals, it does have defined surface normals. It "is" a flat torus in the sense that as metric spaces, it is isometric to a flat square torus. (These infinitely recursive corrugations are used only for embedding into three dimensions; they are not an intrinsic feature of the flat torus.) This is the first time that any such embedding was defined by explicit equations, or depicted by computer graphics.
Genus g surface
In the theory of surfaces there is another object, the "genus" g surface. Instead of the product of n circles, a genus g surface is the connected sum of g two-tori. To form a connected sum of two surfaces, remove from each the interior of a disk and "glue" the surfaces together along the boundary circles. To form the connected sum of more than two surfaces, sum two of them at a time until they are all connected. In this sense, a genus g surface resembles the surface of g doughnuts stuck together side by side, or a 2-sphere with g handles attached.
As examples, a genus zero surface (without boundary) is the two-sphere while a genus one surface (without boundary) is the ordinary torus. The surfaces of higher genus are sometimes called n-holed tori (or, rarely, n-fold tori). The terms double torus and triple torus are also occasionally used.
The classification theorem for surfaces states that every compact connected surface is topologically equivalent to either the sphere or the connect sum of some number of tori, disks, and real projective planes.
Polyhedra with the topological type of a torus are called toroidal polyhedra, and have Euler characteristic V − E + F = 0. For any number holes, the formula generalizes to V − E + F = 2 − 2N, where N is the number of holes.
The term "toroidal polyhedron" is also used for higher-genus polyhedra and for immersions of toroidal polyhedra.
This section needs expansion. You can help by adding to it. (April 2010)
The homeomorphism group (or the subgroup of diffeomorphisms) of the torus is studied in geometric topology. Its mapping class group ( the connected components of the homeomorphism group) is isomorphic to the group GL(n, Z) of invertible integer matrices, and can be realized as linear maps on the universal covering space Rn that preserve the standard lattice Zn (this corresponds to integer coefficients) and thus descend to the quotient.
At the level of homotopy and homology, the mapping class group can be identified as the action on the first homology (or equivalently, first cohomology, or on the fundamental group, as these are all naturally isomorphic; also the first cohomology group generates the cohomology algebra:
Since the torus is an Eilenberg–MacLane space K(G, 1), its homotopy equivalences, up to homotopy, can be identified with automorphisms of the fundamental group); that this agrees with the mapping class group reflects that all homotopy equivalences can be realized by homeomorphisms – every homotopy equivalence is homotopic to a homeomorphism – and that homotopic homeomorphisms are in fact isotopic (connected through homeomorphisms, not just through homotopy equivalences). More tersely, the map Homeo(Tn) → SHE(Tn) is 1-connected (isomorphic on path-components, onto fundamental group). This is a "homeomorphism reduces to homotopy reduces to algebra" result.
Thus the short exact sequence of the mapping class group splits (an identification of the torus as the quotient of Rn gives a splitting, via the linear maps, as above):
so the homeomorphism group of the torus is a semidirect product,
The mapping class group of higher genus surfaces is much more complicated, and an area of active research.
Coloring a torus
If a torus is divided into regions, then it is always possible to color the regions with no more than seven colors so that neighboring regions have different colors. (Contrast with the four color theorem for the plane.)
Cutting a torus
A solid torus of revolution can be cut by n (> 0) planes into maximally
The first 11 numbers of parts, for 0 ≤ n ≤ 10 (including the case of n = 0, not covered by the above formulas), are as follows:
- Algebraic torus
- Angenent torus
- Annulus (mathematics)
- Clifford torus
- Complex torus
- Dupin cyclide
- Elliptic curve
- Irrational cable on a torus
- Joint European Torus
- Klein Bottle
- Loewner's torus inequality
- Maximal torus
- Period lattice
- Real projective plane
- Spiric section
- Toric lens
- Toric section
- Toric variety
- Toroidal and poloidal
- Torus-based cryptography
- Torus knot
- Umbilic torus
- Villarceau circles
- Nociones de Geometría Analítica y Álgebra Lineal, ISBN 978-970-10-6596-9, Author: Kozak Ana Maria, Pompeya Pastorelli Sonia, Verdanega Pedro Emilio, Editorial: McGraw-Hill, Edition 2007, 744 pages, language: Spanish
- Allen Hatcher. Algebraic Topology. Cambridge University Press, 2002. ISBN 0-521-79540-0.
- V. V. Nikulin, I. R. Shafarevich. Geometries and Groups. Springer, 1987. ISBN 3-540-15281-4, ISBN 978-3-540-15281-1.
- "Tore (notion géométrique)" at Encyclopédie des Formes Mathématiques Remarquables
- "A Guide to the Classification Theorem for Compact Surfaces" (PDF). Retrieved 3 March 2018.
- "Equations for the Standard Torus". Geom.uiuc.edu. 6 July 1995. Archived from the original on 29 April 2012. Retrieved 21 July 2012.
- "Torus". Spatial Corp. Archived from the original on 13 December 2014. Retrieved 16 November 2014.
- Weisstein, Eric W. "Torus". MathWorld.
- "poloidal". Oxford English Dictionary Online. Oxford University Press. Retrieved 10 August 2007.
- Tymoczko, Dmitri (7 July 2006). "The Geometry of Musical Chords" (PDF). Science. 313 (5783): 72–74. doi:10.1126/science.1126287. PMID 16825563. Archived (PDF) from the original on 25 July 2011.
- Tony Phillips, Tony Phillips' Take on Math in the Media Archived 5 October 2008 at the Wayback Machine., American Mathematical Society, October 2006
- Filippelli, Gianluigi (27 April 2012). "Doc Madhattan: A flat torus in three dimensional space". Docmadhattan.fieldofscience.com. doi:10.1073/pnas.1118478109. Archived from the original on 25 June 2012. Retrieved 21 July 2012.
- "Mathematicians Produce First-Ever Image of Flat Torus in 3D | Mathematics". Sci-News.com. 18 April 2012. Archived from the original on 1 June 2012. Retrieved 21 July 2012.
- "Mathematics : first-ever image of a flat torus in 3D - CNRS Web site - CNRS". Archived from the original on 5 July 2012. Retrieved 21 July 2012.
- "Flat tori finally visualized!". Math.univ-lyon1.fr. 18 April 2012. Archived from the original on 18 June 2012. Retrieved 21 July 2012.
- Weisstein, Eric W. "Torus Cutting". MathWorld.
|Look up torus in Wiktionary, the free dictionary.|
|Wikimedia Commons has media related to Torus.|
- Creation of a torus at cut-the-knot
- "4D torus" Fly-through cross-sections of a four-dimensional torus.
- "Relational Perspective Map" Visualizing high dimensional data with flat torus.
- Séquin, Carlo H. "Topology of a Twisted Torus - Numberphile" (video). Brady Haran. Retrieved 27 January 2014. | <urn:uuid:56f3a908-9da7-4deb-a263-9b5d549d68a5> | 4.09375 | 4,961 | Knowledge Article | Science & Tech. | 51.712051 | 95,505,068 |
Using Intel’s Secure Key (RDRAND) in MS Visual C++ 2010
Among the features added to Intel’s 3rd-Generation Core i* processors is a Digital Random Number Generator (DRNG) backed by an on-die hardware entropy source. This new hardware feature is made available to software via the also-new RDRAND instruction.
If you’re still using the compiler which shipped with Visual C++ 2010, it seems the only way to leverage the DRNG is either via a third-party library (the one available from Intel’s website was, as of writing, broken) or by dipping into assembly programming. Of these the latter comes with a couple of catches: the mnenomic/intrinsic for the instruction is not available for older assemblers/compilers, and the assembly is slightly different for 32- and 64-bit environments.
This project demonstrates testing whether the host processor supports the RDRAND instruction as well as invoking it (via assembly). When built for 32-bit CPUs, the assembly is inlined; when built for 64-bit CPUs, the assembly is linked in via an exernal module (the 64-bit compiler in Visual Studio 2010 does not support inline assembly).
For the most part, the project simply follows the Software Implementation Guide from Intel. Additionally, it demonstrates invoking the instruction via its opcode, and linking a module implemented in assembly into a VC++ project.
UPDATE (29/05/2016): added a function to use RDRAND to generate a random value within a specified range, and refactored the logic into a static library and wrapped it with a dynamic library for use with P/Invoke. | <urn:uuid:c0e49391-86a1-4e5a-a477-e4d0b39a8a44> | 2.5625 | 352 | Personal Blog | Software Dev. | 36.80021 | 95,505,069 |
After waiting more than six weeks for the optimal weather and auroral conditions to occur, scientists successfully launched four rockets within six minutes from Poker Flat Research Range early Thursday morning.
The first rocket, a single-stage Black Brant, carried about 800 pounds of instruments through the aurora to measure light and small-scale weather in the upper atmosphere. The instrumented payload launched at 12:55 a.m., while the temperature at Poker Flat was 25 below zero. The payload flew 191 kilometers high, separated from the rocket, lowered to the ground by parachute and landed south of the Brooks Range where Poker Flat personnel will fly to retrieve it Friday, weather permitting.
As part of the same experiment, three two-stage Taurus Orion rockets launched at 12:57 a.m., 12:59 a.m. and 1:01 a.m., each releasing brilliant blue-green chemical trails to trace wind in the upper atmosphere. The trails could be seen for 20 minutes from locations in interior Alaska, as far north of the Brooks Range and as far east as the Canadian border.
The trimethyl aluminum released by the three rockets is a benign chemical that breaks down into water, carbon dioxide and aluminum oxide, and was used for the experiment since it glows when it contacts oxygen. When released early Thursday, the chemical trails were visible first as comets, then as corkscrews of light as the releases traced upper atmospheric winds. Another effect of the chemical launch was the creation of a brilliant glob of auroral light caused by the trimethyl aluminum referred to as “artificial aurora.”
With cooperation from the clear early morning sky, the luminous trails were successfully photographed at ground stations at the range and at Coldfoot and Fort Yukon to help scientists determine the speed and direction of wind in the upper atmosphere. Similar in concept to the jet stream, the wind in the upper atmosphere is created by electrical currents in the aurora.
Wind created by the aurora can affect the orbits of satellites and interfere with long range radio transmissions. Information gained from the rocket flights will help scientists design, track and operate satellites and other manmade space systems more effectively.
Clemson University Professor Miguel Larsen, from South Carolina, is the principal investigator for the chemical release rockets. Andrew Christensen, special envoy for the National Oceanic and Atmospheric Administration, is the principal investigator for the instrumented payload.
Five more rockets are scheduled to launch from Poker Flat in March. The research range is located about 30 miles northeast of Fairbanks, and is owned and operated by the Geophysical Institute of the University of Alaska Fairbanks under contract to NASA.
For more information, contact Geophysical Institute Public Relations Specialist Vicki Daniels at (907) 474-5823, or via email at firstname.lastname@example.org.
Principal Investigator Miguel Larsen will be available for comment Friday from 10:00 a.m. until 1:00 p.m. via phone at (864) 656-5309. | <urn:uuid:a2008a51-4489-4861-b23c-5089da5a6403> | 3.296875 | 619 | News (Org.) | Science & Tech. | 46.245211 | 95,505,097 |
> 6) This shows that if we are in a massive computer running in
> a universe, then (supposing we know it or believe it) to
> predict the future of any experiment we decide to carry one
> (for example testing A or B) we need to take into account all
> reconstitutions at any time of the computer (in the relevant
> state) in that universe, and actually also in any other
> universes (from our first person perspective we could not be
> aware of the difference of universes from inside the computer).
Yes, but this is just a fancy version of the good old-fashioned Humean
problem of induction, isn't it?
Indeed, predicting the future on a sound "a priori" basis is not
possible. One must make arbitrary assumptions in order to guide
This is a limitation, not of the "comp" hypothesis specifically, but of
the notion of prediction itself.
You cannot solve the problem of induction with or without "comp", so I
don't think you should use problem-of-induction related difficulties as
an argument against "comp."
In fact, "comp" comes with a kind of workaround to the problem of
induction, which is: To justify induction, make an arbitrary assumption
of a certain universal computer, use this to gauge simplicity, and then
judge predictions based on their simplicity (to use a verbal shorthand
for a lot of math a la Solomonoff, Levin, Hutter, etc.). This is not a
solution to the problem of induction (which is that one must make
arbitrary assumptions to do induction), just an elegant way of
introducing the arbitrary assumptions.
So, in my view, we are faced with a couple different ways of introducing
the arbitrary assumptions needed to justify induction:
1) make an arbitrary assumption that the apparently real physical
universe is real
2) make an arbitrary assumption that simpler hypotheses are better,
where simplicity is judged by some fixed universal computing system
There is no scientific (i.e. inductive or deductive) way to choose
between these. From a human perspective, the choice lies outside the
domain of science and math; it's a metaphysical or even ethical choice.
-- Ben Goertzel | <urn:uuid:aa2cb4b8-9122-4c66-b63e-6c1c143e5f42> | 2.609375 | 479 | Comment Section | Science & Tech. | 27.387635 | 95,505,098 |
In a broad sense, spectral mixing phenomenology in multispectral and hyperspectral imagery can be treated in two ways, depending on how the materials in a scene are presumed to be mixed. A linear mixing model is suitable in cases where materials are presumed to be nonoverlapping areas and can be mathematically expressed as a weighted linear combination, where the weights are associated with the abundances of each material. The endmembers are spectra (real or ideal) representing unique materials in a given image such as water, soil, and vegetation. Abundances are the percentage of each endmember within a given pixel. On the other hand, it is well known that intimately mixed materials frequently exhibit nonlinear spectral mixing. Granular materials, such as soils, are often intimate mixtures of numerous different inorganic (minerals) and organic (humic) substances. And since soils are often significant constituents of spectral remote sensing scenes, intimate mixing may safely be assumed to be a common phenomenon. Another case is the mixing of fluids, such as oil and water, as occurred during the Deepwater Horizon oil spill disaster in 2010. We will consider this second case in more detail later.
Linear mixing is modeled as a linear combination of the spectra of multiple endmembers. The problem is posed in a number of ways using theory from linear statistical models with variations that impose either physical or sparseness constraints.22.214.171.124.6.–7 Sites in a scene are labeled as containing one, two, or perhaps many endmembers. Some approaches begin with what is sometimes referred to as a full model, where the full model contains all possible variables (endmembers), and subsequently eliminates variables that do not contribute to the statistical significance (e.g., using an F-statistic) of the model.8 Alternatively, other approaches are synonymous with stepwise regression, where the process begins with a pair of variables (endmembers) and introduces new variables if they contribute significantly to the model.6,8
Recent research on linear unmixing has resulted in a number of alternative approaches. A stochastic mixture model (SMM) has been proposed with the goal to incorporate inherent endmember variance.9 Related to the SMM is a continuous version of the method, known as the normal compositional model (NCM).10 In the NCM, the mean vectors and covariance matrices of the endmembers are assumed adequate to capture their spectral variability. However, a big challenge to the NCM is parameter estimation. Very recently, a Bayesian algorithm called “normal endmember spectral unmixing” has been proposed to enhance parameter estimation performed in the NCM.11 Research has also been performed on the use of sparse modeling for modeling linear mixtures in hyperspectral imagery,12,13 a data-driven stochastic approach,14 as well as some advanced methods of nonnegative matrix factorization (NMF), such as projection-based NMF,15 and an adaptive sparsity-constrained NMF.16
Intimate mixtures exhibit nonlinear spectral mixing behavior and are modeled as nonlinear combinations of spectra from multiple endmembers. In cases of intimate mixing, a linear spectral unmixing inversion applied to such nonlinear mixtures will yield subpixel material abundance estimates that do not equal the true values of the mixture constituents. An example of this is provided by Keshava and Mustard.17
An intimate mixture model can be described by nonlinear functions, which are justified by Hapke scattering theory18 and photometric phase functions.1920.–21 In such an approach, reflectance is converted to single-scattering albedo (SSA). An exact model can be difficult to obtain, but can show up to 30% improvement in measurements over the linear mixing model when intimate mixtures are present.22 Many years ago studies showed success in using a constrained energy minimization method and other linear methods applied to reflectance spectra transformed to SSA data.2324.–25
More recently, several alternative methods of spectral unmixing of nonlinear spectral mixtures have been proposed. Reviews of nonlinear spectral mixture analysis are given in Refs. 17, 26, 27, and 28 and references cited therein.
Kernel functions provide a way to generalize linear algorithms to nonlinear data.32,33 In the cases of detection and classification applications, they can induce high-dimensional feature spaces. In these spaces, previously nonseparable classes are made linearly separable. Thus, linear methods can be applied in this new feature space that provides nonlinear boundaries back in the original data space. Another example is the kernel principal component analysis method.34 The kernel, in this case, is not used to induce a high-dimensional space, but is used to better match the data structure through nonlinear mappings. It is in this mode that kernels can be applied to obtain abundances in nonlinear mixtures while essentially using a linear mixture model. What is more appealing is that the physics suggests that such a method is ideal if one can model the kernel correctly.
A limitation of earlier kernel algorithms for material detection and scene classification is that they produced abundance estimates that did not meet nonnegativity and sum-to-one constraints. This was overcome by the development of a kernel fully constrained least squares (KFCLS) method that computes kernel-based abundance estimates to meet the physical (nonnegativity and sum-to-one) abundance constraints.35 Further investigation of the KFCLS method has resulted in (1) the development of a generalized kernel for linear and intimate (nonlinear) mixtures37 and (2) an adaptive kernel-based technique for mapping linear and intimate nonlinear mixtures.38 The generalized kernel and adaptive techniques provide a way to adaptively estimate a mixture model suitable to the degree of nonlinearity that may be occurring at each pixel in a scene. This is important because a scene may contain both linear and intimate mixtures and it is not always known a priori which model is appropriate on a pixel-by-pixel basis. This problem was investigated in a study performed by Broadwater and Banerjee.38 Building upon their work, a study investigating the behavior of the generalized KFCLS and adaptive kernel-based techniques was performed using both user-defined and automatically generated endmembers determined by the support vector data description algorithm.39
An investigation using laboratory data was also performed comparing the performance of the fully constrained least squares (FCLS) method applied to spectra converted to SSA with the generalized KFCLS applied to reflectance spectra and another recently published kernel-based method, the “K-Hype” kernel applied to reflectance spectra.40 One conclusion of this study: similar accuracy in abundance estimates can be achieved using the SSA-based method as compared to the generalized KFCLS, but with a much faster computation time. However, not all kernel methods are the same. The study also determined the “K-Hype” kernel method was not worth pursuing further for this type of analysis.
Overall, the impact of nonlinear spectral mixing on algorithm results must be well understood if we are to achieve a major goal of hyperspectral imaging (HSI): the geospatial mapping and quantification of materials that comprise remotely sensed scenes. In this study, we further this understanding by investigating both phenomenology-based SSA and mathematical-based kernel methods; and we investigate some issues noted while conducting earlier studies.39 The experiments include both laboratory and airborne HSI.
There has been a relative paucity of real-spectral image data sets that contain well-characterized intimate mixtures that can be used for algorithm development and testing. An important aspect of this study is the generation of a set of HSI data that are truly nonlinear spectral mixtures for which very precise truth information exists. Our laboratory experiment is performed on highly controlled data containing predetermined, nonlinear (intimate) mixtures of two materials. In addition, airborne experiments are performed on a scene of HyMap data collected over Oahu, Hawaii, and a scene collected by the airborne visible/infrared spectrometer (AVIRIS) instrument over the oil spill region in the Gulf of Mexico during the Deepwater Horizon oil incident.41,42
Description of Algorithms
Fully Constrained Least Squares and Linear Kernel-Based Mixing Models
The fully constrained least squares (FCLS)7 spectral mixing model can be expressed as
Alternatively, the linear model can also be posed as a special case of the kernel-based mixing model derived previously by Broadwater et al.,35 where the abundances of mixture components are estimated through the process
Either way, once is obtained, the estimator vector for a mixed spectral signature is
The root mean-squared error (RMSE) between the observed and estimated spectral signature is measured as
FCLS has been shown to be successful for modeling linear mixing phenomenology.7 In the present study, we will be using FCLS in two ways. The method will be used as a benchmark to compare with the proposed nonlinear methods. The method will also be used in one of the nonlinear approaches, where we will apply the FCLS to spectra that have been converted to SSA, as discussed below in Sec. 2.2.
Using Hapke Theory: Fully Constrained Least Squares Applied to Single-Scattering Albedo Spectra
Previous studies indicate that intimate (nonlinear or macroscopic) spectral mixtures in reflectance space may be linearized when transformed into SSA space. In order to understand this, we first mention the relationship between reflectance and SSA. Reflectance is the ratio of reflected radiance to incident irradiance.43 SSA is the ratio of the total amount of power scattered to the total power removed from the wave. If we assume the materials of interest are Lambertian, then we can consider the case of hemispherical reflectance and the mathematical relationship between reflectance and SSA can then be written19
For a number of intimately mixed materials, the average SSA can be written as38
According to Eq. (7), intimate mixtures are a linear mixture of the albedos and these are nonlinearly mapped to reflectance by Eq. (6). This linear mixing of the SSAs can be seen trivially, as we can write Eq. (7) as38 Noting the form of Eqs. (8) and (9) as compared to Eq. (1) for linear mixing, the terminology for relative geometric cross section and “abundance” can be used interchangeably.
Accordingly, we investigate this behavior by applying a linear mixing method on albedo; specifically, by applying the FCLS method on data that has been converted to SSA.
Conversion to SSA is described further in Resmini et al.23 and Resmini.24 Both studies follow Hapke;18 and Mustard and Pieters1920.–21 assuming the reflectance spectra are bidirectional. The expressions to transform reflectance spectra to SSA are given by Eqs. (10) and (11) for bidirectional (bd) reflectance and for hemispherical-directional (hd) reflectance, respectively. In the derivation of both expressions, the phase angle is large enough that the opposition effect is assumed negligible.2.1, Hapke18); , , , and are the same as in Eq. (6). The two different equations can be used to generate the two sets of SSA spectra for bd and hd reflectance.
In the experiments that follow, we refer to this approach as the “FCLS on SSA” method, or simply denote it as the SSA method. The conversion of reflectance spectra to SSA using Eqs. (10) and (11) is very fast as compared to the generalized kernel least squares (GKLS) method.
Generalized Kernel Fully Constrained Least Squares (Fixed and Automated)
Choosing the linear function for the kernel in Eq. (2) works well for modeling linear mixtures; however, it is not a suitable kernel for intimate mixtures. A physics-inspired kernel was proposed and shown to provide significantly improved behavior to model nonlinear mixtures, but a result of that effort was that although each kernel provides good results for the type of mixing intended, only one kernel or the other could be used, for either linear mixtures or intimate mixtures, but not both.36
The kernel approach was further developed by Broadwater and Banerjee37,38 into a generalized method for adaptive linear and intimate mixtures. This method is motivated by attempting to simulate Hapke theory for SSA by making use of the kernel
In this study, we investigate a fixed- generalized kernel least square” (fixed- GKLS). The computation is similar in form to Eq. (2) except the minimization is done according to
For a specified , we implement this by transforming the observed and into kernel space
Equation (16) has the same constraints as Eq. (1) and the abundance estimates can be computed in the same manner as Eq. (1). The estimated mixed spectra in kernel space is computed as
The estimated error in kernel space is
The estimated error vector in the original space is computed as Eq. (4), where the components of the estimated mixed spectra are computed using the inverse transform of Eq. (14). The RMSE is computed by Eq. (5).
In addition to a fixed- GKLS implementation, an automated GKLS method is investigated that attempts to select the most appropriate gamma according to44 In computing the optimization, we seek to achieve a minimum of the model’s RMSE. In this manner, at least theoretically, the automated GKLS method has the ability to respond differently to differing degrees of nonlinearity to select the best gamma and compute more precise estimates of abundance.
Description of Experiments
Overview of Experiments
Three experiments are performed in this study. The first is an experiment conducted in the laboratory. This is an important aspect of our investigation: a set of HSI data is generated that are truly nonlinear spectral mixtures for which very precise truth information is documented. The laboratory experiment is performed on highly controlled data containing predetermined, nonlinear (intimate) mixtures of two materials. In this experiment, two granular materials were custom fabricated and mechanically mixed to form intimate mixtures. The materials are spherical beads of didymium glass and soda-lime glass, both ranging in particle size from 63 to . The glass beads materials are both translucent. Their chemical composition, densities, and particle size range are well known. The mixtures, which exhibit largely nonlinear spectral mixing, were then observed with a visible/near-infrared (VNIR; 400 to 900 nm) HSI microscope. The second and third experiments are conducted in the field with airborne hyperspectral imagery: the second experiment is performed using data from the HyMap sensor acquired during a campaign by the Navy Research Laboratory (NRL) over Oahu, Hawaii, from January 24 to February 1, 2009. The third experiment utilizes AVIRIS data collected over the oil spill region in the Gulf of Mexico during the Deepwater Horizon oil incident. The SSA, FCLS, and GKLS methods are each applied to data from the three collections.
Experiment 1: Laboratory Experiment
A Resonon Pika II imaging spectrometer with a Xenoplan 1.4/23-0902 objective lens shown in Fig. 1(a) is used to measure the didymium and soda-lime glass bead mixtures.45 We also used an Edmund Optics Gold Series telecentric lens. However, this lens gives data, providing a spatial resolution higher than what was required for the analyses. Consequently, the Edmund data are not reported in our results.
The Pika II is mounted nadir-looking at a mechanical translation table on which the sample to be imaged is placed. This device is a pushbroom sensor with a slit aperture, thus the need for a translation table to move the sample to facilitate hyperspectral image cube formation. The height of the sensor above the table is user selectable; a height was chosen such that all mixtures are captured in the same scene so that the data have a ground sample distance of . Though capable of acquiring 240 bands from 400 to 900 nm, the sensor was configured to acquire 80 bands by binning (spectrally by three) resulting in a sampling interval of and high signal-to-noise ratio spectra. Four quartz–tungsten–halogen (QTH) lamps are used for illumination approximating a hemispherical-directional illumination/viewing geometry.
Sensor and translation table operation, data acquisition, and data calibration are achieved by software that runs on a laptop computer. Calibration consists of a measurement of dark frame data (i.e., acquiring a cube with the lens cap on) and a measurement of a polytetrafluoroethylene (PTFE) reference plaque (large enough to entirely fill the field-of-view). Then, for each HSI cube measured, the sensor’s software first subtracts the dark data and then uses the PTFE data (also dark subtracted) to ratio the spectral measurements to give relative reflectance (also known as reflectance factor: Hapke18 and Schott43).
Three binary mixtures (and the two endmembers) are constructed and emplaced in the wells of a 96-well sample plate: 0/100%, 25/75%, 50/50%, 80/20%, and 100/0% of didymium/soda-lime (percentages by volume). This was done as follows: five cells of a 96-well sample plate, spray-painted flat black, were filled with the various glass bead mixtures; this is shown in Fig. 2. The volume of each cell is (0.33 mL). Three binary mixtures and the two 100% endmembers are constructed and emplaced in five of the wells of the 96-well sample plate: 0/100%, 25/75%, 50/50%, 80/20%, and 100/0% didymium/soda-lime. The glass beads and their mixtures also display subtle, though interesting, gonioapparent changes in color. We remark that the glass bead particle size range is much larger than the VNIR wavelengths used in this analysis.
Fifteen data Pika II HSI cubes were acquired; however, we focus here on the analysis of one cube comprised of 640 samples, 400 lines, and 75 bands ranging from 0.434 to . Of the 80 bands acquired the first five were discarded due to noise content.
Training and test data were extracted from the selected hyperspectral cube. Figure 2 shows polygons defining the training and test regions drawn on top of a red–green–blue (RGB) color composite context image. The training regions are shown in Fig. 2(a) and the test regions are shown in Fig. 2(b). Note that none of the test regions overlapped the training regions. Figure 2(a) shows the small training polygon regions defining the three training endmembers: DiDy (100%), lime (100%), and background. Figure 2(b) shows the test areas used to quantitatively measure the performance of the algorithms. Five regions were defined, corresponding to the five mixtures 100% DiDy, 75/25% DiDy/lime, 50/50% DiDy/lime, 25/75% DiDy/lime, and 0/100% DiDy/lime. Two additional test regions of background spectra were also extracted.
Figure 3 shows plots of the mean vectors of the spectral data extracted within the training polygons shown in Fig. 2(a). These image-derived training endmembers, as just described, are used to investigate the three methods described in Sec. 2: (1) FCLS applied in reflectance space, (2) GKLS applied in reflectance space, and (3) FCLS applied in SSA space. For purposes of conciseness in reporting results, these methods henceforth will be referred to as the FCLS, GKLS, and SSA methods, respectively.
Numerous factors affect the performance of the methods. In particular, three factors affecting the performance of all the methods are: (1) the number the endmembers used in a model, (2) how well these endmembers span the space in which the mixing occurs, and (3) the RMSE threshold for eliminating bad fits between the observed and model-estimated spectra. In addition, the GKLS method uses a parameter to determine the nonlinear behavior introduced by the kernel. We test the GKLS method at fixed values of : 0.1, 0.5, 1.0, 2.0, 3.0, 4.0, 5.0, and 6.0, as well as the automated GKLS method. For the SSA method, performance may be affected by the type of SSA conversion: hemispherical or bidirectional. If bidirectional, the view and illumination angles are factors. Although the laboratory instrument has approximately hemispherical illumination, we will conduct trials for SSA conversions made assuming both bidirectional reflectance with nadir view and illumination angles as defined by Eq. (10), denoted as SSA, as well as hemispherical reflectance as defined by Eq. (11), which we will refer to as SSA-H.
The algorithms are applied to the entire scenes. Both qualitative results (shown by images of the algorithm output) and quantitative results (applied in the test regions) are given.
Experiment 2: Airborne Experiment in Hawaii
Experiment 2 tests the proposed methods using a cube of HyMap data collected during a campaign by the NRL over Oahu, Hawaii. Flight line Az148 (acquired on February 1, 2009) was selected and analysis was performed on a subscene of the original scene with a size of 400 samples by 250 lines over government-owned property near the Waimanalo region of Oahu. The imagery consists of 125 bands, has a spectral range of 0.45 to with a spectral resolution of about , and a ground sampling distance (GSD) of . The calibrated, at-aperture radiance data were converted to reflectance by the NRL using their Tafkaa,46 atmospheric compensation tool. Tafkaa is a heavily modified version of ATREM.47 The reflectance spectra are scaled by a factor of 10,000 and converted to 16-bit integers. An RGB normal-color composite of this scene is shown in Fig. 4.
Ten fabric panels of varying size were placed at three different sites (and in varying local backgrounds) prior to the image collection. These panels are impermeable Tyvek-like house wrap, near white in appearance. The sizes of these panels are listed in Table 1. Figure 5 shows aerial photographs of the three sites. The locations of the panels at each site are annotated. Numerous ground photos were also taken of the panels. Figure 6 shows ground-truth photos for three of these panels (TY-4, TY-5, and TY-7).
Target sizes: the sizes of the 10 target panels are listed in this table. The measurements were made in units of feet, but converted to meters for easier comparison to the estimated 4.0 GSD of the airborne HyMap scene.
Site 1 contains 6 of the 10 panels (TY-4, TY-5, TY-6, TY-7, TY-8, and TY-9). TY-7 is expected to be difficult to detect because of its subpixel size in the HyMap scene and because it is partially occluded by trees. Notice that TY-7 is somewhat difficult to see in the aerial photo shown in Fig. 1; however, it is considerably more difficult to see in the HyMap data because of the reduced spatial resolution. TY-5 is expected to be difficult because of size and shading. Site 2 contains three Tyvek panels (TY-1, TY-2, and TY-3). Site 3 contains one Tyvek panel. During the campaign, spectral measurements were acquired by the USGS and NGA using an ASD spectrometer. However, these field measurements were not used for training in this study. Instead image-derived spectra were used.
Figure 7 shows four of the eight endmember spectra used in experiment 2. Similar to the procedure used in the previous experiment, training regions for eight classes of materials in the scene were defined for use as endmembers. Seven of these belonged to naturally occurring classes, such as sand, gravel, water, and different types of vegetation having regional extent (e.g., multiple pixels). These endmembers were computed as mean spectra within regions defined by polygon overlays (similar to the overlays shown in Fig. 2). However, as shown in Table 1, the Tyvek panels are comparably small and mostly subpixel in extent. TY-6 is the largest panel and is just slightly larger than a full pixel (). The spectral vector from the center point of TY-6 was extracted and used as the Tyvek endmember.
Experiment 3: Airborne Experiment: Deep Water Horizon
Experiment 3 uses a scene that was extracted from AVIRIS data collected during a JPL/USGS airborne campaign during the Deep Water Horizon (DWH) oil spill incident.41,42 As a part of these studies, during the DWH campaign, USGS collected numerous oil samples for laboratory measurements.41 Although precise ground truth is not available due to the dynamically changing environment of an oil spill on water, previous studies using feature-based spectroscopy methods determined the locations of three predominant oil thicknesses in the region.42
Out of the numerous flight lines of AVIRIS acquired on May 17, 2010, run 11 of AVIRIS was selected, which includes the actual incident site of the oil spill. This run was collected at an altitude of 28,000 feet resulting in a GSD of . The AVIRIS scene is 677 samples by 16,835 lines with 224 bands of reflectance data from 0.365 to . As part of the calibration to reflectance, the USGS scaled the imagery by a factor of 20,000. The experiment focused on a subset scene of size 500 pixels by 500 lines by 224 bands. Out of the 224 bands, 97 bands were excluded: The first 39 bands (0.365 to ) were excluded to focus the algorithm on the longer wavelengths away from the visible, the last 10 bands (2.407 to ) were excluded because of low signal-to-noise, and the remaining bands were excluded because of atmospheric absorption.
Figure 8 shows a true-color RGB composite of the subset scene. The scene is mostly water with large portions containing a great diversity of oil thicknesses. Different states of the oil can be seen visibly, appearing in different colors ranging from almost black, to dark brown, to light brown, and to orange.
Figure 9 shows laboratory-measured reflectance spectra of four oil samples at differing thicknesses. Absorption features due to C-H with band centers (bc) at 1.2 and are labeled as F1 and F2.
For purposes of training and defining the endmembers, laboratory spectra for pure oil samples with 4-mm thickness and 1.85-mm thickness are used, as well as an oil/water emulsion having a thickness of 1.0 mm. Because much of the emulsion behavior is difficult to model in the laboratory, user-defined image spectra are also used: orange-colored spectra (in the RGB true-color composite) and water spectra with trace oil.
The FCLS, GKLS, and SSA methods are applied to the AVIRIS data, reporting on the results using the fixed- implementation and the SSA conversion made assuming bidirectional reflectance with nadir input and output angles.
Overview of Results
The results for the three experiments are presented below, showing tables with model diagnostics that provide estimated abundance values for the estimated models and RMSE values for the model errors; abundance maps that provide a visual representation of the abundances of materials found in the scenes; and graphs showing a comparison of the estimated (simulated) spectra estimated by the computed models and the observed spectra at selected points.
Results of Experiment 1
The results for experiment 1 (the laboratory experiment) are shown in Figs. 10 and 11, as well as Tables 1 and 2. Figure 10 shows RGB color abundance maps for four of the trials (, , ). Qualitatively, Figs. 10(a) and 10(b) show poor correspondence to the known mixtures shown in Fig. 2. Figures 10(c) and 10(d) show much better correspondence to the actual abundances given in Fig. 2. The variations in color within the discs containing the three mixtures (75/25, 50/50, and 25/75) are noteworthy. These variations indicate the methods are detecting notable variance in the abundance proportions. This indicates that the attempt to prepare mixture proportions that are as uniform as possible were not completely successful. These variations are real and the abundance estimation methods (particularly, GKLS and SSA) are responding correctly. Static clinging and other interparticle interactions likely account for the clumping and variations observed. Nonetheless, overall population averages captured by regions of interest delineated in Fig. 2(b) covering a large portion of the individual cells yield averages close to those intended in the experiment design phase.
Abundance results: the average estimated abundances are listed for the FCLS, GKLS, and SSA methods in the five test regions. The “truth” for these regions (actual physically measured proportions by volume) is 1.0, 0.788, 0.505, 0.242, and 0.0 for DiDy100, DiD075, DiDy050, DiDy025, and DiDy000, respectively. Results for the GKLS method using a fixed gamma of γ=0.1, γ=0.5, γ=1.0, γ=2.0, γ=3.0, γ=4.0, γ=5.0, and γ=6.0 are given, as well as for the automated GKLS.
Table 2 lists the average estimated abundances for the FCLS, GKLS, SSA, and SSA-H methods in the five test regions. The “truth” (actual physically measured) percent by volume of DiDy for these regions varied slightly from the goal of 100%, 75%, 50% 25%, and 0%. In reality, these were measured as 100%, 78.8%, 50.5%, 24.2%, and 0% for DiDy100, DiD075, DiDy050, DiDy025, and DiDy000, respectively. Results for the GKLS method using a fixed gamma of , , , , , , , and are given, as well as for the automated GKLS. This table shows FCLS to be poor at predicting the abundances for DiDy075 (93.11% predicted versus 78.8% truth) and DiDy050 (74.97% predicted versus 50.5% truth), as well as very poor at predicting DiDy025 (63.88% predicted versus 24.2% truth). The results of GKLS for small values of agree with theoretical expectations of an approximately linear model. Specifically, the prediction of GKLS at is almost exactly the same as the FCLS method.
Out of the eight values tested, GKLS at provides the closest prediction for DiDy50 (49.08% versus 50.5%) and is only slightly worse at predicting the correct abundance than GKLS at .
Figure 11 shows the observed and estimated mixture (averaged) spectra of the 50/50% region using the FCLS, GKLS, and SSA-H methods. Visually, we can see both the GKLS () and SSA-H methods provide a better fit as compared to the FCLS method.
Table 3 lists the RMSE of the fit between the estimated and observed spectral mixtures for selected points in the scene. Except for DiDy at 25% (0.242), the RMSE errors for FCLS were not considerably larger than the errors for the other methods. Yet we know FCLS is poorly predicting the known abundances for these samples. We conclude RMSE is not necessarily a good indicator of a method’s accuracy to predict abundance.
Model diagnostics: the RMSE results of the fit between the estimated and observed spectral mixtures are listed for selected points in the scene. The first column lists the location and planned percentage mix of DiDy for these points. The actual physically measured mixes were 100%, 78.8%, 50.5%, 24.2%, and 0.0%. Bold values indicate lowest RMSE value retrieved for the indicated pixel.
|(x,y) DiDy %||FCLS||GKLS γ=5||GKLS γ=6||SSA||SSA-H|
|(585, 248) 100%||0.0141||0.0293||0.0369||0.0315||0.0320|
|(400, 228) 75%||0.0245||0.0244||0.0248||0.0351||0.0388|
|(324, 049) 50%||0.0357||0.0225||0.0222||0.0129||0.0190|
|(313, 062) 50%||0.0359||0.0318||0.0390||0.0118||0.0175|
|(328, 067) 50%||0.0333||0.0294||0.0533||0.0223||0.0294|
|(223, 314) 25%||0.0526||0.0380||0.0379||0.0140||0.0241|
|(224, 327) 25%||0.0495||0.0383||0.0384||0.0186||0.0298|
|(246, 330) 25%||0.0420||0.0289||0.0636||0.0121||0.0197|
The results in Tables 2 and 3 also show that the automated implementation of the GKLS method was not as successful as the fixed- GKLS (, 4, 5, or 6) for estimating the correct abundance. The automated GKLS method attempts to select the most appropriate based on achieving a minimum of the model’s RMSE. This metric seems to respond to linear and nonlinear mixtures; however, it does not appear to be a reliable metric to determine the degree of nonlinear behavior. RMSE also cannot be used to achieve the most accurate estimate of abundance. Consequently, an alternative approach should be sought for implementing an automated GKLS.
Determining which value of gives the best result for this experiment is difficult. In terms of abundance, clearly provides the best estimate for the DiDy50 mix (49% abundance). However, provides slightly better estimates of abundance for the DiDy75 and DiDy25 mixes, (84% and 0.31%, respectively). In terms of RMSE, provides a better fit than in most (but not all) cases. Noting that provides the highly close prediction of abundance for the DiDy50 mixture and the RMSE fits are better, we henceforth consider to provide the best GKLS result as compared to the others (although only slightly better than ).
The processing times on the data consisting of 256,000 pixel vectors (640 samples by 400 lines) were recorded for each of the methods. The FCLS, SSA, and fixed- GKLS runs were completed in 9 s, 9 s, and 12 s, respectively. The automated GKLS run finished in 228 s. Note the automated GKLS method was slower by a factor of 19.
Results of Experiment 2
Results for experiment 2 are shown in Tables 4Table 5Table 6–7 and Figs. 12 and 13, showing the behavior for the FCLS, GKLS, and SSA methods, quantitatively and visually. The SSA method in this experiment is synonymous to the SSA method discussed in the first experiment, which is defined by Eq. (10) assuming bidirectional reflectance with nadir view and illumination angles.
Model diagnostics of panels for experiment 2. The RMSE values of the FCLS, SSA, and GKLS methods are listed for selected panel pixels in the scene, as well as the corresponding γ value chosen by the GKLS method at these points.
|T01||(369, 126)||0.007||0.017||0.007||6.440||T06A||(211, 62)||0.016||0.018||0.011||6.440|
|T02A||(365, 128)||0.020||0.022||0.017||4.569||T06B||(211, 63)||0.000||0.000||0.000||5.126|
|T02B||(365, 129)||0.021||0.029||0.020||3.413||T06C||(212, 63)||0.010||0.017||0.009||6.440|
|T03A||(363, 182)||0.020||0.014||0.020||6.440||T07||(187, 99)||0.005||0.018||0.005||0.002|
|T03B||(362, 182)||0.039||0.024||0.039||6.440||T08||(201, 64)||0.004||0.014||0.004||0.006|
|T03C||(363, 181)||0.004||0.004||0.004||0.005||T09A||(163, 33)||0.007||0.010||0.007||2.838|
|T04A||(209, 79)||0.005||0.021||0.005||0.002||T09B||(164, 33)||0.007||0.024||0.007||0.002|
|T04B||(209, 80)||0.003||0.007||0.003||1.778||T10A||(43, 106)||0.008||0.014||0.008||3.459|
|T05A||(212, 76)||0.008||0.017||0.007||5.321||T10B||(43, 107)||0.007||0.010||0.006||2.479|
|T05B||(212, 77)||0.003||0.014||0.003||1.714||T10C||(44, 107)||0.006||0.013||0.006||0.002|
Model diagnostics of vegetation mixes for experiment 2. The RMSE values of the FCLS, SSA, and GKLS methods are listed for selected pixels containing vegetation mixtures in the scene, as well as the corresponding γ value chosen by the GKLS method at these points.
Abundance results for Tyvek panels in experiment 2. The abundance values computed by the FCLS, SSA, and GKLS methods are listed for selected locations of the Tyvek panels.
|Panel name||Location (x,y)||WaterDrk||Gravel||SandPit||Grass healthy||Trees||VegNrTyvek||Tyvek||Panel name||Location (x, y)||WaterDrk||Gravel||SandPit||Grass healthy||Trees||VegNrTyvek||Tyvek|
|FCLS||T01||(369, 126)||0.04||0.65||0.05||0.00||0.00||0.00||0.25||T07||(187, 99)||0.18||0.00||0.00||0.00||0.14||0.60||0.08|
|SSA||T01||(369, 126)||0.07||0.00||0.45||0.00||0.01||0.26||0.21||T07||(187, 99)||0.09||0.00||0.00||0.01||0.30||0.39||0.21|
|GKLS||T01||(369, 126)||0.07||0.01||0.39||0.00||0.00||0.31||0.22||T07||(187, 99)||0.18||0.00||0.00||0.00||0.14||0.60||0.08|
|FCLS||T03C||(363, 181)||0.00||0.00||0.98||0.01||0.00||0.00||0.01||T08||(201, 64)||0.00||0.00||0.07||0.25||0.00||0.46||0.22|
|SSA||T03C||(363, 181)||0.00||0.00||0.97||0.00||0.00||0.00||0.03||T08||(201, 64)||0.00||0.00||0.00||0.32||0.07||0.09||0.52|
|GKLS||T03C||(363, 181)||0.00||0.00||0.98||0.01||0.00||0.00||0.01||T08||(201, 64)||0.00||0.00||0.07||0.25||0.00||0.46||0.22|
|FCLS||T04B||(209, 80)||0.00||0.52||0.08||0.11||0.00||0.21||0.09||T09A||(163, 33)||0.00||0.00||0.19||0.16||0.00||0.60||0.02|
|SSA||T04B||(209, 80)||0.01||0.36||0.12||0.17||0.00||0.10||0.21||T09A||(163, 33)||0.00||0.00||0.24||0.19||0.00||0.36||0.22|
|GKLS||T04B||(209, 80)||0.00||0.47||0.09||0.11||0.00||0.21||0.11||T09A||(163, 33)||0.00||0.00||0.22||0.19||0.00||0.51||0.07|
|FCLS||T05B||(212, 77)||0.53||0.07||0.00||0.00||0.07||0.28||0.05||T10A||(43, 106)||0.00||0.67||0.24||0.03||0.00||0.00||0.05|
|SSA||T05B||(212, 77)||0.32||0.00||0.00||0.00||0.25||0.27||0.17||T10A||(43, 106)||0.00||0.27||0.40||0.00||0.00||0.16||0.17|
|GKLS||T05B||(212, 77)||0.49||0.02||0.00||0.00||0.08||0.34||0.07||T10A||(43, 106)||0.00||0.57||0.29||0.04||0.00||0.00||0.11|
|FCLS||T06C||(212, 63)||0.33||0.00||0.25||0.00||0.00||0.00||0.42||T10B||(43, 107)||0.00||0.57||0.17||0.08||0.00||0.00||0.18|
|SSA||T06C||(212, 63)||0.12||0.01||0.35||0.00||0.00||0.00||0.51||T10B||(43, 107)||0.00||0.04||0.35||0.01||0.00||0.27||0.33|
|GKLS||T06C||(212, 63)||0.14||0.01||0.27||0.00||0.00||0.00||0.52||T10B||(43, 107)||0.00||0.44||0.20||0.08||0.00||0.05||0.23|
The bold values emphasize the predicted abundance for “Tyvek,” the primary material of interest.
Abundance results for vegetation mixtures in experiment 2. The abundance values computed by the FCLS, SSA, and GKLS methods are listed for selected locations of vegetation mixes.
|Vegetation mix location (x,y)||WaterDrk||Gravel||Grass healthy||Trees||VegNrTyvek||Blg01||Vegetation mix location (x,y)||WaterDrk||Gravel||Grass healthy||Trees||VegNrTyvek||Blg01|
|FCLS||(277, 50)||0.06||0.00||0.15||0.05||0.74||0.01||(233, 141)||0.03||0.00||0.53||0.00||0.39||0.02|
|SSA||(277, 50)||0.03||0.08||0.29||0.13||0.47||0.00||(233, 141)||0.05||0.00||0.83||0.03||0.00||0.06|
|GKLS||(277, 50)||0.06||0.01||0.19||0.07||0.66||0.01||(233, 141)||0.06||0.00||0.63||0.00||0.26||0.03|
|FCLS||(277, 60)||0.11||0.00||0.11||0.08||0.70||0.00||(234, 141)||0.00||0.00||0.54||0.00||0.38||0.05|
|SSA||(277, 60)||0.08||0.00||0.40||0.11||0.42||(234, 141)||0.04||0.00||0.82||0.00||0.00||0.12|
|GKLS||(277, 60)||0.11||0.00||0.11||0.08||0.70||0.00||(234, 141)||0.01||0.00||0.63||0.00||0.25||0.08|
|FCLS||(248, 68)||0.00||0.00||0.27||0.21||0.52||0.00||(270, 178)||0.00||0.00||0.16||0.18||0.66||0.00|
|SSA||(248, 68)||0.01||0.00||0.51||0.27||0.21||0.00||(270, 178)||0.00||0.00||0.29||0.24||0.48||0.00|
|GKLS||(248, 68)||0.00||0.31||0.27||0.43||0.00||(270, 178)||0.00||0.00||0.19||0.21||0.60||0.00|
|FCLS||(99, 79)||0.00||0.15||0.22||0.00||0.63||0.00||(274, 179)||0.00||0.00||0.16||0.30||0.54||0.00|
|SSA||(99, 79)||0.00||0.06||0.34||0.00||0.60||0.00||(274, 179)||0.00||0.00||0.28||0.36||0.37||0.00|
|GKLS||(99, 79)||0.00||0.07||0.35||0.00||0.58||0.00||(274, 179)||0.00||0.00||0.17||0.31||0.51||0.00|
The bold values emphasize the predicted abundance for three types of vegetation, the primary materials of interest.
The abundance maps for experiment 2 are shown in Figs. 12 and 13. Figure 12 shows maps for the Tyvek panels using the GKLS and SSA methods. The map for the FCLS method is not shown because this map is almost identical to the GKLS method. The colored circles show where the panels are located. Due to limited dynamic range in the printed version, some of the low abundance panels (T02, T03, T04, and T07) cannot be seen, but the detection can be verified by noting the abundances shown in Table 6. The RGB color composite maps in Fig. 13 show the differences in vegetation modeling for the three methods. The overall greater abundance of grass predicted by the GKLS as compared to FCLS method can be seen by comparing Figs. 13(a) and 13(b). An even greater abundance of grass is predicted by the SSA method, as shown by Fig. 13(c).
The abundance retrieval capability of all methods is good; however, a moderate advantage is shown for the SSA and GKLS methods. Tables 4 and 5 list the model diagnostics for selected points in the scene. The entries in Table 4 list the results for many of the Tyvek panels. The entries in Table 5 list results for selected mixes of vegetation with other backgrounds. Note the RMSE for the GKLS is always less than or equal to the RMSE of the FCLS. This is to be expected since the GKLS algorithm iterates with increasing until the RMSE converges to a minimum RMSE. A higher value of corresponds to greater nonlinearity in the model. The RMSE values of the SSA method are almost always higher than either of the other methods; however, they were still in an acceptable range. The higher values may be attributed to SSA spectra having overall higher values than reflectance.
Figure 14 shows plots of observed and estimated spectra, as computed by the three methods, for one of the Tyvek panel (TY-5) mixtures with background at location and one of the vegetation mixtures at . Observed spectra (labeled as “O”) are displayed in dark gray and estimated modeled spectra (labeled as “E”) are displayed in lighter gray.
Figures 14(a)–14(c) show results for the T05A panel mixed with background as computed by the FCLS, GKLS, and SSA methods, respectively. The GKLS result is reported for . Note there are diagnostic Tyvek features located at 1.72 and (see Fig. 9). Overall, the fits between the observed and estimated spectra are good for the three methods, especially near the diagnostic Tyvek features. Note the GKLS improves the fit as compared to the FCLS method in agreement with the numerical results listed in Table 2.
Figures 14(d)–14(f) show results for the vegetation mix as computed by the FCLS, GKLS, and SSA methods. The GKLS result from is shown. The fits between the observed and estimated spectra were good for the three methods. As with the Tyvek panel, the GKLS improves the fit as compared to the FCLS method in agreement with the numerical results listed in Table 2. However, in this case, the SSA method was slightly better.
Tables 6 and 7 show the abundance results of experiment 2 for the three methods. Table 6 shows these results for the panels. At least one pixel location is listed for each Tyvek panel shown. Although most panels are subpixel, the ground resolution distance of the sensor can cause the panel’s signature to occur over multiple pixels. Not all locations are listed due to an uncertainty factor of the actual sample and also because of space limitations. We notice there is not always a significant difference in abundances between the FCLS and GKLS methods, such as with T04A (one of the pixels in TY-4) and T09B (one of the pixels in TY-9). However, at these locations, the SSA method actually does estimate significantly higher abundances. Overall, the abundances estimated by the SSA method are higher, and for the case of T07 it is sufficiently high to trigger the detection of that panel. The abundance estimates from the other methods applied to T07 are too low to be considered significant.
Table 7 shows the results for vegetation mixtures. The vegetation abundance estimates are notably different for each method. It is difficult to quantitatively assess the results in Table 7 because of a lack of ground truth. However, they corroborate with the qualitative results shown in Fig. 13 and indicate that the SSA method is preferable over the FCLS for better characterizing the nonlinear mixtures of vegetation in the scene.
Results of Experiment 3
The results for experiment 3 (airborne HSI data) are shown in Figs. 15Fig. 16–17 and Tables 8 and 9. Figure 15 shows an RGB composite abundance map and five single-layer abundance maps for the fixed- GKLS method (). The composite abundance map visually shows the method’s response to three thicknesses of oil: the R layer depicts the abundance of 4.0-mm-thick oil; the G layer depicts the abundance of 1.85-mm-thick oil; and the B layer depicts the abundance of 1.00-mm-thick oil. The single-layer abundance maps show the results for five of the six endmembers. Due to the small size of these photos in the report, it is difficult to see the detailed patterns, but it is possible to get a sense of the method’s behavior. The oil is expected to spread out in a certain pattern from thick to thin, which is indeed what occurs. In the original photos of the three methods (not shown here but available in supplemental material), there is little difference in the patterns observed between the GKLS and SSA methods. However, there is a notable difference in the patterns seen for the 4.0-mm-thick oil between the FCLS and GKLS (or FCLS and SSA) methods.
Abundance results: estimated abundance values of the methods are listed for selected locations (pixels) in the AVIRIS DWH scene.
|Location||DWH oil||DWH oil||DWH oil||DWH oil||(546, 481)||(1154, 112)|
|Method||(x,y)||4.0 mm||1.85 mm||1.0 mm||0.5 mm||Orange||Water|
The bold values indicate the best fitting model for each location.
Model diagnostics: RMSE values computed for the fit between the observed and estimated mixed spectra are listed for selected locations of the AVIRIS DWH scene.
Figure 16 shows the single-layer abundance maps of the 4.0-mm-thick oil for the FCLS and fixed- GKLS () method. The FCLS method shows much of the same large area of thick oil in the central-right side of the scene as the GKSL (and SSA, not shown). However, these maps also show a notable difference in pattern between the FCLS and GKLS methods, as just mentioned above. The linear FCLS indicates a significant amount of 4.0-mm-thick oil throughout the scene in areas that would not be expected. Physically we expect the oil to spread out in a certain pattern from thick to thin. There is isolated 4.0 mm oil scattered throughout the area in the FCLS photo not seen in the GKLS photo. Neither the GKLS nor the SSA methods indicate much 4.0-mm-thick oil in these regions.
Tables 8 and 9 show the abundance results and RMSE values, respectively, for the three methods at selected locations of the AVIRIS scene. In Table 8, the abundance estimates are not very different for the first three locations. Interestingly, the RMSE values in Table 9 indicate the mixtures are likely to be linear at these locations. The abundance estimates listed in Table 8 for the locations at and are in agreement with Figs. 15 and 16. The smaller RMSE value for the GKLS at this location indicates the mixture is likely to be nonlinear.
Figure 17 shows plots of the observed and estimated spectra for a thinner layer of oil at and a thicker layer of oil at . The fits are qualitatively good except for a significant difference in the F2 diagnostic region for the FCLS method and in the F1 diagnostic region for the SSA method. However, overall the fits are quite good. The larger values of RMSE listed for the SSA method are likely a consequence of the magnitude of the SSA spectra as compared to the normalized reflectance values used in the FCLS and GKLS methods (see range of the -axis in Fig. 17).
This study has investigated the use of a GKLS method applied to reflectance data, and an SSA method for nonlinear mixture analysis. In the case of the SSA method, an FCLS method is applied to data that have been converted from reflectance space to SSA space. Our baseline method was the FCLS method applied to reflectance data. Our hypothesis is that, for intimate (nonlinear) mixtures, both of these methods will provide improved modeling and abundance estimates as compared to the baseline FCLS method. Three scenarios are tested on different scenes: laboratory, aerial over land, and aerial over oil-contaminated seawater.
Overall the results for experiment 1 using laboratory data from the Pika II sensor indicate the FCLS method has a poor capability for modeling intimate mixtures. In contrast, both the GKLS and SSA methods are much better at retrieving actual material abundances. Whether or not one is better than the other is not conclusive. However, we conclude that our hypothesis is confirmed and that both of these methods provide a better estimate of abundance for mixtures exhibiting nonlinearity. For the laboratory experiment of known abundance quantities, the SSA and GKLS methods responded well to the nonlinearity present in a mixture of materials and provide better estimates of abundance than the linear FCLS method for the DiDy and soda-lime glass bead mixtures.
The GKLS parameter determines the degree of nonlinear behavior exhibited by the GKLS method and affects its accuracy for estimating abundances. The automated GKLS method attempts to select the most appropriate gamma based on achieving a minimum of the model’s RMSE. We conclude the RMSE metric seems to respond to a mixture being linear and nonlinear, but is not a reliable metric to determine the degree of nonlinearity present. It could not be used to achieve the most accurate estimate of abundance and an alternative approach should be sought to automate the GKLS method.
For mixtures known to be nonlinear, the fixed- implementation of GKLS with or 6 provides a good estimate of abundance. The fixed- GKLS and the SSA methods can be computed in approximately the same amount of time and provide approximately the same accuracy for estimating abundances. Compared to the fixed- GKLS method, the automated GKLS was much slower to compute by a factor of about 19 and did not really achieve better accuracy.
For experiments 2 and 3 using the airborne data from HyMap (scene over land) and AVIRIS (scene over water), we also conclude that our hypothesis is confirmed and that either of these methods provides a better estimate of abundance for mixtures exhibiting nonlinearity. Unfortunately, the data set used in experiment 2 was from a collection campaign not particularly focused on nonlinear mixing problems, and although it contained many nonlinear mixtures, precise ground truth is not available. However, the data provide good opportunity to observe that linear mixtures did not significantly degrade the performance of the nonlinear GKLS and SSA methods. In experiment 3 (Deepwater Horizon oil spill), the SSA and GKLS methods performed equivalently and appeared to respond better to the nonlinear mixtures of oil thicknesses in the water than the linear method. Specifically, the linear method appears to be predicting too much of the thicker oils. This can only be assessed qualitatively because, here, too, there is a lack of ground truth. Additional analysis should be performed on scenes containing a variety of known nonlinear mixtures that are well documented with ground truth.
Appendix provides information about obtaining data from the first experiment. All of the data as well as dark field data cubes (with 80, 120, and 240 bands) and some additional information [e.g., the ENVI ROI files and an ENVI ‘Spectral Math’ expression file for implementing Eqs. (2) and (3)] are available for download as one compressed (.zip) file.
Additional experiments are recommended that investigate the effects of adding endmembers to the models and also to the physical mixtures in a controlled environment. In experiment 1, the number of controlled endmembers was limited to two.
Obtaining the Data
The focus of the first experiment discussed in this study has been on the analysis of a single Pika II HSI data cube (file: 20130521_sphere_array_2.bil). However, 15 measurements, in total, of the glass beads in the 96-well sample plate were acquired as listed in Table 10. Two are simply redundant measurements ( prior to spectral subsetting in ENVI). Six others are of the same sample plate and glass bead mixtures but with 120 and 240 bands. These data are somewhat noisier than those with 80 bands but were acquired to assess the impact of an increased noise level on abundance estimations. Three cubes are with the sensor moved closer to the sample plate (still with the Xenoplan objective lens) and with 80 bands. Two additional cubes contain two replicate sets of glass bead mixtures placed in adjacent empty cells of the plate (for a total of 15 filled cells). These cubes, too, contain 80 bands. All of the data sets are with four QTH lamps and nadir-looking sensor configuration shown in Fig. 1. To date, no data have yet been acquired with a line-light source to yield true bidirectional reflectance spectra. All of these data sets as well as dark field data cubes (with 80, 120, and 240 bands) and some additional information [e.g., the ENVI ROI files and an ENVI “spectral math” expression file for implementing Eqs. (2) and (3)] are available for download as one compressed (.zip) file. Send an e-mail to email@example.com requesting the data; a link for downloading will be sent in return. All questions and comments about these data should be addressed to R.G. Resmini.
A list is shown of the available Pika II data sets of the glass beads. (File 20130521_sphere_array_shifted_8.bil is the same as file 20130521_sphere_array_8.bil except it has been shifted so that some regions of interest defined in other cubes in the data set also apply.)
References are made to certain commercially available products in this paper to adequately specify the experimental procedures involved. Such identification does not imply a recommendation or endorsement by the U.S. government, nor does it imply that these products are the best for the purpose specified. Thanks are given to Dr. Charles Bachmann and the U.S. Naval Research Laboratory for providing the HyMap imagery, as well as associated aerial photographs and atmospheric calibration for experiment 2. Thanks are also given to Dr. Roger Clark and Eric Livo (USGS) for the data provided in experiment 3, including the laboratory and field measurements, as well as the AVIRIS imagery. The MITRE Innovation Program (MIP) is gratefully acknowledged for funding the HSI microscopy aspect of the project in which the study presented here was conducted. This article has been approved by NGA for public release, Case 16-132.
Robert S. Rand received his BS degree in physics from the University of Massachusetts at Lowell and his PhD in engineering physics from the University of Virginia, Charlottesville. He has worked for agencies with the U.S. Department of Defense since 1977. His recent efforts have focused on hyperspectral exploitation using feature-based methods and spectral mixture models, as well as the use of three-dimensional information from LIDAR to improve the exploitation of hyperspectral imagery.
Ronald G. Resmini received his MS degree in geology from Boston College, Boston, Massachusetts, USA, and his PhD in geology from Johns Hopkins University, Baltimore, Maryland, USA. He is a research scientist at the MITRE Corporation, McLean, Virginia, USA, and an associate professor in the College of Science at George Mason University, Fairfax, Virginia, USA. He specializes in visible to infrared multi- and hyperspectral remote sensing, the geological and geophysical sciences, and algorithm development for remote sensing applications.
David W. Allen received his PhD in earth systems and geoinformation sciences with a concentration in remote sensing from George Mason University. He is a research chemist at the NIST, where he has worked for over 20 years. Current research interests include advancing hyperspectral imaging for applications related to medicine, security, and the environment. This effort encompasses the development of end-to-end analysis methodology that integrates standards and best practices for hyperspectral imager performance metrics. | <urn:uuid:42e841a2-b30d-40dd-88a4-3b9a1672ccf1> | 2.75 | 14,324 | Academic Writing | Science & Tech. | 70.261755 | 95,505,102 |
A new study by Jessica Hellmann, associate professor of biological sciences at the University of Notre Dame, and researchers from Western University found that mild winters, such as the one many of us just experienced, can be taxing for some butterfly or possibly other species.
Hellmann and her fellow researchers studied caterpillars of the Propertius Duskywing butterfly, which feed on Gary Oak trees. This species of caterpillar, like many insects, has a higher metabolic rate and burns more fat during mild winters.
"The energy reserves the caterpillars collect in the summer need to provide enough energy for both overwintering and metamorphosing into a butterfly in the spring," Caroline William, lead author of the study, said.
So a butterfly needs to conserve as much energy as it can during the winter months. In the paper, Hellmann and her colleagues explain for the first time how warmer winters can lead to a decrease in the number of butterflies.
However, Hellmann and the Western University researchers found that warmer winters might not always reduce butterfly populations as much as one might initially think. They reared caterpillars in two different locations: one which often experiences more variable and warmer winter temperatures and one which generally features more stable and generally cooler winter temperatures. The caterpillars that were exposed to the warmer and more variable conditions were better able to withstand the warmer conditions, simply by being exposed to them. They did so by lowering the sensitivity of their metabolism.
However, the ability of even caterpillars accustomed to warmer, more variable winters to cope with such conditions is still limited, according to the researchers. They calculated the energy use of both groups of caterpillars and discovered that the caterpillars that lower their metabolic rates to deal with warmer winters still use significantly more energy to survive them.
"We still have lot to learn about how organisms will respond to climate change," Hellmann said. "Our study shows significant biological effects of climate change, but it also shows that organisms can partially adjust their physiology to compensate. We now need to discover if other species adjust in similar ways to our example species."
So although mild winters may be a cause for celebration for many of us, those who are concerned are biodiversity might find them to be much more somber seasons.
The research was funded by the Natural Sciences and Engineering Council of Canada, the Canadian Foundation for Innovation, the Ontario Ministry of Research and Innovation and the U.S. Department of Energy.
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In search of noise-induced bimodality
Many biological studies are carried out on large populations of cells, often in order to obtain enough material to make measurements. However, we now know that noise is endemic in biological systems and this results in cell-to-cell variability in what appears to be a population of identical cells. Although often neglected, this noise can have a dramatic effect on system responses to environmental cues with significant and often counter-intuitive biological outcomes. A recent study in BMC Systems Biology provides an example of this, documenting a bimodal distribution of activated extracellular signal-regulated kinase in a population of cells exposed to epidermal growth factor and demonstrating that the observed bimodality of the response is induced purely by noise.
See research article: http://www.biomedcentral.com/1752-0509/6/109
KeywordsBimodal Distribution Activation Threshold Empirical Cumulative Distribution Function Synthetic Biologist Epidermal Growth Factor Signal
Noise in biological systems is endemic and can contribute to biological phenotypes. For example, noise can affect fate determination in virus-infected cells by randomly switching between latency and reactivation, and it can also cause Escherichia coli to switch between competency and non-competency for DNA uptake. The origin of biological noise is attributed to randomness in biological reaction events, and leads to cell-to-cell variability in genetically identical cell populations. To understand noise-induced phenotypes, it is important to have the capability to observe behaviors at the single-cell level.
The paper by Birtwistle et al. highlights an interesting issue with respect to noise in the mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) protein signaling pathway. This pathway receives environmental cues, such as changes in epidermal growth factor (EGF) that leads to an amplified signal in the form of activated ERK. The activation of ERK in turn results in diverse cellular responses, such as proliferation, differentiation, and apoptosis. In their study, flow cytometry was used to collect data in the form of fluorescence signals emitted from individual single cells. The signals indicated the activated ERK levels and showed a bimodal histogram in the cell population, which might otherwise have been obscured without adopting single-cell-level measurement techniques.
Bimodality can be the result of positive feedback mechanisms resulting in bistablity, a state very much like a light toggle switch. Such systems have been found in a number of natural systems so it was an obvious choice for Birtwistle et al. to investigate this possibility. However, although the ERK activation pathway has been found to exhibit bistability in some types of cell, it does not in others, and the experimental observations made by Birtwistle el al. in their cell line were not compatible with the mechanisms that used positive feedback. Closer investigation reveals something completely different and novel.
The observed bimodal distributions were shown by computer modeling to be induced purely by noise without being related to positive feedback. There have been many theoretical studies to understand the mechanisms for such noise-induced bimodality. Many of these are related to the close interplay between nonlinearities in the system and noise in biological signals. The signal noise can be processed via a nonlinear input-output response, causing sufficient signal distortion to transform unimodal signal distributions into bimodal ones . In the case of the signaling pathway studied by Birtwistle et al., the EGF signal is transmitted across the membrane to generate activated Ras, and the Ras signal, considered unimodal in the cell line used, activates the MAPK/ERK cascade after passing a threshold. The existence of this threshold means that the system is not purely linear, and signal distortion due to the system nonlinearity can explain the bimodality that emerges. This provides a first in vivo example in protein signaling networks that shows noise-induced bimodality due to its inherent nonlinear signal processing without positive feedback.
Careful attention is, however, needed when computing distributions, more precisely histograms, depending on the choice of the scale of the x-axis. As in the case of flow cytometry, a signal is often visualized in the log scale to show the broad ranges of the signal values. This leads researchers to use the histogram of log-transformed signal values. One pitfall of this procedure is that it is possible to generate transformation artifacts. One example is shown in Figure 1, where a unimodal (or very weakly bimodal) distribution in a linear scale (Figure 1h) becomes a bimodal distribution in a log scale (Figure 1g). This is because the log-scale representation causes larger values of × to be compressed more visually, so that a greater number of samples will be taken at larger × values and fewer at smaller × values (Figure 1c,g). Therefore, flow cytometry data may need to be treated carefully. Birtwistle et al. represented their data in a log scale using Kaplan-Meier empirical cumulative distribution functions, which may reduce the problem. Although this method can visually amplify the bimodal distribution as the conventional log scale representation does, it was confirmed via personal communication that the bimodality robustly appears in the wide range of input doses (0.5 nM and 1 nM EGF in Figure 1B in Birtwistle et al. ) and the transformation artifact does not eliminate the bimodality except for the case of 0.1 nM EGF in the same figure.
From these simulations, we can conclude that the variability in the input signal and the activation threshold both individually enhanced bimodality in the output signal. This is because the value of y is determined by the distance between the value of × and the activation threshold. Thus, the variability in the threshold has the same effect as the input variability by changing the distance between the value of × and the threshold. What happens if variability appears in both × and the threshold simultaneously? If the input signal and the threshold can be assumed to fluctuate independently due to the fact that they can be processed through sufficiently different biological systems, then the presence of the variability in both will enhance the bimodality further.
Finally, we can consider the variability in the saturation levels of the response curves. This variability smoothed the sharp peak appearing at the saturation level (y = 1) for the case that the saturation level was fixed (Figure 2f). Thus, the variability in the saturation level did not have any significant effect on bimodality.
The lesson from this work, and one that we see more and more often, is that the interaction of noise and the underlying deterministic dynamics can result in non-intuitive behavior. We are only beginning to understand how noise is exploited by nature and furthermore by system designers like synthetic biologists , but the influence of noise is likely to be subtle and counter-intuitive to our normal deterministic view of the world.
This work was supported by a National Science Foundation (NSF) Molecular Cell Biology Grant 1158573 and NIGMS grant GM081070.
- 1.Birtwistle MR, Rauch J, Kiyatkin A, Aksamitiene E, Dobrzyński M, Hoek JB, Kolch W, Ogunnaike BA, Kholodenko BN: Emergence of bimodal cell population responses from the interplay between analog single-cell signaling and protein expression noise. BMC Syst Biol. 2012, 6: 109-10.1186/1752-0509-6-109.PubMedPubMedCentralCrossRefGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. | <urn:uuid:223279b2-3807-44f0-b1af-e7bf473a642c> | 2.5625 | 1,682 | Academic Writing | Science & Tech. | 24.171603 | 95,505,155 |
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Discussion and illustration of problems suggested by the analysis of atmospheric cross-sections
Willett, Hurd C.
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The preparation of atmospheric cross-sections, in which the fields of the various meteorological elements are represented in the vertical plane containing a number of synoptic aerological soundings, has long been a part of the technique applied to the investigation of problems in synoptic meteorology. However, owing to the lack of adequate observational material, the number of such cross-sections prepared in the past has been very small. The method was applied only in a few cases chosen for careful analysis and study. Consequently no uniform technique of analysis of such cross-sections has been developed, nor have the possibilities of this method of synoptic investigation received much attention. In the fall of 1933 the author decided that the possibilities of the cross-section method of synoptic representation warranted the systematic preparation and analysis of a large number of cross-sections. For this purpose a number of periods during which the synoptic maps seemed to indicate interesting atmospheric developments, and for which numerous aerological observations were available, were chosen from the maps of the preceding two or three years for detailed cross-sectional study. In all, ten periods of from two to six days each were chosen, a total of 36 days, entailing the preparation of about 90 cross-sections, and the use of about 400 aerological soundings.
Suggested CitationBook: Willett, Hurd C., "Discussion and illustration of problems suggested by the analysis of atmospheric cross-sections", Papers in Physical Oceanography and Meteorology, v.4, no.2, 1935-07, DOI:10.1575/1912/1085, https://hdl.handle.net/1912/1085
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Colour polymorphism in the coconut crab (Birgus latro)
- 362 Downloads
Coconut crabs (Birgus latro) are strikingly variable in coloration, but the significance of this colour diversity has never been investigated. We studied coloration, morphology, behaviour and background matching of adult coconut crabs, the world’s largest terrestrial invertebrate, at the western edge of its distribution on Pemba Island, Tanzania. Adults are evidently polymorphic; they come in red and blue types (3:1 ratio on Pemba). The best predictor of colour morph was ventral hue, which, using a discriminant function analysis, correctly classified 96% of the crabs assigned into a predefined colour group. In contrast, principal component analyses suggested a degree of overlapping colour variation. We found no evidence that coloration was sex or size-linked. Males were larger than females and the Pemba adult population appeared male-biased (3:1). We also report that red adults may match the background better than do blue adults on land, whereas blue match better near shore than do red. We postulate that although colour diversity in coconut crabs may be genetically determined, potentially through a crustacyanin gene polymorphism influencing the stability of integument pigmentation, its maintenance may involve several ecological drivers.
KeywordsBackground matching Colour variation Crustaceans Developmental plasticity Pemba Island Trade-off
We thank the Revolutionary Government of Zanzibar for permission; the University of California, Davis for funding fieldwork; and two anonymous reviewers for comments.
Compliance with ethical statement
Conflict of interest
The authors declare that they have no conflict of interest.
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A hydroxy or hydroxyl group is the entity with the formula OH. It contains oxygen bonded to hydrogen. In organic chemistry, alcohol and carboxylic acids contain hydroxy groups. The anion [OH−], called hydroxide, consists of a hydroxy group.
Water, alcohols, carboxylic acids, and many other hydroxy-containing compounds can be deprotonated readily. This behavior is rationalized by the disparate electronegativities of oxygen and hydrogen. Hydroxy-containing compounds engage in hydrogen bonding, which causes them to stick together, leading to higher boiling and melting points than found for compounds that lack this functional group. Organic compounds, which are often poorly soluble in water, become water soluble when they contain two or more hydroxy groups, as illustrated by sugars and amino acid.
The hydroxy group is pervasive in chemistry and biochemistry. Many inorganic compounds contain hydroxy groups, including sulfuric acid, the chemical compound produced on the largest scale industrially.
Hydroxy groups participate in the dehydration reactions that link simple biological molecules into long chains. The joining of a fatty acid to glycerol to form a triacylglycerol removes the −OH from the carboxy end of the fatty acid. The joining of two aldehyde sugars to form a disaccharide removes the −OH from the carboxy group at the aldehyde end of one sugar. The creation of a peptide bond to link two amino acids to make a protein removes the −OH from the carboxy group of one amino acid.
Hydroxyl radicals are highly reactive and undergo chemical reactions that make them short-lived. When biological systems are exposed to hydroxyl radicals, they can cause damage to cells, including those in humans, where they can react with DNA, lipids, and proteins.
Lunar and other extraterrestrial observations
In 2009, India's Chandrayaan-1 satellite, NASA's Cassini spacecraft and the Deep Impact probe have each detected the presence of water by evidence of hydroxyl fragments on the Moon. As reported by Richard Kerr, "A spectrometer [the Moon Mineralogy Mapper, a.k.a. "M3"] detected an infrared absorption at a wavelength of 3.0 micrometers that only water or hydroxyl—a hydrogen and an oxygen bound together—could have created." NASA also reported in 2009 that the LCROSS probe revealed an ultraviolet emission spectrum consistent with hydroxyl presence. The Venus Express orbiter sent back Venus science data from April 2006 until December 2014. Results from Venus Express include the detection of hydroxyl in the atmosphere.
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- Full paper
- Open Access
Scale analysis of equatorial plasma irregularities derived from Swarm constellation
© The Author(s) 2016
Received: 11 December 2015
Accepted: 28 June 2016
Published: 15 July 2016
In this study, we investigated the scale sizes of equatorial plasma irregularities (EPIs) using measurements from the Swarm satellites during its early mission and final constellation phases. We found that with longitudinal separation between Swarm satellites larger than 0.4°, no significant correlation was found any more. This result suggests that EPI structures include plasma density scale sizes less than 44 km in the zonal direction. During the Swarm earlier mission phase, clearly better EPI correlations are obtained in the northern hemisphere, implying more fragmented irregularities in the southern hemisphere where the ambient magnetic field is low. The previously reported inverted-C shell structure of EPIs is generally confirmed by the Swarm observations in the northern hemisphere, but with various tilt angles. From the Swarm spacecrafts with zonal separations of about 150 km, we conclude that larger zonal scale sizes of irregularities exist during the early evening hours (around 1900 LT).
At the low-latitude ionosphere, the equatorial plasma irregularities (EPIs), also often called equatorial plasma bubbles (EPBs) or equatorial spread-F (ESF), have been a subject of intense research for several decades. Their morphology, including the generation and evolution processes, spatial structures, global distribution, as well as their effects on the global navigation satellite system (GNSS) have widely been investigated by ground-based radars and in situ satellite observations (Tsunoda 1980; Huang et al. 2001; Burke et al. 2004; Kil et al. 2004; Su et al. 2006; Stolle et al. 2006; Yokoyama et al. 2007; Xiong et al. 2010, 2012; Huang et al. 2014; Park et al. 2015a). These plasma irregularities usually cover a range of scale sizes, from thousands of kilometers to a few meters (e.g., Zargham and Seyler 1989; Hysell and Seyler 1998). EPIs with scale sizes larger than about 100 km, between 10 and 100 m, and smaller than 100 m have been classified as large-, intermediate-, and small-scale structures, respectively (Lühr et al. 2014).
Singh et al. (1997) presented some examples of EPIs from the Atmosphere Explorer E (AE-E) satellite observations showing that EPIs can develop from wavy density structures in the bottom side F-layer around sunset. These wavy structures with zonal (east–west) wavelengths of about 150–880 km evolved later into either large-scale depletions or multiple depleted patches. In a fully developed EPI event, structures with wavelengths from 690 km down to about 0.5 km were simultaneously present. Plasma density measurements with high sampling rates (up to 1024 Hz) on board the ROCSAT-1 satellite have provided a comprehensive view of the spectral characteristics of EPIs (Su et al. 2001). They also showed that the plasma irregularity spectrum can be approximated by a power law with piecewise constant spectral indices. In the meridional (north–south) direction, the 50-Hz magnetic field data from the CHAMP satellite revealed EPI structures with scale size as small as 50 m (Stolle et al. 2006).
In situ satellite measurements mainly detect EPI structures along the satellite track. For a single spacecraft mission, the plasma irregularities can only be sampled either in the zonal direction (for low-inclination satellites, such as AE-E, ROCSAT-1, and C/NOFS) or in the meridional direction (for near-polar orbiting satellites, such as CHAMP and GRACE). ESA’s newly launched Swarm constellation comprised of three spacecrafts (with the lower pair flying side-by-side) providing now the opportunity for investigating EPIs in both meridional and zonal directions. Related case studies from Swarm observations focusing on a dayside plasma depletion (Park et al. 2015b) and its relation to GNSS signal losses (Buchert et al. 2015) have already been reported.
In this paper, we present a statistical study of EPI scale sizes, as derived from the Swarm constellation measurements. We aim to reveal the typical scale sizes of EPIs in both the meridional and zonal directions. In the section to follow, we first introduce the data set. Examples and statistical results of EPIs are presented in section “Results.” In section “Discussion and summary,” we will discuss our observations in the context of earlier reports and summarize our findings.
Data set and processing approach
The Swarm fleet, comprising three spacecrafts, was launched on November 22, 2013, into a near-polar (87.5° inclination) orbit with initial altitude of about 500 km. From January 2014 onward, the three spacecrafts were maneuvered apart and achieved their final constellation on April 17, 2014. From then on, the lower pair, Swarm A and C, are flying side-by-side at an altitude of about 470 km, separated by about 1.4° in longitude. The third spacecraft, Swarm B, orbits the Earth at about 520 km with a somewhat higher inclination. The plasma density data set measured by the electric field instrument (EFI) onboard Swarm is available at http://earth.esa.int/swarm, including the Langmuir probe (LP)-derived plasma density data with a time resolution of 2 Hz.
After the final constellation has been completed on April 17, 2014, Swarm A and C started flying side-by-side at an altitude of about 470 km, separated by about 1.4° in longitude, with Swarm C a few seconds ahead of Swarm A. The evolutions of their longitudinal and temporal separations are presented in Fig. 1c, d. For the second period, we used data from April 17, 2014 to September 27, 2015, and consider only observations from the lower spacecraft pair, Swarm C/A.
For detecting EPI events from the Swarm electron density measurements, we used the same approach as described by (Xiong et al. 2010). Electron density (Ne) time series from each equatorial orbital segment (within ±40° magnetic latitude, MLAT) are first high-pass filtered with a cutoff period of 40 s, corresponding to an along-track wavelength of about 300 km. The magnetic latitude we used is calculated by the Apex or Quasi-Dipole magnetic field model, which has been defined by Richmond (1995) and updated by Emmert et al. (2010). Subsequently, the filtered signal is rectified. Values exceeding an upper limit (UL) are identified as an EPI event. For each event, the rectified signal should have multi-peaked values above UL, and this event is limited along the orbit, by rectified signals below a lower limit (LL) for at least 3° north and south of the event. Otherwise, the fluctuations of rectified signal are attributed to enhanced noise and are considered as not significant. The thresholds of UL and LL are set here to 3 × 1010 m−3 and 1.5 × 1010 m−3, respectively, which are mainly estimated from the level of quiet-time Ne variations at Swarm altitudes. Using such a method for detecting plasma irregularities, we focus on EPIs with plasma structures scale lengths less than 300 km.
For deriving EPI scale sizes, we determine their correlation from the electron density recordings between two Swarm spacecrafts. For the early mission phase, we divided the spacecraft into three pairs: Swarm B/A, A/C and B/C. As we are interested in plasma irregularities, the background density of each EPI event has been subtracted to reduce the effect of non-depleted variations on the correlation analysis. For each equatorial orbital segment with EPI, the Ne time series with 2 Hz resolution were first projected onto MLAT, then a multi-wavelet method has been applied to the MLAT profile of Ne. The wavelet function we used is “coiflets,” which is discrete, near symmetric and has scaling functions with vanishing moments (Beylkin et al. 1991). The wavelet coefficients with wavelengths between 240 and 960 km were used to reconstruct the background Ne profile. As reported in previous studies, EPIs exhibit inverted-C shell structures when projected onto the horizontal plane. In case of a westward-tilted inverted-C shell structure, more poleward EPI parts appear further westward (Kelley et al. 2003; Kil et al. 2004; Huba et al. 2009; Park et al. 2015a). This means that the Swarm spacecraft on the westside is expected to observe depletions at higher latitude than the spacecraft on the eastside. We examined such expected feature further by using a cross-correlation analysis. We treated separately the data from the two hemispheric parts (0° to ±20° MLAT). For each hemisphere, the electron density recording of the eastside spacecraft was taken as reference and the other one was time-shifted from −100 to 100 s. The maximum value of the correlation coefficients (R max) and the corresponding time shifts (Δt) were recorded for following statistical analysis. To keep consistency, a positive value of Δt means that the electron density from the westside spacecraft had to be shifted equatorward to get R max in both hemispheres.
Examples of EPIs observed by Swarm
Figure 2b, c presents two examples of EPIs observed by Swarm during the first study period. For both events, the top panel presents the latitudinal profiles of the original electron density time series (Ne, 2 Hz resolution) with different colors for different spacecrafts. The epochs, altitudes and longitudes when the spacecraft passed the geographic equator are listed in the topside. The middle panel shows the reconstructed background Ne and the bottom panel presents ΔNe with background variations subtracted. For the first event observed on December 10, 2013, the three spacecrafts flew at an altitude of about 501 km. Swarm B was the leading spacecraft, crossing the geographic equator 1 min earlier than A and C. In this case, Swarm A was 0.1° (about 11 km) westward of B and 0.2° (about 22 km) eastward of C, respectively. As the three spacecrafts were so close in time and space, they observed similar plasma density irregularities in both hemispheres. In the northern hemisphere, the correlations of the density depletions, R max, attained values of 0.91, 0.83 and 0.76 for the pairs Swarm B/A, A/C and B/C, respectively. As Swarm B/A were most closely spaced, R max is largest, as expected. Furthermore, a correlation less than 1.0 between Swarm B/A also reveals that the EPI had fine structure with zonal extent less than 0.1° (about 11 km) or that the plasma density structure did change within the 27 s leading time between Swarm B/A. As described in section “Processing approach,” to calculate the correlations, for each Swarm pair, we took the ΔNe series from the eastside spacecraft as reference and time-shifted the other one. In the northern hemisphere, we found Δt is about 2.0, 3.5 and 5.0 s for the three pairs, respectively. The positive values of Δt result from the EPI inverted-C shell structure. In fact, from the latitudinal profiles of electron density, we can also see that Swarm C observed the density depletion at highest latitudes, as expected for the most westerly spacecraft.
However, the correlations are significantly reduced in the southern hemisphere with R max of 0.56, 0.55 and 0.42 for the three pairs, compared to the northern hemisphere. The corresponding Δt with values of −10.0, −14.5 and −24.0 s are also somewhat larger in absolute value than those in the northern hemisphere. And we found the electron density latitudinal profiles show much finer structures between 6° and 12°S MLAT, which might contribute to the lower correlations in the southern hemisphere. In addition, the negative values of Δt in the southern hemisphere seem not support the westward-tilted inverted-C shell structure of EPI.
Figure 2c presents another example of EPI on December 19, 2013. The electron densities measured by Swarm also show similar depletions in both hemispheres. In this case, the three spacecrafts were practically at the same altitude of about 501 km, and Swarm B was 1 and 2 min ahead of Swarm A and C, respectively. The longitudinal separation between Swarm B/A increased to 0.2° (about 22 km), and 0.3° (about 33 km) between Swarm A/C. In the northern hemisphere, R max was larger than 0.7 for all the three pairs and again was largest for Swarm B/A. In the southern hemisphere, R max is somewhat smaller but still around 0.7. The time shifts (Δt) were again mainly positive in the northern and negative in the southern hemisphere.
Discussion and summary
In this study, we analyze the scale sizes of equatorial plasma irregularities based on observations from the Swarm constellation.
The examples of EPIs presented in Fig. 2 show that the plasma irregularities usually have various scale sizes in the meridional direction, and these structures sometimes are not well correlated in the zonal direction. As shown in Fig. 5, the correlation rapidly decreases between neighboring measurements over tenths of a degree in longitude. With longitudinal separations larger than 0.4° (about 44 km), no significant correlation was found any more. This result suggests that small-scale structures within EPIs have short correlation lengths in longitude. Numerical model studies also report typical density depletion diameters of 20–30 km (e.g., Huba and Joyce 2007; Retterer 2010; Yokoyama et al. 2015). Swarm observational results can help to constrain the range of EPIs sizes in simulation studies.
From the two examples presented in Fig. 2, we found Swarm C observed density depletions at highest latitude in the northern hemisphere. Considering the spatial formation of Swarm during the early mission phase (Swarm C is on the most westward), this result is consistent with the positive value of Δt for cross-correlation in the northern hemisphere and supports a westward-tilted inverted-C shell structure of EPIs, which is believed to be present for most of observed irregularity structures. However, the negative values of Δt in the southern hemisphere seem not to support such a simple model. Huba et al. (2009) showed that the shell structure of EPIs strongly depends on the vertical profile of zonal wind. For different zonal wind conditions, this shell structure can be westward tilted with different angles and sometimes is even eastwardly tilted (see their Fig. 4). Figure 5a, b illustrates this range of variations. In general, for other examples (not shown here), most of the EPI events presented positive Δt in the northern hemisphere (corresponding to westward tilt), but there were also some events of negative Δt in the northern hemisphere with R max larger than 0.6. The Δt in the southern hemisphere is more scattered toward both positive and negative values. For the interpretation of the observed latitude shift, we may have to consider also the magnetic field declination and meridional drifts, and dedicated study may be needed.
Another point to be considered here is the southern location of the geomagnetic equator. During December solstice month, being local summer in the southern hemisphere, larger electron densities are expected in the regions south of the geographic equator that has the potential to develop steeper ∇n, and thus enhance the EPI growth rate. We therefore explain the observed lower correlations of EPIs between Swarm A and C in the southern than those in the northern hemisphere by these two effects, low geomagnetic field and southern location of the magnetic equator.
In summary, the Swarm constellation mission provides us with the opportunity to study the scale sizes of equatorial plasma irregularities simultaneously in the meridional and zonal directions. We found that with longitudinal separations between Swarm satellites larger than 0.4° no significant correlation was found any more. This result suggests that EPI structures include plasma density scale sizes less than 44 km in the zonal direction. During the earlier mission period, clearly better correlations of EPIs are obtained in the northern hemisphere, implying more fragmented irregularities in the southern hemisphere where the ambient magnetic field is low. The previously reported inverted-C shell structure of EPIs is generally confirmed by the Swarm observations in the northern hemisphere, but with various tilt angles. From the Swarm spacecraft with zonal separations of about 150 km, we conclude that larger zonal scale sizes of irregularities exist in the early evening hours (around 1900 LT) that are interpreted as larger scale lengths, initial perturbations of post-sunset ionospheric plasma irregularities.
CX designed the MATLAB program for finding the equatorial plasma irregularity (EPI) events from Swarm electron density observations, performed the statistical analysis, and drafted the manuscript. CS provided fruitful discussion of the results and drafted the manuscript. HL, JP, BGF, GNK gave constructive suggestions for improving the text. All authors read and approved the final manuscript.
The authors thank J. Rauberg and I. Michaelis for their valuable comments on Swarm data processing. The European Space Agency (ESA) is acknowledged for providing the Swarm data. The official Swarm website is http://earth.esa.int/swarm, and the server for Swarm data distribution is ftp://swarm-diss.eo.esa.int.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
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Albert A. Michelson (1852-1931) was known for his experiments for measuring the speed of light. Here is a short summary of what he did from The University of Chicago.
A master at the art of measurement, Michelson devised experiments noted for their simplicity. Thus measurements of the velocity of light used a rapidly rotating, multifaceted mirror to reflect a beam of light. When the speed was correctly adjusted, light reflected from the rotating mirror struck a mirror held in a fixed position and returned to strike the next face on the spinning mirror. The time needed for the next face to rotate into position to precisely reflect the light was the time required for the light to travel the known distance to the stationary mirror and back. His earliest attempts, made with materials costing barely ten dollars, measured the speed of light at 186,508 miles per second, or within two hundred miles of the actual value.Above is one of the mirrors used by Michelson. Michelson attended the Naval Academy and the apparatus is in the Naval Academy Museum. Here is a link to a short biography of Michelson from the Wikipedia article about his life. | <urn:uuid:0b105f40-91f2-4a09-8c14-af6149be103a> | 3.78125 | 229 | Knowledge Article | Science & Tech. | 44.574091 | 95,505,289 |
Sitting in a museum is not where you would probably imagine discovering eight new species, rather you might imagine intrepid biologists and researchers trekking through the jungles and up mountains all in search of new life much like an 18th Century explorer. However biologists working on a journal for PLoSONE have found eight new species of whip spiders amongst zoological collections.
Whip Spiders are often likened to tailless scorpions, when in fact they are neither scorpion nor spider. You might actually be familiar with them as they guest starred in Harry Potter, as the character Mad Eye Moody demonstrated various curses on them, however for anyone who is not a Potter fan they belong to a group of arachnids called Amblypygi, meaning blunt rump.
They also bare considerably differences to spiders as they only use six legs to walk on rather than the traditional eight. Their eighth pair of legs is instead used as a sensory probe. They also do not have venomous fangs or silk glands.
It is always exciting to find new species of such a unique animal however this discovery comes with a note of caution. The majority of the new species have only been recorded at a handful of sites thus their restricted distribution makes them highly vulnerable to external pressures.
Four of the new species are located in areas of high human exploitation or environmental modification. To give you an example of how imminent extinction could be for some of these species, C. bichuetteae is only found in an area threatened by imminent flooding expected to be caused by the hydroelectric dam of Belo Monte, located on the Xingu River in the Brazilian Amazon.
Gustavo Silva de Miranda of the University of Copenhagen, told the Christian Science Monitor, “Because these are cave animals, they only inhabit these caves and nowhere else. If we destroy their habitats, they are gone forever.” There are now calls for them to be added to the list of threatened species.
Featured Image courtesy of Alessandro Ponce de Leão Giupponi and Gustavo Silva de Miranda, 2016.
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Rooftop bugs may provide valuable insight regarding climate change and how speceis adapt, relocate or die.
Cowbirds lay their eggs in neighboring nests and abandon the to be raised by surrogate bird parents. Researchers recently spent some attempting to figure out whether the birds know they are different from their adopted families and how they handle it in terms of feeding, mating and flocking.
Spruce beetles have often been blamed for increasing the severity of raging wildfires throughout Colorado becuase they kill the trees they call home. But a University of Colorado Boulder study has determined that the insects are innocent of the charge.
University of Exeter reseachers took a closer look at fruit flies and found that while environmental conditions have some influence on their mating decision, it mostly comes down to genetics.
After solving and controlling the mechanisms responsible for a cockroach's mobility, researchers from Case Western Reserve University (CWRU) believe these findings could be applied to helping the handicapped walk, building more stable robots, and improving autonomous vehicles.
Caucasian Parsley Frogs (Pelodytes punctatus) feed on insects breeding in the feces of bats in remote limestone caves near Russia, researchers discovered. Their study sheds light on the importance of species conservation.
While the social roles some insects inhabit are set in stone, dinosaur ants and red paper wasps are far more flexible – and able to adjust their behaviors for varying responsibilities.
Some pollinating leafhoppers may be transmitting deadly bacteria to flowering plants. When infected, the plants are unable to blossom and sexual reproduction is prevented turning them into the living dead.
The ability of spiders to adapt to their local environment and humidity levels by altering the viscosity of their "web glue" may help scientists develop stronger adhesives.
Honey Bees can't resist caffeine, and some plants are even making their nectar more caffeinated to attract them. This dynamic could have serious impacts on pollination and honey production, researchers say.
In recent days, as temperatures increased in Minnesota and Wisconsin, millions of ladybugs crept out from hidden crevices to plague homeowners. Although these tiny insects were once considered cute, their huge swarms are causing more people to view them as mere pests.
Male crickets often use edible gifts to attract females during mating. However, unique proteins contained in this gift may alter female's behavior to ensure reproductive success.
A featherwing beetle was measured to be 0.325mm. This is considered the world's tiniest, free-living insect.
Ants that called Europe their home 45 to 10 million years ago were actually more similar to modern-day ants now living in South East Asia than they are to their European cousins.
The worker ant: even the name of this tiny insect calls fourth impressions of hard work and dedication, the tireless and duty-bound individual laboring for the greater good of his community. However, it turns out that this really doesn't describe your everyday worker ant. In fact, the great majority of the little buggers are actually slackers. | <urn:uuid:5b6a91cd-52b0-4a4f-bb5b-9d14ac91eab9> | 3.484375 | 624 | Content Listing | Science & Tech. | 32.025342 | 95,505,338 |
Astronomers have identified a young star, located almost 11,000 light years away, which could help us understand how the most massive stars in the Universe are formed. This young star, already more than 30 times the mass of our Sun, is still in the process of gathering material from its parent molecular cloud, and may be even more massive when it finally reaches adulthood.
The researchers, led by a team at the University of Cambridge, have identified a key stage in the birth of a very massive star, and found that these stars form in a similar way to much smaller stars like our Sun - from a rotating disc of gas and dust. The results will be presented this week at the Star Formation 2016 conference held at the University of Exeter, and are reported in the Monthly Notices of the Royal Astronomical Society.
In our galaxy, massive young stars - those with a mass at least eight times greater than the Sun - are much more difficult to study than smaller stars. This is because they live fast and die young, making them rare among the 100 billion stars in the Milky Way, and on average, they are much further away.
"An average star like our Sun is formed over a few million years, whereas massive stars are formed orders of magnitude faster -- around 100,000 years," said Dr John Ilee from Cambridge's Institute of Astronomy, the study's lead author. "These massive stars also burn through their fuel much more quickly, so they have shorter overall lifespans, making them harder to catch when they are infants."
The protostar that Ilee and his colleagues identified resides in an infrared dark cloud - a very cold and dense region of space which makes for an ideal stellar nursery. However, this rich star-forming region is difficult to observe using conventional telescopes, since the young stars are surrounded by a thick, opaque cloud of gas and dust. But by using the Submillimeter Array (SMA) in Hawaii and the Karl G Jansky Very Large Array (VLA) in New Mexico, both of which use relatively long wavelengths of light to observe the sky, the researchers were able to 'see' through the cloud and into the stellar nursery itself.
By measuring the amount of radiation emitted by cold dust near the star, and by using unique fingerprints of various different molecules in the gas, the researchers were able to determine the presence of a 'Keplerian' disc - one which rotates more quickly at its centre than at its edge.
"This type of rotation is also seen in the Solar System - the inner planets rotate around the Sun more quickly than the outer planets," said Ilee. "It's exciting to find such a disc around a massive young star, because it suggests that massive stars form in a similar way to lower mass stars, like our Sun."
The initial phases of this work were part of an undergraduate summer research project at the University of St Andrews, funded by the Royal Astronomical Society (RAS). The undergraduate carrying out the work, Pooneh Nazari, said, "My project involved an initial exploration of the observations, and writing a piece of software to 'weigh' the central star. I'm very grateful to the RAS for providing me with funding for the summer project -- I'd encourage anyone interested in academic research to try one!"
From these observations, the team measured the mass of the protostar to be over 30 times the mass of the Sun. In addition, the disc surrounding the young star was also calculated to be relatively massive, between two and three times the mass of our Sun. Dr Duncan Forgan, also from St Andrews and lead author of a companion paper, said, "Our theoretical calculations suggest that the disc could in fact be hiding even more mass under layers of gas and dust. The disc may even be so massive that it can break up under its own gravity, forming a series of less massive companion protostars."
The next step for the researchers will be to observe the region with the Atacama Large Millimetre Array (ALMA), located in Chile. This powerful instrument will allow any potential companions to be seen, and allow researchers to learn more about this intriguing young heavyweight in our galaxy.
This work has been supported by a grant from the European Research Council.
Sarah Collins | EurekAlert!
Computer model predicts how fracturing metallic glass releases energy at the atomic level
20.07.2018 | American Institute of Physics
What happens when we heat the atomic lattice of a magnet all of a sudden?
18.07.2018 | Forschungsverbund Berlin
A new manufacturing technique uses a process similar to newspaper printing to form smoother and more flexible metals for making ultrafast electronic devices.
The low-cost process, developed by Purdue University researchers, combines tools already used in industry for manufacturing metals on a large scale, but uses...
For the first time ever, scientists have determined the cosmic origin of highest-energy neutrinos. A research group led by IceCube scientist Elisa Resconi, spokesperson of the Collaborative Research Center SFB1258 at the Technical University of Munich (TUM), provides an important piece of evidence that the particles detected by the IceCube neutrino telescope at the South Pole originate from a galaxy four billion light-years away from Earth.
To rule out other origins with certainty, the team led by neutrino physicist Elisa Resconi from the Technical University of Munich and multi-wavelength...
For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.
Whirlpools are an everyday experience in a bath tub: When the water is drained a circular vortex is formed. Typically, such whirls are rather stable. Similar...
Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.
A frequently used reaction in organic chemistry is nucleophilic substitution. It plays, for example, an important role in in the synthesis of new chemical...
Optical spectroscopy allows investigating the energy structure and dynamic properties of complex quantum systems. Researchers from the University of Würzburg present two new approaches of coherent two-dimensional spectroscopy.
"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy....
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- David George
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An ant colony’s queen sends out a message encoded on her eggs. The surface of the queen’s eggs is coated with a pheromone that prevents worker ants from laying their own eggs. Each colony includes a fertile queen and her largely infertile offspring, known as workers. If the workers also reproduced, the colony’s productivity might suffer. Pheromones can signal the presence of a fertile queen and thus prevent worker reproduction.It seems that the phermone is a special hydrocarbon blend similar to the one found in the Queen ant's body.
"Nothing in biology makes sense except in the light of evolution"
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Junction is a pseudostate (see Section 20.11, “Pseudostate”) which is used to split an incoming transition into multiple outgoing transition segments with different guard conditions. A Junction is also called a Merge or Static conditional branch. The chosen transition is that whose guard is true at the time of the transition.
A predefined guard denoted
else may be
defined for at most one outgoing transition. This transition is
enabled if all the guards labeling the other transitions are
According the UML standard, its symbol is a small black circle. Alternatively, it may be represented by a diamond shape (in case of "Decision" for Activity diagrams). ArgoUML only represents a junction on the diagram as a solid (white by default) diamond, and does not support the black circle symbol for a junction. | <urn:uuid:acc4fd03-3be0-4fb2-b22d-b603e4b95afa> | 2.53125 | 174 | Documentation | Software Dev. | 44.601136 | 95,505,385 |
Horologium Oscillatorium: sive de motu pendulorum ad horologia aptato demonstrationes geometricae (Latin for The Pendulum Clock: or geometrical demonstrations concerning the motion of pendula as applied to clocks), often abbreviated Horologium Oscillatorium, is a book published by Christiaan Huygens in 1673; it is his major work on pendulums and horology. This work is regarded as one of the three most important work done in mechanics in the 17th century, the other two being Galileo Galilei’s Discourses and Mathematical Demonstrations Relating to Two New Sciences (1638) and Isaac Newton’s Philosophiæ Naturalis Principia Mathematica (1687).
The book is divided into five parts, where the first part contains the descriptions of clock designs, while the rest of the book is devoted to the analysis of pendulum motion and a theory of curves. In the second part of the book, Huygens states three hypotheses on the motion of bodies. They are essentially the law of inertia and the law of composition of "motion". He then uses these three rules to re-derive Galileo's original study of falling bodies, based on clearer logical framework. He then studies constrained fall, obtaining the solution to the tautochrone problem as given by a cycloid curve and not a circle as Galileo had conceived. In the third part of the book, he outlines a theory of evolutes and rectification of curves. The fourth part of the book is concerned with the study of the center of oscillation. The derivations of propositions in this part is based on a single assumption: that the center of gravity of heavy objects cannot lift itself, which Huygens used as a virtual work principle. In the process, Huygens obtained solutions to dynamical problems such as the period of an oscillating simple pendulum as well as a compound pendulum, center of oscillation and its interchangeability with the pivot point, and the concept of moment of inertia. The last part of the book gives propositions regarding bodies in uniform circular motion, without proof, and states the laws of centrifugal force for uniform circular motion.
Levy & Wallach-Levy (2001) write that, as well as being known for its science, the book is also known for its strangely worded dedication to Louis XIV. Joella G. Yoder writes that the appearance of the book in 1673 was a political issue, since at that time Netherlands was at war with France; and Huygens was anxious to show his allegiance to his patron, which can be seen in the obsequious dedication to Louis IV.
- Huygens, Christiaan; Blackwell,, Richard J., trans. (1986). Horologium Oscillatorium (The Pendulum Clock, or Geometrical demonstrations concerning the motion of pendula as applied to clocks). Ames, Iowa: Iowa State University Press. ISBN 0813809339.
- Herivel, John. "Christiaan Huygens". Encyclopædia Britannica. Retrieved 14 November 2013.
- Bell, A. E. (30 Aug 1941). "The Horologium Oscillatorium of Christian Huygens". doi:10.1038/148245a0. Retrieved 14 November 2013.
- Ducheyne, Steffen (2008). "Galileo and Huygens on free fall: Mathematical and methodological differences". Dynamis : Acta Hispanica ad Medicinae Scientiarumque. Historiam Illustrandam. pp. 243–274. ISSN 0211-9536. Retrieved 2013-12-27.
- Mahoney, Michael S. (March 19, 2007). "Christian Huygens: The Measurement of Time and of Longitude at Sea". Princeton University. Archived from the original on 2007-12-04. Retrieved 2013-12-27.
- Bevilaqua, Fabio; Lidia Falomo; Lucio Fregonese; Enrico Gianetto; Franco Giudise; Paolo Mascheretti (2005). "The pendulum: From constrained fall to the concept of potential". The Pendulum: Scientific, Historical, Philosophical, and Educational Perspectives. Springer. pp. 195–200. ISBN 1-4020-3525-X. Retrieved 2008-02-26. gives a detailed description of Huygen's methods
- Huygens, Christian (August 2013). "Horologium Oscillatorium (An English translation by Ian Bruce)". Retrieved 14 November 2013.
- Levy, David H.; Wallach-Levy, Wendee (2001), Cosmic Discoveries: The Wonders of Astronomy, Prometheus Books, ISBN 9781615925667.
- Yoder, Joella G. (2005), "Christiaan Huygens book on the pendulum clock 1673", Landmark Writings in Western Mathematics 1640-1940, Elsevier, ISBN 9780080457444.
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Nitrogen is an essential nutrient. As N2 gas it is a major constituent of the atmosphere, but N2 is chemically inert and therefore unavailable as a source of nitrogen for use by most living organisms. However, some bacteria have the ability to reduce N2 and thereby “fix” atmospheric nitrogen using the enzyme nitrogenase. Many leguminous plants have capitalised on this special bacterial asset by going into partnership with nitrogen-fixing bacteria called rhizobia. In return for supplying nutrients to the bacteria, the plants receive a supply of reduced nitrogen. In essence, the legumes create a highly specialised environment within which the bacteria fix nitrogen. These specialised plant structures are called nodules; usually they are found on roots, but they also occur on the stems of some legumes.
KeywordsNitrogen Fixation Root Hair Nodule Development Infection Thread Legume Root
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- 1.Bergersen FJ (1982) Root nodules of legumes: structure and functions. Research Studies Press, John Wiley and Sons, Chichester.Google Scholar
- Dart PJ (1977) Infection and development of leguminous root nodules. In: Hardy RWF, Silver WS (eds) A treatise on dinitrogen fixation, III, Biology. Wiley, New Nork, pp 367–472.Google Scholar
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- Weeden NF, Kneen BE, LaRue TA (1990) Genetic analysis of sym genes and other related genes in Pisum sativum. In: Gresshoff PM, Roth J, Stacey G, Newton W (eds) Nitrogen fixation: achievements and objectives. Chapman and Hall, New York, pp 323–330.Google Scholar
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When you study galaxies you get to look at the most beautiful things in the universe (at least I think so). As late as 1920 the true nature of the fuzzy "spiral nebulae" was not known. The 60-inch telescope at Mt. Wilson began operating about 1907; it was capable of getting good photographs of the nebulae. Their spiral shape was obvious but their nature was not. There was a great debate about what they were. Were they relatively small objects relatively close or huge systems at very great distances? Edwin Hubble attacked the problem using both the 60-inch telescope and the new 100-inch telescope (1917) at Mt. Wilson. By 1926 or so he had identified and studied a number of Cepheid variables in the Andromeda spiral (M31). His results indicated a distance of about 275,000 parsecs, or about 900,000 light years. This discovery answered the question; the spiral nebulae were giant systems at very large distances. Hubble's photographs also began to show individual stars in the nebulae, refuting the notion that they were really nebulous objects not made of stars.
Spiral galaxies vary in appearance. At one end of the range are those with large central bulges and tightly wrapped arms. Toward the other end we find galaxies with relatively small central bulges and more open arms. The first type he called "Sa"(M104 is an example) and the second is "Sb." (The Whirlpool Galaxy M51 is one). Type "Sc" galaxies (like M74 here) have very open and less well-defined spiral structure.
Elliptical galaxies look elliptical. Of course, this is a projection of a three-dimensional object into two dimensions, so the actual shape is some spherical or oblong solid. A type E0 galaxy appears nearly round. An E1 galaxy, like M87 here,is slightly flattened. An E7 may resemble a fat cigar.
Ellipticals come in a wide range of sizes. Dwarfs can be as small as 1 kpc in diameter and have less than 1,000,000 stars; that is small for a galaxy. Giant ellipticals can be up to several megaparsecs across. Compare this to the Milky Way Galaxy's diameter of 30 kpc; a giant elliptical can be over 50 times the diameter of the Milky Way!
There's not much star formation going on in elliptical galaxies. Someone has detected evidence of a little star formation, but it is small. This type of galaxy is populated mostly with older stars, mostly low mass stars which are more reddish in color.
Irregular galaxies don't have any identifiable shape to them. That's why they are known as irregular. They tend to be smaller than the spirals but larger than dwarf ellipticals. The irregular galaxy M82 is well-known and can be seen in an amateur telescope as an elongated glow. The Small and Large Magellanic Clouds (SMC and LMC) are small irregular systems that are very close to the Milky Way. The SMC is the system in which Henrietta Leavitt discovered the relation of period to luminosity in Cepheid variables.
Photos of spiral galaxies present a somewhat deceptive picture of the galaxy's makeup. Most of the light in a galaxy comes from the brightest stars (O and B), while the huge number of dim low-mass stars (M) makes them comprise most of the galaxy's mass. So - in that beautiful galaxy picture - the spiral arms are outlined by the light of the blue giants, which are comparatively rare, while the mass is made up of the dimmer stars that you can't see in the photo. Also - spiral galaxies are oriented randomly in space; some are seen face-on (the pretty ones) and some are seen edge-on.
Some of the galaxies pictured in Chapter 15 have "M" numbers, like "M51" 1n figure 15.2 part (b) on page 393. Picture (a) is of M81. The "M" comes from the last name of Charles Messier (messiay) (1730-1817), a French observer who specialized in comets. He also discovered several objects which resembled comets but were not comets. After listing 3 or 4 of these things, he undertook a search for other such things using his own observations plus catalogs compiled by others, and out of these compiled a list of 110 objects that were known to be fixed in the sky and therefore not comets.
Consider the Crab Nebula, which you encountered in Lab 10; it is listed as M1. The Pleiades are known as M45. The nearest galaxy to us, the Andromeda Spiral, is M31.
Finding the distance to galaxies is not trivial. They are over 1000 times too far away for parallax distance measurement. The only way to do it is to locate some known standard candle object in the galaxy. Cepheid variables are excellent since we can know their absolute magnitude and they are very luminous. Dim standard candles would not be useful because they cannot be seen at great distances. Be sure to study the distance ladder in Figure 15.11. Type I supernovae also make good standard candles. Since we think that they are all about the same (carbon or carbon/oxygen white dwarf exploding at about 1.4 solar masses), their peak luminosity should be consistent and knowable. These explosions also have the advantage of being VERY bright, and therefore visible very far away. The only problem is that we must wait until one decides to explode; they don't do it on our schedule. Also - recent work indicates that there is some variance in the luminosity of these blasts, so there is an error bar on the distances calculated from them. Here's a good illustration of the distance ladder.
Note one very important property of the distance ladder: each rung is built on the previous one, and the uncertainty increases with each step. At the greatest distances the precision of the measurement may not be much better than 2 significant figures if that good.
Finally, we noted the concept of a fundamental measurement - one for which no assumptions are necessary. The use of radar to find the AU is one of these. The speed of light has been precisely measured and Kepler's Laws tell us about the planet's orbits. Measuring Venus' distance requires no assumptions. Once you have the AU measured precisely, parallax measurement of stellar distance requires no assumptions. With the baseline known, the limit to the method is our ability to measure the tiny parallax angles. | <urn:uuid:c592568e-0f2b-4ab3-a819-1a76001c79c7> | 3.703125 | 1,356 | Academic Writing | Science & Tech. | 54.795903 | 95,505,405 |
The Scientists made a scientific breakthrough that could open the door to a new method of production and source of nuclear fuel – the World ocean, which contains accumulated over a long time chemical. Check a special fiber that allows you to extract from sea water natural traces of uranium allowed scientists to produce the first 5 grams of a radioactive substance – poroshkoobraznogo uranium concentrate used as fuel in the production of nuclear energy.
"This is a very important achievement, which indicates that this approach is able to provide a commercially attractive method for the production of nuclear fuel from the oceans is the largest source of uranium on the Earth," — says biochemist Gary Gill Pacific Northwest national laboratory (PNNL, USA).
produced the First grams of powdered uranium
For the extraction of substances, the team from PNNL (owned by the U.S. Department of energy) has teamed up with scientists from the company LCW Supercritical Technologies. The latter has developed a special acrylic fiber, which is extracted dissolved in water natural uranium.
"We chemically modified a cheap fiber and turned it into an efficient and reusable absorbent, attractive and exciting Uranus" — explains the President of LCW Supercritical Technologies Chien Wai.
"Opportunities for PNNL in the improvement and testing of this material has provided invaluable support in the development of this technology."
In the early studies, the Wai has helped to develop an extraction process in which uranium is absorbed by the ligand (molecule that interacts with a complementary site of a certain structure), chemically bound with acrylic fibers. To collect molecules of uranium fiber is placed directly in ocean water (or the water is pumped to the laboratory), where it is through a certain amount of time starts to extract floating in it uranium.
Scientists have long worked on the optimization of this method is the production of radioactive substances, and believe that one day it will be of great benefit. Not just because to extract uranium from the ocean will be easier – no need to dig ground uranium mines, — but also because in the oceans can contain virtually an endless supply of this substance.
"Concentrate on first glance very small, equivalent to one crystal salt diluted in one liter of water. But the oceans are so huge that if we could economically extract uranium, you get a virtually endless supply of this nuclear fuel," — comment on third-party researchers from Stanford University, did not participate in the research.
Test absorbent in the laboratory
According to Chien Wai absorbent material is inexpensive to manufacture, and the scale of this production cost is below the cost of mining uranium on land. In the future, significantly below scientists estimate that on the bottom of the ocean may be at least 4 billion tons of uranium, which is approximately 500 times greater than all known reserves available in the ground ore.
The researchers plan to continue work and want to see what types of other chemical substances able to absorb the developed fiber.
In a recently published work in Nature Sustainability group scientists concluded that the Earth can support, at best, only 7 billion people on the subsistence level (and in this June we had already 7.6 billion). Achieving "a high level of life satisf...
for the First time in history, scientists have found evidence that a radical change of a volcano in southern Japan were a direct result of the eruption of another volcano 22 kilometers away. Monitoring these two volcanoes, Aira, and Kirishima – showe...
Almost a year ago Antarctic glacier Larsen has created iceberg A-68, one of the largest blocks of ice from everyone that knows the story. Video made using satellite imagery, shows a split, separation and the subsequent journey of the iceberg in the l...
a Message from the English giant of astrophysics Stephen Hawking sent to the nearest black hole during a solemn funeral in Westminster Abbey, which took place on Friday, June 15. A specially written composition with his famous syn...
the world's largest particle accelerator is launching a major update which would allow us to extract 10 times more data and help in revealing the secrets of physics. European organization for nuclear research, aka CERN, began work...
the Scientists were able to collect water from dry desert air. They only require heat from the sun. The invention can change the lives of the 2.1 billion people who lack clean drinking water. The main advantage of the invention is... | <urn:uuid:42b21208-8a23-48b9-9f67-a39932fa5648> | 3.328125 | 895 | News Article | Science & Tech. | 40.841782 | 95,505,420 |
The Land Surface Temperature (LST) raster dataset is a dataset derived from a satellite image from Landsat 8 to show the land surface temperature for the seven-county metropolitan region of the Twin Cities, in degrees Fahrenheit. In essence, this map shows how hot the ground was to the touch at any given location. This satellite image was taken at 11:59 am CDT on July 22, 2016. At this time, the air temperature was 90F with a heat index of 90.3F, as taken at the Minneapolis-St. Paul International Airport (Minneapolis-St Paul International Airport, 2016). This was the third day of a regional heat wave , and while temperatures overnight had dipped down to around 74F, temperatures had climbed up to a maximum temperature of 97F by around 5-6pm, resulting in the hottest day in roughly three years (Midwestern Regional Climate Center, 2016). The original thermal image was taken at a 100 x 100 meter resolution, but was re-scaled and processed at the 30 x 30 meter scale. (Landsat 8 collects image data at a spatial resolution of 30 meters (visible, NIR, SWIR); 100 meters (thermal); and 15 meters (panchromatic).) This Landsat 8 raster image was first used to create a NDVI raster dataset. Using that NDVI dataset with Bands 10 and 11 of the Landsat 8 raster dataset, a Land Surface Temperature raster can be derived. That raster is further processed from celsius to fahrenheit, then clipped to the 7-county metropolitan area, and all water bodies removed from the final image. The LST values without regional water bodies is the primary basis for the Metropolitan Council Climate Vulnerability Assessment Report. Part of this report specifically considers the urban heat island effect, or the effect of human activity and the built environment on increases in urban temperature on human life, and as water has different heat retention properties than most land surfaces, we chose to do our analysis without regional water bodies. It should be noted that the lowest original LST values were water bodies, and so by removing the water bodies from the map the minimum LST value raised 0.6F. This is not particularly significant, except that it would minimally affect which values would be highlighted in the third layer of this package. | <urn:uuid:58142c81-1385-49be-8685-d08884846bb3> | 3.59375 | 477 | Knowledge Article | Science & Tech. | 43.443607 | 95,505,424 |
The previous chapter introduced you to the process of building a GUI-based desktop application using System.Windows.Forms. The point of this chapter is to examine the details of rendering graphics (including stylized text and image data) onto a Form’s surface area. We’ll begin by taking a high-level look at the numerous drawing-related namespaces, and we’ll examine the role of the Paint event, and the almighty Graphics object.
KeywordsSource Code Graphic Class Visual Studio Graphic Object Menu Item
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On January 26, 1700, at about 9 p.m. local time, the Juan de Fuca plate beneath the ocean in the Pacific Northwest suddenly moved, slipping some 60 feet eastward beneath the North American plate in a monster quake of approximately magnitude 9, setting in motion large tsunamis that struck the coast of North America and traveled to the shores of Japan.
Virtual 9.0 Earthquake
Shakes Pacific Northwest
Scientists used a supercomputer-driven “virtual earthquake” to explore likely ground shaking in a magnitude 9.0 megathrust earthquake in the Pacific Northwest. Peak ground velocities are displayed in yellow and red. The legend represents speed in meters per second (m/s) with red equaling 2.3 m/s. Although the largest ground motions occur offshore near the fault and decrease eastward, sedimentary basins lying beneath some cities amplify the shaking in Seattle, Tacoma, Olympia, and Vancouver, increasing the risk of damage. Credit: Kim Olsen, SDSU.
Since then, the earth beneath the region – which includes the cities of Vancouver, Seattle and Portland -- has been relatively quiet. But scientists believe that earthquakes with magnitudes greater than 8, so-called “megathrust events,” occur along this fault on average every 400 to 500 years.
To help prepare for the next megathrust earthquake, a team of researchers led by seismologist Kim Olsen of San Diego State University (SDSU) used a supercomputer-powered “virtual earthquake” program to calculate for the first time realistic three-dimensional simulations that describe the possible impacts of megathrust quakes on the Pacific Northwest region. Also participating in the study were researchers from the San Diego Supercomputer Center at UC San Diego and the U.S. Geological Survey.
What the scientists learned from this simulation is not reassuring, as reported in the Journal of Seismology, particularly for residents of downtown Seattle.
With a rupture scenario beginning in the north and propagating toward the south along the 600-mile long Cascadia Subduction Zone, the ground moved about 1 ½ feet per second in Seattle; nearly 6 inches per second in Tacoma, Olympia and Vancouver; and 3 inches in Portland, Oregon. Additional simulations, especially of earthquakes that begin in the southern part of the rupture zone, suggest that the ground motion under some conditions can be up to twice as large.
“We also found that these high ground velocities were accompanied by significant low-frequency shaking, like what you feel in a roller coaster, that lasted as long as five minutes – and that’s a long time,” said Olsen.
The long-duration shaking, combined with high ground velocities, raises the possibility that such an earthquake could inflict major damage on metropolitan areas -- especially on high-rise buildings in downtown Seattle. Compounding the risks, like Los Angeles to the south, Seattle, Tacoma, and Olympia sit on top of sediment-filled geological basins that are prone to greatly amplifying the waves generated by major earthquakes.
“One thing these studies will hopefully do is to raise awareness of the possibility of megathrust earthquakes happening at any given time in the Pacific Northwest,” said Olsen. “Because these events will tend to occur several hundred kilometers from major cities, the study also implies that the region could benefit from an early warning system that can allow time for protective actions before the brunt of the shaking starts.” Depending on how far the earthquake is from a city, early warning systems could give from a few seconds to a few tens of seconds to implement measures, such as automatically stopping trains and elevators.
Added Olsen, “The information from these simulations can also play a role in research into the hazards posed by large tsunamis, which can originate from such megathrust earthquakes like the ones generated in the 2004 Sumatra-Andeman earthquake in Indonesia.” One of the largest earthquakes ever recorded, the magnitude 9.2 Sumatra-Andeman event was felt as far away as Bangladesh, India, and Malaysia, and triggered devastating tsunamis that killed more than 200,000 people.
In addition to increasing scientific understanding of these massive earthquakes, the results of the simulations can also be used to guide emergency planners, to improve building codes, and help engineers design safer structures -- potentially saving lives and property in this region of some 9 million people.
Even with the large supercomputing and data resources at SDSC, creating “virtual earthquakes” is a daunting task. The computations to prepare initial conditions were carried out on SDSC’s DataStar supercomputer, and then the resulting information was transferred for the main simulations to the center’s Blue Gene Data supercomputer via SDSC’s advanced virtual file system or GPFS-WAN, which makes data seamlessly available on different – sometimes distant – supercomputers.
Coordinating the simulations required a complex choreography of moving information into and out of the supercomputer as Olsen’s sophisticated “Anelastic Wave Model” simulation code was running. Completing just one of several simulations, running on 2,000 supercomputer processors, required some 80,000 processor hours – equal to running one program continuously on your PC for more than 9 years!
“To solve the new challenges that arise when researchers need to run their codes at the largest scales, and data sets grow to great size, we worked closely with the earthquake scientists through several years of code optimization and modifications,” said SDSC computational scientist Yifeng Cui, who contributed numerous refinements to allow the computer model to “scale up” to capture a magnitude 9 earthquake over such a vast area.
In order to run the simulations, the scientists must recreate in their model the components that encompass all the important aspects of the earthquake. One component is an accurate representation of the earth’s subsurface layering, and how its structure will bend, reflect, and change the size and direction of the traveling earthquake waves. Co-author William Stephenson of the USGS worked with Olsen and Andreas Geisselmeyer, from Ulm University in Germany, to create the first unified “velocity model” of the layering for this entire region, extending from British Columbia to Northern California.
Another component is a model of the earthquake source from the slipping of the Juan de Fuca plate underneath the North American plate. Making use of the extensive measurements of the massive 2004 Sumatra-Andeman earthquake in Indonesia, the scientists developed a model of the earthquake source for similar megathrust earthquakes in the Pacific Northwest.
The sheer physical size of the region in the study was also challenging. The scientists included in their virtual model an immense slab of the earth more than 650 miles long by 340 miles by 30 miles deep -- more than 7 million cubic miles -- and used a computer mesh spacing of 250 meters to divide the volume into some 2 billion cubes. This mesh size allows the simulations to model frequencies up to 0.5 Hertz, which especially affect tall buildings.
“One of the strengths of an earthquake simulation model is that it lets us run scenarios of different earthquakes to explore how they may affect ground motion,” said Olsen. Because the accumulated stresses or “slip deficit” can be released in either one large event or several smaller events, the scientists ran scenarios for earthquakes of different sizes.
“We found that the magnitude 9 scenarios generate peak ground velocities five to 10 times larger than those from the smaller magnitude 8.5 quakes.”
The researchers are planning to conduct additional simulations to explore the range of impacts that depend on where the earthquake starts, the direction of travel of the rupture along the fault, and other factors that can vary.
This research was supported by the National Science Foundation, the U.S. Geological Survey, the Southern California Earthquake Center, and computing time on an NSF supercomputer at SDSC.
Paul Tooby | EurekAlert!
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19.07.2018 | Materials Sciences | <urn:uuid:0772bf29-7382-43f9-8553-1be70c680ab1> | 3.625 | 2,280 | Knowledge Article | Science & Tech. | 36.204637 | 95,505,427 |
Species Detail - European Shag (Phalacrocorax aristotelis) - Species information displayed is based on all datasets.
Terrestrial Map - 10kmDistribution of the number of records recorded within each 10km grid square (ITM).
Marine Map - 50kmDistribution of the number of records recorded within each 50km grid square (WGS84).
Protected Species: Wildlife Acts || Threatened Species: Birds of Conservation Concern || Threatened Species: Birds of Conservation Concern >> Birds of Conservation Concern - Amber List
2 January (recorded in 2011)
31 December (recorded in 2012)
National Biodiversity Data Centre, Ireland, European Shag (Phalacrocorax aristotelis), accessed 21 July 2018, <https://maps.biodiversityireland.ie/Species/10023> | <urn:uuid:bb9ef8f1-df9a-477f-b358-9cf8b6f18f4e> | 2.59375 | 178 | Structured Data | Science & Tech. | 20.131053 | 95,505,434 |
What are the coordinates of the coloured dots that mark out the tangram? Try changing the position of the origin. What happens to the coordinates now?
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Geometry problems for inquiring primary learners.
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Wildfires have scorched more than 9 million acres in the United States so far this year, destroying buildings and homes in their paths. However, wildfires affect forests in various lesser-known ways. The following is a list of five different ways that plants, animals and water are affected by wildfires.
1. Animal predators see wildfire areas as an opportunity for food.
Rodents seek shelter from the flames by burrowing into the ground, taking cover in logs or hiding under rocks. Once the fire cools, they emerge and have less places to hide, making them easy targets for predators.
"Raptors will hunt at the edge of fires, so as the rodents are scampering away from the flames, they kind of are flushed out," Dr. Timothy Ingalsbee, co-director of the Association for Fire Ecology said.
Ingalsbee added that hunting is not as active while the fire is still burning because most critters are escaping or seeking shelter. However, right after a fire has been extinguished or controlled that the area becomes a "happy hunting grounds" for predators.
2. Most animals do not wait till the flames start to seek shelter or escape the forest."Rodents will burrow down into their hole in the ground or some of them actually will burrow into big, down logs that even though they are dead and dry on the outside, they can be very moist on the inside," Ingalsbee said.
Large game typically scurry away from the fire while birds will fly away, Ingalsbee added.
Smoke, heat and/or noise associated with fires can be signals for animals to get out of harm's way, Ingalsbee said.
3. Young and small animals are particularly at risk in wildfires.
Often the strategies animals have in place to escape the flames do not work, especially for young and small animals. They may not be able to find shelter or run fast enough to escape the flames.
"Those few who do survive fires usually have prolific reproduction," Ingalsbee said. 4. Bodies of water such as streams and rivers that flow through a fire burned area can be altered.
Impacts from wildfires can be detrimental to aquatic species; however, there are some positive effects.
"Fires can make the streams warmer which is not good for fish," Ingalsbee said.
Changes to the water flow or volume of the water can also occur from wildfires.
"[Fires] can increase volume of water because more is running off the slope or through the soil instead of being drawn up by plants," Ingalsbee said.
As a result, debris will flow or landslides can occur and these may alter the course of a stream.
Following wildfires, harmful sediment can also enter into streams along with any runoff.
However, some of the sediment that infiltrates the water is filled with nutrients for insects, which in turn becomes great food for fish and plants.
5. There are many plant species that need fires to occur as it is part of their life history.
Fire-dependent species such as the giant sequoia and lodgepole and jack pine rely of fires in order to reproduce.
"[A giant sequoia] has the tiniest little seed that will only take root and grow in the ash layer of a fresh fire. It needs the other plants to be cleared out of its way for it to grow," Ingalsbee said.
The ash contains many rich nutrients and when mixed with water can help a new generation of giant sequoia to grow, Ingalsbee added.
"We are missing several generations of giant sequoias because we have put out fires for so many decades. Instead, all these fir trees have grown up right underneath the giant sequoias, and in some cases, put the giant sequoias at risk," Ingalsbee said.
Lodgepole and jack pine use the heat from the flames to melt away the wax that holds their cones closed. This will open up the cones and the seeds will fall into the fresh ash, where they are able to take root without much competition, according to Stephen J. Pyne, professor in the Biology and Society Program at Arizona State University and author of the "Cycle of Fire," book series.
"The biodiversity of even the most severely burned areas are hot spots of all these diverse species of not just plants, but animals that flock to burned forests," Ingalsbee said. "In some case, there's more diversity there [than] before the fire."
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The intense record heat baking the south-central United States is expected to get trimmed back early this week, but a sweep of refreshing air is not on the horizon.
This past weekend's rainstorm was only the start of an abnormally wet pattern that will elevate the flood risk in the eastern United States into the end of the month.
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The remainder of July will be dominated by a resurgence of heat across the northwestern United States.
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In addition to a slash being the divider between the URL marker and the parameters, a question mark can be used. In this case the parameter mapping in the Friendly URL Configuration (FSCOWS@1.1001:WebServiceConfiguration) will be ignored.
If a question mark is used as divider, the following string is interpreted as a URL query string. The implementation then tries to parse the parameters assuming each parameter (or key-value pair) is separated by a "&" character and spitted by “=”. Both key and value are URL-Decoded before they are inserted to the Dictionary. The parsed values are then passed to the DAV action by the transaction variable TV_FRIENDLYURL_URL_PARAMS as a COOSYSTEM@1.1:DICTIONARY.
Value of dictionary TV_FRIENDLYURL_URL_PARAMS:
key: param1value: 123
key: param2value: value2
key: param3value: COO.126.96.36.199
Values in this dictionary are of type COOSYSTEM@1.1:STRING
Request parameters can be sent in a HTTP POST request in the HTTP body. This is usually used when a HTML POST form is sent to the web server. The values are provided as key/value pairs similar to a HTTP GET request. The HTTP body can be encoded in two ways:
URL encoded form data
The parameters are encoded similar to those of the HTTP GET request.
Multipart form data
The parameters are encoded as multipart type. This is usually used when a file is to be uploaded to the web server. The file is stored at the web server and deleted when the HTTP request is finished. The absolute path to the file is provided as value in the resulting dictionary.
In both cases it is assumed that non ASCII characters are properly encoded as UTF-8. By specifying the attribute accept-encoding=”UTF-8” in the HTML form it is ensured that the characters are encoded properly.
For Microsoft Internet Explorer, a workaround is necessary to ensure the correct behavior. A hidden form field has to be added to the HTML form. In this case the Microsoft Internet Explorer recognizes a non ASCII character and encodes the parameters in the HTTP POST request properly.
<input name="ieworkaround" type="hidden" value="☠"/>
To utilize certain features of the vApp Engine, a certain URL marker is used to mark certain URL parts as only relevant for the vApp Engine itself.
Everything in the URL following the defined parameter “&vappquerystring=true“ is considered as an argument for the vApp Engine and therefore not interpreted by the Friendly URL parser.
Only the value of “param1” and “param2” is passed to the friendly URL implementation. | <urn:uuid:481b6eb7-3df1-4548-a9c4-308945e08e03> | 2.59375 | 598 | Documentation | Software Dev. | 48.040471 | 95,505,476 |
Corrosion Product Release into Sodium from Austenitic Stainless Steel
During operation of a sodium-cooled nuclear reactor, the fuel cladding and in-core materials will undergo material loss (corrosion) by the flowing sodium. An experimental program to characterize the release and transport of corrosion products has been active at our laboratory since 1966. The purpose of this paper is to describe the results of radioactive and non-radioactive corrosion product release from austenitic stainless steel obtained during the study and to identify some release mechanisms. The previous excellent reviews of material corrosion in sodium (1, 2) did not discuss release of manganese and cobalt, the two most significant radioactive species. Discussion of control measures against radioactive material transport, and general summaries of that problem have also been presented elsewhere (3, 4, 5).
KeywordsAustenitic Stainless Steel Diffusion Profile 60Co Release Weight Loss Data Ferrite Layer
Unable to display preview. Download preview PDF.
- 1.J. R. Weeks and H. S. Isaacs, Advances in Corrosion Science and Technology, Vol. 3, pp 1–66 (1973).Google Scholar
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- 7.W. F. Brehm, et al, “Techniques for Studying Corrosion and Deposition of Radioactive Materials in Sodium Loops”, paper 18, Proc. 1 AEA, Specialists Meeting on Fission and Corrosion Product Behaviour in Primary Systems of LWFBRfs, Dimitrovgrad USSR (1975) IWGFR-7.Google Scholar
- 8.L. E.Chulos, “Operational Techniques Employed for the Liquid Sodium Source Term Control Loops”, paper 1l-A-8 in Ref.4 proceedings.Google Scholar
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Tropical Depression Gil is more than halfway to Hawaii from Mexico and continues to hold onto depression strength. Meanwhile, NOAA's GOES-15 satellite captured both storms in one image on Aug. 5 at 1200 UTC (8 a.m. EDT). The imagery shows Tropical Storm Henriette dwarfed the smaller Gil.
In an infrared image on Aug. 5 at 8 a.m. EDT, NOAA's GOES-15 satellite imagery showed Tropical Depression Gil approaching the Central Pacific Ocean, and Tropical Storm Henriette located to Gil's east.
Credit: NASA GOES Project
The infrared image was created by NASA's GOES Project at NASA's Goddard Space Flight Center in Greenbelt, Md. GOES satellite imagery showed a circular concentration of thunderstorms tightly around Gil's small center, while Henriette, located to the east, is several times the size of Gil.Gil is Tropically "Depressed"
The National Hurricane Center noted that Gil is weakening and will likely cease to qualify as a tropical cyclone in the next day or two. However, if Gil survives the adverse atmospheric environment and holds together, it would track far south of the Hawaiian Island chain over the weekend of Aug. 10 and 11.NASA Infrared Data Shows a Developing Eye in Henriette
Henriette developed from the System 90E which became the eighth tropical depression of the eastern Pacific Ocean hurricane season over the weekend of Aug. 3 and 4.
On Aug. 5 at 11 a.m. EDT (1500 UTC), Tropical Storm Henriette's maximum sustained winds had increased to near 60 mph/95 kph. Further strengthening is forecast by the National Hurricane Center and Henriette is expected to become a hurricane in the next day.
Henriette was located near latitude 12.1 north and longitude 128.2 west, about 1,415 miles (2,280 km west-southwest of the southern tip of Baja California, Mexico. Henriette is moving toward the west near 6 mph/9 kph and is expected to turn west-northwestward Minimum central pressure is near 999 millibars.
Rob Gutro | EurekAlert!
Further reports about: > AIRS > Aqua satellite > Depression > Eastern > GOES satellite > Goddard Space Flight Center > Hurricane > Hurricane Center > NASA > National Hurricane Center > Pacific Ocean > Pacific coral > Tropical Depression > UTC > infrared image > infrared light > satellite imagery > tropical diseases
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The researchers found evidence suggesting that when resources for conservation enforcement are lacking, management strategies designed to meet community goals can succeed in compliance and conservation to a greater degree than strategies that are based on different priorities. The findings are reported by Timothy McClanahan and Michael Marnane of the Wildlife Conservation Society and colleagues at James Cook University and Fagatele Bay National Marine Sanctuary in American Samoa. The work appears in the July 25th issue of Current Biology, published by Cell Press.
In their work, the researchers examined three different types of reef management systems in Indonesia and Papua New Guinea. The sites included four national parks, which encompassed large areas and were managed and enforced by the national government with the explicit goal of sustainable use and improvement of reef conditions; four "co-managed" reserves, which encompassed small areas and were managed and enforced by the community in partnership with non-governmental organizations, tourism operators, and universities, with a range of social and economic goals; and three traditionally managed areas, which also encompassed small areas but were instigated and maintained by the community with the goals of providing food for celebratory events or goals that in some other way provided a social benefit to the community.
The researchers performed underwater censuses of key reef features, and found that two--the average size and biomass of targeted fish species--were found to be different in managed areas compared to similar non-managed areas. A key finding was that three of the four sites that exhibited the greatest average size and biomass of fishes within the management areas were the self-governing, traditionally managed systems. The other site was one of the co-managed systems. The authors note that, contrary to the idea that permanent reef closures are the most effective ways to improve reef ecosystem health, none of the traditional management regimes involved permanent reef closures--instead, fishing was limited in other ways. The fourth, co-managed site did implement a permanent reef closure.
The authors also noted that the traditionally managed sites were implemented to meet community goals, rather than goals that explicitly reflect western concepts of ecological conservation. In addition, the fourth, co-managed site, was designed largely from a social perspective after community consultation, and was chosen because of its high visibility to the community. In their socioeconomic survey of the various sites under study, the authors found that the sites that were effective at conserving resources had higher compliance with conservation rules, were visible to the local community, and had been under management for a longer period of time than the less successful protected areas. The areas associated with the successful sites also tended to have less involvement in formal or professional economic activities, had lower populations, and less overall wealth. The authors' findings indicated that high compliance at the traditionally managed areas--despite a lack of formal enforcement patrols--was probably influenced by the locations of the areas near the village, the existence of traditional social barriers that limited use by outsiders, and understanding of the relationship between human-environment interactions and local benefits.
On the basis of their findings, the authors propose that while large, permanent marine-protected areas may provide the best protection for species that are at particular risk from overfishing, a combination of such large marine protected areas and traditionally managed systems may represent the best overall solution for meeting conservation and community goals and reversing the degradation of reef ecosystems.
Heidi Hardman | EurekAlert!
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Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.
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20.07.2018 | Power and Electrical Engineering
20.07.2018 | Information Technology
20.07.2018 | Materials Sciences | <urn:uuid:8b11cbe1-bcb8-49ab-bb59-a6cd7b7b1127> | 3.171875 | 1,274 | Content Listing | Science & Tech. | 24.902861 | 95,505,555 |
The fossilised structures -- called stromatolites -- preserved in ancient rocks along the edge of Greenland's ice cap were 220 million years older than the previous record holders.
They prove that life emerged just a few hundred million years after the Earth was formed some 4.5 billion years ago, said lead researcher Allen Nutman of the University of Wollongong.
"This discovery represents a new benchmark for the oldest preserved evidence of life on Earth," Professor Martin Julian Van Kranendonk, a geology expert at the University of New South Wales and one of the study's co-authors, said in a statement.
"The structures and geochemistry from the newly exposed outcrops in Greenland display all of the features used in younger rocks to argue for a biological origin. It points to a rapid emergence of life on Earth."
The one-to-four centimetre (0.4-1.6 inch) high Isua stromatolites -- exposed after the melting of a snow patch in the Isua Greenstone Belt -- matched other biological evidence on the evolution of the genetic code that placed the origins of life in a similar period, Nutman said.
He added that the discovery could help the hunt for life on Mars, considered the most likely location for microbial life-forms among other planets in the Solar System.
"The significance for Mars is that 3,700 million years ago, Mars was probably still wet and probably still had oceans and so on, so if life develops so quickly on Earth to be able to form things like stromatolites -- it might be more easy to detect signs of life on Mars," Nutman told AFP.
"Instead of looking at just the chemical signature, we might be able to see things like stromatolites in images (from Mars) sent back to Earth."
The earliest evidence of life on Earth ahead of the Greenland discovery was made in 2006 when Australian and Canadian researchers dated microfossils in rocks from Pilbara's Strelley Pool Chert formation at more than 3.4 billion years old.
The findings are published in the journal Nature. | <urn:uuid:2f709617-333b-48da-bd78-63d4a1479f3f> | 4.03125 | 438 | News Article | Science & Tech. | 44.516295 | 95,505,571 |
Raspberry-Pi Bare Metal Tutorial
This repository contains the code for the Raspberry-Pi bare-metal programming in C series. The home of the tutorial and all of the articles relating to the code is at valvers.com
You can build on linux, windows, or MAC - all you need is the [https://launchpad.net/gcc-arm-embedded](arm-none-eabi toolchain).
For all platforms, you can get the required toolchain used in this tutorial from the GCC-ARM-embedded project - I recommend getting the 4.7 toolchain because I've had problems with the 4.9 toolchain.
Have fun, and remember to experiment!
Tutorial links (online)
Some interesting links:
Original Cambridge Tutorial that inspired this tutorial series: http://www.cl.cam.ac.uk/projects/raspberrypi/tutorials/os/index.html
ARM Instruction Reference: http://infocenter.arm.com/help/topic/com.arm.doc.qrc0001l/QRC0001_UAL.pdf
GNU ARM Embedded Toolchain: https://launchpad.net/gcc-arm-embedded
Newlib C-Library documentation: https://sourceware.org/newlib/libc.html | <urn:uuid:fa441616-bbd3-4935-b773-c53f6b9b9ef2> | 2.6875 | 276 | Documentation | Software Dev. | 66.694246 | 95,505,584 |
"Over the last eight years we have been looking for new natural products in the DNA sequence of the antibiotic-producing bacterium Streptomyces coelicolor," said Professor Gregory Challis from the University of Warwick. "In the last 15 years it became accepted that no new natural products remained to be discovered from these bacteria. Our work shows this widely-held view to be incorrect."
In 1928 Alexander Fleming discovered penicillin, which was subsequently developed into a medicine by Florey and Chain in the 1940s. The antibiotic was hailed as a 'miracle cure' and a golden age of drug discovery followed. However, frequent rediscovery of known natural products and technical challenges forced pharmaceutical companies to retreat and stop looking for new molecules.
Currently the complete genetic sequences of more than 580 microbes are known. It is possible to identify pathways that produce new compounds by looking at the DNA sequences and many gene clusters likely to encode natural products have been analysed. 'Genome mining' has become a dynamic and rapidly advancing field.
Professor Challis and his colleagues have discovered the products of two cryptic gene clusters. One of the clusters was found to produce several compounds that inhibit the proliferation of certain bacteria. Three of these compounds were new ones, named isogermicidin A, B and C. "This discovery was quite unexpected," said Professor Challis. "Our research provides important new methodology for the discovery of new natural products with applications in medicine, such as combating MRSA infections."
The other product they discovered is called coelichelin. Iron is essential for the growth of nearly all micro-organisms. Although it is the fourth most abundant element in the Earth's crust it often exists in a ferric form, which microbes are unable to use. "The gene cluster that directs production of coelicehlin was not known to be involved in the production of any known products," said Professor Challis. "Our research suggests that coelichelin helps S. coelicolor take up iron."
Many researchers have followed Professor Challis and his colleagues into the exciting field of genome mining. "In the near future, compounds with useful biological activities will be patented and progressed into clinical or agricultural trials, depending on their applications" said Professor Challis.
Lucy Goodchild | alfa
The secret sulfate code that lets the bad Tau in
16.07.2018 | American Society for Biochemistry and Molecular Biology
Colorectal cancer risk factors decrypted
16.07.2018 | Max-Planck-Institut für Stoffwechselforschung
For the first time ever, scientists have determined the cosmic origin of highest-energy neutrinos. A research group led by IceCube scientist Elisa Resconi, spokesperson of the Collaborative Research Center SFB1258 at the Technical University of Munich (TUM), provides an important piece of evidence that the particles detected by the IceCube neutrino telescope at the South Pole originate from a galaxy four billion light-years away from Earth.
To rule out other origins with certainty, the team led by neutrino physicist Elisa Resconi from the Technical University of Munich and multi-wavelength...
For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.
Whirlpools are an everyday experience in a bath tub: When the water is drained a circular vortex is formed. Typically, such whirls are rather stable. Similar...
Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.
A frequently used reaction in organic chemistry is nucleophilic substitution. It plays, for example, an important role in in the synthesis of new chemical...
Optical spectroscopy allows investigating the energy structure and dynamic properties of complex quantum systems. Researchers from the University of Würzburg present two new approaches of coherent two-dimensional spectroscopy.
"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy....
Ultra-short, high-intensity X-ray flashes open the door to the foundations of chemical reactions. Free-electron lasers generate these kinds of pulses, but there is a catch: the pulses vary in duration and energy. An international research team has now presented a solution: Using a ring of 16 detectors and a circularly polarized laser beam, they can determine both factors with attosecond accuracy.
Free-electron lasers (FELs) generate extremely short and intense X-ray flashes. Researchers can use these flashes to resolve structures with diameters on the...
13.07.2018 | Event News
12.07.2018 | Event News
03.07.2018 | Event News
16.07.2018 | Physics and Astronomy
16.07.2018 | Transportation and Logistics
16.07.2018 | Agricultural and Forestry Science | <urn:uuid:b27efb1f-0768-40e0-9058-5c0196426ab2> | 3.65625 | 1,111 | Content Listing | Science & Tech. | 35.504952 | 95,505,587 |
1S–2S Transition-Frequency Calibration
Recent experiments on atomic hydrogen have shown that it is now possible to make measurements with a reproducibility far exceeding the accepted absolute precision of the available visible frequency standards. For example, the recent work of ALLEGRINI et al. has shown that the overall systematic error of their measurements of the Rydberg states in hydrogen is about 4 parts in 1011. Their measurements have had to be referred to the visible iodine stabilised HeNe laser which has an internationally accepted absolute accuracy of only 1.6 parts in 1010. If any progress is to be made in making precision measurements of fundamental atomic constants and in tests of QED, new visible frequency standards and new ways of comparing optical frequencies will have to be developed.
KeywordsFrequency Modulate Mode Spacing Frequency Chirp Pulse Amplifier Optical Frequency Standard
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Sandstorm that stopped the Rover Opportunity has covered almost the entire Red Planet.
Over the past two weeks a huge sand storm that erupted on Mars where NASA put the Rover Opportunity into sleep mode increased so that it now covers almost the entire Mars in a circle, enveloping the planet “fur coat”, very slightly pervious to sunlight. Note that the Rover Curiosity, powered by built-in nuclear thermoelectric generator, as well was feeling the effects of this storm, but he remains able to observe the environment and transmit data to Earth on this large-scale phenomenon.
Over the past few days the amount of dust deposited on the surface of the body of the Rover Curiosity, has increased almost twice. We can assume that in the case of the Mars Rover Opportunity, located at a distance of 8,200 km from Curiosity, the situation is not much better, and even worse, which is cause for concern mission specialists.
The density of sand fog in the Gale crater where the Curiosity Rover is 8.0 units (Tau), and the place where Opportunity is now even more “dark”, the density of the fog there is 11 Tau. Despite the fact that lowering the level of lighting requires longer exposures when shooting, the Curiosity Rover managed to make a series of pictures, even though it compromises the optical system cameras.
Fortunately for us, such a sand storm surrounding the entire planet, can not be formed on the Ground due to the thick and dense atmosphere of our planet, the force of gravity, a sufficiently large number of water surfaces and surfaces covered with vegetation. However, on Mars, where the vast majority of these constraints, not as a class, sandstorms are quite frequent phenomenon, especially during spring and summer in the southern hemisphere, when Mars is as close as possible to the Sun. Frozen in the polar regions carbon dioxide evaporates, which leads to thickening of the planet’s atmosphere and increase the pressure on the surface. This, plus some other factors raised in the air, clouds of dust, the height of which sometimes can reach 60 kilometers.
The sand storms on Mars last for a few weeks, and some storms can rage for several years. The main factors that ensure the continued existence of these phenomena is still not fully known to our scientists. Note that the current sand storm is not yet the strongest, the strongest is storm of 2007, which scientists continue to study today. In this light, the data that is now collecting Curiosity Rover, complementing the existing array of information and, thanks to them, scientists will be able to find answers to some of their questions.
In conclusion, we note that Mars is the only planet in the Solar system which periodically “storm”. In 2013, astronomers discovered a giant, illuminated by lightning storm on Saturn, which captured a continuous ring all this gas giant planet. The length of this storm was 300 thousand kilometers, which is a lot compared to the current Martian storm, whose length is about 21 thousand kilometers. | <urn:uuid:722bf789-61c7-4098-8de4-4cee0f10560b> | 3.125 | 608 | Truncated | Science & Tech. | 39.584386 | 95,505,635 |
Researchers plan to launch a tiny spacecraft to orbit Earth and beyond sometime in the next 18 months. The launch will serve as a key test of new propulsion technology that could help cut the cost of planetary exploration by a factor of 1,000.
The scientists and engineers are developing a new plasma propulsion system designed for ultrasmall CubeSats. If all goes well, they say, it may be possible to launch a life-detection mission to Jupiter's ocean-harboring moon, Europa, or other intriguing worlds for as little as $1 million in the not-too-distant future.
"We want to enable new missions that right now cost about $1 billion, or maybe $500 million — to go, for example, explore the moons of Jupiter and Saturn," said project leader Ben Longmier, a plasma physicist and assistant professor at the University of Michigan.
To get the ball rolling, Longmier and his team launched a crowdfunding campaign on the website Kickstarter last Thursday. They hope to raise a minimum of $200,000 by Aug. 5, which should be enough to loft the miniature thruster on its maiden space voyage.
Miniature Thruster Technology
CubeSats are cheap and tiny spacecrafts that weigh just 11 pounds (five kilograms) or so. At present, they're generally restricted to Earth orbit, where they circle passively until their orbits decay and they die a fiery death in the planet's atmosphere.
But the new propulsion system — which the team calls the CubeSat Ambipolar Thruster, or CAT — could change all that, turning such bantam spacecrafts into interplanetary probes, Longmier and his colleagues say.
CAT is a plasma engine, generating thrust by accelerating superheated ionized gas out of a discharge chamber. The CAT thruster is powered by solar panels, and permanent magnets will guide the plasma out the back of the spacecraft.
CAT is similar in concept to the ion engine that powers NASA's Dawn spacecraft, which orbited the protoplanet Vesta for more than a year and is now on its way to study Ceres, the largest body in the main asteroid belt between Mars and Jupiter. Over long periods of time, such thrusters can accelerate spacecrafts to higher speeds than typical chemical rockets can achieve.
But with CAT, everything must work on the micro scale. The thruster and power systems will weigh less than 1 pound (0.5 kg), while the supply of propellant — likely either iodine or water, though many different substances could be used — will be capped at about 5.5 pounds (2.5 kg), researchers said.
Most of the CAT components have been built and tested individually, and the team is making good progress toward incorporating them into a unified whole, researchers said.
"The hurdles that exist right now are getting our newly designed thruster up and running. We think we're about three weeks from that," Longmier told SPACE.com. "We're really sort of ramping up and hitting full tilt right now."
SEE ALSO: NASA EDGE: CubeSat Launch Initiative
To Earth Orbit and Beyond
The main goal of the new CAT Kickstarter campaign is to raise enough money to space-test the engine in Earth orbit. The team is planning to launch its first probe within the next 18 months, though it may be possible to get off the ground even sooner, Longmier said.
The team plans to send the maiden CAT-equipped probe out into deep space as well — not all the way to Europa or Saturn's geyser-spewing moon, Enceladus, but far enough to demonstrate CAT's capabilities.
"Our secondary goal is getting it out of Earth orbit and proving to the community that this thing works," Longmier said. "If it does work, it's a lot easier to get funding and write grants in the traditional sense."
Raising $200,000 should make all of this possible, while meeting other funding milestones will allow the CAT team to tackle "stretch goals." If the Kickstarter campaign nets $500,000, for example, the team will fast-track its space trip by purchasing a commercial launch, while raising $900,000 will enable a two-CubeSat "space race" to escape Earth orbit.
SEE ALSO: Photos: The Galilean Moons of Jupiter
Longmier and his core team at the University of Michigan are working with experts at a variety of institutions, including three different NASA centers — Ames Research Center in Moffett Field, Calif., the Jet Propulsion Laboratory in Pasadena, Calif., and Glenn Research Center in Cleveland, Ohio.
The asteroid-mining firm Planetary Resources is another partner. The billionaire-backed company, which counts Google execs Larry Page and Eric Schmidt among its investors, is interested in possibly using CAT-equipped probes to do up-close asteroid reconnaissance on the cheap, Longmier said.
"That's sort of where we come in — sending that small spacecraft out as a scout, a radio beacon, to go radiotag it," he said.
Asteroid tagging is just one of many potential applications for the technology, CAT team members say. A fleet of CAT-powered CubeSats could also provide cheap global Internet access, for instance, or study the impacts of solar eruptions on Earth's neighborhood, helping scientists better understand and predict space weather.
And then there's the lure of mounting stripped-down, $1 million life-detection missions to Europa, Enceladus or other intriguing and farflung worlds.
"I think we have the opportunity — for the first time, more or less, in history — to go and see if we can make these detections of life within our own solar system," Longmier said. "Not just looking at them, but going and taking sensors, doing in situ measurements, flying through the plumes of Enceladus with small spacecraft. We think we can do that in the relatively near future."
Image courtesy of Ben Longmier, University of Michigan
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This article originally published at Space.com here | <urn:uuid:5a8eef9d-4f54-49ab-b6bd-c853242084f5> | 3.5 | 1,307 | Truncated | Science & Tech. | 48.682561 | 95,505,660 |
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A recent report in the Seismological Research Letters journal identifies nearly 730 earthquakes that may have been triggered by human activity.
The Human-Induced Earthquake Database, (HiQuake), is the largest and most complete database of earthquake sequences proposed to have been induced by human activity, according to the HiQuake webpage.
The database resulted from a 2016 Dutch oil and gas company, Nederlandse Aardolie Maatschappij (NAM), meeting. NAM held the meeting to discuss induced earthquakes in the Groningen gas field in the Netherlands, said Miles Wilson, a University of Durham in North East England geophysicist who collected the study's data.
Wilson and his research team were invited to provide a global overview of induced seismicity at the meeting, Wilson said.
Induced seismicity refers to seismic events, typically earthquakes, caused either in part or completely by human activities.
There are a number of human activities that are linked to induced seismicity, according to the HiQuake site.
The most commonly reported human activities proposed to have caused earthquakes are mining and water reservoir impoundment. In recent years, the number of earthquakes proposed to have been induced by fluid‐injection activities has grown.
The most commonly reported maximum magnitude in an induced earthquake sequence is between three and four.
The largest earthquake in HiQuake had a magnitude of 7.9 and occurred in China in 2008. Such large earthquakes release mostly stress of natural tectonic origin but are conceivably triggered by small anthropogenic stress changes.
The HiQuake database will help to improve the overall understanding of induced earthquakes and to manage their impact on society, Wilson said.
“It’s very easy to become focused on a particular earthquake, especially if it directly affects us. HiQuake, although incomplete, provides a bigger global picture than people have ever had access to before,” Wilson said.
There is controversy surrounding cases of induced seismicity and the overall role of human behavior.
“It’s almost impossible to scientifically prove that an earthquake is caused by human activity, which is why many cases are highly debated,” said Wilson.
For example, in the United States, there has been large debate over the role of fracking for natural gas in seismic events.
The U.S. Geological Survey (USGS) addresses some of the myths and misconceptions surrounding fracking and wastewater disposal.
"Fracking is not causing most of the induced earthquakes. Wastewater disposal is the primary cause of the recent increase in earthquakes in the central United States," the USGS website reads.
Fracking is more likely to induce seismic activity indirectly through the disposing of wastewater used in the process. Wastewater disposal is the byproduct of water, sand and chemicals used to hydraulically fracture hydrocarbons from rock. That high-pressure wastewater can crack rocks and lubricate faults.
The HiQuake data shows 29 project sites where earthquakes were induced by fracking itself, 36 sites where quakes were induced by wastewater disposal, and 12 sites where temblors induced by unspecific oil, gas, and wastewater disposal.
The potential risks of fracking have led to political and social debate.
“Whether or not a nation allows fracking is up to the governing body, but that decision needs to be based on scientific research and not on public perception,” Wilson said.
Quiz Maker - powered by Riddle
Many proposed cases of induced seismicity are controversial. When figuring out which cases to contain on the database, HiQuake decided to include all induced cases proposed on scientific grounds to the database, Wilson said.
“Filtering HiQuake using our judgment would not be a rigorous scientific method,” Wilson explained. “Any judgment calls we leave to users.”
HiQuake includes data drawn from publications that span almost a century. The authors estimate under-reporting to be about 30 percent for magnitude 4 events, about 60 percent for magnitude 3 events, and about 90 percent for magnitude 2 events.
While it is rare for human-induced earthquakes to cause disruptions, some cases may be a significant problem, such as the hydrocarbon-producing areas of Oklahoma, according to the HiQuake abstract.
The potential disruptions of induced earthquakes is increasing as the size of projects and density of populations increase. Therefore, effective management strategies are needed, according to the HiQuake abstract.
“To completely eliminate induced earthquakes, you would have to stop any human activity which influences the forces acting in the Earth’s crust,” Wilson said.
This is not practical in a world where resource demands are ever growing in response to population growth and development, Wilson said.
“However, the risk of induced earthquakes can be reduced by continued research into their causes and by assessing project sites as best as possible before operations commence,” Wilson said.
Wilson and his research team plan to continue to keep HiQuake updated with new scientific research as it continues to evolve. NAM has recently agreed to continue funding the project for another two and half years, according to Wilson.
“We plan to keep HiQuake as an up-to-date resource which is accessible to anyone, anywhere and at any time,” Wilson said.
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An uptick in monsoon rainfall is expected to heighten the flood threat across eastern and northern India this week.
The threat for damaging thunderstorms will shift into the southeastern United States as the weekend kicks off.
Three people were injured after severe weather tore from Indiana to Kentucky and Tennessee to end the week.
A new round of severe weather is threatening lives from Ohio through Tennessee and will continue into Saturday morning.
In select regions of the world, people can live long enough to make some wonder if these countries have discovered the heavily sought-after fountain of youth.
A town in Iowa was severely damaged by a tornado on Thursday, while strong storms led to a tour boat disaster in Missouri that killed 17. | <urn:uuid:819dc602-535b-4191-91ce-121eb7f90276> | 3.6875 | 1,270 | News Article | Science & Tech. | 28.276105 | 95,505,662 |
Biologists at the Farber Institute for Neurosciences at Thomas Jefferson University have shown for the first time in the laboratory that they can convert some adult human neural stem cells to brain cells that can produce dopamine, the brain chemical missing in Parkinsons disease. If the researchers can better understand the process and harness this ability, the work may someday lead to new strategies in treating neurodegenerative diseases such as Parkinson’s.
Developmental biologist Lorraine Iacovitti, Ph.D., professor of neurology at Jefferson Medical College of Thomas Jefferson University in Philadelphia, is searching for ways to convert stem cells into dopamine-making neurons to replace those lost in Parkinson’s. In previous work, she and her co-workers showed that mouse neural stem cells placed in rats with Parkinson’s disease could develop into brain cells that produced tyrosine hydroxylase (TH), the enzyme needed to make dopamine.
Dr. Iacovitti, who also is associate director of the Farber Institute for Neurosciences at Jefferson, wanted to see if human neural stem cells could become dopamine-producing brain cells as well. She and her colleagues grew neural stem cells in a laboratory dish. Using a cocktail of protein growth factors and nutrients, the researchers found they could coax approximately 25 percent of the stem cells to make TH in the dish, proving the stem cells had the capacity to manufacture dopamine. What’s more, when they removed the growth factor-cocktail, the cells continued to produce the enzyme. She reports her team’s findings November 5 at the annual meeting of the Society for Neuroscience in Orlando.
Steve Benowitz | EurekAlert!
Scientists uncover the role of a protein in production & survival of myelin-forming cells
19.07.2018 | Advanced Science Research Center, GC/CUNY
NYSCF researchers develop novel bioengineering technique for personalized bone grafts
18.07.2018 | New York Stem Cell Foundation
A new manufacturing technique uses a process similar to newspaper printing to form smoother and more flexible metals for making ultrafast electronic devices.
The low-cost process, developed by Purdue University researchers, combines tools already used in industry for manufacturing metals on a large scale, but uses...
For the first time ever, scientists have determined the cosmic origin of highest-energy neutrinos. A research group led by IceCube scientist Elisa Resconi, spokesperson of the Collaborative Research Center SFB1258 at the Technical University of Munich (TUM), provides an important piece of evidence that the particles detected by the IceCube neutrino telescope at the South Pole originate from a galaxy four billion light-years away from Earth.
To rule out other origins with certainty, the team led by neutrino physicist Elisa Resconi from the Technical University of Munich and multi-wavelength...
For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.
Whirlpools are an everyday experience in a bath tub: When the water is drained a circular vortex is formed. Typically, such whirls are rather stable. Similar...
Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.
A frequently used reaction in organic chemistry is nucleophilic substitution. It plays, for example, an important role in in the synthesis of new chemical...
Optical spectroscopy allows investigating the energy structure and dynamic properties of complex quantum systems. Researchers from the University of Würzburg present two new approaches of coherent two-dimensional spectroscopy.
"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy....
13.07.2018 | Event News
12.07.2018 | Event News
03.07.2018 | Event News
20.07.2018 | Power and Electrical Engineering
20.07.2018 | Information Technology
20.07.2018 | Materials Sciences | <urn:uuid:fcd40221-87cc-44fc-bc74-c4e34ef021b9> | 3.171875 | 924 | Content Listing | Science & Tech. | 37.021384 | 95,505,664 |
The work, presented online May 27 in Nature Geoscience, shows that once the angle of a slope exceeds 30 degrees – whether from uplift, a rushing stream carving away the bottom of the slope or a combination of the two – landslide erosion increases significantly until the hillside stabilizes.
The Landsat satellite image at left shows a huge lake on the Tsangpo River behind a dam created by a landslide (in red, lower right of the lake) in early 2000. The image at right shows the river following a catastrophic breach of the dam in June 2000. Credit: U.S. Geological Survey/NASA
"I think the formation of these landscapes could apply to any steep mountain terrain in the world," said lead author Isaac Larsen, a University of Washington doctoral student in Earth and space sciences.
The study, co-authored by David Montgomery, a UW professor of Earth and space sciences and Larsen's doctoral adviser, focuses on landslide erosion along rivers in the eastern Himalaya region of southern Asia.
The scientists studied images of more than 15,000 landslides before 1974 and more than 550 more between 1974 and 2007. The data came from satellite imagery, including high-resolution spy satellite photography that was declassified in the 1990s.
They found that small increases in slope angle above about 30 degrees translated into large increases in landslide erosion as the stress of gravity exceeded the strength of the bedrock.
"Interestingly, 35 degrees is about the same angle that will form if sand or other coarse granular material is poured into a pile," Larsen said. "Sand is non-cohesive, whereas intact bedrock can have high cohesion and should support steeper slopes.
"The implication is that bedrock in tectonically active mountains is so extensively fractured that in some ways it behaves like a sand pile. Removal of sand at the base of the pile will cause miniature landslides, just as erosion of material at the base of hill slopes in real mountain ranges will lead to landslides."
The researchers looked closely at an area of the 150-mile Tsangpo Gorge in southeast Tibet, possibly the deepest gorge in the world, downstream from the Yarlung Tsangpo River where the Po Tsangpo River plunges more than 6,500 feet, about 1.25 miles. It then becomes the Brahmaputra River before flowing through the Ganges River delta and into the Bay of Bengal.
The scientists found that within the steep gorge, the rapidly flowing water can scour soil from the bases, or toes, of slopes, leaving exposed bedrock and an increased slope angle that triggers landslides to stabilize the slopes.
From 1974 through 2007, erosion rates reached more than a half-inch per year along some 6-mile stretches of the river within the gorge, and throughout that active landslide region erosion ranged from 0.15 to 0.8 inch per year. Areas with less tectonic and landslide activity experienced erosion rates of less than 0.15 inch a year.
Images showed that a huge landslide in early 2000 created a gigantic dam on a stretch of the Po Tsangpo. The dam failed catastrophically in June of that year, and the ensuing flood caused a number of fatalities and much property damage downstream.
That event illustrates the processes at work in steep mountain terrain, but the processes happen on a faster timescale in the Tsangpo Gorge than in other steep mountain regions of the world and so are more easily verified.
"We've been able to document the role that landslides play in the Tsangpo Gorge," Larsen said. "It explains how steep mountain topography evolves over time."
The work was financed by NASA, the Geological Society of America, Sigma Xi (the Scientific Research Society) and the UW Quaternary Research Center and Department of Earth and Space Sciences.
For more information, contact Larsen at 206-265-0473 or firstname.lastname@example.org, or Montgomery at 206-685-2560 or email@example.com
Vince Stricherz | EurekAlert!
Global study of world's beaches shows threat to protected areas
19.07.2018 | NASA/Goddard Space Flight Center
NSF-supported researchers to present new results on hurricanes and other extreme events
19.07.2018 | National Science Foundation
A new manufacturing technique uses a process similar to newspaper printing to form smoother and more flexible metals for making ultrafast electronic devices.
The low-cost process, developed by Purdue University researchers, combines tools already used in industry for manufacturing metals on a large scale, but uses...
For the first time ever, scientists have determined the cosmic origin of highest-energy neutrinos. A research group led by IceCube scientist Elisa Resconi, spokesperson of the Collaborative Research Center SFB1258 at the Technical University of Munich (TUM), provides an important piece of evidence that the particles detected by the IceCube neutrino telescope at the South Pole originate from a galaxy four billion light-years away from Earth.
To rule out other origins with certainty, the team led by neutrino physicist Elisa Resconi from the Technical University of Munich and multi-wavelength...
For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.
Whirlpools are an everyday experience in a bath tub: When the water is drained a circular vortex is formed. Typically, such whirls are rather stable. Similar...
Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.
A frequently used reaction in organic chemistry is nucleophilic substitution. It plays, for example, an important role in in the synthesis of new chemical...
Optical spectroscopy allows investigating the energy structure and dynamic properties of complex quantum systems. Researchers from the University of Würzburg present two new approaches of coherent two-dimensional spectroscopy.
"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy....
13.07.2018 | Event News
12.07.2018 | Event News
03.07.2018 | Event News
20.07.2018 | Power and Electrical Engineering
20.07.2018 | Information Technology
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The information is contained in the resin found within trees and on their bark. Resin is produced in large quantities by a tree when it's under attack by insects.
Normally, to assess if a tree is under an attack from boring insects researchers have sometimes had to rip patches of bark from healthy trees. But now forestry workers looking for the telltale sign of insect borings in tree trunks have a far less invasive method—they can just examine the resin that collects in clumps on the tree trunk.
An attack by boring beetles typically affects trees in two ways. The boring action damages the phloem layer just under the bark, which cuts off the passage of nutrients within the trunk. Also, beetles often introduce a fungus that spreads into the woody xylem tissue of the tree and starves the treetop of water. A side-effect of insect invasion and water stress is a reduction in the tree's ability to absorb carbon dioxide from the atmosphere. Carbon dioxide is necessary for life-sustaining photosynthesis.
The research team, including U of A paleontology graduate student Ryan McKellar, looked for subatomic-sized isotopic evidence that indicates water stress levels in trees as a result of an insect attack.
The team discovered a common marker in carbon isotopes found in the resin of living trees under insect attack and in the fossilized resin or amber produced by ancient trees going as far back as the age of dinosaurs: they both contain elevated levels of carbon-13.
McKellar's group also found evidence of boring beetles and the increased presence of carbon-13 within amber fossils dating back in the geological record to 90 million and 17 million years ago. The locations are as geographically removed as present-day New Jersey and the Dominican Republic.
With this finding the researchers suggest that two or the world's major amber deposits may have been produced by insect attacks like mountain pine beetle that are seen in modern ecosystems.
This discovery will help researchers understand the history of insect infestations.
McKellar's research will be published March 23 in Proceedings of the Royal Society B: Biological Sciences.
Brian Murphy | EurekAlert!
Upcycling of PET Bottles: New Ideas for Resource Cycles in Germany
25.06.2018 | Fraunhofer-Institut für Betriebsfestigkeit und Systemzuverlässigkeit LBF
Dry landscapes can increase disease transmission
20.06.2018 | Forschungsverbund Berlin e.V.
For the first time ever, scientists have determined the cosmic origin of highest-energy neutrinos. A research group led by IceCube scientist Elisa Resconi, spokesperson of the Collaborative Research Center SFB1258 at the Technical University of Munich (TUM), provides an important piece of evidence that the particles detected by the IceCube neutrino telescope at the South Pole originate from a galaxy four billion light-years away from Earth.
To rule out other origins with certainty, the team led by neutrino physicist Elisa Resconi from the Technical University of Munich and multi-wavelength...
For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.
Whirlpools are an everyday experience in a bath tub: When the water is drained a circular vortex is formed. Typically, such whirls are rather stable. Similar...
Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.
A frequently used reaction in organic chemistry is nucleophilic substitution. It plays, for example, an important role in in the synthesis of new chemical...
Optical spectroscopy allows investigating the energy structure and dynamic properties of complex quantum systems. Researchers from the University of Würzburg present two new approaches of coherent two-dimensional spectroscopy.
"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy....
Ultra-short, high-intensity X-ray flashes open the door to the foundations of chemical reactions. Free-electron lasers generate these kinds of pulses, but there is a catch: the pulses vary in duration and energy. An international research team has now presented a solution: Using a ring of 16 detectors and a circularly polarized laser beam, they can determine both factors with attosecond accuracy.
Free-electron lasers (FELs) generate extremely short and intense X-ray flashes. Researchers can use these flashes to resolve structures with diameters on the...
13.07.2018 | Event News
12.07.2018 | Event News
03.07.2018 | Event News
16.07.2018 | Physics and Astronomy
16.07.2018 | Life Sciences
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This Is Why Physicists Think String Theory Might Be Our ‘Theory Of Everything’
In 2015, Ed Witten, the greatest living string theorist, wrote a piece on why. Here’s the version for everyone.
It’s one of the most brilliant, controversial and unproven ideas in all of physics: string theory. At the heart of string theory is the thread of an idea that’s run through physics for centuries, that at some fundamental level, all the different forces, particles, interactions and manifestations of reality are tied together as part of the same framework. Instead of four independent fundamental forces — strong, electromagnetic, weak and gravitational — there’s one unified theory that encompasses all of them.
In many regards, string theory is the best contender for a quantum theory of gravitation, which just happens to unify at the highest-energy scales. Even though there’s no experimental evidence for it, there are compelling theoretical reasons to think it might be true. Back in 2015, the top living string theorist, Ed Witten, wrote a piece on what every physicist should know about string theory. Here’s what that means, even if you’re not a physicist.
When it comes to the laws of nature, it’s remarkable how many similarities there are between seemingly unrelated phenomena. The mathematical structure underlying them is often analogous, and occasionally even identical. The way that two massive bodies gravitate, according to Newton’s laws, is almost identical to the way that electrically charged particles attract-or-repel. The way a pendulum oscillates is completely analogous to the way a mass on a spring moves back-and-forth, or the way a planet orbits a star. Gravitational waves, water waves, and light waves all share remarkably similar features, despite arising from fundamentally different physical origins. And in the same vein, although most don’t realize it, the quantum theory of a single particle and how you’d approach a quantum theory of gravity are similarly analogous.
The way quantum field theory works is that you take a particle and you perform a mathematical “sum over histories.” You can’t just calculate where the particle was and where it is and how it got to be there, since there’s an inherent, fundamental quantum uncertainty to nature. Instead, you add up all the possible ways it could have arrived at its present state (the “past history” part), appropriately weighted probabilistically, and then you can calculate the quantum state of a single particle.
If you want to work with gravitation instead of quantum particles, you have to change the story a little bit. Because Einstein’s General Relativity isn’t concerned with particles, but rather the curvature of spacetime, you don’t average over all possible histories of a particle. In lieu of that, you average instead over all possible spacetime geometries.
Working in three spatial dimensions is very difficult, and when a physics problem is challenging, we often try and solve a simpler version first. If we go down to one dimension, things become very simple. The only possible one-dimensional surfaces are an open string, where there are two separate, unattached ends, or a closed string, where the two ends are attached to form a loop. In addition, the spatial curvature — so complicated in three dimensions — becomes trivial. So what we’re left with, if we want to add in matter, is a set of scalar fields (just like certain types of particles) and the cosmological constant (which acts just like a mass term): a beautiful analogy.
The extra degrees of freedom a particle gains from being in multiple dimensions don’t play much of a role; so long as you can define a momentum vector, that’s the main dimension that matters. In one dimension, therefore, quantum gravity looks just like a free quantum particle in any arbitrary number of dimensions.
The next step is to incorporate interactions, and to go from a free particle with no scattering amplitudes or cross-sections to one that can play a physical role, coupled to the Universe. Graphs, like the one above, allow us to describe the physical concept of action in quantum gravity. If we write down all the possible combinations of such graphs and sum over them — applying the same laws like conservation of momentum that we always enforce — we can complete the analogy. Quantum gravity in one dimension is very much like a single particle interacting in any number of dimensions.
The next step would be to move from one spatial dimension to 3+1 dimensions: where the Universe has three spatial dimensions and one time dimension. But this theoretical “upgrade” for gravity may be very challenging. Instead, there might be a better approach, if we chose to work in the opposite direction.
Instead of calculating how a single particle (a zero-dimensional entity) behaves in any number of dimensions, maybe we could calculate how a string, whether open or closed (a one-dimensional entity) behaves. And then, from that, we can look for analogies to a more complete theory of quantum gravity in a more realistic number of dimensions.
Instead of points and interactions, we’d immediately start working with surfaces, membranes, etc. Once you have a true, multi-dimensional surface, that surface can be curved in non-trivial ways. You start getting very interesting behavior out; behavior that just might be at the root of the spacetime curvature we experience in our Universe as General Relativity.
While 1D quantum gravity gave us quantum field theory for particles in a possibly curved spacetime, it didn’t describe gravitation itself. The subtle piece of the puzzle that was missing? There was no correspondence between operators, or the functions that represent quantum mechanical forces and properties, and states, or how the particles and their properties evolve over time. This “operator-state” correspondence was a necessary, but missing, ingredient.
But if we move from point-like particles to string-like entities, that correspondence shows up.
As soon as you upgrade from particles to strings, there’s a real operator-state correspondence. A fluctuation in the spacetime metric (i.e., an operator) automatically represents a state in the quantum mechanical description of a string’s properties. So you can get a quantum theory of gravity in spacetime from string theory.
But that’s not all you get: you also get quantum gravity unified with the other particles and forces in spacetime, the ones that correspond to the other operators in the field theory of the string. There’s also the operator that describes the spacetime geometry’s fluctuations, and the other quantum states of the string. The biggest news about string theory is that it can give you a working quantum theory of gravity.
That doesn’t mean it’s a foregone conclusion, however, that string theory is the path to quantum gravity. The great hope of string theory is that these analogies will hold up at all scales, and that there will be an unambiguous, one-to-one mapping of the string picture onto the Universe we observe around us.
Right now, there are only a few sets of dimensions that the string/superstring picture is self-consistent in, and the most promising one doesn’t give us the four-dimensional gravity of Einstein that describes our Universe. Instead, we find a 10-dimensional Brans-Dicke theory of gravity. In order to recover the gravity of our Universe, you must “get rid of” six dimensions and take the Brans-Dicke coupling parameter, ω, to infinity.
If you’ve heard of the term compactification in the context of string theory, that’s the hand-waving word to acknowledge that we must solve these puzzles. Right now, many people assume that there exists a complete, compelling solution to the need for compactification. But how you get Einstein’s gravity and 3+1 dimensions from the 10-dimensional Brans-Dicke theory remains an open challenge for string theory.
String theory offers a path to quantum gravity, which few alternatives can truly match. If we make the judicious choices of “the math works out this way,” we can get both General Relativity and the Standard Model out of it. It’s the only idea, to date, that gives us this, and that’s why it’s so hotly pursued. No matter whether you tout string theory’s successes or failure, or how you feel about its lack of verifiable predictions, it will no doubt remain one of the most active areas of theoretical physics research. At its core, string theory stands out as the leading idea of a great many physicists’ dreams of an ultimate theory.
Starts With A Bang is now on Forbes, and republished on Medium thanks to our Patreon supporters. Ethan has authored two books, Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive. | <urn:uuid:a22ab9aa-6dc6-4bf6-9f02-aae17c285069> | 2.5625 | 1,893 | Nonfiction Writing | Science & Tech. | 39.837742 | 95,505,712 |
Scientists at the University of Michigan and the University of California at Berkeley have taken a step forward on that route by developing small molecules that mimic the behavior and function of a much larger and more complicated natural regulator of gene expression. The research, by associate professor of chemistry Anna Mapp and coworkers, is described in the current issue of the journal ACS Chemical Biology.
Molecules that can prompt genes to be active are called transcriptional activators because they influence transcription---the first step in the process through which instructions coded in genes are used to produce proteins. Transcriptional activators occur naturally in cells, but Mapp and other researchers have been working to develop artificial transcription factors (ATFs)---non-natural molecules programmed to perform the same function as their natural counterparts. These molecules can help scientists probe the transcription process and perhaps eventually be used to correct diseases that result from errors in gene regulation.
In previous work, Mapp and coworkers showed that an ATF they developed was able to turn on genes in living cells, but they weren't sure it was using the same mechanism that natural activators use. Both natural transcriptional activators and their artificial counterparts typically have two essential parts: a DNA-binding domain that homes in on the specific gene to be regulated, and an activation domain that attaches itself to the cell's machinery through a key protein-to-protein interaction and spurs the gene into action. The researchers wanted to know whether their ATFs attached to the same sites in the transcriptional machinery that natural activators did.
In the current work, the team showed that their ATFs bind to a protein called CBP, which interacts with many natural activators, and that the specific site where their ATFs bind is the same site utilized by the natural activators, even though the natural activators are much larger and more complex.
Then the researchers altered their ATFs in various ways and looked to see how those changes affected both binding and ability to function as transcriptional activators. Any change that prevented an ATF from binding to CBP also prevented it from doing its job. This suggests that, for ATFs as for natural activators, interaction with CBP is key to transcriptional activity.
"Taken together, the evidence suggests that the small molecules we have developed mimic both the function and the mechanism of their natural counterparts," said Mapp, who has a joint appointment in the College of Pharmacy's Department of Medicinal Chemistry. Next the researchers want to understand in more detail exactly how the small molecules bind to that site. "Then we'll use that information to design better molecules."
In addition to Mapp, the study's authors are former graduate students Sara Buhrlage, Brian Brennan, Aaron Minter and Chinmay Majmudar, graduate student Caleb Bates, postdoctoral fellow Steven Rowe, associate professor of chemistry and biophysics Hashim Al-Hashimi, and David Wemmer of the University of California, Berkeley.
Funding was provided by the National Institutes of Health, the National Science Foundation, Novartis, the U-M Chemistry Biology Interface Training Program, Wyeth and the U-M Pharmaceutical Sciences Training Program
For more information:
Anna Mapp: https://www.chem.lsa.umich.edu/chem/faculty/facultyDetail.php?Uniqname=amapp
ACS Chemical Biology: http://pubs.acs.org/journal/acbcct
World’s Largest Study on Allergic Rhinitis Reveals new Risk Genes
17.07.2018 | Helmholtz Zentrum München - Deutsches Forschungszentrum für Gesundheit und Umwelt
Plant mothers talk to their embryos via the hormone auxin
17.07.2018 | Institute of Science and Technology Austria
For the first time ever, scientists have determined the cosmic origin of highest-energy neutrinos. A research group led by IceCube scientist Elisa Resconi, spokesperson of the Collaborative Research Center SFB1258 at the Technical University of Munich (TUM), provides an important piece of evidence that the particles detected by the IceCube neutrino telescope at the South Pole originate from a galaxy four billion light-years away from Earth.
To rule out other origins with certainty, the team led by neutrino physicist Elisa Resconi from the Technical University of Munich and multi-wavelength...
For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.
Whirlpools are an everyday experience in a bath tub: When the water is drained a circular vortex is formed. Typically, such whirls are rather stable. Similar...
Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.
A frequently used reaction in organic chemistry is nucleophilic substitution. It plays, for example, an important role in in the synthesis of new chemical...
Optical spectroscopy allows investigating the energy structure and dynamic properties of complex quantum systems. Researchers from the University of Würzburg present two new approaches of coherent two-dimensional spectroscopy.
"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy....
Ultra-short, high-intensity X-ray flashes open the door to the foundations of chemical reactions. Free-electron lasers generate these kinds of pulses, but there is a catch: the pulses vary in duration and energy. An international research team has now presented a solution: Using a ring of 16 detectors and a circularly polarized laser beam, they can determine both factors with attosecond accuracy.
Free-electron lasers (FELs) generate extremely short and intense X-ray flashes. Researchers can use these flashes to resolve structures with diameters on the...
13.07.2018 | Event News
12.07.2018 | Event News
03.07.2018 | Event News
17.07.2018 | Information Technology
17.07.2018 | Materials Sciences
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ENVIRONMENT- WATER CONSERVATION
ENVIRONMENT- WATER CONSERVATION
Date: March 23, 2015
water conservation Climate change Water resource Management Groundwater recharge Water harvesting
Question- What is water conservation? why in recent days the need of water has risen so enormously? also discuss the methods of conservation of water? (200 words)
Water conservation encompasses the policies, strategies & activities to manage fresh water as a sustainable resource, to protect the environment, and to meet current and future human demand. Water conservation means protecting our water from pollution and being wasted. It is vital becauseall living beings including plants, humans and animals all need water to survive. Without water, the earth would have no life. As per scientists, earth is three fourths covered by water which makes ocean, seas, rivers, lakes, snow, glaciers and ground water. Only one per cent of this is available as fresh water.
Water conservation will include-
Reducing water waste
Protecting the clean water we have. If water is polluted by harmful chemicals or garbage, we can’t use it to drink, bathe in or water crops
Helping water management plants minimize the amount of water they need to use on a daily basis.
Encouraging companies to make devices that do not use as much water as they did before.
Causes for increased demand of water-
1- High population growth
2- Incresaing Industrial demand
3- Irrigation and other agricultural uses
4- for production of energy and electricity
Methods for Conservation of Water-
1- Reforestation:- Forests play a very important role in maintaining water balance of the soil and atmosphere. Reforestation has become important. Forests provide major ecological services. They
reduce soil erosion.
absorb and release water.
purify water and air.
Influence local and regional climate.
Store atmospheric carbon.
Recycling of water
2- Water harvesting:- Water harvesting is the process of collecting rainwater that falls on a house or on any building to put it to use later or simply to replenish the ground water by allowing the water to reach underground. Rainwater harvesting essentially means collecting rainwater on the roofs of building and storing it underground for later use. Not only does this recharging arrest groundwater depletion, it also raises the declining water table and can help augment water supply.
3- Groundwater recharge :- The available groundwater is about 13-20 times as much water available on the surface.
Flood water may be injected into aquifers through a series of deep pits or ditches.
Small reservoirs and percolation tanks can be dug to hold runoff water to replenish ground water.
Storm water, used water, domestic drains can be fed into pits, trenches, depressions to be filtered and percolated through the soil for recharging ground water.
Desiltation of canals and tanks should be done regularly.
Pre-monsoon tillage of fields helps to conserve soil moisture
4- Water resource Management :- Water has to be used optimally, and by spreading awareness through campaigns via various media will help in reduction of water wastage.
Water management agencies have to install efficient meters and decide to charge a rate which will force the public to reduce use of municipal water.
Tap, shower flow restrictors and low volume toilet flushes can help in reducing water use.
Leakages in water pipes have to be checked regularly.
Lawns and gardens should be watered either in the morning or evening to avoid evaporation.
Rain water is the main source of water. We only receive three months of rainfall hence the water has to be stored. Water lost through seepage and evaporation, water wasted on weeds and cost of bringing water from ponds to the place of use should be minimised.
Harvesting, collecting, recharging ground water and water storage
Replenishing ground water
Flood waters may be injected into aquifers through series of deep pits or ditches.
Small reservoirs and percolation tanks can be dug to hold run off water recharging ground water.
Rain water harvesting carried out by building power for recharging ground water.
Additional Information to enrich your answer-
Water and Climate change
Water is the primary medium through which climate change influences Earth’s ecosystem and thus the livelihood and well-being of societies.
Global climate change is expected to exacerbate current and future stresses on water resources from population growth and land use, and increase the frequency and severity of droughts and floods.
It is anticipated that climate change will affect the availability of water resources through changes in rainfall distribution, soil moisture, glacier and ice/snow melt, and river and groundwater flows.
Water-related hazards account for 90% of all natural hazards and their frequency and intensity is generally rising, with serious consequences on the economic development. Between 1990 and 2000, natural disasters in several developing countries had caused damage representing between 2 and 15% of their annual GDP.
For instance, South Asia and Southern Africa are predicted to be the most vulnerable regions to climate change-related food shortages by 2030. Water stress is also expected to increase in central and southern Europe and by the 2070s, the number of people affected will rise from 28 million to 44 million. Summer flows are likely to drop by up to 80% in southern Europe and some part of central and Eastern Europe.
The cost of adapting to the impact of a 2°C rise in global average temperature could range from US$70 to US$100 billion per year between 2020 and 2050. Of these costs, between US$13.7 billion (drier scenario) and US$19.2 billion (wetter scenario) will be related the water sector, predominantly through water supply and flood management. | <urn:uuid:8af2235e-aad0-4d78-92e5-561e348973e8> | 3.53125 | 1,203 | Knowledge Article | Science & Tech. | 37.609038 | 95,505,741 |
SØG - mellem flere end 8 millioner bøger:
Viser: Complex Analysis
Detaljer Om Varen
- 4. Udgave
- Udgiver: Springer (Juli 2003)
- ISBN: 9780387985923
Now in its fourth edition, the first part of this book is devoted to the basic material of complex analysis, while the second covers many special topics, such as the Riemann Mapping Theorem, the gamma function, and analytic continuation. Power series methods are used more systematically than is found in other texts, and the resulting proofs often shed more light on the results than the standard proofs. While the first part is suitable for an introductory course at undergraduate level, the additional topics covered in the second part give the instructor of a gradute course a great deal of flexibility in structuring a more advanced course.
I: BASIC THEORY.
1: Complex Numbers and Functions.
2: Power Series.
3: Cauchy's Theorem, First Part.
4: Winding Numbers and Cauchy's Theorem.
5: Applications of Cauchy's Integral Formula.
6: Calculus of Residues.
7: Conformal Mappings.
8: Harmonic Functions.
II: GEOMETRIC FUNCTION THEORY.
9: Schwarz Reflection.
10: The Riemann Mapping Theorem.
11: Analytic Continuation Along Curves.
III: VARIOUS ANALYTIC TOPICS.
12: Applications of the Maximum Modulus Principle and Jensen's Formula.
13: Entire and Meromorphic Functions.
14: Elliptic Functions.
15: The Gamma and Zeta Functions.
16: The Prime Number Theorem.
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“Scientists have been trying to bring these space phenomena down to earth for a decade. With our new method we can enter a new era, and investigate what was previously impossible to study. It will tell us more about how these events occur,” says Longqing Yi, researcher at the Department of Physics at Chalmers.
The research concerns so-called ‘magnetic reconnection’ – the process which gives rise to these phenomena. Magnetic reconnection causes sudden conversion of energy stored in the magnetic field into heat and kinetic energy. This happens when two plasmas with anti-parallel magnetic fields are pushed together, and the magnetic field lines converge and reconnect. This interaction leads to violently accelerated plasma particles that can sometimes be seen with the naked eye – for example, during the northern lights.
Magnetic reconnection in space can also influence us on earth. The creation of solar flares can interfere with communications satellites, and thus affect power grids, air traffic and telephony.
In order to imitate and study these spectacular space plasma phenomena in the laboratory, you need a high-power laser, to create magnetic fields around a million times stronger than those found on the surface of the sun. In the new scientific article, Longqing Yi, along with Professor Tünde Fülöp from the Department of Physics, proposed an experiment in which magnetic reconnection can be studied in a new, more precise way. Through the use of 'grazing incidence' of ultra-short laser pulses, the effect can be achieved without overheating the plasma. The process can thus be studied very cleanly, without the laser directly affecting the internal energy of the plasma. The proposed experiment would therefore allow us to seek answers to some of the most fundamental questions in astrophysics.
“We hope that this can inspire many research groups to use our results. This is a great opportunity to look for knowledge that could be useful in a number of areas. For example, we need to better understand solar flares, which can interfere with important communication systems. We also need to be able to control the instabilities caused by magnetic reconnection in fusion devices,” says Tünde Fülöp.
The study on which the new results are based was financed by the Knut and Alice Wallenberg foundation, through the framework of the project ‘Plasma-based Compact Ion Sources’, and the ERC project ‘Running away and radiating'.
Text: Mia Halleröd Palmgren, email@example.com
Portrait pictures: Peter Widing (Tünde Fülöp) and Mia Halleröd Palmgren (Longqing Yi) A new way of studying magnetic reconnection.
The picture shows the experiment setup. The laser (the red triangle on the right) hits the micro-scale film (the grey slab), which splits the beam like a knife. Electrons accelerate on both sides of the ‘knife’ and produce strong currents, along with extremely strong, anti-parallel magnetic fields. Magnetic reconnection occurs beyond the end of the film (the blue frame). The magnetic field is illustrated with black arrows. The boomerang-like structures illustrate the electrons in the different stages of the simulation. The rainbow colours represent the electron transverse momenta.
Illustration: Longqing Yi
The scientific article was published in the journal Nature Communications.
Tünde Fülöp, Professor,
Department of Physics, Chalmers University of Technology, +46 72 986 74 40, firstname.lastname@example.org | <urn:uuid:4f2694d4-0264-4cef-b608-f3fcf6879a98> | 3.625 | 756 | News (Org.) | Science & Tech. | 37.435846 | 95,505,766 |
Tree taxa shifted latitude or elevation range in response to changes in Quaternary climate. Because many modern trees display adaptive differentiation in relation to latitude or elevation, it is likely that ancient trees were also so differentiated, with environmental sensitivities of populations throughout the range evolving in conjunction with migrations. Rapid climate changes challenge this process by imposing stronger selection and by distancing populations from environments to which they are adapted. The unprecedented rates of climate changes anticipated to occur in the future, coupled with land use changes that impede gene flow, can be expected to disrupt the interplay of adaptation and migration, likely affecting productivity and threatening the persistence of many species.
Mendeley saves you time finding and organizing research
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electrical resistance(redirected from Electrical resistence)
Also found in: Dictionary, Thesaurus.
Related to Electrical resistence: Ohm's law, Electrical capacitance, Electrical resistance thermometer
Opposition of a circuit to the flow of electric current. Ohm's law states that the current I flowing in a circuit is proportional to the applied potential difference V. The constant of proportionality is defined as the resistance R. Hence, Eq. (1)
(1)holds. If V and I are measured in volts and amperes, respectively, R is measured in ohms. Microscopically, resistance is associated with the impedance to flow of charge carriers offered by the material. For example, in a metallic conductor the charge carriers are electrons moving in a polycrystalline material in which their journey is impeded by collisions with imperfections in the local crystal lattice, such as impurity atoms, vacancies, and dislocations. In these collisions the carriers lose energy to the crystal lattice, and thus Joule heat is liberated in the conductor, which rises in temperature. The Joule heat P is given by
(2)Eq. (2). See Crystal defects, Electrical resistivity, Joule's law, Ohm's law
electrical resistance[i′lek·trə·kəl ri′zis·təns]
The physical property of a device, conductor, element, branch, or system, by virtue of which power is lost as heat when current flows through it; the physical property which an electric conductor exhibits to the flow of current; measured in ohms. | <urn:uuid:fe2d7670-53b8-43e8-b05e-17ef6ec7dabf> | 3.09375 | 344 | Structured Data | Science & Tech. | 32.151231 | 95,505,777 |
Scientists Dig Into the Origin of Organics on Ceres
wRI scientists are studying the geology associated with the organic-rich areas on Ceres. Dawn spacecraft data show a region around the Ernutet crater where organic concentrations have been discovered (background image). The color coding shows the surface concentration of organics, as inferred from the visible and near infrared spectrometer. The inset shows a higher resolution enhanced color image of the Ernutet crater acquired by Dawn’s framing camera. Regions in red indicate higher concentration of organics. Credit: Image courtesy of NASA/JPL-Caltech/UCLA/ASI/INAF/MPS/DLR/IDA.
Since NASA’s Dawn spacecraft detected localized organic-rich material on Ceres, Southwest Research Institute (SwRI) has been digging into the data to explore different scenarios for its origin. After considering the viability of comet or asteroid delivery, the preponderance of evidence suggests the organics are most likely native to Ceres.
“The discovery of a locally high concentration of organics close to the Ernutet crater poses an interesting conundrum,” said Dr. Simone Marchi, a principal scientist at SwRI. He is discussing his team findings today at a press conference at the American Astronomical Society’s 49th Division for Planetary Sciences Meeting in Provo. “Was the organic material delivered to Ceres after its formation? Or was it synthesized and/or concentrated in a specific location on Ceres via internal processes? Both scenarios have shortfalls, so we may be missing a critical piece of the puzzle.”
Ceres is believed to have originated about 4.5 billion years ago at the dawn of our solar system. Studying its organics can help explain the origin, evolution, and distribution of organic species across the solar system. The very location of Ceres at the boundary between the inner and outer solar system and its intriguing composition characterized by clays, sodium- and ammonium-carbonates, suggest a very complex chemical evolution. The role of organics in this evolution is not fully understood, but has important astrobiological implications.
“Earlier research that focused on the geology of the organic-rich region on Ceres were inconclusive about their origin,” Marchi said. “Recently, we more fully investigated the viability of organics arriving via an asteroid or comet impact.”
Scientists explored a range of impact parameters, such as impactor sizes and velocities, using iSALE shock physics code simulations. These models indicated that comet-like projectiles with relatively high impact velocities would lose almost all of their organics due to shock compression. Impacting asteroids, with lower incident velocities, can retain between 20 and 30 percent of their pre-impact organic material during delivery, especially for small impactors at oblique impact angles. However, the localized spatial distribution of organics on Ceres seems difficult to reconcile with delivery from small main belt asteroids.
“These findings indicate that the organics are likely to be native to Ceres,” Marchi said.
This article has been republished from materials provided by Southwest Research Institute. Note: material may have been edited for length and content. For further information, please contact the cited source.
‘Good Cholesterol’ May Not Always be Good for Postmenopausal WomenNews
Postmenopausal factors may have an impact on the heart-protective qualities of high-density lipoproteins (HDL) – also known as ‘good cholesterol’ – according to a study led by researchers in the University of Pittsburgh Graduate School of Public Health.READ MORE
What Makes Good Brain Proteins Turn Bad?News
The protein FUS is implicated in two neurodegenerative diseases: amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). Using a newly developed fruit fly model, researchers have zoomed in on the protein structure of FUS to gain more insight into how it causes neuronal toxicity and disease. | <urn:uuid:22197418-bbc3-459a-8b02-6adaa8c2eac4> | 3.34375 | 831 | Truncated | Science & Tech. | 21.089968 | 95,505,787 |
Cockroaches get everywhere. There they are, somehow, against all odds, in that room that looked to be totally sealed from the outside world, in that cupboard you swore was tightly shut. Now, Kaushik Jayaram and Robert Full from the University of California, Berkeley have discovered the secret behind their feats of infiltration.
By confronting American cockroaches with an ever-narrower series of crevices, the duo found that although this insect typically stands 12 millimeters tall, it can squeeze through gaps of 3 millimeters—the height of two stacked U.S. pennies. It does this by squatting down and then compressing its body by half. It is the world’s worst Transformer: instantly changing shape from a cockroach into a much flatter cockroach. Delightful.
Even worse, the compressed cockroaches are still disarmingly fast. Even though their legs are splayed and their bodies are squished, they can still scuttle at 60 centimeters per second. “Scale that up to human size, and it’s like 70 miles per hour,” says Full. “They can run at high speed inside your walls and ceilings.” Hallelujah.
This ability seems doubly extraordinary because cockroaches, like all insects, have rigid exoskeletons. Soft-bodied animals like worms or octopuses can intuitively squeeze through tight spaces—just watch this octopus go—but it’s less obvious how a roach does it. “It’s not just crunchy, rigid parts,” explains Full. The exoskeleton consists of hard plates connected by soft, flexible membranes that act as hinges. Even the solid parts are variable, with some sections being 10 times less stiff than others. The result is a creature that can change shape without sacrificing its infamous indestructibility. Joy untold.
To test just how compressible the exoskeleton is, Jayaram and Full “performed a series of dynamic compressive cycle tests on living animals.” In other words, they squished roaches beneath a metal piston. They found that when the insects squeeze through the tiniest of crevices, they experience compressive forces around 300 times their own body weight. They can actually withstand 900 times without suffering any damage whatsoever, or even slowing down. Good job, evolution; now, go home.
“It’s a nice lesson in humility,” says Daniel Goldman from the Georgia Institute of Technology. “These animals are not simple in any way, shape, or form—and especially not in their shape or form.”
Full specializes in the physics of animal movements. He discovered how geckos stick to walls and steer while falling, and how octopuses walk on their tentacles to mimic coconuts. And he has been investigating cockroaches for over 14 years, looking at how they run, fly, and climb. He has shown that they can swing under ledges like a pendulum to instantly disappear from sight, run upside-down on a branch, flip themselves up with their wings if they land on their backs, run having lost four of their legs, and climb having lost their feet. Full has even stuck tiny jetpacks onto them to see how they cope with being blasted to the side while climbing—very well, it seems.
Full’s team then takes what they learn from real animals, and builds robots that have the same skills. For example, Jayaram has now constructed a compressible cockroach bot, which looks like a yellow woodlouse, and folds in much the same way as its living counterpart. It stands 75 millimeters tall, and can be squashed to just 35 millimeters.
These bioinspired bots are physical models that allow the researchers to check that they’ve understood the animals they’re studying. “We can try a bunch of different legs and bodies and say: What does it tell us about the animal? Are we missing anything?” says Full.
But the robots also have practical uses. Picture RoboRoach scuttling through the rubble of a collapsed building to track down survivors, squeezing through cracks without taking damage or losing speed. Jayaram is now working on making RoboRoach smaller and autonomous, and Full has been talking to first-responders at disaster sites to see what their needs are. “Imagine having a swarm of these and throwing them out to get information as fast as possible,” he says.
The study helps to rethink locomotion in robots, says Cecilia Laschi from the Biorobotics Institute in Italy. She and others have been building soft robots, inspired by flexible animals like octopuses and jellyfish. Eschewing hard shells and skeletons, these machines use floppy and stretchy materials instead.
But these creations often sacrifice speed and strength in the service of pliability. Cockroaches don’t. “People are modeling slugs and worms for soft robotics, but we think these cockroaches are the way to go,” says Full. “They have appendages like ours, and muscles that can generate a lot of force.”
Jayaram and Full “are serving as biology translators, who can give us roboticists ideas on how to really build systems with some of biology’s features,” says Howie Choset from the Georgia Institute of Technology. “When I was done reading their paper, I wanted to build a robot with these capabilities for search and rescue!”
We want to hear what you think. Submit a letter to the editor or write to email@example.com. | <urn:uuid:4f724278-2ec1-473d-9366-a82e18a339d1> | 2.921875 | 1,183 | News Article | Science & Tech. | 53.078852 | 95,505,797 |
At the end of the article, you will able to describe – Bohr Model Of Atom, Neil Bohr Atomic Theory, Model, Discovery, Limitations. Let’s start discussing one by one.
http://freejobseeker.com/pesco-chandigarh-recruitment-2017-apply-online/ Neil Bohr Atomic Model
To overcome the limitations of Rutherford Model, Niels Bohr proposed a new model of an atom. The explanation of this theory called http://hargapintugarasi.com/bireu/2483 Bohr Atomic Theory. He had a great contribution towards the understanding concept of atomic structure. It was on the basis of Planck’s Quantum Theory.
Niels Bohr model of an atom is made up of three particles electron, proton, and neutron. Electron have the negative charge, Proton has a positive charge, where is neutron has no charge. Due to the presence of an equal number of negative electrons and positive protons. The atom on the whole electrically neutral.
- The protons and neutrons are located in a small nucleus at the center of the atom. Due to the presence of protons nucleus is having a positive charge.
- There is a limit to the number of electrons with each energy level(or shell).
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- The first energy level can hold the maximum eur usd chart 2 electrons.
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- The third shell can hold a maximum of http://sumarplant.ro/franciye/2334 18 electrons.
- The fourth level can hold Par quelles lois sont régies les opzione binarie consob en Suisse ? Quel régulateur prend en charge ce produit ? Quel taux d'imposition ? Réponses ici. 32 electrons.
get link Neil Bohr Atomic Theory
1. In atom, electrons revolve around the nucleus in a certain circular path called http://www.ivst-vz.de/?debin=bdswiss-kom orbits.
2. Each orbit is having a definite energy. So, these are known as follow url energy levels or energy shell. These are numbered as 1,2,3,4 or k, L, M, N…… and so on…
3.When electrons keep revolving in the same energy level. There is no change in the energy of the electron. The atom remains stable. According to Bohr atomic theory, the change in the energy of electron takes place only when it jumps from lower energy shell to higher energy shell. When it comes down from higher energy level to lower energy level it http://tennisclubpaimpol.fr/bisese/1543 loses energy in the form of radiation(visible light, UV etc.)
4. As long as an electron present in a particular orbit, it never absorbs or loses energy. Therefore, remain constant. These energy orbits are therefore known as ground states(or stationary state). However, the electrons are not stationary, they revolve around nucleus very fast.
5.When energy is supplied to the electron it absorbs only in a fixed amount (as celexa 10 mg quanta). This enables to jump to higher energy state away from nucleus known as the dispersible aspirin 300 mg dosage excited state. Since excited state is unstable the electron made a jump to lower energy state and in doing it emit of energy which is the same amount of energy.
6.The electron revolving in an orbit with the permitted angular momentum a whole number zestril price in philippines multiple of h/2π.
Niels Bohr Discovery- Bohr Model of Atom
Neils Bohr discovery has a many has a number of achievement to its credit. These are described as follow:
1.The theory has explained the stability of the atom. According to Bohr’s theory, the electrons present in the particular shell cannot lose the energy of its own. It can do so when jump from higher energy to lower energy level. Thus, in the ground state, the electron keep on revolving in the same circular orbit and does not come close to the nuclear portion( proton).
2.Bohr’s has helped in calculating the energy of an electron. In hydrogen atom and one electron species (He+,Li+2) it is possible to calculate the energy of hydrogen electron and also one electron species(He+,Li+2)
3.Bohr atomic theory has explained the atomic spectra of the hydrogen atom. Normally, electron tends to be in lower energy state called ground state. In case energy is made available to an electron from outside source, it absorbs energy in quanta and jumps to higher energy state called excited state. For example – In hydrogen atom only electron present in K shell in the ground state with energy E1. If it absorbs energy equal E2. It jumps to its first excited state (energy level 2). Since the excited state is unstable the electron will jump back to the ground state by losing a quantum of energy is radiation which appears in spectra.
Bohr Model Limitations
Bohr model was quite successful in explaining the atomic spectra of the hydrogen atom and many other one electron species. It also helps to calculate the energy of an electron in different energy levels. However, suffered from certain limitation when is described as follow:
- The theory could not explain the atomic spectra of an atom containing one or more electrons.
- This theory fails to explain the shape of the molecule by the combination of atoms.
- According to Bohr theory, an electron follows a fixed circular path with definite energy and thus, both his position and directions can be well defined. However, few principles stated that the path of the electron is not definite. It is probable in nature. The serious blow from certain new principle people such as de-Broglie and Heisenberg uncertainty principle.
Must Read: Quantum number.
This is all about the basics of Bohr Model Of Atom, Neil Bohr Atomic Model, Theory, Discovery, Limitations. | <urn:uuid:de16c41f-20c6-4ec9-8112-ab9229d16c7b> | 3.140625 | 1,296 | Knowledge Article | Science & Tech. | 51.054456 | 95,505,811 |
filter function selects an iterable's items based on a test function.
filter function requires a list call to display all its results.
For example, the following filter call picks out items in a sequence that are greater than zero:
d=list(range(-5, 5)) # An iterable in 3.X print( d ) d=list(filter((lambda x: x > 0), range(-5, 5))) # An iterable in 3.X print( d )
Items in the sequence or iterable for which the function returns a true result are added to the result list.
It is roughly equivalent to a for loop as follows:
res = for x in range(-5, 5): # The statement equivalent if x > 0: res.append(x) # ww w .j ava2 s . c o m print( res )
filter can be emulated by list comprehension syntax.
d=[x for x in range(-5, 5) if x > 0] # Use () to generate items print( d ) | <urn:uuid:f4f3dfe5-c5ee-43f1-b850-f7c4a1fc1481> | 4.03125 | 221 | Documentation | Software Dev. | 83.322333 | 95,505,815 |
The regulation of genetically engineered animals: going from bad to worse
MetadataShow full item record
As of this NABC meeting (May, 1992), the regulation of genetically engi-neered animals is hopelessly inadequate, with little hope for improvement. As long as the Council on Competitiveness sets policy, existing statutes are unlikely to be implemented to regulate genetically engineered animals and no new legislation will be sought to provide the new authority needed. From an environmental standpoint, the current situation means that the risks posed by engineered animals to the environment—whether from acci-dental or deliberate release—will go unassessed and uncontrolled. Moreover, without regulation there will be few opportunities for the public to know what is coming or to participate in decisions about the technology. The bot-tom line is that the new policy leaves it up to industry and scientists to decide what kind of animals to make and when and how they should be released. The rest of us must simply hope that their choices will not lead to environ-mental degradation and disaster. This policy of secrecy and exclusion of the public is a recipe for disaster- both for the environment and for the biotechnology industry.
Agricultural biotechnology; animal biotechnology; bioethics; animal well-being; food safety; science communication; agricultural indistry; consumers sentimen;
Except where otherwise noted, this item's license is described as CC BY-NC-ND | <urn:uuid:0a0a9799-31fd-43ae-aa7c-606a63bbc90a> | 2.5625 | 289 | Academic Writing | Science & Tech. | 6.859783 | 95,505,818 |
- Research news
- Open Access
Two breaks make a translocation
© BioMed Central Ltd 2000
Published: 16 June 2000
There are multiple ways in which double-stranded breaks (DSBs) in DNA can be repaired or recombine with other DNA molecules. Under some of these conditions it is theoretically possible that a single DSB could invade a region of homology and cause a translocation. But in the 8 June Nature Richardson and Jasin find that mouse cells with a single DSB often repair the break with homologous sequences from another location, but only cells with two DSBs experience translocation events (Nature 2000, 205:697-700). Richardson and Jasin introduce DSBs by adding a rare-cutting restriction enzyme gene and allowing the enzyme to act on a site within an introduced drug-resistance gene. This system should help in studies of how to suppress translocation events. | <urn:uuid:f68dd4c1-52a9-4659-8db7-76b5840718cd> | 2.953125 | 186 | Truncated | Science & Tech. | 35.023085 | 95,505,819 |
4. Version Control Systems¶
4.1. Quick Reference¶
4.1.1. Commit and Upload¶
Do this whenever you are done with a session of programming:
- Open “git bash” on Windows, or “terminal” on MacOS.
cd mydirectorynamewhere the name of the directory will be the same as the name of your repository on BitBucket. You can usually type in the first few letters and hit <tab> to fill in the rest of the directory name.
git add *
git commit -m "Work on lab 1"Update the comment to whatever you did.
4.1.2. Turn In Your Work¶
- Go to BitBucket
- Click on “Source”
- Find the folder with your lab
- Copy link
- Go to Scholar for the lab
- Paste link, and turn in.
4.2. What is a Distributed Version Control System¶
Now we need to set up the computer to manage the code that we type in. This will allow you to upload the code so that I can see it and give feedback.
No serious development should be done without version control. In fact, version control is so important, many developers would argue that almost no development should be done without version control. Even all my notes for class I keep in version control.
Version control allows developers to:
- Get any prior version of a project.
- Released version 1.5 of your program, and now it is crashing? Quick! Go back to version 1.4.
- Did the ‘new guy’ mess up the project? Revert back!
- Know exactly what changed in the code, when, and by who. See who is actually doing the work. If a mistake gets added in, see when it was added and by whom.
- Easily share code between developers.
- Easily work independently of other developers.
- Recover an accidentally deleted or overwritten file.
- Go back and create a bug-fix release on prior versions of a program.
- Work on multiple computers and keep files in sync.
Version control saves untold time and headaches. It used to be that version control had enough of a learning curve that some developers refused to use it. Thankfully today’s version control tools are so easy to use there’s no excuse not to.
There are two main types of version control. The original version control systems were “centralized.” Subversion (SVN) is a very popular piece of software that supports this type of version control. The other type is a “Distributed Version Control Systems” (DVCS). There are two popular versions of DVCS in use today, Git and Mercurial. Mercurial is sometimes also known as Hg. Get it? Hg is the symbol for Mercury. Either Git or Hg works fine, but for this tutorial we will standardize on Git.
4.3. The Interactive Git Tutorial¶
This is a great interactive tutorial to learn how to use
git. Go through it now:
4.4. Installing Git¶
Now that you’ve learned how to use
git, let’s install it on your computer.
If you are using a school computer with
git pre-installed, you can skip
Click the link below and download and install the 64-bit version of the
4.5. Forking the Repository¶
You should only have to fork the code once during class. If you do it more than once, something is wrong. Stop before you do this and see the instructor. It is a big headache for everyone if your fork more than once.
- We are going to store our programs on-line with a website called BitBucket.
BitBucket and a program called SourceTree are owned by a company called
Atlassian. They offer enhanced
accounts for e-mail addresses ending in
.edu. To use BitBucket, create an account https://bitbucket.org/account/signup/
- Go to this web address which has a template for the labs we’ll create in class: https://bitbucket.org/pcraven/arcade-games-work
- We need to “fork” the repository. This will create your own copy of the repository that will be independent of mine. Changes you make to a “fork” aren’t automatically sent to the original. Fork the repository by clicking on the plus button:
- Then select “Fork”:
- Next, select a name for your fork. Use your last name and first name. Also, select that your repository is private, so that you don’t share your homework answers with the world.
- Now you have your own fork. It exists on the BitBucket server only.
4.7. Cloning the Repository¶
Every time you start working on a new computer, you’ll need to create a clone. (Unless you use a flash drive.)
- Run the program “Git Bash” on Windows. Or, if you are on the mac, go under “Applications”, find “Utilities” and in that run “Terminal”.
- Figure out where you want to store your files. You might want to store the files on your laptop, a flash drive, or a networked drive.
- Figure out what directory your “Bash” window is in. Do this by typing
pwd, which is short for “print working directory”.
- You can see what files are in the directory by typing
ls, short for “list files”.
- You can change directories using the
cdcommand. You should default to your “home” directory, which is a great place to put your files. But if you want them in a different location, change to that location now. There’s a lot to the
cdcommand, but there are a few variations you need to know:
cdChange to your “home” directory.
cd mydirChange to
mydirdirectory. That directory must be in the same directory you are in now. Also, if you don’t want to type the full directory name, you can type the first few letters and hit <tab>.
cd ..Go up one directory.
- We want to copy the repository you created to your computer. We’ll call this a “clone.” A “clone” is a copy we normally try to keep synced up, which is different than a “fork.” To clone the repository, hit the “plus” and then select “Clone Repository”
- Copy the address that it gives you. It should have your name, and not my name. If you get this wrong, you’ll have to restart everything back at the clone section. (Not the fork section.)
- Paste the command it gives you in your command prompt:
- There you go! You now have a directory set up where you can do your work.
4.8. Open Project in Pycharm¶
Go ahead and start PyCharm, then select “File…Open” and select that directory.
Your project should look like the image below. If this isn’t what you have, you might have opened the wrong folder. Hit “File…Open” and try again.
If you click the arrow next to the folder name, you can see all the folders in the project folder.
If you move from computer to computer hand have a flash drive, you can reopen your project be just doing “File…Open”. If you don’t have your flash drive, you’ll need to re-clone your repository.
4.9. Change a File¶
Let’s practice making a quick change to one of our files. Open your project folder, open the lab 1 folder, then open lab one. Type in “Hi” or something similar.
Hit Ctrl-S to save.
4.10. Commit Your Code¶
It is time to commit. Wait! You are young and don’t want to commit yet?
The cool thing with version control, is that every time you commit, you can go back to the code at that point in time. Version control lets you take it all back! It is the best type of commitment ever!
First, open Git Bash, and switch to the directory with your project using the
craven@DESKTOP-RAUFKMA MINGW64 ~ $ cd arcade-games-work2/
Optionally, we can use
git status to see what files have changed:
craven@DESKTOP-RAUFKMA MINGW64 ~/arcade-games-work2 (master) $ git status On branch master Your branch is up-to-date with 'origin/master'. Changes not staged for commit: (use "git add <file>..." to update what will be committed) (use "git checkout -- <file>..." to discard changes in working directory) modified: Lab 01 - First Program/lab_01.py no changes added to commit (use "git add" and/or "git commit -a")
Now, add all the files that have changed. The asterisk (
*) is a wild card character
that means get all changes. Optionally, we could list out each file, but that’s a lot
of work and we don’t want to leave anything behind anyway.
craven@DESKTOP-RAUFKMA MINGW64 ~/arcade-games-work2 (master) $ git add *
Commit the changes:
craven@DESKTOP-RAUFKMA MINGW64 ~/arcade-games-work2 (master) $ git commit -m "Work on lab 1" [master 45028a5] Work on lab 1 1 file changed, 1 insertion(+)
You might get an error, if the computer doesn’t know who you are yet. If you get this error, it will tell you the commands you need to run. They will look like:
git config --global user.email "email@example.com" git config --global user.name "Jane Smith"
Then you can re-run your commit command. You can use the “up” arrow to get commands you typed in previously so you don’t need to retype anything.
4.11. Push Your Code¶
And push them to the server:
craven@DESKTOP-RAUFKMA MINGW64 ~/arcade-games-work2 (master) $ git push Counting objects: 4, done. Delta compression using up to 8 threads. Compressing objects: 100% (2/2), done. Writing objects: 100% (4/4), 329 bytes | 0 bytes/s, done. Total 4 (delta 1), reused 0 (delta 0) To bitbucket.org:pcraven/arcade-games-work2.git 519c361..45028a5 master -> master craven@DESKTOP-RAUFKMA MINGW64 ~/arcade-games-work2 (master) $
Look to see if the message says that there is an “error.” The message will probably look a little different than what you see above, with other objects or threads, but there should not be any errors. If there are errors, skip down to What If You Can’t Push?.
4.12. Turning In Your Programs¶
When it comes time to turn in one of your programs, go back to BitBucket. Click on “source”, find the lab file, copy the URL:
Now go to Scholar and paste the link into the text field for the lab you are are working on.
4.13. What If You Can’t Push?¶
What happens if you can’t push to the server? If you get an error like what’s below? (See highlighted lines.)
$ git push To bitbucket.org:pcraven/arcade-games-work2.git ! [rejected] master -> master (fetch first) error: failed to push some refs to 'firstname.lastname@example.org:pcraven/arcade-games-work2.git' hint: Updates were rejected because the remote contains work that you do hint: not have locally. This is usually caused by another repository pushing hint: to the same ref. You may want to first integrate the remote changes hint: (e.g., 'git pull ...') before pushing again. hint: See the 'Note about fast-forwards' in 'git push --help' for details.
4.13.1. Step 1: Make Sure You Have No Pending Changes¶
git status and make sure you have nothing to commit.
It should look like this:
craven@DESKTOP-RAUFKMA MINGW64 ~/arcade-games-work2 (master) $ git status On branch master Your branch is up-to-date with 'origin/master'. nothing to commit, working tree clean
If you do hove code to commit, jump up to Commit Your Code and then come back here.
4.13.2. Step 2: Pull Changes From The Server¶
Pull changes from the server:
$ git pull
Normally, this will work fine and you’ll be done. But if you have other computers that you are coding on, the computer will automatically try to merge.
188.8.131.52. Step 2A: Merging¶
If you get a screen like the image below, the computer automatically
merged your code bases. It now wants you to type in a comment for the
merge. We’ll take the default comment.
Hold down the shift key and type
If that doesn’t work, hit escape, and then try again.
(You are in an editor called vim and it is asking you for a comment about merging the files. Unfortunately vim is really hard to learn. Shift-ZZ is the command to save, and all we want to do is get out of it and move on.)
It should finish with something that looks like:
craven@DESKTOP-RAUFKMA MINGW64 ~/arcade-games-work2 (master) Merge made by the 'recursive' strategy. Lab 01 - First Program/lab_01.py | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
If instead you get this:
Then we edited the same file in the same spot. We have to tell the computer if we want our changes, or the changes on the other computer.
184.108.40.206. Step 2B: Resolving a Merge Conflict¶
git status. It should look something like this:
$ git status On branch master Your branch and 'origin/master' have diverged, and have 1 and 1 different commits each, respectively. (use "git pull" to merge the remote branch into yours) You have unmerged paths. (fix conflicts and run "git commit") (use "git merge --abort" to abort the merge) Unmerged paths: (use "git add <file>..." to mark resolution) both modified: Lab 01 - First Program/lab_01.py no changes added to commit (use "git add" and/or "git commit -a")
The key thing to look for is any file that says
If you want your copy, type:
$ git checkout --ours "Lab 01 - First Program/lab_01.py"
If instead you want their copy (or the copy on the other computer) type
$ git checkout --theirs "Lab 01 - First Program/lab_01.py"
Then when you are all done with all merges, type:
craven@DESKTOP-RAUFKMA MINGW64 ~/arcade-games-work2 (master|MERGING) $ git add * craven@DESKTOP-RAUFKMA MINGW64 ~/arcade-games-work2 (master|MERGING) $ git commit -m"Merged" [master e083f36] Merged craven@DESKTOP-RAUFKMA MINGW64 ~/arcade-games-work2 (master) $ git push Counting objects: 5, done. Delta compression using up to 8 threads. Compressing objects: 100% (5/5), done. Writing objects: 100% (5/5), 531 bytes | 0 bytes/s, done. Total 5 (delta 2), reused 0 (delta 0) To bitbucket.org:pcraven/arcade-games-work2.git 6a8f398..e083f36 master -> master
4.13.3. Step 3: Try Pushing Again¶
$ git push Counting objects: 6, done. Delta compression using up to 8 threads. Compressing objects: 100% (4/4), done. Writing objects: 100% (6/6), 604 bytes | 0 bytes/s, done. Total 6 (delta 2), reused 0 (delta 0) To bitbucket.org:pcraven/arcade-games-work2.git d66b008..aeb9cf3 master -> master | <urn:uuid:0f3ae9fb-5c45-4b3b-bf34-c845abad51ea> | 2.546875 | 3,775 | Tutorial | Software Dev. | 73.882072 | 95,505,836 |
The structure and composition of granites provide clues to the nature of silicic volcanism, the formation of continents, and the rheological and thermal properties of the Earth's upper crust as far back as the Hadean eon during the nascent stages of the planet’s formation1,2,3,4. The temperature of granite crystallization underpins our thinking about many of these phenomena, but evidence is emerging that this temperature may not be well constrained. The prevailing paradigm holds that granitic mineral assemblages crystallize entirely at or above about 650–700 degrees Celsius5,6,7. The granitoids of the Tuolumne Intrusive Suite in California tell a different story. Here we show that quartz crystals in Tuolumne samples record crystallization temperatures of 474–561 degrees Celsius. Titanium-in-quartz thermobarometry and diffusion modelling of titanium concentrations in quartz indicate that a sizeable proportion of the mineral assemblage of granitic rocks (for example, more than 80 per cent of the quartz) crystallizes about 100–200 degrees Celsius below the accepted solidus. This has widespread implications. Traditional models of magma formation require high-temperature magma bodies, but new data8,9 suggest that volcanic rocks spend most of their existence at low temperatures; because granites are the intrusive complements of volcanic rocks, our downward revision of granite crystallization temperatures supports the observations of cold magma storage. It also affects the link between volcanoes, ore deposits and granites: ore bodies are fed by the release of fluids from granites below them in the crustal column; thus, if granitic fluids are hundreds of degrees cooler than previously thought, this has implications for research on porphyry ore deposits. Geophysical interpretations of the thermal structure of the crust and the temperature of active magmatic systems will also be affected.
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All data for this manuscript are reported in the Extended Data. This research was supported by the Carnegie Institution of Washington’s Postdoctoral Fellowship Program and the NASA Astrobiology Institute (grant NNA09DA80A) to the New York Center for Astrobiology. We thank N. Ackerson, M. J. Ackerson and V. Ackerson for assistance in the field, E. Bullock and J. Armstrong for their technical assistance in analysing quartz, and D. Coleman, J. Reimink, L. Wagner, H. Le Mével, S. Shirey and P. Ulmer for conversations during manuscript preparation.
Extended data figures and tables
Geological map modified from ref. 10.
Extended Data Fig. 2 Secondary fluorescence analyses of Ti concentrations in quartz as a function of distance from an adjacent titanite crystal.
Analyses of the Ti of a quartz crystal from the Glen Aulin tonalite (Kga) show a rimward increase in Ti towards an adjacent titanite crystal. This increase in Ti content is due to a secondary fluorescence of Ti from the nearby titanite crystal. Error bars are 1σ.
Extended Data Fig. 3 Blue-filtered cathodoluminescence images of quartz from the TIS demonstrate both primary crystallization features and features that suggest alteration of the initial Ti content.
a, Quartz from the Glen Aulin tonalite. b, Quartz from the equigranular Half Dome granodiorite has a centre-to-rim decrease in Ti content consistent with lower-temperature crystallization. c, d, In contrast, other quartz crystals from the equigranular Half Dome granodiorite exhibit complex cathodoluminescence features suggesting that the primary Ti content of the quartz has been altered by syn-magmatic deformation. e, f, Quartz from the Cathedral Peak granodiorite displays a series of primary magmatic growth features suggesting thermal cycling and dissolution-recrystallization of quartz. Lines are microprobe traverses. Source Data
a, Calibration by Thomas et al.18; b, calibration by Huang and Audétat20. Although the Huang and Audétat20 calibration could yield near-solidus temperatures for 20–40 p.p.m. Ti in quartz, these high temperatures cannot explain the sharp Ti concentration zones in quartz crystals, which could only be retained if the quartz crystallized at low temperatures (Fig. 3).
Extended Data Fig. 5 Core–rim Ti concentration profiles in quartz compared with a constant-cooling rate diffusion model.
Diffusion model from Fig. 3b. a, Cathodoluminescence (452 nm) map image of a quartz crystal from the Cathedral Peak granodiorite from approximately the core (upper left) to the rim (right and bottom). Coloured lines are profiles seen in b. b, Normalized concentration gradients from profiles across the grain combined with the diffusion model from Fig. 3b indicate that initial crystallization temperatures must have been below the traditional solidus at about 700 °C for almost the entirety of the observed quartz crystal. For simplicity, high concentrations are all plotted on the left, regardless of position within the crystal. At the scale of these profiles, this simplification does not noticeably change the modelled profiles (that is, it does not matter if the high concentration is coreward or rimward). A second example from the Cathedral Peak granodiorite also contains core (top) to rim (bottom) cathodoluminescence (c) and corresponding concentration gradient profiles that indicate low temperature crystallization for the entire crystal (d).
Extended Data Fig. 6 The influence of cooling rate on the results of constant-cooling rate diffusion models.
a–c, Diffusion modelling at 10 °C per million years (10 °C Myr−1; a), 36.36 °C Myr−1 (b) and 100 °C Myr−1 (c). Faster cooling rates will lead to less diffusion. However, even at rapid cooling rates (for example, 100 °C Myr−1), the modelled profiles do not match the observed Ti concentrations.
Extended Data Fig. 7 Estimating the volume of low-temperature quartz crystallization when diffusion modelling is intractable.
a, Cathodoluminescence (452 nm) image of a centre-cut quartz crystal from the Half Dome granodiorite. This crystal has internal Ti concentrations as high as 101 p.p.m. Concentric circles are contours showing the percentage of total volume crystallized at that radius, assuming spherical quartz growth. Using 60 p.p.m. Ti as a rough approximation for high-T quartz crystallization (denoted with a bold dot-dashed polygon), this volume approximation demonstrates that only around 10% of the quartz could have crystallized at solidus or supersolidus temperatures. b, Radius of crystal (approximately 590 µm) versus the total volume of quartz crystallized at a given radius for a spherical quartz crystal with r = 590 µm.
In addition to quartz, feldspar compositions and oxygen isotope fractionation between quartz and zircon results in subsolidus temperatures. Two-feldspar temperatures are calculated using three different calibrations (equations (27a), (27b) and a global regression from ref. 49). Zircon–quartz oxygen isotope fractionation temperatures are calculated using averaged oxygen isotope compositions from quartz and zircon from individual units in the TIS19 and an experimentally calibrated zircon–quartz fractionation relationship50. | <urn:uuid:c202926a-44dc-4734-8ed5-fe9f898bc3aa> | 3.171875 | 1,643 | Truncated | Science & Tech. | 36.441383 | 95,505,872 |
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Series: NATO Security Through Science Series: Sub-Series C: Environmental Security Volume: 58
Edited By: Igor Linkov and William R Schell
Concentrations of pollutants in the atmosphere have increased dramatically over the last century and many of these changes are attributable to anthropogenic activities. The influence of acid rain has been well studied, but there has been no extensive exploration of other pollutants, such as toxic chemicals, heavy metals and radionuclides. Natural ecosystems, especially forests, tend to accumulate many of these pollutants which subsequently can affect ecosystem health. These contaminants may be very damaging to the environment in Eastern Europe, where the rapid disappearance of forest is the result not only of contamination but also of poor forest management practices. This book provides a study of this complex subject.
Contaminated forests - processes, measurements and methods; radionuclide fate and transport modelling; remedial policies and risk assessment; conclusion.
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strtod, strtof, strtold - convert ASCII string to floating-point number
double strtod(const char *nptr, char
float strtof(const char *nptr, char
long double strtold(const char *nptr, char
Feature Test Macro Requirements for glibc (see feature_test_macros(7)
_POSIX_C_SOURCE >= 200112L
(), and strtold
() functions convert the
initial portion of the string pointed to by nptr
, and long double
The expected form of the (initial portion of the) string is optional leading
white space as recognized by isspace(3)
, an optional plus ('+') or
minus sign ('-') and then either (i) a decimal number, or (ii) a hexadecimal
number, or (iii) an infinity, or (iv) a NAN (not-a-number).
A decimal number
consists of a nonempty sequence of decimal digits
possibly containing a radix character (decimal point, locale-dependent,
usually '.'), optionally followed by a decimal exponent. A decimal exponent
consists of an 'E' or 'e', followed by an optional plus or minus sign,
followed by a nonempty sequence of decimal digits, and indicates
multiplication by a power of 10.
A hexadecimal number
consists of a "0x" or "0X"
followed by a nonempty sequence of hexadecimal digits possibly containing a
radix character, optionally followed by a binary exponent. A binary exponent
consists of a 'P' or 'p', followed by an optional plus or minus sign, followed
by a nonempty sequence of decimal digits, and indicates multiplication by a
power of 2. At least one of radix character and binary exponent must be
is either "INF" or "INFINITY",
is "NAN" (disregarding case) optionally followed by a
, where n-char-sequence
specifies in an
implementation-dependent way the type of NAN (see NOTES).
These functions return the converted value, if any.
is not NULL, a pointer to the character after the last
character used in the conversion is stored in the location referenced by
If no conversion is performed, zero is returned and (unless endptr
null) the value of nptr
is stored in the location referenced by
If the correct value would cause overflow, plus or minus HUGE_VAL
) is returned (according to the sign of the
value), and ERANGE
is stored in errno
. If the correct value
would cause underflow, zero is returned and ERANGE
is stored in
- Overflow or underflow occurred.
For an explanation of the terms used in this section, see attributes(7)
|strtod (), strtof (), strtold ()
POSIX.1-2001, POSIX.1-2008, C99.
() was also described in C89.
Since 0 can legitimately be returned on both success and failure, the calling
program should set errno
to 0 before the call, and then determine if an
error occurred by checking whether errno
has a nonzero value after the
In the glibc implementation, the n-char-sequence
that optionally follows
"NAN" is interpreted as an integer number (with an optional '0' or
'0x' prefix to select base 8 or 16) that is to be placed in the mantissa
component of the returned value.
See the example on the strtol(3)
manual page; the use of the functions
described in this manual page is similar.
This page is part of release 4.16 of the Linux man-pages
description of the project, information about reporting bugs, and the latest
version of this page, can be found at | <urn:uuid:2766cd6f-78ee-4588-bf02-e9e8b324e544> | 2.65625 | 836 | Documentation | Software Dev. | 34.24409 | 95,505,882 |