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When temperatures fall low enough for troilite formation and hence for iodine retention in the solids, some substantial fraction of the original radioiodine has already decayed into <sup>129</sup>Xe. Thus only the remnant of undecayed <sup>129</sup>I is incorporated into the sulfide grains with the rest of the iodine. ... | {
"Header 1": "Taxonomy and Composition of Irons",
"Header 2": "**Isotopic Composition of Meteorites**",
"token_count": 1133,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
We must consider several distinct but related issues. First, what meteorite classes have compositional relationships that suggest that there may be genetic relationships between them? What parent body conditions are required to effect the necessary transformations? What was the agent responsible for these transformatio... | {
"Header 1": "Genetic Relationships between Meteorite Classes",
"token_count": 1361,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Asteroids are small solid bodies, ranging in size from meters to 1000 km, that orbit the Sun in the prograde direction. Larger bodies in heliocentric orbit are called planets; smaller rocky bodies are termed meteoroids. The large majority of all known asteroids orbit in a region extending from approximately 2.2 to 3.3 ... | {
"Header 1": "Introduction to Asteroids",
"token_count": 2028,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
A satellite with a short orbital period may have a well-determined orbit before its parent asteroid does. When an asteroid that has not yet won a catalog number is found to have a satellite, designations such as S/2000 (1998 WW31)1 or S/2001 (1998 SM165)1 must be used. Since 1998 SM165 has recently received a catalog n... | {
"Header 1": "Introduction to Asteroids",
"token_count": 1456,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
The simplest indication of the complexity of the spatial distribution of asteroids comes from examining their distribution over orbital semimajor axis (Fig. VIII.19). The strong concentration of asteroids into the range from 2.06 to 3.65 AU, usually called the Main Belt, and the sharp peaks and valleys in the distribut... | {
"Header 1": "Asteroid Orbits",
"token_count": 2032,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Some of these bodies pursue eccentric paths, highly vulnerable to perturbations, that cross the orbits of one or more of the Jovian planets. These bodies, termed Centaurs, including (2060) Chiron, (5145) Pholus, and dozens of others, are certainly transient visitors to that region, derived from other, more stable locat... | {
"Header 1": "Asteroid Orbits",
"token_count": 467,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
The first treatment of orbits of this type was published by J.-L. Lagrange in 1772, long before the discovery of the first Trojan asteroid, (617) Patroclus, in 1906. Lagrange considered the stability of a three-body system in which M<sup>1</sup> >> M<sup>2</sup> >> M<sup>3</sup> and in which Jupiter (M2) follows a circ... | {
"Header 1": "Stability of Trojan and Plutino Orbits",
"token_count": 2048,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Even if the L4 and L5 points were potential minima (and they are not), it is not obvious howcapture could take place without some form of energy change. This could be a collision with an existing Trojan asteroid, a ''propulsive maneuver'' executed by an expiring comet, or a complex multibody gravitational interaction w... | {
"Header 1": "Stability of Trojan and Plutino Orbits",
"token_count": 512,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Factual knowledge of the sizes, shapes, and absolute reflectivities of asteroids dates back only to the early 1970s. Although reference books published prior to that time often contained tables of asteroid diameters and masses, close examination of the sources of these ''data'' reveals that some authors simply guessed ... | {
"Header 1": "Sizes, Shapes, and Albedos of Asteroids",
"token_count": 2027,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
The conditions necessary for determination of the mass of an asteroid are very stringent. Indeed, the first realization that there were close flybys of a large asteroid by a smaller one came as late as 1949, when G. Fayet found that (197) Arete had been very close to (4) Vesta in 1939. In 1968 it was discovered that Ar... | {
"Header 1": "Masses and Densities of Asteroids",
"token_count": 1095,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
The simplest, and first-tried, source of spectral information on asteroids was UBV photometry, which has been a standard tool of astronomers for decades. UBV photometric data exist for more than 800 asteroids, and numerous attempts have been made to display and interpret this vast body of low-resolution evidence. The v... | {
"Header 1": "Photometry and Spectroscopy of Asteroids",
"token_count": 2034,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
The region of the flux minimum at the crossover of the reflected and emitted fluxes, usually near $4 \mu m$ , is much harder to observe than the $1-\mu m$ or 10-µm regions, and is often missed in spectral studies of small or distant asteroids.
Figure VIII.28 Reflected sunlight and ree... | {
"Header 1": "Photometry and Spectroscopy of Asteroids",
"token_count": 2027,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
The overall reflectivity will not be high, because the only important mechanism for sending light back toward the observer is reflection off of the vacuum–pyroxene interfaces, where there are very large differences in refractive index. The second sample will be very bright because photons will constantly be encounterin... | {
"Header 1": "Photometry and Spectroscopy of Asteroids",
"token_count": 1979,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
The most recent (2001) data on the statistics of the NEAs suggest a total NEA population of about
Figure VIII.35 Spectral discrimination of subcategories within the S asteroid family. This diagram, based on work by Michael Gaffey of R.P.I., separates the S asteroids into eight groups ba... | {
"Header 1": "Photometry and Spectroscopy of Asteroids",
"token_count": 297,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
The radiometric age data reviewed earlier show convincingly that meteorites formed by agglomeration of solid grains about 4.56 Ga ago. Most of the known meteorite classes were at least mildly thermally altered in a short period of time after accumulation. Except for those fewmeteorites that seem to have originated on M... | {
"Header 1": "Thermal Evolution of Asteroids",
"token_count": 2032,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
This is equivalent to assuming that the time scale for conduction is short compared to the time scale for heat generation. For short-lived radioactive decay, the opposite will be true for large bodies. Let us define a conductive cooling time scale $t_{\rm cond}$ as the ratio of the heat content of the solid body to i... | {
"Header 1": "Thermal Evolution of Asteroids",
"token_count": 2037,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
A piece of metal cooling through the 1200 K temperature region will consist of a single phase, taenite (gamma iron). Kamacite is never stable above 1170 K, the point at which iron and nickel become completely mutually soluble. A metal grain containing 10% Ni will first nucleate kamacite at about 960 K, and the mass fra... | {
"Header 1": "Thermal Evolution of Asteroids",
"token_count": 1695,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
One of the most venerable questions about asteroids is whether they are the results of incomplete accretion or of the disruption of a few larger bodies. There was a close coincidence in time between the first few discoveries of asteroids and the realization that meteorites were of extraterrestrial origin (both occurrin... | {
"Header 1": "Thermal Evolution of Asteroids",
"Header 2": "Dynamical Evolution of the Asteroid Belt",
"token_count": 2038,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Meteorites and Asteroids
tend to retain the incident angular momentum because even splashed material will be gravitationally bound in orbits that will mostly reintersect the surface. Wellcompacted asteroids of even small size will lose less material upon impact, and hence their angular momenta will be more strongly a... | {
"Header 1": "Thermal Evolution of Asteroids",
"Header 2": "Dynamical Evolution of the Asteroid Belt",
"token_count": 2025,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
In the past decade the number of known Centaurs and trans-Neptunian objects has grown from one to roughly 300. These classes are distinguished primarily on the basis of orbital properties. The Centaurs are in modest-inclination orbits with sufficient eccentricity to cross the orbits of two or more of the giant planets.... | {
"Header 1": "Thermal Evolution of Asteroids",
"Header 2": "Centaurs and Trans-Neptunian Objects",
"token_count": 2042,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Another interesting dynamical feature of these remote bodies was remarked earlier: despite the infancy of the search for satellites of these bodies, a number of cubewanos and plutinos have already been found to have companions within a few thousand kilometers.
Figure VIII.39 Semimajo... | {
"Header 1": "Thermal Evolution of Asteroids",
"Header 2": "Centaurs and Trans-Neptunian Objects",
"token_count": 719,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
The most reasonable correlations to make between asteroid and meteorite compositions have already been suggested: C chondrites are C asteroids; ordinary chondrites are rare in the Belt, constituting only a small subset of the S asteroids; most S asteroids are unfamiliar classes of chondrites, stony irons, or achondrite... | {
"Header 1": "Relationships among Asteroids, Meteorites, and Comets",
"token_count": 2011,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Region A contains transjovian comets and asteroids, B is a region of weak Jupiter-related cometary activity, C is the region of strong Jupiterrelated comet activity (most of the short-period comets), D is the ''normal'' asteroid regime, including the Belt, and E contains the Apollo and Aten asteroids. The Amors lie jus... | {
"Header 1": "Relationships among Asteroids, Meteorites, and Comets",
"token_count": 2013,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Given average eccentricities of 0.1 and average orbital velocities of 20 km s<sup>-1</sup> for belt asteroids, mutual collisions of these bodies must occur with typical relative speeds of 2 km<sup>s-1</sup>. Since a 100-km asteroid with a density of $3.5 \,\mathrm{g \, cm^{-3}}$ has an escape velocity of only 70 m s<... | {
"Header 1": "Relationships among Asteroids, Meteorites, and Comets",
"token_count": 2046,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
For several months the miner picks up regolith dust and ices with a scoop (or a magnet, if only metals are desired), heats the dust/ice mixture gently with the solar collector to release water, and loads it into a return container. The return vehicle, carrying perhaps 100 tonnes of water or metals, is lifted free of th... | {
"Header 1": "Relationships among Asteroids, Meteorites, and Comets",
"token_count": 960,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
#### Introduction to Meteorites
VIII.1 Use any newspaper index or online clipping service to find an eyewitness account of the fall of a meteorite. Write a brief report on all the observed phenomena described in the article. If there are any unjustified assumptions or conclusions given in the report, explain why you ... | {
"Header 1": "Exercises",
"token_count": 2018,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Howmuch 26Al must be present initially for its complete decay to heat the rock up to its melting temperature (assume this is 1273 K)?
- b. What would the isotopic composition of magnesium be after complete decay of radioaluminum if it was pure 24Mg originally?
422 VIII. Meteorites and Asteroids
#### Genetic Relatio... | {
"Header 1": "Exercises",
"token_count": 1014,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
- VIII.31 It is sometimes argued that the cost of launching payloads into space must always remain high because of the enormous amount of energy needed to get a payload into orbit. Suppose you had a method of using electric power to accelerate payloads and launch them into space:
- a. Using a market price of \$0.10 per... | {
"Header 1": "Exercises",
"Header 2": "Asteroid Resources",
"token_count": 384,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
We have followed the evolution of matter from the hydrogen and helium ashes of the Big Bang through galaxy and star formation, heavy-element nucleosynthesis and supernova explosions, the formation of the Solar System, and evolutionary processes in the realm of the outer planets. We then examined the chemical and physic... | {
"Header 1": "Introduction",
"token_count": 1662,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Phobos and Deimos are difficult to study from Earth because of their faintness and proximity to Mars. Imaging by Earth-based telescopes cannot resolve their sizes and shapes, and spectra are heavily contaminated by light from Mars that is scattered during passage through Earth's atmosphere.
However, both satellites h... | {
"Header 1": "Orbits and Physical Structure of Phobos and Deimos",
"token_count": 1988,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
The volume over which the band will disperse is then 6 10<sup>10</sup> km<sup>3</sup> , the spatial density of debris is 3 10<sup>7</sup> times as high as in the asteroidal case, and the synodic period is about 80 h, not centuries. The sweepup time would then be roughly 3 ka. Under these circumstances, low-velocity rea... | {
"Header 1": "Orbits and Physical Structure of Phobos and Deimos",
"token_count": 813,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
The most intensely disputed of the many controversies concerning Jupiter's innermost Galilean satellite, Io, is the proper pronunciation of its name. English-speaking modernists generally prefer EYE-oh, whereas classicists favor EE-oh, apparently under the supposition that the Greeks and Romans had a poor command of En... | {
"Header 1": "Io: General Properties",
"token_count": 846,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Craters a few kilometers in diameter, with depths of a few hundred meters, should be formed at a rate of about one every 10,000 years, but are not seen; this argument crudely suggests that the surface is buried or relaxes viscously at a rate on the order of 100 m per 10,000 years, or about 1 cm a1. But kilometers-high ... | {
"Header 1": "Io: Surface Processes",
"token_count": 514,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
The present rate of radiogenic heat production in Io, estimated by assuming solar proportions of rock-forming elements, is about 6 1011 W. This is small compared
Figure IX.6 Reflectance of the white frost on Io. The dashed curve is a laboratory reflection spectrum of sulfur dioxide fros... | {
"Header 1": "Io: Internal Energy Sources",
"token_count": 1658,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Most of the surface of Io is dominated by obviously volcanic landforms. Volcanic collapse craters (calderas), multicolor flows emanating from surface vents, and extremely bright local frost deposits, presumably of SO2,
Figure IX.9 High-resolution color image of Io. The association of s... | {
"Header 1": "Io: Geology",
"token_count": 808,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
The first observational evidence for a possible atmosphere on Io was reported in 1964 by Alan Binder and Dale Cruikshank, who observed a startling excess
Figure IX.11 Io's internal structure. This schematic rendering shows a solid silicate mantle surmounting an Fe–S core. Tidal and ele... | {
"Header 1": "Io: Atmospheric and Volcanic Gases",
"token_count": 2033,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
UV spectroscopy of the space about Io reveals the presence of a number of different lower ionization states of oxygen and sulfur. Photographs of the Jovian system at the sodium D line show a remarkable, very large comet-like
Figure IX.13 Schematic rendering of the atmosphere of Io. Almo... | {
"Header 1": "Io: Escape and the Plasma Torus",
"token_count": 666,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
The presence of a steep temperature gradient in the Jovian subnebula is required by the large density differences of the Galilean satellites. Inward extrapolation of this gradient into the realm of the inner four small satellites suggests Venus- or Mercury-like formation temperatures with a large proportion of reduced ... | {
"Header 1": "Io: Escape and the Plasma Torus",
"Header 3": "Io: Genetic Relationships",
"token_count": 2007,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
The simplest measures of craters are the diameter, $D_{\rm r}$ (from rim crest to rim crest), and the depth of the crater floor relative to the mean rim height, $R_i$ . The ratio of the two, called the depth–diameter ratio, varies slowly with crater size and the surface gravity, g, of the target body according to ... | {
"Header 1": "Io: Escape and the Plasma Torus",
"Header 3": "Io: Genetic Relationships",
"token_count": 1991,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
\tag{IX.9}$$
The velocity at impact is related to the approach velocity by conservation of angular momentum,
$$mxV_{a} = mr_{p}V_{i}, (IX.10)$$
where r<sup>p</sup> is the radius of the target planet. The velocity at the moment of impact, Vi, has a tangential component, Vt, and a radial component, Vr,
$$V_{\rm t... | {
"Header 1": "Io: Escape and the Plasma Torus",
"Header 3": "Io: Genetic Relationships",
"token_count": 1398,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
The Moon orbits at a mean distance of 384,402 km from Earth (0.00256 AU) with a mean orbital eccentricity of e ¼ 0:0549 and an inclination (to the ecliptic) of 5.142. Under the influence of Solar and other gravitational forces, the instantaneous eccentricity can range from about 0.04 to 0.07.
The Moon is a triaxial e... | {
"Header 1": "Motions of the Moon",
"token_count": 1330,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
The mean radius of the Moon is 1738 km. Its mass is almost exactly 1/81.3 of Earth's mass, making the Earth–Moon system in a meaningful sense a double planet, almost as alike in mass as the members of the Pluto–Charon system. The relatively large mass of the Moon contributes to the fact that its orbital angular momentu... | {
"Header 1": "Physical Properties of the Moon",
"token_count": 551,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Spacecraft exploration of the Moon has established that there is indeed a major chemical difference between the mare basins and the highlands. The first analytical data on the lunar surface were returned by a gamma-ray spectrometer aboard the Soviet Luna 10 Orbiter in early 1966. This instrument measured with very low ... | {
"Header 1": "Elemental Composition of the Moon's Surface",
"token_count": 1920,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Rocks are assigned to various types according to their elemental composition, mineralogy, and texture. Because of the enormous geological diversity of Earth, it is not surprising that names already existed for most of the important lunar rock types before chemical investigation of lunar materials became possible. In ma... | {
"Header 1": "Lunar Rock Types",
"token_count": 2039,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
The bulk of the incompatible material consists of relatively abundant, low-density oxides, and hence the rare earths are enriched in low-density, crust-forming liquids. The REEs generally exist in the 3+ valence state; however, europium (Z = 63) is extensively reduced to the 2+ state under lunar conditions. Eu<sup>2+</... | {
"Header 1": "Lunar Rock Types",
"token_count": 1150,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
A list of lunar minerals is given in Table IX.2. Only seven minerals are found in abundances greater than about 1% in lunar rocks. These include pyroxene,
Table IX.2 Selected Native Lunar Minerals
| | | Notes |
|----------------------------|------------------|-------|
| M... | {
"Header 1": "Lunar Minerals",
"token_count": 2021,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
The general trends exhibited by elemental abundance patterns in lunar basalts contain important clues to the origin and history of the Moon. The first noteworthy pattern is that many refractories are enriched in the lunar basalts relative to CI chondritic material by a factor of up to 100. These highly enriched element... | {
"Header 1": "Lunar Elemental Abundance Patterns",
"token_count": 629,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
On the global scale, the most striking feature of the Moon's geology is the dichotomy between the near side and the far side. The near side contains numerous deep, basalt-flooded basins and a generally lower range of elevations than the far side. The far side, dominated by a thick anorthosite crust, lies generally seve... | {
"Header 1": "Geology of the Moon",
"token_count": 1375,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
We have few effective tools at our disposal for probing the lunar interior. Seismic studies were conducted by seismometers deployed during the Apollo program (Fig. IX.24). The Moon is extremely inactive compared to Earth, and the few weak quakes that are observed can originate down to depths of 800 to 900 and even 1000... | {
"Header 1": "Geophysics of the Moon",
"token_count": 1990,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
However, constraining the temperatures using the seismic structural data and the observed topography (which would quickly relax through viscous relaxation if the temperatures were too high) leads to models in which the interior temperature asymptotically approaches 1200 to 1500 Cin the deep interior. This cool interior... | {
"Header 1": "Geophysics of the Moon",
"token_count": 202,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Observations of the present orbit of the Moon show a low eccentricity and a modest inclination (5) to the Earth's equator. The rotation of the Earth and the orbital motion of the Moon are, however, coupled weakly by tidal interactions between the two bodies. The familiar oceanic tides cause the figure of Earth's ocean ... | {
"Header 1": "History of the Earth–Moon System",
"token_count": 1400,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Speculations regarding the origin and evolutionary history of the Moon have been rife since well before the Apollo program. Before any chemical data were available on the Moon there was considerable uncertainty about how large a Solar System body could be without melting and differentiating. The reasoning was simple. C... | {
"Header 1": "Origin and Internal Evolution of the Moon",
"token_count": 2033,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Walker has pointed out that the essential element of these explanations is global homogeneity, not simultaneous global melting. He has devised a scenario in which numerous sequential episodes of melting generate upwardly mobile plagioclase-rich melts and dense deposits of olivine-rich cumulates that sink to the floors ... | {
"Header 1": "Origin and Internal Evolution of the Moon",
"token_count": 405,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Surface magnetic fields on both the Moon and Mercury are weak. Except for small local "bubbles" in which the field offers local partial protection, it is likely that the solar wind strikes the surface of the Sunward side of the Moon unimpeded. Protons and other ions in the solar wind strike the lunar regolith at severa... | {
"Header 1": "Solar Wind Interaction with the Moon and Mercury",
"token_count": 1581,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Mercury is the high-temperature end member of the Solar System; at least, it is presently by far the closest planet to the Sun. What do we truly know and understand about its nature?
Because of Mercury's small size and angular proximity to the Sun (always less than about 28°), it is never high in the sky unless the S... | {
"Header 1": "Solar Wind Interaction with the Moon and Mercury",
"Header 2": "The Planet Mercury",
"token_count": 266,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Mercury orbits the Sun as the innermost planet $(a = 0.387 \,\mathrm{AU})$ , with the shortest orbital period of the known members of the Solar System $(P = 86 \,\mathrm{days})$ . We have seen that its orbit is quite eccentric (e = 0.206), taking it from a perihelion distance of $q = a(1-e) = 0.387(1-0.206) = 0.307 ... | {
"Header 1": "Solar Wind Interaction with the Moon and Mercury",
"Header 2": "Motions of Mercury",
"token_count": 1513,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
No spacecraft has ever orbited about or landed on Mercury, Phobos, or Deimos. All of our meager compositional data on these bodies are derived from a very limited body of remote-sensing observations from Earth or from a few spacecraft flyby missions. In comparison with these bodies, the Moon is extremely well studied. ... | {
"Header 1": "Solar Wind Interaction with the Moon and Mercury",
"Header 2": "Composition and Structure of Mercury",
"token_count": 848,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Mercury's reputation as a heavily cratered planet will not be seriously endangered if we briefly survey the geological features that are not directly due to normal cratering events. As far back as we can see in Mercury's tectonic history the planet has had a very thick, rigid crust. Global-scale features, and features ... | {
"Header 1": "Noncrater Geology of Mercury",
"token_count": 524,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Temperatures on the surface of Mercury vary in a complex manner that reflects the peculiar variation of solar illumination imposed by the spin–orbit resonance and the orbital eccentricity. Daytime temperatures near the subsolar point at perihelion reach about 700 K, but near aphelion the lower solar flux maintains a te... | {
"Header 1": "Geophysics of Mercury",
"token_count": 2040,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
A somewhat simplified structural model of Mercury as it is today is given in Fig. IX.30.
The presence and nature of a magnetosphere on Mercury is clearly of great importance in constraining the interior structure of the planet, and the only source of data on Mercury's magnetic field and magnetosphere is the Mariner 1... | {
"Header 1": "Geophysics of Mercury",
"token_count": 638,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Given that this chapter is devoted to ''airless'' planets, the inclusion of a discussion of atmospheres may seem gratuitous. But in fact both Mercury and the Moon have extremely tenuous, essentially collisionless traces of atmosphere (Table IX.4). Both planets are bombarded by the solar wind, and some proportion of the... | {
"Header 1": "Atmospheres of Mercury and the Moon",
"token_count": 1083,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
The search for polar volatiles on the Moon and Mercury sounds at first like an exercise in futility. I refer not to the expected very low abundance of volatiles in both bodies, but to the logic of detection: the only places on the Moon and Mercury that are cold enough to permit the trapping and long-term retention of v... | {
"Header 1": "Polar Deposits on Mercury and the Moon",
"token_count": 2032,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Unfortunately, our knowledge of the velocity distribution of Mercury-crossing asteroids is certainly very incomplete. Recent NEA discoveries, mentioned in Chapter VIII, show a surprising number of 10- to 500-m bodies in extremely Earth-like orbits of low eccentricity and modest inclination. The origin of this newly d... | {
"Header 1": "Polar Deposits on Mercury and the Moon",
"token_count": 493,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
The only missions that promised a better look at the Martian moons Phobos and Deimos than had been provided by the Viking Orbiter spacecraft were the Russian Phobos 1 and 2 missions of 1988. These were ambitious, well-conceived missions with broad scientific interest and international participation. The in-flight failu... | {
"Header 1": "Unfinished Business",
"token_count": 2034,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Experiments with fusion reactors suggest that the controlled fusion of <sup>3</sup> He with D, which produces no neutrons, might be a clean source of power for 21st-century Earth. Understanding the distribution of implanted helium over the lunar surface and its concentration in regolith grains of different size and com... | {
"Header 1": "Unfinished Business",
"token_count": 418,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
#### Introduction
IX.1 Some theories of the accretion of the terrestrial planets predict that each planet will sample preplanetary solids from a very wide range of heliocentric distances and formation temperatures, and that bodies up to the size of Mars may wander widely throughout the inner Solar System under the ef... | {
"Header 1": "Exercises",
"token_count": 734,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
IX.25 Estimate the aberration angle of the solar wind at Mercury (the angular offset in its apparent direction caused by Mercury's high orbital velocity).
#### Motions of Mercury
- IX.26 a. Write a program that calculates the elevation of the Sun as viewed from any fixed point on Mercury's equator, assuming zero ax... | {
"Header 1": "Exercises",
"Header 2": "Motions of the Moon",
"Header 3": "Solar Wind Interaction with the Moon and Mercury",
"token_count": 939,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Mars is, as the traditional description has it, a brother of Earth. For a century writers of fiction and fact alike have tried to draw a moral from the study of Mars. Some saw it as a parable for the old age and decline of Earth: a still-habitable (but ancient) or once-habitable planet exhausted by the decline of its p... | {
"Header 1": "Introduction",
"token_count": 1047,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Mars presents a small target to terrestrial telescopes. Observations are very difficult except at the time when Earth passes directly between Mars and the Sun, and Mars is opposite the Sun in the sky (i.e., when Mars is at opposition). Even then the planet is most easily observed only at times when Mars is near perihel... | {
"Header 1": "Mars",
"token_count": 2005,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
The present large density differences among these planets were then assumed to be due to extreme heating, devolatilization, and atmospheric escape, which most severely depleted the volatiles from the largest planets. In this view, Earth has the most devolatilized interior and hence the lowest FeO content. Ringwood pict... | {
"Header 1": "Mars",
"token_count": 1801,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Two seismometers have landed on the surface of Mars, both during the Viking program in the 1970s. The seismometer on the Viking 1 lander failed to deploy properly because of the failure of the packing restraints, which protected it from vibration during launch, the flight to Mars, and landing, to retract and ''uncage''... | {
"Header 1": "Geophysical Data on Mars",
"token_count": 1476,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Very precise tracking of the Mariner 9, Viking 1 and 2, and MGS Orbiters and the Mars Pathfinder lander have permitted determination of the gravity field of Mars up to spherical harmonics of degree 12 and order 12 (see Appendix V). Generally the gravity field is well compensated on large scales, but some prominent loca... | {
"Header 1": "Gravity and Tectonics of Mars",
"token_count": 789,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Another crucial line of evidence, not available until the spacecraft missions of recent years, concerns the surface morphology of the planet. One of the principal contributions of the Mariner and Viking missions was the compilation of a vast library of photographs and maps, which reveal a great wealth of detail concern... | {
"Header 1": "Geology of Mars",
"token_count": 2017,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
This crater, with a rim-to-rim diameter of 25 km, has terraced, severely collapsed walls and a large central peak, as shown on this Voyager 1 image. The floor of the largest known crater on Mars, the Hellas basin, which lies fully 6 km below the level of the surrounding rim, appears nearly featureless in many photograp... | {
"Header 1": "Geology of Mars",
"token_count": 2022,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Morphologically, the volcanoes seen by Mariner 9 seem more analogous to intraplate (continental) volcanism on Earth than to activity above crustal subduction zones, and they qualitatively suggest the presence of a very few extremely intense ''hot spots'' in the Martian mantle. (As important as these hot spots are to ... | {
"Header 1": "Geology of Mars",
"token_count": 2034,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Prior to the Viking landing missions, our main source of information concerning the chemical composition of the crust of Mars, and hence the best available indicator of
Figure X.12 Continued
Surface Composition 497
Figure X.13 Possible water seepage ... | {
"Header 1": "Surface Composition",
"token_count": 2025,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
This argues against rock types with high magnesium and iron contents (mafic rocks, from the words magnesium and ferric) and especially against rocks dominated by olivine (ultramafic composition). The production of silica-rich rocks requires extensive geochemical evolution of the Martian crust.
The observed seasonal a... | {
"Header 1": "Surface Composition",
"token_count": 700,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
These remote, global-scale investigations of the geology and tectonic evolution of Mars were given a wholly new dimension by the in situ surface observations of the Viking landers. Viking 1 landed on volcanic terrain in the Chryse region, where the terrain is broken and blocky, a consequence of both faulting and impact... | {
"Header 1": "Viking Lander Investigations",
"token_count": 1086,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Three rare classes of meteorites, the shergottites, nakhlites, and chassignites (the SNC meteorites), represented by more than a dozen different individual stones, have been found to have extremely small formation ages of about 1.3 Ga or less, compared to the normal age of asteroidal meteorites, about 4.55 Ga. Further,... | {
"Header 1": "The Shergottite, Nakhlite, and Chassignite Meteorites",
"token_count": 2042,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
A small portion of the potassium is converted to <sup>39</sup>Ar by neutron capture followed by beta decay. The sample is then melted in a vacuum system attached to a mass spectrometer, and the 40:39 ratio in the released gases gives the ratio of radiogenic argon to potassium, and hence the age of the sample. However, ... | {
"Header 1": "The Shergottite, Nakhlite, and Chassignite Meteorites",
"token_count": 2037,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Before the spacecraft exploration of Mars, the surface pressure had been deduced to be less than about 10 mbar. The thermal structure of the Martian atmosphere was investigated at four points by the Mariner 6 and 7 radio occultations, and Earth-based high- resolution IR spectroscopy of the dayside provided mean rotatio... | {
"Header 1": "Atmospheric Structure",
"token_count": 2020,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Recalling that CO2 features in Mars' IR spectrum have been under study for decades, we should remind the reader that the deduced surface pressures were in the range 90–200 mbar until the early 1960s. The reinterpretation of the CO2 spectrum first suggested low surface pressures, and the radio occultation experiments on... | {
"Header 1": "Atmospheric Composition",
"token_count": 2033,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
If one allows for the lower formation temperature, higher oxidation state, and higher volatile content of preplanetary Mars material favored by many authors, then even more H<sub>2</sub>O and CO<sub>2</sub> must have been released relative to <sup>40</sup>Ar. But such enormous masses of volatiles are hard to find on Ma... | {
"Header 1": "Atmospheric Composition",
"token_count": 1739,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
It seems likely that the Martian atmosphere has always consisted largely of CO<sub>2</sub> with minor amounts of H<sub>2</sub>O. We must then address the question of the stability of such an atmosphere against photochemical decomposition and escape of volatiles.
The abundance and stability of carbon dioxide present s... | {
"Header 1": "Atmospheric Composition",
"Header 2": "Photochemical Stability and Atmospheric Escape",
"token_count": 1999,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
In particular, they compute $K = 3 \times 10^8 \,\mathrm{cm}^2 \,\mathrm{s}^{-1}$ in a mixing layer below 12 km for early afternoon in the equatorial equinox. There are, however, very substantial diurnal, seasonal, latitudinal, and height variations in these computed K values. For example, they are orders of magnitud... | {
"Header 1": "Atmospheric Composition",
"Header 2": "Photochemical Stability and Atmospheric Escape",
"token_count": 2017,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
The odd hydrogen sinks are
Figure X.23 Cycles and loss mechanisms for water on Mars. The reactive minor constituents are indicated with circles. Both oxidation and hydration reactions with the surface are important. The rate of oxygen escape is coupled with the rate of hydrogen product... | {
"Header 1": "Atmospheric Composition",
"Header 2": "Photochemical Stability and Atmospheric Escape",
"token_count": 2006,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Thus CO<sup>+</sup> yields preferential escape of C atoms, O<sup>2+</sup> yields enhanced escape of <sup>16</sup>O, and N<sup>2+</sup> favors loss of <sup>14</sup>N relative to <sup>15</sup>N. The atmospheric reservoirs of C and O are so vast compared to the C and O escape fluxes that no isotopic fractionation is expec... | {
"Header 1": "Atmospheric Composition",
"Header 2": "Photochemical Stability and Atmospheric Escape",
"token_count": 1838,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
An early study of the effects of large comet and asteroid impacts on the atmospheric evolution of Earth led G. Hampton Watkins and me to suggest that impacts could erode atmosphere from Mars with high efficiency. Indeed, if early melting, differentiation, and outgassing of Mars occurred simultaneously with terminal acc... | {
"Header 1": "Atmospheric Composition",
"Header 2": "Photochemical Stability and Atmospheric Escape",
"Header 3": "**Explosive Blowoff**",
"token_count": 218,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
The above discussion of the bulk composition of Mars, the photochemical and aeronomical evolution of its atmosphere, volatile-element budgets, and the geological evidence for massive deposits of mineral ice has covered most of the evidence and interpretation appropriate to this subject. We will therefore conclude with ... | {
"Header 1": "Atmospheric Composition",
"Header 2": "Origin and Evolution of the Atmosphere",
"token_count": 2014,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
This assumption is defended by pointing out that the rare gas composition on Mars is very similar to that on Earth, and any thermal loss process would introduce very large (and quite unobserved) mass fractionation effects in, say, depleting 36Ar by a factor of 10 while leaving the xenon abundance unchanged. This argume... | {
"Header 1": "Atmospheric Composition",
"Header 2": "Origin and Evolution of the Atmosphere",
"token_count": 2014,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
At the beginning of the era of spacecraft exploration of the planets it was widely, but by no means universally, accepted that Mars was inhabited by at least primitive life forms. The spirited debate concerning the possible biological significance of the IR ''Sinton bands'' was finally resolved in the negative by the d... | {
"Header 1": "Organic Matter and the Origin of Life",
"token_count": 2022,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Our first knowledge of any of the intrinsic properties of Venus dates back to observations made by the Russian astronomer M. V. Lomonosov in 1761. He observed refraction of sunlight around the disk of Venus while observing a transit of Venus across the face of the Sun, thus establishing for the first time the presence ... | {
"Header 1": "Venus",
"token_count": 2016,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Thus the Sun as seen from the surface of Venus (if it could be seen from the surface!) makes roughly one complete circuit of the sky in half a Venus year. The length of the Venus day is given more precisely by 1/t = (1/224.7 + 1/243.0) or t = 116.75 Earth days.
The position of Venus in Earth's sky, and hence its obse... | {
"Header 1": "Venus",
"token_count": 2044,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Earth-based delay-Doppler mapping of Venus has revealed two large elevated regions, Ishtar Terra and Aphrodite Terra, and several much smaller elevated areas, most notably Alpha Regio, Beta Regio, and Tellus Regio (Fig. X.25). Accurate altimetry was obtained by a radar altimeter on the Pioneer Venus Orbiter, which prov... | {
"Header 1": "Geology of Venus",
"token_count": 2032,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
The projectile fragments if p is greater than the crushing strength of the projectile, which can be as low as $10^4 \, \mathrm{dyn \, cm^{-2}}$ for fluffy cometary debris to as high as $3 \, \mathrm{kbar} \, (3 \times 10^9 \, \mathrm{dyn \, cm^{-2}})$ for an iron meteorite with no cracks. For a typical meter-size... | {
"Header 1": "Geology of Venus",
"token_count": 1345,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
The early indications of high surface temperatures from the radio-wavelength thermal emission data already implied a massive atmosphere on Venus. The infrared emission from the cloudtops fixes their temperature at about 240 K, and the surface temperature is close to 750 K. Consider an atmosphere with adiabatic structur... | {
"Header 1": "Geology of Venus",
"Header 2": "Venus: Atmospheric Structure and Motions",
"token_count": 2013,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
The radial component simply corresponds to a slight reduction in the effective gravitational acceleration of Venus; but the horizontal component, which points toward the equator, must force the lower atmosphere toward the equator until the pressure gradient between pole and equator becomes large enough to balance the e... | {
"Header 1": "Geology of Venus",
"Header 2": "Venus: Atmospheric Structure and Motions",
"token_count": 2017,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
Thus the amount of exogenic water brought into Venus by cometary impactors in the last 4.5 Ga must be at least several times the present water content of the atmosphere. Any theory that purports to explain the present water content and isotopic composition without quantitative inclusion of the water impact flux cannot ... | {
"Header 1": "Geology of Venus",
"Header 2": "Venus: Atmospheric Structure and Motions",
"token_count": 2030,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
At the very high temperature and pressure of the Venus surface, chemical reactions between the atmosphere and the surface may be very important. Equilibration times are difficult to calculate because of our ignorance of the surface grain size distribution and the surface abundance of reactive polar molecules (especiall... | {
"Header 1": "Venus: Atmosphere–Lithosphere Interactions",
"token_count": 1988,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
From the data tables, $K_{70}$ at 750 K is 0.07, and the CO mole fraction in equilibrium at this boundary would be $0.1K_{70}$ , or $7\times10^{-3}$ . The observed CO mole fraction is $2\times10^{-5}$ , so the gas at the surface of Venus is too CO-poor (too oxidizing) for this high an activity of FeO. Finally, mag... | {
"Header 1": "Venus: Atmosphere–Lithosphere Interactions",
"token_count": 1966,
"source_pdf": "datasets/websources/Astronomy_v1/Astronomy/Lewis_2004.pdf"
} |
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