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context. the change in rotation period and the orientation of the rotation axis of comet 67p/churyumov-gerasimenko (67p) can be deduced with high precision from images taken by the scientific imaging instruments on board the rosetta mission. non-gravitational forces are a natural explanation for these data.aims: we describe observed changes in orientation of the rotation axis and the rotation period of 67p. we explain them based on a sublimation model with a best fit for the surface active fraction (model p). torque effects of periodically changing gas emissions on the surface are considered.methods: we solved the equation of state for the angular momentum in the inertial and the body-fixed frames and provide an analytic theory of the rotation changes in terms of fourier coefficients, which are generally applicable to periodically forced rigid-body dynamics.results: the torque-induced changes in rotation state constrain the physical properties of the surface, the sublimation rate, and the local active fraction of the surface.conclusions: we determine a distribution of the local surface active fraction in agreement with the rotation properties, period, and orientation of 67p. the torque movement confirms that the sublimation increases faster than the insolation toward perihelion. the derived relatively uniform activity pattern is discussed in terms of related surface features. | comet 67p/churyumov-gerasimenko rotation changes derived from sublimation-induced torques |
context. it has recently been proposed that the surface composition of icy main-belt asteroids (b-, c-, cb-, cg-, p-, and d-types) may be consistent with that of chondritic porous interplanetary dust particles (cp idps).aims: in the light of this new association, we re-examine the surface composition of a sample of asteroids belonging to the themis family in order to place new constraints on the formation and evolution of its parent body.methods: we acquired near-infrared spectral data for 15 members of the themis family and complemented this dataset with existing spectra in the visible and mid-infrared ranges to perform a thorough analysis of the composition of the family. assuming end-member minerals and particle sizes (<2 μm) similar to those found in cp idps, we used a radiative transfer code adapted for light scattering by small particles to model the spectral properties of these asteroids.results: our best-matching models indicate that most objects in our sample (12/15) possess a surface composition that is consistent with the composition of cp idps. we find ultra-fine grained (<2 μm) fe-bearing olivine glasses to be among the dominant constituents. we further detect the presence of minor fractions of mg-rich crystalline silicates (enstatite and forsterite). the few unsuccessfully matched asteroids may indicate the presence of interlopers in the family or objects sampling a distinct compositional layer of the parent body.conclusions: the composition inferred for the themis family members suggests that the parent body accreted from a mixture of ice and anhydrous silicates (mainly amorphous) and subsequently underwent limited heating. by comparison with existing thermal models that assume a 400-km diameter progenitor, the accretion process of the themis parent body must have occurred relatively late (>4 myr after cais) so that only moderate internal heating occurred in its interior, preventing aqueous alteration of the outer shell. | compositional characterisation of the themis family |
epic 204278916 has been serendipitously discovered from its k2 light curve that displays irregular dimmings of up to 65 per cent for ≈25 consecutive days out of 78.8 d of observations. for the remaining duration of the observations, the variability is highly periodic and attributed to stellar rotation. the star is a young, low-mass (m-type) pre-main-sequence star with clear evidence of a resolved tilted disc from atacama large millimeter/submillimeter array (alma) observations. we examine the k2 light curve in detail and hypothesize that the irregular dimmings are caused by either a warped inner disc edge or transiting cometary-like objects in either circular or eccentric orbits. the explanations discussed here are particularly relevant for other recently discovered young objects with similar absorption dips. | the peculiar dipping events in the disc-bearing young-stellar object epic 204278916 |
beginning in 2014 march, the osiris (optical, spectroscopic, and infrared remote imaging system) cameras began capturing images of the nucleus and coma (gas and dust) of comet 67p/churyumov-gerasimenko using both the wide angle camera (wac) and the narrow angle camera (nac). the many observations taken since july of 2014 have been used to study the morphology, location, and temporal variation of the comet's dust jets. we analysed the dust monitoring observations shortly after the southern vernal equinox on 2015 may 30 and 31 with the wac at the heliocentric distance rh = 1.53 au, where it is possible to observe that the jet rotates with the nucleus. we found that the decline of brightness as a function of the distance of the jet is much steeper than the background coma, which is a first indication of sublimation. we adapted a model of sublimation of icy aggregates and studied the effect as a function of the physical properties of the aggregates (composition and size). the major finding of this paper was that through the sublimation of the aggregates of dirty grains (radius a between 5 and 50 μm) we were able to completely reproduce the radial brightness profile of a jet beyond 4 km from the nucleus. to reproduce the data, we needed to inject a number of aggregates between 8.5 × 1013 and 8.5 × 1010 for a = 5 and 50 μm, respectively, or an initial mass of h2o ice around 22 kg. | sublimation of icy aggregates in the coma of comet 67p/churyumov-gerasimenko detected with the osiris cameras on board rosetta |
after coming to rest on the night side of comet 67p/churyumov-gerasimenko, the rolis camera on-board rosetta's philae lander acquired five images of the surface below the lander, four of which were with the aid of led illumination of different colors. the images confirm that philae was perched on a sloped surface. a local horizon is visible in one corner of the image, beyond which we can see the coma. having spent a full day on the surface philae was commanded to lift and rotate, after which a final, sixth, led image was acquired. the change in perspective allowed us to construct a shape model of the surface. the distance to the foreground was about 80 cm, much larger than the nominal 30 cm. this caused stray light, rather than directly reflected led light, to dominate the image signal, complicating the analysis. the images show a lumpy surface with a roughness of apparently fractal nature. its appearance is completely different from that of the first landing site, which was characterized by centimeter to meter-sized debris (mottola et al., 2015). we recognize neither particles nor pores at the image resolution of 0.8 mm per pixel and large color variations are absent. the surface has a bi-modal brightness distribution that can be interpreted in terms of the degree of consolidation, a hypothesis that we support with experimental evidence. we propose the surface below the lander to consist of smooth, cracked plates with unconsolidated edges, similar to terrain seen in civa images. | close-up images of the final philae landing site on comet 67p/churyumov-gerasimenko acquired by the rolis camera |
context. the population of comets hosted by the oort cloud is heterogeneous. most studies in this area have focused on highly active objects, those with small perihelion distances or examples of objects with peculiar physical properties and/or unusual chemical compositions. this may have produced a biased sample of oort cloud comets in which the most common objects may be rare, particularly those with perihelia well beyond the orbit of the earth. within this context, the known oort cloud comets may not be representative of the full sample meaning that our current knowledge of the appearance of the average oort cloud comet may not be accurate. comet c/2018 f4 (panstarrs) is an object of interest in this regard.aims: here, we study the spectral properties in the visible region and the cometary activity of c/2018 f4, and we also explore its orbital evolution with the aim of understanding its origin within the context of known minor bodies moving along nearly parabolic or hyperbolic paths.methods: we present observations obtained with the 10.4 m gran telescopio canarias (gtc) that we use to derive the spectral class and visible slope of c/2018 f4 as well as to characterise its level of cometary activity. direct n-body simulations are carried out to explore its orbital evolution.results: the absolute magnitude of c/2018 f4 is hr > 13.62 ± 0.04 which puts a strong limit on its diameter, d < 10.4 km, assuming a pv = 0.04 cometary-like value of the albedo. the object presents a conspicuous coma, with a level of activity comparable to those of other comets observed at similar heliocentric distances. comet c/2018 f4 has a visible spectrum consistent with that of an x-type asteroid, and has a spectral slope s' = 4.0 ± 1.0%/1000 å and no evidence of hydration. the spectrum matches those of well-studied primitive asteroids and comets. the analysis of its dynamical evolution prior to discovery suggests that c/2018 f4 is not of extrasolar origin.conclusions: although the present-day heliocentric orbit of c/2018 f4 is slightly hyperbolic, both its observational properties and past orbital evolution are consistent with those of a typical dynamically old comet with an origin in the oort cloud. based on observations made with the gtc telescope, in the spanish observatorio del roque de los muchachos of the instituto de astrofísica de canarias, under director's discretionary time (program id gtc2018-096). | spectroscopic and dynamical properties of comet c/2018 f4, likely a true average former member of the oort cloud |
the origin of the chicxulub impactor, which is attributed as the cause of the k/t mass extinction event, is an unsolved puzzle. the background impact rates of main-belt asteroids and long-period comets have been previously dismissed as being too low to explain the chicxulub impact event. here, we show that a fraction of long-period comets are tidally disrupted after passing close to the sun, each producing a collection of smaller fragments that cross the orbit of earth. this population could increase the impact rate of long-period comets capable of producing chicxulub impact events by an order of magnitude. this new rate would be consistent with the age of the chicxulub impact crater, thereby providing a satisfactory explanation for the origin of the impactor. our hypothesis explains the composition of the largest confirmed impact crater in earth's history as well as the largest one within the last million years. it predicts a larger proportion of impactors with carbonaceous chondritic compositions than would be expected from meteorite falls of main-belt asteroids. | breakup of a long-period comet as the origin of the dinosaur extinction |
context. detection of abundant o2 at 1-10% relative to h2o ice in the comae of comets 1p/halley and 67p/churyumov-gerasimenko has motivated attempts to explain the origin of the high o2 ice abundance. recent chemical modelling of the outer, colder regions of a protoplanetary disk midplane has shown production of o2 ice at the same abundance as that measured in the comet.aims: we aim to carry out a thorough investigation to constrain the conditions under which o2 ice could have been produced through kinetic chemistry in the pre-solar nebula midplane.methods: we have utilised an updated chemical kinetics code to evolve chemistry under pre-solar nebula midplane conditions. four different chemical starting conditions and the effects of various chemical parameters have been tested.results: using the fiducial network, and for either reset conditions (atomic initial abundances) or atomic oxygen only conditions, the abundance level of o2 ice measured in the comets can be reproduced at an intermediate time, after 0.1-2 myr of evolution, depending on ionisation level. when including o3 chemistry, the abundance of o2 ice is much lower than the cometary abundance (by several orders of magnitude). we find that h2o2 and o3 ices are abundantly produced (at around the level of o2 ice) in disagreement with their respective abundances or upper limits from observations of comet 67p. upon closer investigation of the parameter space, and varying parameters for grain-surface chemistry, it is found that for temperatures 15-25 k, densities of 109-1010 cm-3, and a barrier for quantum tunnelling set to 2 å, the measured level of o2 ice can be reproduced with the new chemical network, including an updated binding energy for atomic oxygen (1660 k). however, the abundances of h2o2 and o3 ices still disagree with the observations. a larger activation energy for the o + o2 → o3 reaction (eact > 1000 k) helps to reproduce the non-detection of o3 ice in the comet, as well as reproducing the observed abundances of h2o2 and o2 ices. the only other case in which the o2 ice matches the observed abundance, and o3 and h2o2 ice are lower, is the case when starting with an appreciable amount of oxygen locked in o2.conclusions: the parameter space investigation revealed a sweet spot for production of o2 ice at an abundance matching those in 67p and 1p, and o3 and h2o2 ice abundances matching those in 67p. this means that there is a radial region in the pre-solar nebula from 120-150 au, within which o2 could have been produced in situ via ice chemistry on grain surfaces. however, it is apparent that there is a high degree of sensitivity of the chemistry to the assumed chemical parameters (e.g. binding energy, activation barrier width, and quantum tunnelling barrier). hence, because the more likely scenario starting with a percentage of elemental oxygen locked in o2 also reproduces the o2 ice abundance in 67p at early stages, this supports previous suggestions that the cometary o2 ice could have a primordial origin. | formation of cometary o2 ice and related ice species on grain surfaces in the midplane of the pre-solar nebula |
the discrete element method (dem) is frequently used to model complex granular systems and to augment the knowledge that we obtain through theory, experimentation, and real-world observations. numerical simulations are a particularly powerful tool for studying the regolith-covered surfaces of asteroids, comets, and small moons, where reduced-gravity environments produce ill-defined flow behaviours. in this work, we present a method for validating soft-sphere dem codes for both terrestrial and small-body granular environments. the open-source code chrono is modified and evaluated first with a series of simple two-body-collision tests, and then, with a set of piling and tumbler tests. in the piling tests, we vary the coefficient of rolling friction to calibrate the simulations against experiments with 1 mm glass beads. then, we use the friction coefficient to model the flow of 1 mm glass beads in a rotating drum, using a drum configuration from a previous experimental study. we measure the dynamic angle of repose, the flowing layer thickness, and the flowing layer velocity for tests with different particle sizes, contact force models, coefficients of rolling friction, cohesion levels, drum rotation speeds, and gravity levels. the tests show that the same flow patterns can be observed at the earth and reduced-gravity levels if the drum rotation speed and the gravity level are set according to the dimensionless parameter known as the froude number. chrono is successfully validated against known flow behaviours at different gravity and cohesion levels, and will be used to study small-body regolith dynamics in future works. | validating n-body code chrono for granular dem simulations in reduced-gravity environments |
a deep understanding of the origin of life requires the physical, chemical, and biological study of prebiotic systems and the comprehension of the mechanisms underlying their evolutionary steps. in this context, great attention is paid to the class of interstellar molecules known as "complex organic molecules" (coms), considered as possible precursors of prebiotic species. although coms have already been detected in different astrophysical environments (such as interstellar clouds, protostars, and protoplanetary disks) and in comets, the physical–chemical mechanisms underlying their formation are not yet fully understood. in this framework, a unique contribution comes from laboratory experiments specifically designed to mimic the conditions found in space. we present a review of experimental studies on the formation and evolution of coms in the solid state, i.e., within ices of astrophysical interest, devoting special attention to the in situ detection and analysis techniques commonly used in laboratory astrochemistry. we discuss their main strengths and weaknesses and provide a perspective view on novel techniques, which may help in overcoming the current experimental challenges. | astrochemical pathways to complex organic and prebiotic molecules: experimental perspectives for in situ solid-state studies |
the calcium-aluminum-rich inclusions (cais) found in chondritic meteorites are probably the oldest solar system solids, dating back to 4567.30 ± 0.16 million years ago. they are thought to have formed in the protosolar nebula within a few astronomical units of the sun, and at a temperature of around 1300 k. the stardust mission found evidence of cai-like material in samples recovered from comet wild 2. the appearance of cais in comets, which are thought to be formed at lower temperatures and larger distances from the sun, is only explicable if some mechanism allows the efficient transfer of such objects from the inner solar nebula to the outer solar nebula. such mechanisms have been proposed such as an x-wind or turbulence. in this work, particles collected from within the coma of comet 67p/churyumov-gerasimenko are examined for compositional evidence of the presence of cais. cosima (the cometary secondary ion mass analyzer) uses secondary ion mass spectrometry to analyze the composition of cometary dust captured on metal targets. while cais can have a radius of centimeters, they are more typically a few hundred microns in size, and can be smaller than 1 μm, so it is conceivable that particles visible on cosima targets (ranging in size from about 10 μm to hundreds of microns) could contain cais. using a peak fitting technique, the composition of a set of 13 particles was studied, looking for material rich in both calcium and aluminum. one such particle was found. | searching for calcium-aluminum-rich inclusions in cometary particles with rosetta/cosima |
this paper discusses an algorithm for detecting single transits in photometric time-series data. specifically, we aim to identify asymmetric transits with ingress that is more rapid than egress, as expected for cometary bodies with a significant tail. the algorithm is automated, so can be applied to large samples and only a relatively small number of events need to be manually vetted. we applied this algorithm to all long-cadence light curves from the kepler mission, finding 16 candidate transits with significant asymmetry, 11 of which were found to be artefacts or symmetric transits after manual inspection. of the five remaining events, four are the 0.1 per cent depth events previously identified for kic 3542116 and 11084727. we identify hd 182952 (kic 8027456) as a third system showing a potential comet transit. all three stars showing these events have h-r diagram locations consistent with ∼100 myr-old open cluster stars, as might be expected given that cometary source regions deplete with age, and giving credence to the comet hypothesis. if these events are part of the same population of events as seen for kic 8462852, the small increase in detections at 0.1 per cent depth compared to 10 per cent depth suggests that future work should consider whether the distribution is naturally flat, or if comets with symmetric transits in this depth range remain undiscovered. future searches relying on asymmetry should be more successful if they focus on larger samples and young stars, rather than digging further into the noise. | an automated search for transiting exocomets |
three different measurement campaigns have resulted in three drastically different sets of color measurements of comet 41p/tuttle-giacobini-kresák, ranging from a strongly red to a strongly blue color. although the color slope is normalized to the wavelength range between the filters used, this only serves to partially normalize the resulting color, as the reflectance of cometary dust has a very strong dependence on particle absorption, which may change significantly over the wavelength range of measurement. we demonstrate that the different measurements are physical and are consistent with real materials; for example, we are able to reproduce the color measured during one epoch in which both strong blue and red color slopes were measured almost simultaneously in different filter sets with the mineral dust pyroxene. such measurements with different filter sets serve as an additional constraint in modeling dust properties. | resolving color differences of comet 41p/tuttle-giacobini-kresák |
the rosetta spacecraft observations revealed that the nucleus of comet 67p/churyumov-gerasimenko consists of two similarly sized lobes connected by a narrow neck. here, we evaluate the possibility that 67p is a collapsed binary. we assume that the progenitor of 67p was a binary and consider various physical mechanisms that could have brought the binary components together, including small-scale impacts and gravitational encounters with planets. we find that 67p could be a primordial body (i.e., not a collisional fragment) if the outer planetesimal disk lasted ≲10 myr before it was dispersed by migrating neptune. the probability of binary collapse by impact is ≃30% for tightly bound binaries. most km-class binaries become collisionally dissolved. roughly 10% of the surviving binaries later evolve to become contact binaries during the disk dispersal, when bodies suffer gravitational encounters with neptune. overall, the processes described in this work do not seem to be efficient enough to explain the large fraction (∼67%) of bi-lobed cometary nuclei inferred from spacecraft imaging. | bi-lobed shape of comet 67p from a collapsed binary |
we report results of polarimetric observations of comet 21p/giacobini-zinner made at phase angles, α ≈ 76-78°, between 10 and 17 of september 2018, and compare them with previous measurements. we find significant variations in the polarimetric signals that appear consistent with those reported previously. these variations and subsequent modeling suggest that the particles in the coma are replenished within a period of approximately one day. this period is significantly shorter for highly absorbing carbonaceous particles than for non-absorbing mg-rich silicate particles. such a difference in the relative abundances of these components can lead to variations in the polarization response of the coma. the strong positive polarization in the subsolar direction suggests a large relative abundance of carbonaceous material, which may be an indicator of jet-type activity. | imaging polarimetry and photometry of comet 21p/giacobini-zinner |
despite extensive effort during the last four decades, no clear signature of a lunar indigenous noble gas component has been found. in order to further investigate the possible occurrence of indigenous volatiles in the moon, we have re-analyzed the noble gas and nitrogen isotopic compositions in three anorthosite samples. lunar anorthosites 60025, 60215 and 65315 have the lowest exposure duration (∼2 ma) among apollo samples and consequently contain only limited cosmogenic (e.g. 124,126xe) and solar wind (sw) noble gases. furthermore, anorthosites have negligible contributions of fissiogenic xe isotopes because of their very low pu and u contents. as observed in previous studies (lightner and marti, 1974; leich and niemeyer, 1975), lunar anorthosite xe presents an isotopic composition very close to that of terrestrial atmospheric xe, previously attributed to "anomalous adsorption" of terrestrial xe after sample return. the presumed atmospheric xe contamination can only be removed by heating the samples at medium to high temperatures under vacuum, and is therefore different from common adsorption. to test this hypothesis, we monitored the adsorption of xe onto lunar anorthositic powder using infrared reflectance spectroscopy. a clear shift in the anorthosite ir absorbance peaks is detected when comparing the ir absorbance spectra of the lunar anorthositic powder before and after exposure to a neutral xe-rich atmosphere. this observation accounts for the chemical bonding (chemisorption) of xe onto anorthosite, which is stronger than the common physical bonding (physisorption) and could account for the anomalous adsorption of xe onto lunar samples. our high precision xe isotope analyses show slight mass fractionation patterns across 128-136xe isotopes with systematic deficits in the heavy xe isotopes (mostly 136xe and marginally 134xe) that have not previously been observed. this composition could be the result of mixing between an irreversibly adsorbed terrestrial contaminant that is mostly released at high temperature and an additional signature. solar wind (sw) xe contents, estimated from sw-ne and sw-ar concentrations and sw-ne/ar/xe elemental ratios, do not support sw as the additional contribution. using a χ2 test, the latter is best accounted for by cometary xe as measured in the coma of comet 67p/churyumov-gerasimenko (marty et al., 2017) or by the primordial u-xe composition inferred to be the precursor of atmospheric xe (pepin, 1994; avice et al., 2017). it could have been contributed to the lunar budget by volatile-rich bodies that participated to the building of the terrestrial atmosphere inventory (marty et al., 2017). | stepwise heating of lunar anorthosites 60025, 60215, 65315 possibly reveals an indigenous noble gas component on the moon |
since 1950 when oort published his paper on the structure of the cloud of comets it is believed that stars passing near this hypothetical cometary reservoir play an important role in the dynamical evolution of long-period comets and injecting them into the observability region of the solar system. the aim of this paper is to discuss two cases in which the data obtained from observations were used and stellar perturbations (of different intensity, strong case of c/2002 a3 linear and weaker case of c/2013 f3 panstarrs) on cometary motion were detected. using the best available data from the gaia dr2 catalogue and some other sources, we searched for close stellar passages near the sun. our study took into account that some of the stars are parts of multiple systems. over 600 stars or systems that approached or will approach the sun closer than 4.0 pc were found. having the list of perturbers completed, we studied their influence on a sample of 277 oort spike comets that were observed since 1901 and discovered that two comets might have their orbits fundamentally changed due to a close stellar encounter. our results show how much different the dynamical evolution of comets would have looked when their motion was considered only in the galactic potential. uncertainties both in stellar and cometary data were carefully taken into account. our analysis indicates that the occurrence of stellar perturbations on cometary motions is very rare and the uncertainties of these effects are hard to estimate. | first stars that could significantly perturb comet motion are finally found |
the earth's extraterrestrial dust flux includes a wide variety of dust particles that include feni metallic grains. during their atmospheric entry iron micrometeoroids melt and oxidize to form cosmic spherules termed i-type spherules. these particles are chemically resistant and readily collected by magnetic separation and are thus the most likely micrometeorites to be recovered from modern and ancient sediments. understanding their behavior during atmospheric entry is crucial in constraining their abundance relative to other particle types and the nature of the zodiacal dust population at 1 au. this article presents numerical simulations of the atmospheric entry heating of iron meteoroids to investigate the abundance and nature of these materials. the results indicate that iron micrometeoroids experience peak temperatures 300-800 k higher than silicate particles explaining the rarity of unmelted iron particles which can only be present at sizes of <50 μm. the lower evaporation rates of liquid iron oxide leads to greater survival of iron particles compared with silicates, which enhances their abundance among micrometeorites by a factor of 2. the abundance of i-types is shown to be broadly consistent with the abundance and size of metal in ordinary chondrites and the current day flux of ordinary chondrite-derived mms arriving at earth. furthermore, carbonaceous asteroids and cometary dust are suggested to make negligible contributions to the i-type spherule flux. events involving such objects, therefore, cannot be recognized from i-type spherule abundances in the geological record. | the origins of i-type spherules and the atmospheric entry of iron micrometeoroids |
shortly after the rosetta mission's rendezvous with 67p/churyumov-gerasimenko the rpc/ies instrument intermittently detected negative particles that were identified as singly charged nano-dust grains. these grains were recorded as a nearly mono-energetic beam of particles in the 200-500 ev range arriving from the direction of the comet. occasionally, another population of particles in the energy range of 1-20 kev were also noticed arriving from the approximate direction of the sun. in this paper we review the processes that can explain the energization and the directionality of the observed nano-dust populations. we show that the observations are consistent with gas-drag acceleration of the outflowing particles with radii of 3-4 nm, and with the returning fragments of bigger particles accelerated by radiation pressure with approximate radii of 30-80 nm. in addition to gas drag and radiation pressure, we also examine the role of the solar wind induced motional electric field, and its possible role in explaining the intermittency of the detection of a nano-grain population arriving from the solar direction. | negatively charged nano-grains at 67p/churyumov-gerasimenko |
context. the scattering of light by an asteroid's surface depends on the properties of its particles, volume density, and roughness. it is described by the reflection coefficient which, upon integration over the illuminated and observed part of the surface, yields the disk-integrated photometric brightness of the asteroid. the lommel-seeliger reflection coefficient is applicable to dark, low-albedo c-class asteroids, with prospects for moderate-albedo s-class and m-class asteroids.aims: we calculate the disk-integrated brightness for an ellipsoidal asteroid with a lommel-seeliger reflection coefficient (ls ellipsoid). furthermore, we calculate the photocenter for the ls ellipsoid, that is, the distance of the center of light from the barycenter.methods: because of their analytical nature, the closed-form expressions can be readily utilized in numerical simulations.results: we show lightcurves and photocenter variations for realistic examples of ellipsoidal shapes for a number of pole orientations. the results highlight the reciprocity principle of the radiative-transfer theory and suggest a nontrivial dependence of the photocenter on the pole orientation and viewing geometry.conclusions: finally, we outline a number of applications and future prospects. | disk-integrated brightness of a lommel-seeliger scattering ellipsoidal asteroid |
we previously showed that farmers in bolivia are exposed to many pesticides, some at elevated levels, and that this was associated with increased risk of genetic damage. to improve the understanding of possible mixture effects, the cytotoxicity and genotoxicity of pesticides were studied in vitro using human liver hepg2 cells. the studied pesticides were 2,4‑d, chlorpyrifos, cypermethrin, glyphosate, methamidophos, paraquat, profenofos, and tebuconazole. three mixtures (u1, u2, and u3) were based on profiles of urinary pesticide metabolites and one mixture on the most frequently used pesticides (s1). the results showed that paraquat and methamidophos were the most cytotoxic pesticides (ec50 ≤0.3 mm). paraquat, chlorpyrifos, tebuconazole, and the u1, u2, and u3 mixtures, which contained a large proportion of either chlorpyrifos or tebuconazole, significantly increased intracellular ros levels. most pesticides activated dna damage signaling through proteins chk1 and h2ax. strongest responses were elicited by paraquat, profenofos, chlorpyrifos, cypermethrin, and the s1 mixture, which contained 25% paraquat. comet assay revealed significant increases of dna damage in response to paraquat, cypermethrin, and u2 and s1 mixtures, which contained high levels of cypermethrin and paraquat, respectively. in summary, we showed that the tested pesticides, alone or in mixtures, in general induced oxidative stress and that most pesticides, and especially paraquat and cypermethrin, were genotoxic in hepg2 cells. we could also show that mixtures dominated by these two pesticides displayed a marked genotoxic potency, which agreed with our previous population studies. | in vitro cytotoxicity and genotoxicity of single and combined pesticides used by bolivian farmers |
alteration processes on asteroid and comet surfaces, such as thermal fracturing, (micrometeorite) impacts or volatile outgassing, are complex mechanisms that form diverse surface morphologies and roughness on various scales. these mechanisms and their interaction may differ on the surfaces of different bodies. asteroid ryugu and comet 67p/churyumov-gerasimenko, both, have been visited by landers that imaged the surfaces in high spatial resolution. we investigate the surface morphology and roughness of ryugu and 67p/churyumov-gerasimenko based on high-resolution in situ images of 0.2 and 0.8 mm pixel resolution over an approximately 25 and 80 cm wide scene, respectively. to maintain comparability and reproducibility, we introduce a method to extract surface roughness descriptors (fractal dimension, hurst exponent, joint roughness coefficient, root-mean-square slope, hemispherical crater density, small-scale roughness parameter, and hapke mean slope angle) from in situ planetary images illuminated by leds. we validate our method and choose adequate parameters for an analysis of the roughness of the surfaces. we also derive the roughness descriptors from 3d shape models of ryugu and orbiter camera images and show that the higher spatially resolved images result in a higher roughness. we find that 67p/churyumov-gerasimenko is up to 6 per cent rougher than ryugu depending on the descriptor used and attribute this difference to the different intrinsic properties of the materials imaged and the erosive processes altering them. on 67p/churyumov-gerasimenko sublimation appears to be the main cause for roughness, while on ryugu micrometeoroid bombardment as well as thermal fatigue and solar weathering may play a significant role in shaping the surface. | surface roughness of asteroid (162173) ryugu and comet 67p/churyumov-gerasimenko inferred from in situ observations |
we present high spectral resolution (r ∼ 110 000) ca ii k-line (3933 å) absorption profiles observed towards 20 nearby a-type stars thought to possess circumstellar debris discs. the stars were repeatedly observed over three nights on the 3.6m eso telescope (chile) during 2016 november. these spectra have revealed four stars (hd 24966, hd 38056, hd 79469 and hd 225200) that exhibit transient and weak absorption features at velocities in excess of 15 km s-1 from the cirumstellar radial velocity. these absorption features mostly appear at redshifted velocities with respect to the stellar radial velocity, some evolving towards bluer velocities as a function of time. we identify these absorption events with the evaporation of gas liberated during the infall of star grazing exocomets / planetesimals, a process often termed that of `falling evaporating bodies' (febs) (beust et al. 1990). five of the stars (hd 2262, hd 30422, hd 45557, hd 181296 and hd 188228) exhibit weak absorption features in at least one of their nightly spectra. however, none of these targets show any significant absorption centred near the stellar radial velocity that could be confidently associated with a circumstellar gas disc. the origin of these sporadic absorptions is unclear at present. finally, we present a list of 22 stars with known feb absorption components in their ca ii k-line spectra that will hopefully prove useful as a target list for the possible detection, by high contrast imaging, of exoplanets in orbit around young a-type stars. | further detections of exocomet absorbing gas around southern hemisphere a-type stars with known debris discs |
we show observational evidence for a new form of collisionless shock in interplanetary space near mars, small-scale shocks with periodic spacings. pickup of new ionized hydrogen atoms in a magnetic field aligned with the solar wind direction causes the generation of a magnetosonic wave train through an ion beam instability. the waves have a frequency close to the local proton gyrofrequency. this is a similar physical process as for the formation of cometary plasma waves/turbulence. however, for the case of proton pickup near mars, each individual magnetosonic wave cycle develops into a small-scale shock. so there is a string of fast mode shocks formed with proton gyroperiod spacings. these small-scale shocks display dissipation in the ions and dispersive whistlers. a fraction of ions trapped/reflected at the small-scale shocks are accelerated by the motional electric field. observational results demonstrate that periodic shocks can perform the same functions as a single supercritical shock in a high-speed flow. | observational evidence for fast mode periodic small-scale shocks: a new type of plasma phenomenon |
on 8 october 2011, the draconid meteor shower (iau, dra) was predicted to cause two brief outbursts of meteors, visible from locations in europe. for the first time, a european airborne meteor observation campaign was organized, supported by ground-based observations. two aircraft were deployed from kiruna, sweden, carrying six scientists, 19 cameras and eight crew members. the flight geometry was chosen such that it was possible to obtain double-station observations of many meteors. the instrument setup on the aircraft as well as on the ground is described in full detail. the main peak from 1900-dust ejecta happened at the predicted time and at the predicted rate. the second peak was observed from the earlier flight and from the ground, and was caused most likely by trails ejected in the nineteenth century. a total of 250 meteors were observed, for which light curve data were derived. the trajectory, velocity, deceleration and orbit of 35 double station meteors were measured. the magnitude distribution index was high, as a result of which there was no excess of meteors near the horizon. the light curve proved to be extremely flat on average, which was unexpected. observations of spectra allowed us to derive the compositional information of the draconids meteoroids and showed an early release of sodium, usually interpreted as resulting from fragile meteoroids. lessons learned from this experience are derived for future airborne meteor shower observation campaigns. | the 2011 draconids: the first european airborne meteor observation campaign |
context. an open question in the study of comets is the so-called cohesion bottleneck, that is, how dust particles detach from the nucleus.aims: we test whether the co pressure buildup inside the pebbles of which cometary nuclei consist can overcome this cohesion bottleneck.methods: a recently developed pebble-diffusion model was applied here to comet c/2017k2 panstarrs, assuming a co-driven activity.results: (i) the co-gas pressure inside the pebbles erodes the nucleus into the observed dust, which is composed of refractories, h2o ice and co2 ice. (ii) the co-driven activity onset occurs up to heliocentric distances of 85 au, depending on the spin orientation of the comet nucleus. (iii) the activity onset observed at ≈26 au suggests a low obliquity of the nucleus spin axis with activity in a polar summer. (iv) at 14 au, the smallest size of the ejected dust is ≈0.1 mm, consistent with observations. (v) the observed dust-loss rate of ≈200 kg s-1 implies a fallout ≥30%, a nucleus surface active area ≥10 km2, a co-gas loss rate ≥10 kg s-1, and a dust-to-gas ratio ≤20. (vi) the co-driven activity never stops if the average refractory-to-all-ices mass ratio in the nucleus is ≤4.5 for a nucleus all-ices-to-co mass ratio ≈4, as observed in comets hale-bopp and hyakutake. these results make comet c/2017k2 similar to the rosetta target comet 67p/churyumov-gerasimenko. (vii) the erosion lifetime of cometary planetesimals is a factor 103 shorter than the timescale of catastrophic collisions. this means that the comets we observe today cannot be products of catastrophic collisions. | co-driven activity constrains the origin of comets |
in recent decades, light pollution of the night sky has become a burning issue for all humanity. the negative impact of this state of affairs is particularly burdensome for the living organisms such as humans, animals, and plants, as well as for the whole ecosystems. in addition, it has a particularly negative impact on astronomical observations carried out by professional researchers as well as by amateurs. the night sky polluted by artificial electric light makes the observations of faint astronomical bodies very difficult or even impossible. a special type of such faint celestial bodies comprises comets. however, sometimes comets manifest very large, sudden increases in their luminosity called outbursts of brightness. during the outburst of their brightness, there is a chance to record comets, which are normally objects of low brightness. in this paper, we discuss the conditions of observability of comets during their outbursts of brightness in its relation to the light pollution of the night sky. numerical calculations are carried out for a hypothetical comet belonging to the jupiter family. the obtained results are consistent with the observations of outbursts of real comets orbiting in the solar system. | visibility of comets during their outbursts and the night sky light pollution—use the bortle scale |
the so-called “early activity” of comet 67p/churyumov-gerasimenko has been observed to originate mostly in parts of the concave region or “neck” between its two lobes. since activity is driven by the sublimation of volatiles, this is a puzzling result because this area is less exposed to the sun and is therefore expected to be cooler on average. we used a thermophysical model that takes into account thermal inertia, global self-heating, and shadowing, to compute surface temperatures of the comet. we found that, for every rotation in the 2014 august-december period, some parts of the neck region undergo the fastest temperature variations of the comet’s surface precisely because they are shadowed by their surrounding terrains. our work suggests that these fast temperature changes are correlated to the early activity of the comet, and we put forward the hypothesis that erosion related to thermal cracking is operating at a high rate on the neck region due to these rapid temperature variations. this may explain why the neck contains some ice—as opposed to most other parts of the surface—and why it is the main source of the comet’s early activity. in a broader context, these results indicate that thermal cracking can operate faster on atmosphereless bodies with significant concavities than implied by currently available estimates. | rapid temperature changes and the early activity on comet 67p/churyumov-gerasimenko |
caiman latirostris is one of the two species of the order crocodylia that inhabit argentina and is considered a species of vital ecological and economic importance in the north-east of argentina. in this region, pesticides are the most common contaminants in natural environments and wild caiman populations are subject to this contamination constantly. the aim of this study was to evaluate the effects the main pesticides used in the region: glyphosate (gly), cypermethrin (cyp) and chlorpyrifos (cpf) -based formulations, as well as the mixture of them, on c. latirostris juveniles under semi-controlled condition of exposure (ex-situ) during 75 days. one hundred yearling caimans (10-month-old) were equally distributed into five experimental groups (20 animals per group): a negative control (nc -tap water), gly 2% (roundup® full ii formulation -ru), cyp 0.12% (atanor® formulation), cpf 0.8% (lorsban® formulation), and a mixture of the three pesticides (mx3: gly 2% + cyp 0.12% + cpf 0.8%). we applied early warning biomarkers to detect damage induced by these chemicals in peripheral blood: activity of the antioxidant enzymes catalase (cat) and superoxide dismutase (sod), analysis of lipid peroxidation (lpo) by the thiobarbituric acid reactive substances (tbars), dna damage and specific base oxidation through the standard and modified comet assay (ca), chromosome damage by micronucleus (mn) test and other nuclear abnormalities (nas), hematological and growth parameters. results showed a statistically significant increase in mn and nas frequency, dna damage, with an important contribution of base oxidation for all exposed groups compared to the nc. total white blood cells count (twbcc), and growth parameters showed effects mainly at the mx3. the principal component analysis (pca) demonstrated more sensitivity for biomarkers associated to genetic damage, including base oxidation to dna than lpo, antioxidant enzyme modulation, immunotoxicity or growth parameters, to detect pesticides effects, applied under conditions similar to that found in natural environments. | a comprehensive approach using multiple biomarkers to detect damage induced by pesticides in broad-snouted caiman (caiman latirostris) under ex-situ conditions |
between 14 and 20 july 2018, small unmanned aircraft systems (uass) were deployed to the san luis valley of colorado (usa) alongside surface-based remote sensors, in situ sensors, and radiosonde systems as part of the lower atmospheric profiling studies at elevation - a remotely-piloted aircraft team experiment (lapse-rate). the measurements collected as part of lapse-rate targeted quantities related to enhancing our understanding of boundary layer structure, cloud and aerosol properties and surface-atmosphere exchange and provide detailed information to support model evaluation and improvement work. additionally, intensive intercomparison between the different unmanned aircraft platforms was completed. the current paper describes the observations obtained using three different types of surface-based mobile observing vehicles. these included the university of colorado mobile uas research collaboratory (murc), the national oceanic and atmospheric administration national severe storms laboratory mobile mesonet, and two university of nebraska combined mesonet and tracker (comet) vehicles. over the 1-week campaign, a total of 143 h of data were collected using this combination of vehicles. the data from these coordinated activities provide detailed perspectives on the spatial variability of atmospheric state parameters (air temperature, humidity, pressure, and wind) throughout the northern half of the san luis valley. these datasets have been checked for quality and published to the zenodo data archive under a specific "community" setup for lapse-rate (https://zenodo.org/communities/lapse-rate/, last access: 21 january 2021) and are accessible at no cost by all registered users. the primary dataset dois are https://doi.org/10.5281/zenodo.3814765 (cu murc measurements; de boer et al., 2020d), https://doi.org/10.5281/zenodo.3738175 (nssl mm measurements; waugh, 2020), and https://doi.org/10.5281/zenodo.3838724 (unl comet measurements; houston and erwin, 2020). | measurements from mobile surface vehicles during the lower atmospheric profiling studies at elevation - a remotely-piloted aircraft team experiment (lapse-rate) |
in this work sublimation of porous water ice in vacuum is investigated. the experiments were performed at low temperature gradient of the order 10 k m-1. it was found, that the surface of porous granular ice can recede much slower than of compact ice. when the porosity is 0.75, as relevant for the nuclei of comets, then the rate of recession can be about 60% lower than in the case of compact ice. in addition, preliminary experiments performed at moderate and high temperature gradient are described. | sublimation of porous granular ice in vacuum |
scientific studies using spacecraft radio links, which are shared with communications and navigation functions, have been carried out on almost every solar system exploration mission in the past five decades. they have led to numerous scientific discoveries as well as technological advances. these radio science experiments have investigated the interior structure of every planet in the solar system and several of their satellites, the moon, and several comets and asteroids and sounded the atmospheres of every planet in the solar system except mercury, and the atmospheres of pluto and several large satellites, the io plasma torus, saturn's and uranus' rings, and the solar corona. future experiments at mercury, the jovian system, asteroid psyche, and other targets are currently in various planning and development phases. over the next 30 years, significant advances in radio and laser link science technologies, including one order of magnitude improvement achievable in range rate and range accuracy and advanced calibration techniques, could enable many additional scientific breakthroughs. future exploration concepts focus on applications of small spacecraft and entry probes and can include constellations for studies of atmospheric dynamics, interior structures, and surface properties. selected science-enabling technologies specific to small spacecraft instrumentation on future missions are under study. examples include field tests of radio scattering to determine surface properties, constellations for high-resolution spatial and temporal atmospheric sounding, small science-quality software-defined transponders, miniature ultrastable oscillators, and advanced radio-metric calibrations at the deep space network. | future of planetary atmospheric, surface, and interior science using radio and laser links |
the alice far-ultraviolet imaging spectrograph onboard rosetta observed emissions from atomic and molecular species from within the coma of comet 67p/churyumov-gerasimenko during the entire escort phase of the mission from 2014 august to 2016 september. the initial observations showed that emissions of atomic hydrogen and oxygen close to the surface were produced by energetic electron impact dissociation of h2o. following delivery of the lander, philae, on 2014 november 12, the trajectory of rosetta shifted to near-terminator orbits that allowed for these emissions to be observed against the shadowed nucleus that, together with the compositional heterogeneity, enabled us to identify unique spectral signatures of dissociative electron impact excitation of h2o, co2, and o2. co emissions were found to be due to both electron and photoexcitation processes. thus, we are able, from far-ultraviolet spectroscopy, to qualitatively study the evolution of the primary molecular constituents of the gaseous coma from start to finish of the escort phase. our results show asymmetric outgassing of h2o and co2 about perihelion, h2o dominant before and co2 dominant after, consistent with the results from both the in situ and other remote sensing instruments on rosetta. | fuv spectral signatures of molecules and the evolution of the gaseous coma of comet 67p/churyumov-gerasimenko |
rotational spin-up due to outgassing of comet nuclei has been identified as a possible mechanism for considerable mass-loss and splitting. we report a search for spin changes for three large jupiter-family comets (jfcs): 14p/wolf, 143p/kowal-mrkos, and 162p/siding spring. none of the three comets has detectable period changes, and we set conservative upper limits of 4.2 (14p), 6.6 (143p), and 25 (162p) min per orbit. comparing these results with all eight other jfcs with measured rotational changes, we deduce that none of the observed large jfcs experiences significant spin changes. this suggests that large comet nuclei are less likely to undergo rotationally driven splitting, and therefore more likely to survive more perihelion passages than smaller nuclei. we find supporting evidence for this hypothesis in the cumulative size distributions of jfcs and dormant comets, as well as in recent numerical studies of cometary orbital dynamics. we added 143p to the sample of 13 other jfcs with known albedos and phase-function slopes. this sample shows a possible correlation of increasing phase-function slopes for larger geometric albedos. partly based on findings from recent space missions to jfcs, we hypothesize that this correlation corresponds to an evolutionary trend for jfcs. we propose that newly activated jfcs have larger albedos and steeper phase functions, which gradually decrease due to sublimation-driven erosion. if confirmed, this could be used to analyse surface erosion from ground and to distinguish between dormant comets and asteroids. | implications of the small spin changes measured for large jupiter-family comet nuclei |
recently acquired bathymetric and high-resolution seismic data from the upper slope of santos basin, southern brazilian margin, reveal a major geomorphological feature in the sw atlantic that is interpreted as a carbonate ridge - the alpha crucis carbonate ridge (accr). the accr is the first megastructure of this type described on the sw atlantic margin. the 17 × 11-km-wide ring-shaped accr features tens of >100-m-high steep-sided carbonate mounds protruding from the surrounding seabed and flanked by elongated depressions. comet-like marks downstream of the mound structures indicate that the area is presently influenced by the northward flow of the intermediate western boundary current (iwbc), a branch of the subtropical gyre that transports antarctic intermediate water. abundant carbonate sands and gravels cover the mounds and are overlain by a biologically significant community of living and dead ramified corals and associated invertebrates. the iwbc acts as a hydrodynamic factor that is responsible for both shaping the bottom and transporting coral larvae. we contend that the accr was formed by upward fluid flow along active sub-surface faults and fractures that formed by lateral extension generated by the ascending movement of salt diapirs at depth. the accr provides an important modern and accessible analogue for a seabed carbonate build-up related to sub-surface hydrocarbon systems. | the alpha crucis carbonate ridge (accr): discovery of a giant ring-shaped carbonate complex on the sw atlantic margin |
the rosetta orbiter witnessed several hundred diamagnetic cavity crossings (unmagnetized regions) around comet 67p/churyumov-gerasimenko during its two year survey of the comet. the characteristics of the plasma environment inside these diamagnetic regions are studied using in situ measurements by the rosetta plasma consortium instruments. although the unmagnetized plasma density has been observed to exhibit little dynamics compared to the very dynamical magnetized cometary plasma, we detected several localized dynamic plasma structures inside those diamagnetic regions. these plasma structures are not related to the direct ionization of local cometary neutrals. the structures are found to be steepened, asymmetric plasma enhancements with typical rising-to-descending slope ratio of ∼2.8 (±1.9), skewness ∼0.43 (±0.36), mean duration of ∼2.7 (±0.9) min and relative density variation δn/n of ∼0.5 (±0.2), observed close to the electron exobase. similar steepened plasma density enhancements were detected at the magnetized boundaries of the diamagnetic cavity as well as outside the diamagnetic region. the plausible scalelength and propagation direction of the structures are estimated from simple plasma dynamics considerations. it is suggested that they are large-scale unmagnetized plasma enhancements, transmitted from the very dynamical outer magnetized region to the inner magnetic field-free cavity region. | dynamic unmagnetized plasma in the diamagnetic cavity around comet 67p/churyumov-gerasimenko |
kepler's observation of comet tails initiated the research on the radiation pressure of celestial objects and 250 years later they found new incarnation after the maxwell's equations were formulated to describe a plethora of light-matter coupling phenomena. further, quantum mechanics gave birth to the photon drag effect. here, we develop a microscopic theory of this effect which can occur in a general system containing bose-einstein-condensed particles, which possess an internal structure of quantum states. by analyzing the response of the system to an external electromagnetic field we find that such a drag results in a flux of particles constituting both the condensate and the excited states. we show that in the presence of the condensed phase, the response of the system acquires steplike behavior as a function of the electromagnetic field frequency with the elementary step determined by the internal energy structure of the particles. | photon drag of a bose-einstein condensate |
we provide observational evidence for the existence of large-scale cylindrical (or conic-like) current sheets (ccss) at high heliolatitudes. long-lived ccss were detected by ulysses during its passages over the south solar pole in 1994 and 2007. the characteristic scale of these tornado-like structures is several times less than a typical width of coronal holes within which the ccss are observed. ccs crossings are characterized by a dramatic decrease in the solar wind speed and plasma beta typical for predicted profiles of ccss. ulysses crossed the same ccs at different heliolatitudes at 2-3 au several times in 1994, as the ccs was declined from the rotation axis and corotated with the sun. in 2007, a ccs was detected directly over the south pole, and its structure was strongly highlighted by the interaction with comet mcnaught. restorations of solar coronal magnetic field lines reveal the occurrence of conic-like magnetic separators over the solar poles in both 1994 and 2007. such separators exist only during solar minima. interplanetary scintillation data analysis confirms the presence of long-lived low-speed regions surrounded by the typical polar high-speed solar wind in solar minima. energetic particle flux enhancements up to several mev/nuc are observed at edges of the ccss. we built simple mhd models of a ccs to illustrate its key features. the ccss may be formed as a result of nonaxiality of the solar rotation axis and magnetic axis, as predicted by the fisk-parker hybrid heliospheric magnetic field model in the modification of burger and coworkers. | high-latitude conic current sheets in the solar wind |
the landing of philae, the lander of esa's rosetta-mission, on november 12th 2014 on comet 67p/churyumov-gerasimenko, was planned as a descent with passive landing activating a damper system and anchoring by harpoons at touch-down. the lander was not fixed to the ground at touch-down due to failing harpoons. the lander damper, however, was actuated for a length of 42.6 mm with a maximal speed of 0.08 m/s, while the lander speed was 1 m/s. based on the damper data and a detailed mechanical model of philae, an estimate can be made for the forces acting and the energy dissipated at touch-down inside the lander and the energy dissipated by ground penetration. the forces acting at ground penetration provide constraints on the mechanical strength of the soil. two different soil models are investigated. assuming constant compressive strength σ, one obtains σ ≈ 2 kpa. assuming an increasing σs strength with penetration depth with results in σs = 3 kpa/m fits the damper data best. | rosetta lander philae - soil strength analysis |
we present the first accurate rate coefficients for the rotational excitation of co by h2o in the kinetic temperature range 5-100 k. the statistical adiabatic channel method (sacm) is combined with a high-level rigid-rotor co-h2o intermolecular potential energy surface. transitions among the first 11 rotational levels of co and the first 8 rotational levels of both para-h2o and ortho-h2o are considered. our rate coefficients are compared to previous data from the literature and they are also incorporated in a simple non-lte model of cometary coma including collision-induced transitions, solar radiative pumping and radiative decay. we find that the uncertainties in the collision data have significant influence on the co population distribution for h2o densities in the range 103-108 cm-3. we also show that the rotational distribution of h2o plays an important role in co excitation (owing to correlated energy transfer in both co and h2o), while the impact of the ortho-to-para ratio of h2o is found to be negligible. | the effect of co-h2o collisions in the rotational excitation of cometary co |
we present the first ultraviolet spectra of jupiter trojans. these observations were carried out using the space telescope imaging spectrograph on the hubble space telescope and cover the wavelength range 200-550 nm at low resolution. the targets include objects from both of the trojan color subpopulations (less-red and red). we do not observe any discernible absorption features in these spectra. comparisons of the averaged uv spectra of less-red and red targets show that the subpopulations are spectrally distinct in the uv. less-red objects display a steep uv slope and a rollover at around 450 nm to a shallower visible slope, whereas red objects show the opposite trend. laboratory spectra of irradiated ices with and without h2s exhibit distinct uv absorption features; consequently, the featureless spectra observed here suggest h2s alone is not responsible for the observed color bimodality of trojans, as has been previously hypothesized. we propose some possible explanations for the observed uv-visible spectra, including complex organics, space weathering of iron-bearing silicates, and masked features due to previous cometary activity. | hubble ultraviolet spectroscopy of jupiter trojans |
the origin of planetary rings is one of the least understood processes related to planet formation and evolution. whereas rings seem ubiquitous around giant planets, their great diversity of mass, structure, and composition is a challenge for any formation scenario. satellite destruction by cometary impacts and meteoroid bombardment seem to be key processes leading to the very low-mass rings of uranus, neptune, and jupiter. by contrast, moon destruction is unlikely to have produced saturn's much more massive rings recently, so they still represent a strong challenge for astronomers. recent advances in our understanding of ring and satellite formation and destruction suggest that these processes are closely interconnected, so that rings and satellites may be two aspects of the same geological system. indeed, rings may not be only beautiful planetary ornaments, but, possibly, an essential step in the process of satellite formation, at least for the small and mid-sized moons. these recent advances have taken advantage of the many tantalizing results from the cassini mission, as well as advances in numerical simulation techniques. however, no single theory seems able to explain the origin of the different planetary rings known in our solar system, and it now seems evident that rings may result from a variety of processes like giant collisions, tidal stripping of comets or satellites, as well as planet formation itself. understanding rings appears to be an important step toward understanding the origin and evolution of planetary environments. most work on the origin of rings has been devoted to saturn, and somewhat less to the rings of jupiter, uranus, and neptune. so our chapter will be mainly focused on the case of saturn. however, processes that are common to all rings or particularly to those of saturn will be clearly delineated. in order to build any theory of ring formation it is important to specify physical processes that affect the long-term evolution of rings, as well as to describe the different observations that any ring formation model should explain. this is the topic of section 18.2. in section 18.3, we focus our attention on saturn's rings and their main properties, and then discuss the pros and cons of a series of ring formation models. we also discuss the link between rings and satellites. in section 18.4, we extend the discussion to the other giant planets (jupiter, uranus, and neptune). | the origin of planetary ring systems |
we report the discovery of a new small magellanic cloud pulsar wind nebula (pwn) at the edge of the supernova remnant (snr) dem s5. the pulsar powered object has a cometary morphology similar to the galactic pwn analogues psr b1951+32 and `the mouse'. it is travelling supersonically through the interstellar medium. we estimate the pulsar kick velocity to be in the range of 700-2000 km s-1 for an age between 28 and 10 kyr. the radio spectral index for this snr-pwn-pulsar system is flat (-0.29 ± 0.01) consistent with other similar objects. we infer that the putative pulsar has a radio spectral index of -1.8, which is typical for galactic pulsars. we searched for dispersion measures up to 1000 cm-3 pc but found no convincing candidates with an s/n greater than 8. we produce a polarization map for this pwn at 5500 mhz and find a mean fractional polarization of p ∼ 23 per cent. the x-ray power-law spectrum (γ ∼ 2) is indicative of non-thermal synchrotron emission as is expected from pwn-pulsar system. finally, we detect dem s5 in infrared (ir) bands. our ir photometric measurements strongly indicate the presence of shocked gas that is expected for snrs. however, it is unusual to detect such ir emission in an snr with a supersonic bow shock pwn. we also find a low-velocity h i cloud of ∼107 km s-1 that is possibly interacting with dem s5. snr dem s5 is the first confirmed detection of a pulsar-powered bow shock nebula found outside the galaxy. | discovery of a pulsar-powered bow shock nebula in the small magellanic cloud supernova remnant dem s5 |
here, we explore some peculiar orbital features of the recently discovered asteroid a/2017 u1, which is a clear outlier when considering the average value of the eccentricity of known hyperbolic comets. as for the orientation of its orbit in space, the orbital plane of a/2017 u1 seems to be away from any obvious clusters present for this population. the orbital nodes of a/2017 u1 are well away from the paths of the planets of the solar system and the sun. all these orbital properties appear to confirm a/2017 u1 as the first known interstellar asteroid. | pole, pericenter, and nodes of the interstellar minor body a/2017 u1 |
context. results from previous searches for new meteor showers in the combined croatian meteor network and sonotaco meteor databases suggested possible parent bodies for several newly identified showers.aims: we aim to perform an analysis to validate the connection between the identified showers and candidate parent bodies.methods: simulated particles were ejected from candidate parent bodies, a dynamical modeling was performed and the results were compared to the real meteor shower observations.results: from the 13 analysed cases, three were found to be connected with comets, four with asteroids which are possibly dormant comets, four were inconclusive or negative, and two need more observational data before any conclusions can be drawn. | dynamical modeling validation of parent bodies associated with newly discovered cmn meteor showers |
the nature of 1i/’oumuamua (henceforth, 1i), the first interstellar object (iso) known to pass through the solar system, remains mysterious. feng & jones noted that the incoming 1i velocity vector “at infinity” ({v}∞ ) is close to the motion of the pleiades dynamical stream (or local association), and suggested that 1i is a young object ejected from a star in that stream. micheli et al. subsequently detected nongravitational acceleration in the 1i trajectory; this acceleration would not be unusual in an active comet, but 1i observations failed to reveal any signs of activity. bialy & loeb hypothesized that the anomalous 1i acceleration was instead due to radiation pressure, which would require an extremely low mass-to-area ratio (or area density). here i show that a low area density can also explain the very close kinematic association of 1i and the pleiades stream, as it renders 1i subject to drag capture by interstellar gas clouds. this supports the radiation pressure hypothesis and suggests that there is a significant population of low area density isos in the galaxy, leading, through gas drag, to enhanced iso concentrations in the galactic dynamical streams. any iso entrained in a dynamical stream will have a predictable incoming {{\boldsymbol{v}}}∞ ; targeted deep surveys using this information should be able to find dynamical stream objects from months to as much as a year before their perihelion, providing the lead time needed for fast-response missions for the future in situ exploration of such objects. | high-drag interstellar objects and galactic dynamical streams |
we critically examine what hyperactivity on a comet entails, fully develop the a'hearn model for hyperactivity based on the analyses of data collected for the deep impact encounter of comet 103p/hartley 2, describe manifestations of hyperactivity suggested on many, if not all, comets, and give implications of hyperactivity for future cometary exploration. the a'hearn model requires a highly volatile ice reservoir within a comet to undergo sublimation, escape the nucleus, and drive out less volatile ices along its path to the surface. once in the coma, the less volatile ice eventually sublimates, creating a secondary source of that gas in the coma, which is generally displaced anti-sunward and not distributed symmetrically about the nucleus. the secondary source of gas increases the total production of the less volatile species in the coma, sometimes well above that expected if the total surface was undergoing sublimation. we argue that based on the simple assumptions of the a'hearn model and the fact that several comets display one or more of the characteristics of hyperactivity detailed here, it is probable that nearly all comets experience some degree of hyperactivity. of significance, the ice that is brought from deep within the nucleus into the coma via the process described by the a'hearn model is the least thermally altered and is thus the most pristine ice in the comet. therefore, it behooves future mission teams to consider cryogenically sampling coma ice, rather than or in addition to attempting a direct nucleus sample, for a better understanding of the unaltered ices and conditions present in the protoplanetary disk. | all comets are somewhat hyperactive and the implications thereof |
we present visible spectroscopic and albedo data of the 2.3 gyr old themis family and the <10 myr old beagle sub-family. the slope and albedo variations between these two families indicate c-complex asteroids become redder and darker in response to space weathering. our observations of themis family members confirm previously observed trends where phyllosilicate absorption features are less common among small diameter objects. similar trends in the albedos of large (>15 km) and small (⩽15 km) themis members suggest these phyllosilicate feature and albedo trends result from regolith variations as a function of diameter. observations of the beagle asteroids show a small, but notable fraction of members with phyllosilicate features. the presence of phyllosilicates and the dynamical association of the main-belt comet 133p/elst-pizarro with the beagle family imply the beagle parent body was a heterogenous mixture of ice and aqueously altered minerals. | space weathering trends among carbonaceous asteroids |
methane is the second most important greenhouse gas in terms of anthropogenic radiative forcing. since pre-industrial times, the globally averaged dry mole fraction of methane in the atmosphere has increased considerably. emissions from coal mining are one of the primary anthropogenic methane sources. however, our knowledge about different sources and sinks of methane is still subject to great uncertainties. comprehensive measurement campaigns and reliable chemistry-climate models, are required to fully understand the global methane budget and to further develop future climate mitigation strategies. the comet 1.0 campaign (may to june 2018) combined airborne in situ, as well as passive and active remote sensing measurements to quantify the emissions from coal mining in the upper silesian coal basin (uscb, poland). roughly 502 kt of methane is emitted from the ventilation shafts per year. in order to help with the flight planning during the campaigns, we performed 6 d forecasts using the online coupled, three-time nested global and regional chemistry-climate model meco(n). we applied three-nested cosmo/messy instances going down to a spatial resolution of 2.8 km over the uscb. the nested global-regional model system allows for the separation of local emission contributions from fluctuations in the background methane. here, we introduce the forecast set-up and assess the impact of the model's spatial resolution on the simulation of methane plumes from the ventilation shafts. uncertainties in simulated methane mixing ratios are estimated by comparing different airborne measurements to the simulations. results show that meco(3) is able to simulate the observed methane plumes and the large-scale patterns (including vertically integrated values) reasonably well. furthermore, we obtain reasonable forecast results up to forecast day four. | hindcasting and forecasting of regional methane from coal mine emissions in the upper silesian coal basin using the online nested global regional chemistry-climate model meco(n) (messy v2.53) |
we present the code catalogue, the new cometary catalogue containing data for almost 300 long-period comets that were discovered before 2018. this is the first catalogue containing cometary orbits in the five stages of their dynamical evolution and covering three successive passages through the perihelion, with the exception of the hyperbolic comets which are treated in a different manner. non-gravitational orbits are given for about 100 of these long-period comets, and their orbits obtained while neglecting the existence of non-gravitational acceleration are included for comparison. for many of the presented comets, different orbital solutions, based on the alternative force models or various subsets of positional data, are also provided. the preferred orbit is always clearly indicated for each comet. this catalogue (the code catalogue) is publicly available at https://pad2.astro.amu.edu.pl/comets | the catalogue of cometary orbits and their dynamical evolution |
debris disks are the dust disks found around ∼20% of nearby main sequence stars in far-ir surveys. they can be considered as descendants of protoplanetary disks or components of planetary systems, providing valuable information on circumstellar disk evolution and the outcome of planet formation. the debris disk population can be explained by the steady collisional erosion of planetesimal belts; population models constrain where (10-100 au) and in what quantity (>1m ⊕) planetesimals (>10 km in size) typically form in protoplanetary disks. gas is now seen long into the debris disk phase. some of these are secondary implying planetesimals have a solar system comet-like composition, but some systems may retain primordial gas. ongoing planet formation processes are invoked for some debris disks, such as the continued growth of dwarf planets in an unstirred disk or the growth of terrestrial planets through giant impacts. planets imprint structure on debris disks in many ways; images of gaps, clumps, warps, eccentricities and other disk asymmetries are readily explained by planets at ≫5 au. hot dust in the region planets are commonly found (<5 au) is seen for a growing number of stars. this dust usually originates in an outer belt (e.g., from exocomets), although an asteroid belt or recent collision is sometimes inferred. | debris disks: probing planet formation |
we present the experimental phase functions of three types of millimeter-sized dust grains consisting of enstatite, quartz, and volcanic material from mount etna, respectively. the three grains present similar sizes but different absorbing properties. the measurements are performed at 527 nm covering the scattering angle range from 3° to 170°. the measured phase functions show two well-defined regions: (i) soft forward peaks and (ii) a continuous increase with the scattering angle at side- and back-scattering regions. this behavior at side- and back-scattering regions is in agreement with the observed phase functions of the fomalhaut and hr 4796a dust rings. further computations and measurements (including polarization) for millimeter-sized grains are needed to draw some conclusions about the fluffy or compact structure of the dust grains. | experimental phase functions of millimeter-sized cosmic dust grains |
the near-earth object wide-field infrared survey explorer (neowise) spacecraft has been conducting a two-band thermal infrared survey to detect and characterize asteroids and comets since its reactivation in 2013 december. using the observations collected during the fourth and fifth years of the survey, our automated pipeline detected candidate moving objects that were verified and reported to the minor planet center. using these detections, we perform thermal modeling of each object from the near-earth object (neo) and main belt asteroid (mba) populations to constrain their sizes. we present thermal model fits of asteroid diameters for 189 neos and 5831 mbas detected during the fourth year of the survey, and 185 neos and 5776 mbas from the fifth year. to date, the neowise reactivation survey has provided thermal model characterization for 957 unique neos. including all phases of the original wide-field infrared survey explorer survey brings the total to 1473 unique neos that have been characterized between 2010 and the present. | asteroid diameters and albedos from neowise reactivation mission years 4 and 5 |
aims: we present a multi-instrumental, multidecadal analysis of the activity of the eta-aquariid and orionid meteor showers for the purpose of constraining models of 1p/halley's meteoroid streams.methods: the interannual variability of the showers' peak activity and period of duration is investigated through the compilation of published visual and radar observations prior to 1985 and more recent measurements reported in the international meteor organization (imo) visual meteor database, by the imo video meteor network and by the canadian meteor orbit radar (cmor). these techniques probe the range of meteoroid masses from submilligrams to grams. the η-aquariids and orionids activity duration, shape, maximum zenithal hourly rates values, and the solar longitude of annual peaks since 1985 are analyzed. when available, annual activity profiles recorded by each detection network were measured and are compared.results: observations from the three detection methods show generally good agreement in the showers' shape, activity levels, and annual intensity variations. both showers display several activity peaks of variable location and strength with time. the η-aquariids are usually two to three times stronger than the orionids, but the two showers display occasional outbursts with peaks two to four times their usual activity level. cmor observations since 2002 seem to support the existence of an ~12 yr cycle in orionids activity variations; however, additional and longer term radar and optical observations of the shower are required to confirm such periodicity. | activity of the eta-aquariid and orionid meteor showers |
we apply the analytical disturbing function for arbitrary inclination derived in our previous work to characterize resonant width and libration of mean motion resonances at arbitrary inclination obtained from direct numerical simulations of the three-body problem. we examine the 2:1 and 3:1 inner jupiter and 1:2 and 1:3 outer neptune resonances and their possible asymmetric librations using a new analytical pendulum model of resonance that includes the simultaneous libration of multiple arguments and their second harmonics. the numerically derived resonance separatrices are obtained using the mean exponential growth factor of nearby orbits (megno chaos indicator). we find that the analytical and numerical estimates are in agreement and that resonance width is determined by the first few fundamental resonance modes that librate simultaneously on the resonant time-scale. our results demonstrate that the new pendulum model may be used to ascertain resonance width analytically, and more generally, that the disturbing function for arbitrary inclination is a powerful analytical tool that describes resonance dynamics of low as well as high inclination asteroids in the solar system. | resonance libration and width at arbitrary inclination |
aims: we investigated wosret, a region located on the small lobe of the 67p/churyumov-gerasimenko comet subject to strong heating during the perihelion passage. this region includes the two last landing sites of the philae lander as well as, notably the final one, abydos, where the lander performed most of its measurements. we study wosret in order to constrain its compositional properties and its surface evolution. by comparing them with those of other regions, we aim to identify possible differences among the two lobes of the comet.methods: we analyzed high-resolution images of the wosret region acquired between 2015 and 2016 by the optical, spectroscopic, and infrared remote imaging system (osiris) on board the rosetta spacecraft, at a resolution ranging from 2 to 10 m px−1 before and close to perihelion, up to 0.07-0.2 m px−1 in the post-perihelion images. the osiris images were processed with the osiris standard pipeline, then converted into i∕f radiance factors and corrected for the viewing and illumination conditions at each pixel using the lommel-seeliger disk function. spectral slopes were computed in the 535-882 nm range.results: we observed a few morphological changes in wosret, related to local dust coating removal with an estimated depth of ~1 m, along with the formation of a cavity measuring 30 m in length and 6.5 m in depth, for a total estimated mass loss of 1.2 × 106 kg. the spectrophotometry of the region is typical of medium-red regions of comet 67p, with spectral slope values of 15-16%/(100 nm) in pre-perihelion data acquired at phase angle 60°. as observed globally for the comet, also wosret shows spectral slope variations during the orbit linked to the seasonal cycle of water, with colors getting relatively bluer at perihelion. wosret has a spectral phase reddening of 0.0546 × 10−4 nm−1 deg−1, which is about a factor of 2 lower than what was determined for the nucleus northern hemisphere regions, possibly indicating a reduced surface micro-roughness due to the lack of widespread dust coating. a few tiny bright spots are observed and we estimate a local water-ice enrichment up to 60% in one of them. morphological features such as "goosebumps" or clods are widely present and larger in size than similar features located in the big lobe.conclusions: compared to anhur and khonsu, two southern hemisphere regions in the big lobe which have been observed under similar conditions and also exposed to high insolation during perihelion, wosret exhibits fewer exposed volatiles and less morphological variations due to activity events. considering that the high erosion rate in wosret unveils part of the inner layers of the small lobe, our analysis indicates that the small lobe has different physical and mechanical properties than the big one and a lower volatile content, at least in its uppermost layers. these results support the hypothesis that comet 67p originated from the merging of two distinct bodies in the early solar system. | small lobe of comet 67p: characterization of the wosret region with rosetta-osiris |
the cosmic dust particles found in space are mainly porous aggregates of smaller grains. theoretically, these aggregates are replicated using fractal geometry, assuming a cluster of spheres. although the light scattering response of cosmic dust aggregates has been thoroughly studied using clusters of spherical grains in the past few decades, the effect of irregularities on the surface of each grain in an entire aggregate has mostly been neglected. we introduce, for the first time, a visually realistic cosmic dust model that incorporates a mixture of rough fractal aggregates (rfa) and agglomerated debris (solids) to replicate the unusual polarization-phase curve observed in the case of the interstellar comet 2i/borisov at multiple wavelengths. the authenticity of the rfa structures has been verified by replicating light scattering results of circumstellar dust analogs from the granada amsterdam light scattering database. we demonstrate that the light scattering response from the rfa structures has a very close resemblance to the experimental values. finally, we model the observed polarization-phase curve of the interstellar comet 2i/borisov using a mixture of rfa and solid particles. the best-fit data indicate the presence of a higher percentage of porous rfa structures (80%) owing to the fact that the comet carries a higher percentage of small and highly porous pristine cosmic dust particles. further, the model indicates that the unusually steep polarimetric slope and the high dust-to-gas ratio in newer comets are mainly due to a higher porous-to-compact ratio. | a comprehensive model of morphologically realistic cosmic dust particles: an application to mimic the unusual polarization properties of the interstellar comet 2i/borisov |
this paper introduces an algorithm to perform optimal reorientation of a spacecraft during a high speed flyby mission that maximizes the time a certain target is kept within the field of view of scientific instruments. the method directly handles the nonlinear dynamics of the spacecraft, sun exclusion constraint, torque and momentum limits on the reaction wheels as well as potential faults in these actuators. a sequential convex programming approach was used to reformulate non-convex pointing objectives and other constraints in terms of a series of novel convex cardinality minimization problems. these subproblems were then efficiently solved even on limited hardware resources using convex programming solvers implementing second-order conic constraints. the proposed method was applied to a scenario that involved maximizing the science time for the upcoming comet interceptor flyby mission developed by the european space agency. extensive simulation results demonstrate the capability of the approach to generate viable trajectories even in the presence of reaction wheel failures or prior dust particle impacts. | optimal science-time reorientation policy for the comet interceptor flyby via sequential convex programming |
we study the long-term radial transport of micron to millimeter-size grains in protostellar disks (psds) based on diffusion and viscosity coefficients measured from 3d global stratified-disk simulations with a lagrangian hydrodynamic method. while gas drag tends to transport dust species radially inwards, stochastic diffusion can spread a considerable fraction of dust radially outwards (upstream) depending on the nature of turbulence. in gravitationally unstable disks, we measure a high radial diffusion coefficient d r ~ h 2ω with little dependence on altitude. this leads to strong and vertically homogeneous upstream diffusion in early psds. in the solar nebula, the robust upstream diffusion of micron to millimeter-size grains not only efficiently transports highly refractory micron-size grains (such as those identified in the samples of comet 81p/wild 2) from their regions of formation inside the snow line out to the kuiper belt, but can also spread millimeter-size calcium-aluminum-rich inclusions formed close to the sun to distances where they can be assimilated into chondritic meteorites. in disks dominated by magnetorotational instability, the upstream diffusion effect is generally milder, with a separating feature due to diffusion being stronger in the surface layer than in the midplane. this variation becomes much more pronounced if we additionally consider a quiescent midplane with lower turbulence and larger characteristic dust size due to nonideal mhd effects. this segregation scenario helps to account for the dichotomy of the spatial distribution of two dust populations as observed in scattered light and atacama large millimeter/submillimeter array images. | turbulent transport of dust particles in protostellar disks: the effect of upstream diffusion |
context. a cornucopia of rosetta and ground-based observational data sheds light on the evolution of the characteristics of dust particles from comet 67p/churyumov-gerasimenko (hereafter 67p) with seasons, implying the different dust environments in the source regions on the surface of the comet.aims: we aim to constrain the properties of the dust particles of 67p and therefrom diagnose the dust environment of its coma and near-surface layer at around the end of the southern summer of the comet.methods: we performed spectropolarimetric observations for 67p dust over 4000-9000 å using the eso/very large telescope from january-march 2016 (phase angle ranging from ~26° -5°). we examined the optical behaviours of the dust, which, together with rosetta colour data, were used to search for dust evolution with cometocentric distance. modelling was also conducted to identify the dust attributes compatible with the results.results: the spectral dependence of the polarisation degree of 67p dust is flatter than that found in other dynamical groups of comets in similar observing geometry. the depth of its negative polarisation branch appears to be a bit shallower than in long-period comets and might be getting shallower as 67p repeats its apparitions. its dust colour shows a change in slope around 5500 å, (17.3 ± 1.4) and (10.9 ± 0.6)% (1000 å)-1, shortward and longward of the wavelength, respectively, which are slightly redder but broadly consistent with the average of jupiter-family comets.conclusions: observations of 67p dust in this study can be attributed to dust agglomerates of ~100 μm in size detected by rosetta in early 2016. a porosity of 60% shows the best match with our polarimetric results, yielding a dust density of ~770 kg m-3. the compilation of rosetta and our data indicates the dust's reddening with increasing nucleus distance, which may be driven by water-ice sublimation as the dust moves out of the nucleus. we estimate the possible volume fraction of water ice in the initially ejected dust as ~6% (i.e. the refractory-to-ice volume ratio of ~14). | vlt spectropolarimetry of comet 67p: dust environment around the end of its intense southern summer |
in this work, we introduce a comet dust model that incorporates multiple dust morphologies along with inhomogeneous mixture of silicate minerals and carbonaceous materials under power-law size distribution, to replicate the standard polarization-phase curve observed in several comets in the narrow-band continuum. following the results from rosetta/midas and cosima, we create high porosity hierarchical aggregates (ha) and low porosity (<10 per cent) solids in the form of agglomerated debris. we also introduce a moderate porosity structure with solids in the core, surrounded by fluffy aggregates called fluffy solids (fs). we study the mixing combinations, (ha and solids), (ha and fs), and (ha, fs, and solids) for a range of power-law index n= 2.0 to 3.0 for different sets of mixing percentage of silicate minerals and carbonaceous materials. polarimetry of the short period comets 1p/halley and 67p/churyumov-gerasimenko match best with the polarization resulting from the combination of ha and solids while the combinations (ha and fs) and (ha, fs, and solids) provide the best-fitting results for the long period comets c/1995 o1 (hale-bopp) and c/1996 b2 (hyakutake). the best-fitting model results also recreate the observed wavelength dependence of polarization. our dust model agree with the idea that the long period comets may have high percentage of loose particles (ha and fs) compared to those in the case of short period comets as the short period comets experience more frequent and/or higher magnitude of weathering. | modelling heterogeneous dust particles: an application to cometary polarization |
we announce the discovery of activity, in the form of a distinct cometary tail, emerging from main-belt asteroid 2015 va108. activity was first identified by volunteers of the citizen science project active asteroids (a nasa partner). we uncovered one additional image from the same observing run which also unambiguously shows 2015 va108 with a tail oriented between the anti-solar and anti-motion vectors that are often correlated with activity orientation on sky. both publicly available archival images were originally acquired ut 2015 october 11 with the dark energy camera (decam) on the blanco 4 m telescope at the cerro tololo inter-american observatory (chile) as part of the dark energy camera legacy survey. activity occurred near perihelion and, combined with its residence in the main asteroid belt, 2015 va108 is a candidate main-belt comet, an active asteroid subset known for volatile sublimation. | new active asteroid 2015 va108: a citizen science discovery |
interstellar objects (isos) are fascinating and under-explored celestial objects, providing physical laboratories to understand the formation of our solar system and probe the composition and properties of material formed in exoplanetary systems. this paper will discuss the accessibility of and mission design to isos with varying characteristics, including a discussion of state covariance estimation over the course of a cruise, handoffs from traditional navigation approaches to novel autonomous navigation for fast flyby regimes, and overall recommendations about preparing for the future in situ exploration of these targets. the lessons learned also apply to the fast flyby of other small bodies including long-period comets and potentially hazardous asteroids, which also require a tactical response with similar characteristics | interstellar object accessibility and mission design |
formamide is a potentially important molecule in the context of pre-biotic chemistry, since reactions involving it can lead to precursors of genetic and metabolic molecules. being abundant in cometary material and in star-forming regions, the formation and destruction routes of interstellar formamide have been the focus of several studies. in this work, we focus on the electron recombination of protonated formamide, an important step of its destruction routes, by performing rigorous ab initio calculations of this process. we found that our values are in good agreement with previous qualitative estimates of the global rate coefficients. on the contrary, we propose a substantial revision of the products and branching ratios. finally, we justify and emphasize the importance of carrying out similar theoretical calculations on the largest possible number of complex species of astrochemical interest. | dissociative electron recombination of nh2choh+ and implications for interstellar formamide abundance |
we report that a major outburst of comet 29p/schwassmann-wachmann 1 was detected by using the images acquired with the slt (16" ritchey-chretien telescope) at lulin observatory. | another major outburst of comet 29p/schwassmann-wachmann 1 |
the paper considers the free molecular flow of gas through the dusty porous surface layer of a comet nucleus. the study is based on computer models of generation of a porous medium and knudsen gas diffusion. we consider various types of homogeneous and heterogeneous layers constructed from nonintersecting spheres, including layers that contain microcracks or inner cavities. using the test-particle method, we quantitatively estimate the free path distribution function, layer permeability, and effective kinetic characteristics of sublimation products passing through a nonisothermal porous layer. in addition, in this approach, we consider the volumetric absorption of visible solar radiation in the near-surface absorbing layer. simple approximation expressions are obtained for all the transport characteristics under study, which makes it possible to estimate the characteristics with sufficient accuracy for practical applications in the physics of comets. the results will be used to construct new consistent models of energy transfer in the near-surface layer of a comet nucleus and, first of all, to analyze the results of the observations of comet 67p/churyumov−gerasimenko. | transport characteristics of the near-surface layer of the nucleus of comet 67p/churyumov-gerasimenko |
consideration is given to the estimates for the strength of the consolidated material forming the bulk of the nucleus of comet 67p churyumov-gerasimenko and those for the strength of the surface material overlying the consolidated material at the sites of the first and last contact of the philae lander with the nucleus. the strength of the consolidated material was estimated by analyzing the terrain characteristics of the steep cliffs, where the material is exposed on the surface. based on these estimates, the tensile strength of the material is in the range from 1.5 to 100 pa; the shear strength, from ∼13 to ⩾30 pa; and the compressive strength, from 30 to 150 pa, possibly up to 1.5 kpa. these are very low strength values. given the dependence of the measurement results on the size of the measured object, they are similar to those of fresh dry snow at -10°c. the (compressive) strength of the surface material at the site of the first touchdown of philae on the nucleus is estimated from the measurements of the dynamics of the surface impact by the spacecraft's legs and the geometry of the impact pits as 1-3 kpa. for comparison with the measurement results for ice-containing materials in terrestrial laboratories, it needs to be taken into account that the rate of deformation by philae's legs is four orders of magnitude higher than that in typical terrestrial measurements, leading to a possible overestimation of the strength by roughly an order of magnitude. there was an attemp to put one of the mupus sensors into the surface material at the site of the last contact of philae with the nucleus. noticeable penetration of the tester probe was not achieved that led to estimation of the minimum compressive strength of the material to be ⩾4 mpa4 this fairly high strength appears to indicate the presence of highly porous ice with grains "frozen" at contacts. | estimating the strength of the nucleus material of comet 67p churyumov-gerasimenko |
astrophysical shocks are commonly revealed by the non-thermal emission of energetic electrons accelerated in situ1-3. strong shocks are expected to accelerate particles to very high energies4-6; however, they require a source of particles with velocities fast enough to permit multiple shock crossings. while the resulting diffusive shock acceleration4 process can account for observations, the kinetic physics regulating the continuous injection of non-thermal particles is not well understood. indeed, this injection problem is particularly acute for electrons, which rely on high-frequency plasma fluctuations to raise them above the thermal pool7,8. here we show, using laboratory laser-produced shock experiments, that, in the presence of a strong magnetic field, significant electron pre-heating is achieved. we demonstrate that the key mechanism in producing these energetic electrons is through the generation of lower-hybrid turbulence via shock-reflected ions. our experimental results are analogous to many astrophysical systems, including the interaction of a comet with the solar wind9, a setting where electron acceleration via lower-hybrid waves is possible. | electron acceleration by wave turbulence in a magnetized plasma |
aims: we derive the size-frequency distribution of boulders on comet 103p/hartley 2, which are computed from the images taken by the deep impact/hri-v imaging system. we indicate the possible physical processes that lead to these boulder size distributions.methods: we used images acquired by the high resolution imager-visible ccd camera on 4 november 2010. boulders ≥10 m were identified and manually extracted from the datasets with the software arcgis. we derived the global size-frequency distribution of the illuminated side of the comet (~50%) and identified the power-law indexes characterizing the two lobes of 103p. the three-pixel sampling detection, together with the shadowing of the surface, enables unequivocally detection of boulders scattered all over the illuminated surface.results: we identify 332 boulders ≥10 m on the imaged surface of the comet, with a global number density of nearly 140/km2 and a cumulative size-frequency distribution represented by a power law with index of -2.7 ± 0.2. the two lobes of 103p show similar indexes, i.e., -2.7 ± 0.2 for the bigger lobe (called l1) and -2.6+ 0.2/-0.5 for the smaller lobe (called l2). the similar power-law indexes and similar maximum boulder sizes derived for the two lobes both point toward a similar fracturing/disintegration phenomena of the boulders as well as similar lifting processes that may occur in l1 and l2. the difference in the number of boulders per km2 between l1 and l2 suggests that the more diffuse h2o sublimation on l1 produce twice the boulders per km2 with respect to those produced on l2 (primary activity co2 driven). the 103p comet has a lower global power-law index (-2.7 vs. -3.6) with respect to 67p. the global differences between the two comets' activities, coupled with a completely different surface geomorphology, make 103p hardly comparable to 67p. a shape distribution analysis of boulders ≥30 m performed on 103p suggests that the cometary boulders show more elongated shapes when compared to collisional laboratory fragments as well as to the boulders present on the surfaces of 25 143 itokawa and 433 eros asteroids. consequently, this supports the interpretation that cometary boulders have different origins with respect to the impact-related asteroidal boulders. | size-frequency distribution of boulders ≥10 m on comet 103p/hartley 2 |
the structure and composition of cometary constituents, down to their microscopic scale, are critical witnesses of the processes and ingredients that drove the formation and evolution of planetary bodies toward their present diversity. on board rosetta’s lander philae, the comet infrared and visible analyser (civa) experiment took a series of images to characterize the surface materials surrounding the lander on comet 67p/churyumov-gerasimenko. images were collected twice: just after touchdown, and after philae finally came to rest, where it acquired a full panorama. these images reveal a fractured surface with complex structure and a variety of grain scales and albedos, possibly constituting pristine cometary material. | 67p/churyumov-gerasimenko surface properties as derived from civa panoramic images |
cosmic dust is a polydisperse mixture of irregular, often aggregated, particles. previous attempts have tried to simulate polarimetric properties of this dust using aggregate dust models, but it has not been possible to consider particle sizes larger than a couple of microns due to limitations of computer memory and processing power. attempts have also been made to replace aggregates by polydisperse regular particles (spheres, spheroids, cylinders), but those models could not consistently reproduce the observed photopolarimetric characteristics. in this study, we introduce to the astronomical community the software package developed by dubovik et al. (2006) for modeling light scattering by a polydisperse mixture of randomly oriented smooth and rough spheroids of a variety of aspect ratios. the roughness of spheroids is defined by a normal distribution of the surface slopes, and its degree depends on the standard deviation of the distribution (which is zero for smooth surface and greater than zero for rough surface). the pre-calculated kernels in the software package allow for fast, accurate, and flexible modeling of different size and shape distributions. we present our results of a systematic investigation of polarization obtained with the rough and smooth spheroid models; we study differences in their phase angle dependence and how those differences change with the particle size distribution. we found that the difference between smooth and rough particles increases with increasing effective size parameter and affects mainly the value and position of the maximum polarization. negative polarization was found to be typical only for silicate-like refractive indexes and only when the particles have size parameters within 2.5-25. as an example of an application of the rough spheroid model, we made computations for rough spheroids that have a size distribution and composition typical for cometary dust. we found that a mixture of porous rough spheroids made of absorbing material compositionally similar to comet halley's dust and solid silicate spheroids, dominated by particles of size parameter 5<x<20, can reproduce angular and spectral characteristics of the brightness and polarization observed for cometary dust. | polarization of cosmic dust simulated with the rough spheroid model |
the comet assay is a sensitive method for the evaluation of dna damages and dna repair capacity at single‑cell level. allium cepa is a well‑established plant model for toxicological studies. the aim of this scoping review was to investigate the recent application of the comet assay in allium cepa root cells to assess the genotoxicity. to explore the literature a search was performed selecting articles published between january 2015 and february 2023 from web of science, pubmed, and scopus databases using the combined search terms "comet assay" and "allium cepa". all the original articles that applied the comet assay to allium cepa root cells were included. of the 334 records initially found, 79 articles were identified as meeting the inclusion criteria. some studies reported results for two or more toxicants. in these cases, the data for each toxicant were treated separately. thus, the number of analyzed toxicants (such as chemicals, new materials, and environmental matrices) was higher than the number of selected papers and reached 90. the current use of the allium‑comet assay seems to be directed towards two types of approach: the direct study of the genotoxicity of compounds, mainly biocides (20% of analyzed compounds) and nano‑ and microparticles (17%), and assessing a treatment's ability to reduce or eliminate genotoxicity of known genotoxicants (19%). although the genotoxicity identified by the allium‑comet assay is only one piece of a larger puzzle, this method could be considered a useful tool for screening the genotoxic potential of compounds released into the environment. | a scoping review of recent advances in the application of comet assay to allium cepa roots |
the international rosetta mission was launched on 2nd march 2004 on its 10 year journey to rendezvous with comet 67p churyumov-gerasimenko. rosetta performed comet orbit insertion on the 6th of august 2014, after which it characterised the nucleus and orbited it at altitudes as low as a few kilometres. in november 2014 rosetta delivered the lander philae to perform the first soft landing ever on the surface of a comet. the critical landing operations have been conducted with remarkable accuracy and will constitute one of the most important achievements in the history of spaceflight. after this critical operation, rosetta began the escort phase of the comet in its journey in the solar system heading to the perihelion, reached in august 2015. throughout this period, the comet environment kept changing with increasing gas and dust emissions. a first phase of bound orbits was followed by a sequence of complex flyby segments which allowed the scientific instruments to perform in depth investigation of the comet environment and nucleus. the unpredictable nature of the comet activity forced the mission control team to implement unplanned changes to the flight plan prepared for this mission phase and to plan the whole mission in a more dynamic way than originally conceived. this paper describes the details of the landing operations and of the main comet escort phase. it also includes the mission status as achieved after perihelion and the findings about the evolution of the comet and its environment from a mission operations point of view. the lessons learned from this unique and complex operations phase and the plans for the next mission phases, which include a mission extension into 2016, are also described. | rosetta following a living comet |
disinfection with performic acid (pfa) represents an emerging technology in wastewater treatment. many recent studies indicate its effectiveness and suitability as a disinfectant for different applications; several have demonstrated its reliability as an alternative to chlorine for disinfecting secondary effluents from urban wastewater treatment plants (wwtps). some disinfection technologies, in relation to their oxidative power, lead to the formation of disinfection by-products (dbps), some of which are of concern for their toxic and carcinogenic potential. the aim of this study was to investigate potential genotoxic, cytotoxic, and mutagenic effects of this disinfection agent on treated secondary effluent coming from a municipal wwtp. a strategy with multiple short-term tests and different target cells (bacterial, plant, and mammalian) was adopted to explore a relatively wide range of potential genotoxic events. the ames test (point mutation in salmonella), the micronucleus (chromosomal damage) and comet tests (primary dna damage) on human hepatic cells (hepg2) were conducted to detect mutagenicity and chromosomal dna alterations. dna fragmentation and mitochondrial potential assays were conducted to evaluate apoptosis in the same kinds of cells. mutagenic and clastogenic effect potentials were evaluated by examining micronucleus formation in allium cepa root cells. in all the in vitro tests, carried out on both disinfected and non-disinfected effluents, negative results were always obtained for mutagenic and genotoxic effects. in the allium cepa tests, however, some non-concentrated wastewater samples after pfa treatment induced a slight increase in micronucleus frequencies in root cells, but not in a dose-related manner. in conclusion, pfa applied for disinfection to a secondary effluent from a municipal wastewater treatment plant did not contribute to the release of genotoxic or mutagenic compounds. further studies are required to establish to which extent these findings can be generalized to support pfa for other disinfection applications. | evaluation of cytotoxicity, genotoxicity, and apoptosis of wastewater before and after disinfection with performic acid |
the chondritic-porous subset of interplanetary dust particles (cp-idps) are thought to have a cometary origin. since the cp-idps are anhydrous and unaltered by aqueous processes that are common to chondritic organic matter (om), they represent the most pristine material of the solar system. however, the study of idp om might be hindered by their further alteration by flash heating during atmospheric entry, and we have limited understanding on how short-term heating influences their organic content. in order to investigate this problem, five cp-idps were studied for their om contents, distributions, and isotopic compositions at the submicro- to nanoscale levels. the om contained in the idps in this study spans the spectrum from primitive om to that which has been significantly processed by heat. similarities in the raman d bands of the meteoritic and idp oms indicate that the overall gain in the sizes of crystalline domains in response to heating is similar. however, the raman γg values of the om in all of the five idps clearly deviate from those of chondritic om that had been processed during a prolonged episode of parent body heating. such disparity suggests that the nonaromatic contents of the om are different. short duration heating further increases the h/c ratio and reduces the δ13c and δd values of the idp om. our findings suggest that idp om contains a significant proportion of disordered c with low h content, such as sp2 olefinic c=c, sp3 c-c, and/or carbonyl contents as bridging material. | organics preserved in anhydrous interplanetary dust particles: pristine or not? |
we have investigated through simulation the electrostatic charging of the nucleus of comet 67p/churyumov-gerasimenko during periods of weak outgassing activity. specifically, we have modeled the surface potential and electric field at the surface of the nucleus during the initial rosetta rendezvous at 3.5 au and the release of the philae lander at 3 au. we have also investigated the possibility of dust acceleration and ejection above the nucleus due to electrostatic forces. finally, we discuss these modeling results in the context of possible observations by instruments on both the rosetta orbiter and the philae lander. | surface charging and electrostatic dust acceleration at the nucleus of comet 67p during periods of low activity |
numerical simulations show that impactors can penetrate europa's ice, creating conduits to the underlying ocean. breaching becomes inevitable when transient cavity depth exceeds 90% of ice thickness. results indicate that a 0.5 km comet would penetrate 5 km ice, and a 5 km comet could breach 40 km ice. thinner ice would be breached more frequently, thicker ice less so, but even the 40 km upper estimates for ice thickness would be penetrated by comets with recurrence intervals less than 250 ma. if actual ice thickness is 8-13 km (indicated by comparing europan and simulated crater geometries), the ocean could be exposed by impactors in the range 0.7-1.5 km, which have recurrence intervals ≈3 to 7 ma. thus, it seems that europa's ice has been penetrated often in the past and possibly in geologically recent time. the largest known impact sites, callanish and tyre, probably represent transition from craters to penetrating impacts. the geomorphic expression of full penetration must exist on the surface; chaos terrain is a candidate. astrobiological materials could be transported to the ocean via these impact-created conduits. | impact breaching of europa's ice: constraints from numerical modeling |
analyzing data from the ion composition analyzer on board the rosetta spacecraft, we studied a flow pattern of accelerated cometary ions (40-80 ev) inside and outside the diamagnetic cavity of comet 67p/churyumov-gerasimenko (67p). we found that the accelerated ions are intermittently observed and are ten times more frequently observed outside the cavity than inside, and they mainly flow tailward with an aberration ( 20-40°). we suggest that they are accelerated by the tailward polarization electric field upstream of the comet. because their occurrence frequency becomes lowest near perihelion where the water production rate is highest at 67p, ion-neutral collisions and/or charge exchange may play a role in controlling the occurrence frequency. the aberration pattern is different inside and outside the cavity in the cometocentric solar equatorial (cseq) frame but it is consistent in the comet-sun electric (cse) frame; the latter is rotated from the cseq frame about the comet-sun line so that the z-axis is aligned with the local motional electric field. because the flow pattern of the accelerated ions inside the cavity in the cse frame is the same as outside, we suggest that the flow pattern inside is determined by the flow outside, depending on the local plasma and magnetic field. near the cse polar plane the aberration is in the opposite direction of the motional electric field, while it is in the anti-cometward direction near the cse equator plane. the aberration in the anti-electric-field direction near the cse polar plane suggests that the accelerated ions are mass-loaded by local cold cometary ions, just like the mass-loading of the solar wind by cold cometary ions. the cause of the anti-cometward aberration near the cse equator plane is still unknown, but this may indicate that the tailward-flowing cometary ions are deflected across the upstream boundaries or by an outward-pointing ambipolar electric field. | flow pattern of accelerated cometary ions inside and outside the diamagnetic cavity of comet 67p/churyumov-gerasimenko |
the topological aspects of fluid flows have long been fascinating subjects in the study of the physics of fluids. in this study, the topology of the second-order boltzmann-curtiss constitutive model beyond the conventional navier-stokes-fourier equations and stokes's hypothesis was investigated. in the case of velocity shear, the topology of the second-order constitutive model was shown to be governed by a simple algebraic form. the bulk viscosity ratio in diatomic and polyatomic gases was found to play an essential role in determining the type of topology: from an ellipse to a circle, to a parabola, and then finally to a hyperbola. the topology identified in the model has also been echoed in other branches of science, notably in the orbits of planets and comets and dirac cones found in electronic band structures of two-dimensional materials. the ultimate origin of the existence of the topology was traced to the coupling of viscous stress and velocity gradient and its subtle interplay with the bulk viscosity ratio. in the case of compression and expansion, the topology of the second-order constitutive model was also found to be governed by a hyperbola. the trajectories of solutions of two representative flow problems—a force-driven poiseuille gas flow and the inner structure of shock waves—were then plotted on the topology of the constitutive model, demonstrating the indispensable role of the topology of the constitutive model in fluid dynamics. | topology of the second-order constitutive model based on the boltzmann-curtiss kinetic equation for diatomic and polyatomic gases |
the spectra emitted by highly charged ions, which are abundant in laboratory plasma and stellar systems, cover a wide range of wavelengths from near-infrared to x-rays. measurements on such spectral lines can provide plasma information such as the electron temperature, electron/ion density, chemical composition, and the evolution of these parameters. to simulate the x-ray emission from comets and other celestial bodies irradiated by solar wind ions in the laboratory, we established an experimental platform for x-ray measurements at institute of modern physics, chinese academy of sciences, and measured the x-ray emission spectra from a series of metal and silicate minerals bombarded by slow highly charged nitrogen and oxygen ions. in this paper, we describe the x-ray measurement platform in detail and report the x-ray emission caused by the interaction of 1.5-20 kev/q oq+ (q = 3, 5, 6) and nq+ (q = 3, 5) ions with nickel surface. it is discovered that the measured x-ray yield and production cross section increase rapidly as increasing the impact energy, but have no discernible dependence on the charge state of incident ions. the experimental results reveal that the incident ions have been neutralized and achieved charge state equilibration before the k-shell electron is ionized. when the incident energy is greater than 5 kev/q, the binary encounter approximation calculations are consistent well with the experimental data, indicating that the k-shell ionization of incident ions is due to direct coulomb excitation processes that occur below the target surface. however, the experimental ionization cross sections clearly deviate from the binary encounter approximation calculations when the impact energy is less than 5 kev/q. the discrepancy at the low collision energy is discussed and explained based on a multi-electron excitation model. | k-x-ray emission of 1.5-20 kev/q oq+ (q = 3, 5, 6) and nq+ (q = 3, 5) ions impinging on nickel surface |
the response of the mutual impedance probe rpc-mip on board rosetta orbiter electrostatically modeled considering an unmagnetized and collisionless plasma with two maxwellian electron populations. a vacuum sheath surrounding the probe was considered in our model in order to take the ion sheath into account that is located around the probe, which is immersed in the cometary plasma. for the first time, the simulated results are consistent with the data collected around comet 67p/churyumov-gerasimenko (67p), but strong discrepancies were identified with the previous simulations that neglected the plasma sheath around the probe. we studied the influence of the sheath thickness and of the electron populations. this work helps to better understand the initially unexpected responses of the mutual impedance probe that were acquired during the rosetta mission. it suggests that two electron populations exist in the cometary plasma of 67p. | rpc-mip observations at comet 67p/churyumov-gerasimenko explained by a model including a sheath and two populations of electrons |
in this paper, we present a study of the dynamical effects of the galaxy on the external region of the oort cloud. the aims of this paper are: (i) to determine an outer limit for the oort cloud; and (ii) to analyse the dynamical behaviour of the most external objects of the oort cloud and how they are ejected from the solar system. to achieve these aims, we follow the temporal evolution of massless test particles in the galactic environment of the solar neighbourhood. we show that the effect of the perturbations from the galactic tide in the particles is similar to that found for the evolution of the population of wide binary stars. moreover, in the oort cloud, we find a dynamical structure around 105 au conformed by objects unbound from the sun. this structure allows us to define a transition region of stability and an outer boundary for the oort cloud, and it is also in agreement with previous results about the disruption of wide binary stars. | stability in the most external region of the oort cloud: evolution of the ejected comets |
a number of studies have assessed or modeled the distribution of the radionuclides released by the accident at the fukushima-daiichi nuclear power plant (fdnpp). few studies however have investigated its consequences for the local biota. we tested whether exposure of barn swallow (hirundo rustica) nestlings to low dose ionizing radiation increased genetic damage to their peripheral erythrocytes. we estimated external radiation exposure by using thermoluminescent dosimeters, and by measuring radioactivity of the nest material. we then assessed dna damage by means of the neutral comet assay. in addition, we conducted standard point-count censuses of barn swallows across environmental radiation levels, and estimated their abundance and local age ratio. radioactivity of nest samples was in the range 479-143,349 bq kg-1, while external exposure varied between 0.15 and 4.9 mgy. exposure to radioactive contamination did not correlate with higher genetic damage in nestlings. however, at higher levels of radioactive contamination the number of barn swallows declined and the fraction of juveniles decreased, indicating lower survival and lower reproduction and/or fledging rate. thus, genetic damage to nestlings does not explain the decline of barn swallows in contaminated areas, and a proximate mechanism for the demographic effects documented here remains to be clarified. | abundance and genetic damage of barn swallows from fukushima |
water ices are a common component of cold space environments, including molecular and protostellar clouds, and the frozen surfaces of moons, planets, and comets. when exposed to ionizing and/or thermal processing, they become a nursery for new molecular species and are also responsible for their desorption to the gas-phase. crystalline water ice, produced by the deposition of gaseous water at warm (80-150 k) surfaces or by the heating of cold amorphous water ice (up to ~150 k), is also regularly detected by astronomical observations. here, we employed the procoda code to map the chemical evolution of 5 kev electron-bombarded crystalline water-ices at different temperatures (12, 40, 60 and 90 k). the chemical network considered a total of 61 coupled reactions involving nine different chemical species within the ice. among the results, we observe that the average calculated effective rate constants for radiation-induced dissociation decrease as the icés temperature increases. the abundance of molecular species in the ice at chemical equilibrium and its desorption to gas-phase depend on both the temperature of the ice. h2o molecules are the dominant desorbed species, with a desorption yield of about 1 molecule per 100 electrons, which seems to be enhanced for warmer crystalline ices. the obtained results can be employed in astrochemical models to simulate the chemical evolution of interstellar and planetary environments. these findings have implications for astrochemistry and astrobiology, providing insight into crucial chemical processes and helping us understand the chemistry in cold regions in space. | chemical evolution of electron-bombarded crystalline water ices at different temperatures using the procoda code |
we analyzed images of comet 49p/arend-rigaux on 33 nights between 2012 january and may and obtained r-band lightcurves of the nucleus. through usual phasing of the data, we found a double-peaked lightcurve having a synodic rotation period of 13.450 ± 0.005 hr. similarly, phase dispersion minimization and the lomb-scargle method both revealed rotation periods of 13.452 hr. throughout the 2011/2012 apparition, the rotation period was found to increase by a small amount, consistent with a retrograde rotation of the nucleus. we also reanalyzed the publicly available data from the 1984/1985 apparition by applying the same techniques, finding a rotation period of 13.45 ± 0.01 hr. based on these findings, we show that the change in rotation period is less than 14 s per apparition. furthermore, the amplitudes of the lightcurves from the two apparitions are comparable, to within reasonable errors, even though the viewing geometries differ, implying that we are seeing the comet at a similar sub-earth latitude. we detected the presence of a short-term jet-like feature in 2012 march, which appears to have been created by a short-duration burst of activity on march 15. production rates obtained in 2004/2005, along with reanalysis of the previous results from 1984/1985, imply a strong seasonal effect and a very steep fall-off after perihelion. this, in turn, implies that a single source region, rather than leakage from the entire nucleus, dominates activity. | the rotation and other properties of comet 49p/arend-rigaux, 1984-2012 |
this paper is a review of molecules with internal rotors detected or tentatively detected with upper limits in the interstellar medium. internal rotor molecules containing one or two methyl groups are present at each of the different phases leading to stellar and planetary formation, from the molecular clouds to the star and its planets. they are also found in comets and asteroids. they represent a ubiquitous hydrogen-rich substituent in many complex organic molecules. this paper shows how high-resolution spectra (mainly in the microwave, millimeter, and submillimeter wavelength ranges) are analyzed in the laboratory and which theoretical methods can be used to properly reproduce the spectra. we also present a range of examples of important methyl rotors studied and how they can be used as tools to study the chemistry of interstellar medium. | spectroscopy of interstellar internal rotors: an important tool for investigating interstellar chemistry |
context. the first 1000 km of the ion tail of comet 67p/churyumov-gerasimenko were explored by the european rosetta spacecraft, 2.7 au away from the sun.aims: we characterised the dynamics of both the solar wind and the cometary ions on the night-side of the comet's atmosphere.methods: we analysed in situ ion and magnetic field measurements and compared the data to a semi-analytical model.results: the cometary ions are observed flowing close to radially away from the nucleus during the entire excursion. the solar wind is deflected by its interaction with the new-born cometary ions. two concentric regions appear, an inner region dominated by the expanding cometary ions and an outer region dominated by the solar wind particles.conclusions: the single night-side excursion operated by rosetta revealed that the near radial flow of the cometary ions can be explained by the combined action of three different electric field components, resulting from the ion motion, the electron pressure gradients, and the magnetic field draping. the observed solar wind deflection is governed mostly by the motional electric field -uion × b. | the root of a comet tail: rosetta ion observations at comet 67p/churyumov-gerasimenko |
radar-bright deposits at the poles of mercury are located in permanently shadowed regions, which provide thermally stable environments for hosting and retaining water ice on the surface or in the near subsurface for geologic timescales. while the areal distribution of these radar-bright deposits is well characterized, their thickness, and thus their total mass and volume, remain poorly constrained. here we derive thickness estimates for selected water-ice deposits using small, simple craters visible within the permanently shadowed, radar-bright deposits. we examine two endmember scenarios: in case i, these craters predate the emplacement of the ice, and in case ii, these craters postdate the emplacement of the ice. in case i, we find the difference between estimated depths of the original unfilled craters and the measured depths of the craters to find the estimated infill of material. the average estimated infilled material for 9 craters assumed to be overlain with water ice is ∼ 41-14+30 m, where 1-σ standard error of the mean is reported as uncertainty. reported uncertainties are for statistical errors only. additional systematic uncertainty may stem from georeferencing the images and topographic datasets, from the radial accuracy of the altimeter measurements, or from assumptions in our models including (1) ice is flat in the bowl-shaped crater and (2) there is negligible ice at the crater rims. in case ii, we derive crater excavation depths to investigate the thickness of the ice layer that may have been penetrated by the impact. while the absence of excavated regolith associated with the small craters observed suggests that impacts generally do not penetrate through the ice deposit, the spatial resolution and complex illumination geometry of images may limit the observations. therefore, it is not possible to conclude whether the small craters in this study penetrate through the ice deposit, and thus case ii does not provide a constraint on the ice thickness. for mercury's polar deposits, we argue that case i of the small craters predating the emplacement of the ice deposits is more likely, given other geologic evidence that suggests that these ice deposits are relatively young. using the ice thickness estimates from case i to calculate the total amount of water ice currently contained in mercury's polar deposits results in a value of ∼1014-1015 kg. this is equivalent to ∼100-1000 km3 ice in volume. this volume of water ice is consistent with delivery via micrometeorite bombardment, jupiter-family comets, or potentially a single impactor. | constraining the thickness of polar ice deposits on mercury using the mercury laser altimeter and small craters in permanently shadowed regions |
tin order to improve our current knowledge on the budgets of the two most important anthropogenic greenhouse gases, co2 and ch4, an airborne mission on board the german research aircraft halo in coordination with two smaller cessna aircraft is going to be conducted in april/may 2017. the goal of comet is to combine a suite of the best currently available active (lidar) and passive remote sensors as well as in-situ instruments to provide regional-scale data of greenhouse gases which are urgently required. | comet: an airborne mission to simultaneously measure co2 and ch4 using lidar, passive remote sensing, and in-situ techniques |
plumes produced by the impacts of asteroids and comets consist of rock vapor and heated air. they emit visible light, ultraviolet, and infrared radiation, which can greatly affect the environment. we have carried out numerical simulations of the impacts of stony and cometary bodies with a diameter of 0.3, 1, and 3 km, which enter the atmosphere at various angles, using a hydrodynamic model supplemented by radiation transfer. we assumed that the cosmic object has no strength, and deforms, fragments, and vaporizes in the atmosphere. after the impact on the ground, the formation of craters and plumes was simulated, taking the internal friction of destroyed rocks and the trail formed in the atmosphere into account. the equation of radiative transfer, added to the equations of gas dynamics, was used in the approximation of radiative heat conduction or, if the rosseland optical depth of a radiating volume of gas and vapor was less than unity, in the volume-emission approximation. we used temperature and density distributions obtained in these simulations to calculate radiation fluxes on the earth's surface by integrating the equation of radiative transfer along rays passing through a luminous region. we used tables of the equation of state of dunite and quartz (for stony impactors and a target) and air, as well as tables of absorption coefficients of air, vapor of ordinary chondrite, and vapor of cometary material. we have calculated the radiation impulse on the ground and the impact radiation efficiency (a ratio of thermal radiation energy incident on the ground to the kinetic energy of a body), which ranges from 0.5% to 9%, depending on the impactor size and the angle of entry into the atmosphere. direct thermal radiation from fireballs and impact plumes, poses a great danger to people, animals, plants, and economic objects. after the impacts of asteroids at a speed of 20 km s-1 at an angle of 45°, a fire can occur at a distance of 250 km if the asteroid has a diameter of 0.3 km, and at a distance of 2000 km if the diameter is 3 km. | thermal radiation from impact plumes |
long term evolution of an initial set of 107 oort cloud comets is performed for the age of the solar system taking into account the action of passing stars using 10 different sequences of stellar encounters, galactic tides and the gravity of the giant planets. the initial conditions refer to a disk-shaped oort cloud precursor, concentrated toward the ecliptic with perihelia in the region of uranus and neptune. our results show that the shape of the oort cloud quickly reach a kind of steady state beyond a semi-major axis greater than about 2000 au (this threshold depending on the evolution time-span), with a boltzmann distribution of the orbital energy. the stars act in an opposite way to what was found in previous papers, that is they emptied an initial tidal active zone that is overfilled with respect to the isotropic case. consequently, the inclusion of stellar perturbations strongly affect the shape of the oort spike. on the contrary, the oort spike shape appears to be poorly dependent on the stellar sequences used, whereas the total flux of observable comets and the proportion of retrograde comets for the inner part of the spike are significantly dependent of it. then it has been highlighted that the total flux, the shape of the oort spike and the shape of the final oort cloud are almost independent of the initial distribution of orbital energy considered. | on the present shape of the oort cloud and the flux of ;new; comets |
the recent successes of the european rosetta mission have shown the possibility of a close observation with one of the most evasive celestial bodies in the solar system, the comets, and the practical feasibility of a comet rendezvous to obtain detailed information and in situ measurements. this paper discusses a preliminary study of the transfer trajectory toward the comet 67p/churyumov-gerasimenko (the same target used by rosetta) for a spacecraft whose primary propulsion system is an electric solar wind sail. the use of a propellantless propulsion system with a continuous thrust is theoretically able to simplify the transfer trajectory by avoiding the need of intermediate flyby maneuvers. the problem is addressed in a parametric way, by looking for the possible optimal launch windows as a function of the propulsion system performance. the study is completed by a mass breakdown analysis of the spacecraft, for some mission scenarios of practical interest, based on the actual payload mass of the spacecraft rosetta. | electric sail option for cometary rendezvous |
a theoretical study on the ultra-low frequency small but finite amplitude solitary waves has been carried out in an electron depleted magnetized nonthermal dusty plasma consisting of both polarity (positively charged as well as negatively charged) inertial massive dust particles and nonextensive q distributed ions. the reductive perturbation technique is employed to derive the zakharovkuznetsov (zk) equation. the basic features of low frequency solitary wave are analyzed via the solution of zk equation. it is observed that the intrinsic properties (e.g., polarity, amplitude, width, etc.) of dust-acoustic (da) solitary waves (sws) are significantly influenced by the effects external magnetic field, obliqueness, nonextensivity of ions, and the ratio of ion number density to the product of electron and negative dust number density. the findings of our results may be useful to explain the low frequency nonlinear wave propagation in some plasma environments like cometary tails, the earth polar mesosphere, jupiter's magnetosphere, etc. | low frequency nonlinear waves in electron depleted magnetized nonthermal plasmas |
we report a theoretical study of the infrared (ir) vibrational spectra of ch3 + and its deuterated isotopologues. we also report exothermicities for reactions of these species with hd and d2. the calculations are performed using the two-layer lanzcos algorithm with an exact full-dimensional molecular hamiltonian, based on a slightly adjusted potential energy surface of yu and sears [h.-g. yu and t. j. sears, j. chem. phys. 117, 666 (2002)]. the transition strengths are evaluated within the traditional dipole approach. the full-dimensional dipole moment surfaces are calculated at ccsd(t)/cc-pvtz level and fitted by integrity basis functions using a bond vector method. the results should be helpful for understanding the roles of ch3 + in the interstellar medium, especially in the coma of comet halley. | infrared vibrational spectra of ch3 + and its deuterated isotopologues |
water worlds are water-rich (>1 wt% h2o) exoplanets. the classical models of water worlds considered layered structures determined by the phase boundaries of pure water. however, water worlds are likely to possess comet-like compositions, with between ∼3 and 30 mol% co2 relative to water. in this study, we build an interior structure model of habitable (i.e., surface liquid ocean-bearing) water worlds using the latest results from experimental data on the co2-h2o system to explore the co2 budget and localize the main co2 reservoirs inside of these planets. we show that co2 dissolved in the ocean and trapped inside of a clathrate layer cannot accommodate a cometary amount of co2 if the planet accretes more than 11 wt% of volatiles (co2 + h2o) during its formation. if the atmosphere holds more than a negligible amount of the co2 (>0.01% of the planet mass), the planet will not have a habitable surface temperature. we propose a new, potentially dominant, co2 reservoir for water worlds: co2 buried inside of the high-pressure water ice mantle as co2 ices or (h2co3 × h2o), the monohydrate of carbonic acid. if insufficient amounts of co2 are sequestered in either this reservoir or the planet's iron core, habitable-zone water worlds could generically be stalled in their cooling before liquid oceans have a chance to condense. | internal structure and co2 reservoirs of habitable water worlds |
context. cometary meteoroid trails exist in the vicinity of comets, forming a fine structure of the interplanetary dust cloud. the trails consist predominantly of the largest cometary particles (with sizes of approximately 0.1 mm-1 cm), which are ejected at low speeds and remain very close to the comet orbit for several revolutions around the sun. in the 1970s, two helios spacecraft were launched towards the inner solar system. the spacecraft were equipped with in situ dust sensors which measured the distribution of interplanetary dust in the inner solar system for the first time. recently, when re-analysing the helios data, a clustering of seven impacts was found, detected by helios in a very narrow region of space at a true anomaly angle of 135 ± 1°, which the authors considered as potential cometary trail particles. however, at the time, this hypothesis could not be studied further.aims: we re-analyse these candidate cometary trail particles in the helios dust data to investigate the possibility that some or all of them indeed originate from cometary trails and we constrain their source comets.methods: the interplanetary meteoroid environment for exploration (imex) dust streams in space model is a new and recently published universal model for cometary meteoroid streams in the inner solar system. we use imex to study the traverses of cometary trails made by helios.results: during ten revolutions around the sun, the helios spacecraft intersected 13 cometary trails. for the majority of these traverses the predicted dust fluxes are very low. in the narrow region of space where helios detected the candidate dust particles, the spacecraft repeatedly traversed the trails of comets 45p/honda-mrkos-pajdušáková and 72p/denning-fujikawa with relatively high predicted dust fluxes. the analysis of the detection times and particle impact directions shows that four detected particles are compatible with an origin from these two comets. by combining measurements and simulations we find a dust spatial density in these trails of approximately 10-8-10-7 m-3.conclusions: the identification of potential cometary trail particles in the helios data greatly benefited from the clustering of trail traverses in a rather narrow region of space. the in situ detection and analysis of meteoroid trail particles which can be traced back to their source bodies by spacecraft-based dust analysers provides a new opportunity for remote compositional analysis of comets and asteroids without the necessity to fly a spacecraft to or even land on those celestial bodies. this provides new science opportunities for future missions like destiny+ (demonstration and experiment of space technology for interplanetary voyage with phaethon flyby and dust science), europa clipper, and the interstellar mapping and acceleration probe. | helios spacecraft data revisited: detection of cometary meteoroid trails by following in situ dust impacts |
the atacama large millimeter/submillimeter array (alma) is a powerful tool for high-resolution mapping of comets, but the main interferometer (comprised of 50 × 12 m antennas) is insensitive to the largest coma scales due to a lack of very short baselines. in this letter, we present a new technique employing alma autocorrelation data (obtained simultaneously with the interferometric observations), effectively treating the entire 12 m array as a collection of single-dish telescopes. using combined autocorrelation spectra from 28 active antennas, we recovered extended hcn coma emission from comet c/2012 s1 (ison), resulting in a fourteen-fold increase in detected line brightness compared with the interferometer. this resulted in the first detection of rotational emission from h13cn in this comet. using a detailed coma radiative transfer model accounting for optical depth and non-local thermodynamic equilibrium excitation effects, we obtained an h12cn/h13cn ratio of 88 ± 18, which matches the terrestrial value of 89. this is consistent with a lack of isotopic fractionation in hcn during comet formation in the protosolar accretion disk. the possibility of future discoveries in extended sources using autocorrelation spectroscopy from the main alma array is thus demonstrated. | alma autocorrelation spectroscopy of comets: the hcn/h13cn ratio in c/2012 s1 (ison) |
the effect of the radial drift rate on mean motion resonance capture is studied for prograde, polar and retrograde orbits. we employ the numerical framework of our earlier exploration of resonance capture at arbitrary inclination. randomly constructed samples of massless particles are set to migrate radially from outside the orbit of a jupiter-mass planet at different drift rates totalling more than 1.6 × 106 numerical simulations. slower drift rates reduce overall capture probability especially for prograde orbits and enhance capture at specific initial inclinations of high-order resonances such as the outer 1:5, 1:4, 1:3, 2:5, 3:7, 5:7. global capture is reduced with increasing eccentricity at all inclinations as high-order resonances capture more particles that are subsequently lost by disruptive close encounters with the planet. the relative efficiency of retrograde resonances at long-lived capture with respect to prograde resonances is explained by the reduced effect of planet encounters as such events occur with a shorter duration and a higher relative velocity for retrograde motion. capture in the co-orbital 1:1 resonance is marginally affected by the radial drift rate except for nearly co-planar retrograde eccentric orbits whose capture likelihood is increased significantly with slower drift rates. an unexpected finding is the presence of a dynamical corridor for capture in high-order inner prograde resonances with initial inclinations in the range [50°, 80°] especially at the inner 5:2 resonance whose capture likelihood peaks at 80 per cent to 90 per cent depending on the initial eccentricity. | resonance capture at arbitrary inclination - ii. effect of the radial drift rate |
this paper discusses the influence of light pollution of the night sky on the conditions of visibility of astronomical objects such as planets, stars and comets. this phenomenon has a huge impact on the observability of astronomical objects, especially in cities, where the brightness of the sky makes it difficult or even impossible to conduct astronomical observations. the main purpose of this article is to measure and analyse the surface brightness of the night sky in rzeszów and its surroundings. a device called the sky quality meter was used to measure the brightness of the night sky. this paper presents measurement results for the years 2015 and 2018, from which it is clear that the quality of the night sky has been deteriorated in terms of the observability of celestial bodies. as an example, the numerical value of the measurement for the centre of rzeszów has been taken. in 2015, this value was 18.70± 1.87 mag/arcsec2, while in 2018, it was equal to 16.73± 1.67 mag/arcsec2. the results obtained were used to analyse the visibility of celestial bodies. here, particular attention was paid to the analysis of the visibility of comets (also during the outburst), in the context of increasing light pollution of the night sky. observers in neighboring villages have also experienced a change in the sky quality from bortle class v to class vii, requiring objects to be approximately one magnitude brighter in order to be visible. | impact of light pollution on the visibility of astronomical objects in medium-sized cities in central europe on the example of the city of rzeszów, poland |
the compositional and morphological evolution of minor bodies in the solar system is primarily driven by the evolution of their heliocentric distances, as the level of incident solar radiation regulates cometary activity. we investigate the dynamical transfer of centaurs into the inner solar system, facilitated by mean motion resonances with jupiter and saturn. the recently discovered object p/2019 ld2 will transition from the centaur region to the inner solar system in 2063. in order to contextualize ld2, we perform n-body simulations of a population of centaurs and jupiter-family comets. objects between jupiter and saturn with tisserand parameter tj~ 3 are transferred onto orbits with perihelia q < 4 au within the next 1000 yr with notably high efficiency. our simulations show that there may be additional ld2-like objects transitioning into the inner solar system in the near future, all of which have low δv with respect to jupiter. we calculate the distribution of orbital elements resulting from a single jovian encounter and show that objects with initial perihelia close to jupiter are efficiently scattered to q < 4 au. moreover, approximately 55% of the transitioning objects in our simulated population experience at least one jovian encounter prior to reaching q < 4 au. we demonstrate that a spacecraft stationed near jupiter would be well positioned to rendezvous, orbit-match, and accompany ld2 into the inner solar system, providing an opportunity to observe the onset of intense activity in a pristine comet in situ. finally, we discuss the prospect of identifying additional targets for similar measurements with forthcoming observational facilities. | a sublime opportunity: the dynamics of transitioning cometary bodies and the feasibility of in situ observations of the evolution of their activity |
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