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aims: the osiris camera onboard the rosetta spacecraft obtained close-up views of the dust coma of comet 67p. the jet structures can be used to trace their source regions and to examine the possible effect of gas-surface interaction.methods: we analyzed the wide-angle images obtained in the special dust observation sequences between august and september 2014. the jet features detected in different images were compared to study their time variability. the locations of the potential source regions of some of the jets are identified by ray tracing. we used a ring-masking technique to calculate the brightness distribution of dust jets along the projected distance.results: the jets detected between august and september 2014 mostly originated in the hapi region. morphological changes appeared over a timescale of several days in september. the brightness slope of the dust jets is much steeper than the background coma. this might be related to the sublimation or fragmentation of the emitted dust grains. interaction of the expanding gas flow with the cliff walls on both sides of hapi could lead to erosion and material down-fall to the nucleus surface. appendix a is available in electronic form at http://www.aanda.org | morphology and dynamics of the jets of comet 67p/churyumov-gerasimenko: early-phase development |
the reaction of the nitrogen atom (n) in its first electronically excited state (2d) with cyanoacetylene (hc3n) has been investigated under single-collision conditions by using the crossed molecular beam method with mass spectrometric detection at a collision energy of 31 kj mol-1. with the support of electronic structure calculations, we found that this reaction proceeds via the barrierless addition of the n(2d) atom to the carbon-carbon triple bond of hc3n, followed by the formation of a cyclic intermediate adduct hc(n)ccn, which dissociates to c(n)ccn + h products or isomerises to a more stable intermediate hncccn by h-migration and ring-opening processes. the long-lived hncccn complex produces the linear 3σg - ground state dicyanocarbene (ncccn) radical plus atomic hydrogen through a barrierless unimolecular dissociation accompanied by a negligible competitive channel forming the ncccn radical (1a1) with a bent c 2vstructure plus h. the main product of this neutral-neutral reaction is the 3ncccn radical that could be a potential precursor to form other nitriles (c2n2, c3n) or more complex organic species in planetary atmospheres, such as that of titan and pluto, in cometary comas, and in uv irradiated interstellar environments. | combined crossed molecular beams and computational study on the n(2d) + hcccn(x1σ+) reaction and implications for extra-terrestrial environments |
transmission electron microscope examination of more than 250 fragments, >1 μm from comet wild 2 and a giant cluster interplanetary dust particle (gcp) of probable cometary origin has revealed four new calcium-aluminum-rich inclusions (cais), an amoeboid olivine aggregate (aoa), and an additional aoa or al-rich chondrule (arc) object. all of the cais have concentric mineral structures and are composed of spinel + anorthite cores surrounded by al,ti clinopyroxenes and are similar to two previous cais discovered in wild 2. all of the cometary refractory objects are of moderate refractory character. the mineral assemblages, textures, and bulk compositions of the comet cais are similar to nodules in fine-grained, spinel-rich inclusions (fgis) found in primitive chondrites and like the nodules may be nebular condensates that were altered via solid-gas reactions in the solar nebula. oxygen isotopes collected on one wild 2 cai also match fgis. the lack of the most refractory inclusions in the comet samples may reflect the higher abundances of small moderately refractory cai nodules that were produced in the nebula and the small sample sizes collected. in the comet samples, approximately 2-3% of all fragments larger than 1 μm, by number, are cais and nearly 50% of all bulbous stardust tracks contain at least one cai. we estimate that 0.5 volume % of wild 2 material and 1 volume % of gcp is in the form of cais. arcs and aoas account for <1% of the wild 2 and gcp grains by number. | refractory materials in comet samples |
previous work on protoplanetary dust growth shows a halt at centimeter sizes owing to the occurrence of bouncing at velocities of ≳0.1 m s-1 and fragmentation at velocities ≳1 m s-1. to overcome these barriers, spatial concentration of centimeter-sized dust pebbles and subsequent gravitational collapse have been proposed. however, numerical investigations have shown that dust aggregates may undergo fragmentation during the gravitational collapse phase. this fragmentation in turn changes the size distribution of the solids and thus must be taken into account in order to understand the properties of the planetesimals that form. to explore the fate of dust pebbles undergoing fragmenting collisions, we conducted laboratory experiments on dust-aggregate collisions with a focus on establishing a collision model for this stage of planetesimal formation. in our experiments, we analyzed collisions of dust aggregates with masses between 0.7 and 91 g mass ratios between target and projectile from 1 to 126 at a fixed porosity of 65%, within the velocity range of 1.5-8.7 m s-1, at low atmospheric pressure of ∼10-3 mbar, and in free-fall conditions. we derived the mass of the largest fragment, the fragment size/mass distribution, and the efficiency of mass transfer as a function of collision velocity and projectile/target aggregate size. moreover, we give recipes for an easy-to-use fragmentation and mass-transfer model for further use in modeling work. in a companion paper, we use the experimental findings and the derived dust-aggregate collision model to investigate the fate of dust pebbles during gravitational collapse. | the role of pebble fragmentation in planetesimal formation. i. experimental study |
laboratory spectral measurements of relevant analogue materials were performed in the framework of the rosetta mission in order to explain the surface spectral properties of comet 67p. fine powders of coal, iron sulphides, silicates and their mixtures were prepared and their spectra measured in the vis-ir range. these spectra are compared to a reference spectrum of 67p nucleus obtained with the virtis/rosetta instrument up to 2.7 μm, excluding the organics band centred at 3.2 μm. the species used are known to be chemical analogues for cometary materials which could be present at the surface of 67p. grain sizes of the powders range from tens of nanometres to hundreds of micrometres. some of the mixtures studied here actually reach the very low reflectance level observed by virtis on 67p. the best match is provided by a mixture of sub-micron coal, pyrrhotite, and silicates. grain sizes are in agreement with the sizes of the dust particles detected by the giada, midas and cosima instruments on board rosetta. the coal used in the experiment is responsible for the spectral slope in the visible and infrared ranges. pyrrhotite, which is strongly absorbing, is responsible for the low albedo observed in the nir. the darkest components dominate the spectra, especially within intimate mixtures. depending on sample preparation, pyrrhotite can coat the coal and silicate aggregates. such coating effects can affect the spectra as much as particle size. in contrast, silicates seem to play a minor role. | laboratory simulations of the vis-nir spectra of comet 67p using sub-μm sized cosmochemical analogues |
nucleobases are the informational subunits of rna and dna and are essential to all known forms of life. the nucleobases can be divided into two groups of molecules: the pyrimidine-based compounds that include uracil, cytosine, and thymine, and the purine-based compounds that include adenine and guanine. previous work in our laboratory has demonstrated that uracil, cytosine, thymine, and other nonbiological, less common nucleobases can form abiotically from the uv photoirradiation of pyrimidine in simple astrophysical ice analogues containing combinations of h2o, nh3, and ch4. in this work, we focused on the uv photoirradiation of purine mixed with combinations of h2o and nh3 ices to determine whether or not the full complement of biological nucleobases can be formed abiotically under astrophysical conditions. room-temperature analyses of the resulting photoproducts resulted in the detection of adenine, guanine, and numerous other functionalized purine derivatives. | the formation of nucleobases from the ultraviolet photoirradiation of purine in simple astrophysical ice analogues |
stars which pass close to the sun can perturb the oort cloud, injecting comets into the inner solar system where they may collide with the earth. using van leeuwen's re-reduction of the hipparcos data complemented by the original hipparcos and tycho-2 catalogues, along with recent radial velocity surveys, i integrate the orbits of over 50 000 stars through the galaxy to look for close encounters. the search uses a monte carlo sampling of the covariance of the data in order to properly characterize the uncertainties in the times, distances, and speeds of the encounters. i show that modelling stellar encounters by assuming instead a linear relative motion produces, for many encounters, inaccurate and biased results. i find 42, 14, and 4 stars which have encounter distances below 2, 1, and 0.5 pc respectively, although some of these stars have questionable data. of the 14 stars coming within 1 pc, 5 were found by at least one of three previous studies (which found a total of 7 coming within 1 pc). the closest encounter appears to be hip 85605, a k or m star, which has a 90% probability of coming between 0.04 and 0.20 pc between 240 and 470 kyr from now (90% bayesian confidence interval). however, its astrometry may be incorrect, in which case the closest encounter found is the k7 dwarf gl 710, which has a 90% probability of coming within 0.10-0.44 pc in about 1.3 myr. a larger perturbation may have been caused by gamma microscopii, a g6 giant with a mass of about 2.5 m⊙, which came within 0.35-1.34 pc (90% confidence interval) around 3.8 myr ago. full table 3 is only available at the cds via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?j/a+a/575/a35 | close encounters of the stellar kind |
in 1977, while apple ii and atari computers were being sold, a tiny dot was observed in an inconvenient orbit. the minor body 1977 ub, to be named (2060) chiron, with an orbit between saturn and uranus, became the first centaur, a new class of minor bodies orbiting roughly between jupiter and neptune. the observed overabundance of short-period comets lead to the downfall of the oort cloud as exclusive source of comets and to the rise of the need for a trans-neptunian comet belt. centaurs were rapidly seen as the transition phase between kuiper belt objects, also known as trans-neptunian objects (tnos) and the jupiter-family comets (jfcs). since then, a lot more has been discovered about centaurs: they can have cometary activity and outbursts, satellites, and even rings. over the past four decades since the discovery of the first centaur, rotation periods, surface colors, reflectivity spectra, and albedos have been measured and analyzed. however, despite such a large number of studies and complementary techniques, the centaur population remains a mystery as they are in so many ways different from the tnos and even more so from the jfcs. | from centaurs to comets - 40 years |
context. in 2020, the initial version of the stellar potential perturbers database (stepped) was presented with the aim to deliver up-to-date information on the stars and stellar systems that may perturb a long-period comet motion. we used the minimal distance between a star and the sun as a selecting tool when compiling a list of interesting objects with close encounters with the solar system, and our selection for that study was based on gαiα dr2 data.aims: when the gaia edr3 data release was published, it became necessary to update this database. additionally, we performed monte carlo simulations to obtain uncertainties on the parameters of the closest approach to the sun of each object.methods: we recalculated the close approach parameters of all stars in the previous stepped release, which resulted in removing approximately one-third of the total. then we searched for new candidates in the whole gaia edr3 catalogue. we also take into account the duplicity of the found stars and additionally searched for double stars passing near the sun which had been overlooked in previous papers. we also found the necessary mass estimates for new objects and updated this information for previously selected stars.results: after a careful checking of all the collected data, we composed a new list of 155 potential stellar perturbers of the long-period comet motion. we applied a new threshold of 2 pc for the minimum star-sun distance. this list consists of 146 single stars and nine multiple systems. for each object, we also estimated the uncertainty of the parameters of their closest approach to the sun. among these stars, we found a new potential strong past perturber, hd 7977, and confirmed the plausibility of a similar action on the part of gliese 710 in the future. | important stellar perturbers found during the stepped database update based on gaia edr3 data |
the electron-ion-positively charged dust plasma system containing boltzmann distributed electron species, cold inertial ion species, and stationary positively charged dust (pcd) species are considered. the roles of pcd species in the formation of ion-acoustic (ia) subsonic solitary waves (sws) are investigated by the pseudo-potential approach, which is valid for arbitrary amplitude time-independent subsonic sws, as well as by the reductive perturbation method, which is valid for the time-dependent small amplitude subsonic sws. it is observed that the presence of the pcd species reduces the phase speed of the ia waves, and consequently supports the ia subsonic sws with the positive wave potential in such electron-ion-pcd plasmas. this is due to the reduction of the space charge electric field by the presence of the pcd species. the applications of the work in space environments (viz. earth's mesosphere, cometary tails, jupiter's magnetosphere, etc.), where pcd species have been detected, are briefly discussed. | roles of positively charged dust in the formation of ion-acoustic subsonic solitary waves in electron-ion-positively charged dust plasmas |
context. the esa/rosetta mission has been orbiting comet 67p/churyumov-gerasimenko since august 2014, measuring its dayside plasma environment. the ion spectrometer onboard rosetta has detected two ion populations, one energetic with a solar wind origin (h+, he2+, he+), the other at lower energies with a cometary origin (water group ions such as h2o+). he+ ions arise mainly from charge-exchange between solar wind alpha particles and cometary neutrals such as h2o.aims: the he+ and he2+ ion fluxes measured by the rosetta plasma consortium ion composition analyser (rpc-ica) give insight into the composition of the dayside neutral coma, into the importance of charge-exchange processes between the solar wind and cometary neutrals, and into the way these evolve when the comet draws closer to the sun.methods: we combine observations by the ion spectrometer rpc-ica onboard rosetta with calculations from an analytical model based on a collisionless neutral haser atmosphere and nearly undisturbed solar wind conditions.results: equivalent neutral outgassing rates q can be derived using the observed rpc-ica he+/he2+ particle flux ratios as input into the analytical model in inverse mode. a revised dependence of q on heliocentric distance rh in au is found to be rh-7.06 between 1.8 and 3.3 au, suggesting that the activity in 2015 differed from that of the 2008 perihelion passage. conversely, using an outgassing rate determined from optical remote sensing measurements from earth, the forward analytical model results are in relatively good agreement with the measured rpc-ica flux ratios. modelled ratios in a 2d spherically-symmetric plane are also presented, showing that charge exchange is most efficient with solar wind protons. detailed cometocentric profiles of these ratios are also presented.conclusions: in conclusion, we show that, with the help of a simple analytical model of charge-exchange processes, a mass-capable ion spectrometer such as rpc-ica can be used as a "remote-sensing" instrument for the neutral cometary atmosphere. | the atmosphere of comet 67p/churyumov-gerasimenko diagnosed by charge-exchanged solar wind alpha particles |
meteor showers and their outbursts are the dominant source of meteoroid impact risk to spacecraft on short time-scales. meteor shower prediction models depend on historical observations to produce accurate forecasts. however, the current lack of quality and persistent world-wide monitoring at optical meteoroid sizes has left some recent major outbursts poorly observed. a novel method of computing meteor shower flux is developed and applied to global meteor network data. the method is verified against previously published observations of the perseids and the geminids. the complete mathematical and algorithmic details of computing meteor shower fluxes from video observations are described. as an example application of our approach, the flux measurements of the 2021 perseid outburst, the 2020-2022 quadrantids, and 2020-2021 geminids are presented. the flux of the 2021 perseids reached similar levels to the 1991-1994 and 2016 outbursts (zhr ~ 280). the flux of the quadrantids shows high year-to-year variability in the core of the stream while the longer lasting background activity is less variable, consistent with an age difference between the two components. the geminids show a double peak in flux near the time of peak. | computing optical meteor flux using global meteor network data |
context. ices are present in comets and in the mantles of interstellar grains. their chemical composition has been indirectly derived by observing molecules released in the gas phase, when comets approach the sun and when ice mantles are sublimated or destroyed, e.g. in the hot cores present in high-mass, star-forming regions. comparison of these chemical compositions sheds light on the formation of comets and on the evolution of interstellar matter from the molecular cloud to a protoplanetary disk, and it shows, to first order, a good agreement between the cometary and interstellar abundances. however, a complex o-bearing organic molecule, ethylene glycol (ch2oh)2, seems to depart from this correlation because it was not easily detected in the interstellar medium (sgr b2) although it proved to be rather abundant with respect to other o-bearing species in comet c/1995 o1 (hale-bopp). ethylene glycol thus appears, together with the closely related molecules glycolaldehyde ch2ohcho and ethanol ch3ch2oh, as a key species in the comparison of interstellar and cometary ices as well as in any discussion on the formation of cometary matter.aims: it is important to measure the molecular abundances in various hot cores to see if the observed differences between the interstellar medium and the comets are general. we focus here on the analysis of ethylene glycol in the nearest and best studied hot core-like region, orion-kl.methods: we use alma interferometric data because high spatial resolution observations allow us to reduce the line confusion problem with respect to single-dish observations since different molecules are expected to exhibit different spatial distributions. furthermore, a large spectral bandwidth is needed because many individual transitions are required to securely detect large organic molecules. confusion and continuum subtraction are major issues and have been handled with care.results: we have detected the agg' conformer of ethylene glycol in orion-kl. the emission is compact and peaks towards the hot core close to the main continuum peak, about 2″ to the south-west; this distribution is notably different from other o-bearing species. assuming optically thin lines and local thermodynamic equilibrium, we derive a rotational temperature of 145 ± 30 k and a column density of 4.6 ± 0.8 × 1015 cm-2. the limit on the column density of the ggg' conformer is five times lower. based on observations carried out with alma and the iram plateau de bure interferometer. iram is supported by insu/cnrs (france), mpg (germany) and ign (spain).appendix a is available in electronic form at http://www.aanda.org | antifreeze in the hot core of orion. first detection of ethylene glycol in orion-kl |
the tunguska event remained enigmatic for almost 100 years until the collision of comet shoemaker-levy 9 with jupiter in 1994 helped to resolve this enigma and allowed us to adequately interpret the more recent chelyabinsk event. airbursts typically occur if a meteoroid entering earth's atmosphere is 10-100 m in diameter, i.e., its energy ranges from 0.5 (chelyabinsk) to 20 (tunguska) mt tnt. all this energy is released in the atmosphere with strong shock waves generated during the entry reaching the surface and causing substantial damage. atmospheric plumes are capable of dispersing extraterrestrial materials worldwide. modern civilization is extremely vulnerable to those relatively small disturbances that recur on a decadal timescale and are still difficult to predict. | from tunguska to chelyabinsk via jupiter |
selection of a suitable low-pressure sprinkler can be an effective measure to reduce energy consumption, wind drift and evaporation losses. the crop canopy often redistributes the applied water during sprinkler irrigation. therefore, we investigated the water distributions above and below the canopy as well as their differences in heermann and hein uniformity coefficient (cuh) under three commonly used sprinkler types (nelson d3000, nelson r3000, and komet kpt) and two growth stages of summer maize (6-leaf stage and tasselling stage) to evaluate the effect of crop canopy on the water distribution of low-pressure sprinklers. the variations in the soil water content (swc) and heermann and hein uniformity coefficient of soil water content (cuhs) were investigated further at 1 h before, 1 h after, and 24 h. additionally, the effects of growth indices and sprinkler types on cuh and cuhs values were analysed. both the water application depths and cuh values above the canopy were significantly higher than those below the canopy and the differences above and below the canopy increased significantly with the growth of maize. the cuhs after irrigation mainly depended on the initial swc and cuhs values but was minimally correlated with the plant height, leaf area index, and sprinkler application uniformity. although the cuh above the canopy (78.1%) of d3000 was significantly lower than r3000 and kpt sprinklers (both exceeding 90%), the cuhs of d3000 reached more than 93% after irrigation. this result indicated that the effect of sprinkler application uniformity was not important as predicted, and a good cuhs would be achieved after canopy interception and soil water redistribution irrespective of sprinkler types. this study provides an insight into the selection of best low-pressure sprinklers in centre pivot irrigation systems. | water distributions of low-pressure sprinklers as affected by the maize canopy under a centre pivot irrigation system |
the solar wind is a highly turbulent medium exhibiting scalings of the fluctuations ranging over several decades of scales from the correlation length down to proton and electron gyroradii, thus suggesting a self-similar nature for these fluctuations. during its journey, the solar wind encounters the region of space surrounding earth dominated by the geomagnetic field which is called magnetosphere. the latter is exposed to the continuous buffeting of the solar wind which determines its characteristic comet-like shape. the solar wind and the magnetosphere interact continously, thus constituting a coupled system, since perturbations in the interplanetary medium cause geomagnetic disturbances. however, strong variations in the geomagnetic field occur even in absence of large solar perturbations. in this case, a major role is attributed to solar wind turbulence as a driver of geomagnetic activity especially at high latitudes. in this review, we report about the state-of-art related to this topic. since the solar wind and the magnetosphere are both high reynolds number plasmas, both follow a scale-invariant dynamics and are in a state far from equilibrium. moreover, the geomagnetic response, although closely related to the changes of the interplanetary magnetic field condition, is also strongly affected by the intrinsic dynamics of the magnetosphere generated by geomagnetic field variations caused by the internal conditions. | the effect of solar-wind turbulence on magnetospheric activity |
the lander philae, part of the esa rosetta mission successfully landed on comet 67p/churyumov- gerasimenko on november 12th, 2014. after several (unplanned) bounces it performed a first scientific sequence (fss), based on the energy stored in its on board batteries. all ten instruments of the payload aboard philae have been operated at least once. due to the fact that the final landing site was poorly illuminated, philae went into hibernation on november 15th. signals from the lander were received again in june and july 2015, which indicated multiple awakening episodes of the lander. however, various attempts to re-establish reliable and stable communications links failed. based on the analysis of the data gained during fss, and during the contacts in june and july 2015 we draw conclusions on the state of philae. in addition, images from the osiris camera aboard the rosetta orbiter have allowed the identification of the exact position of philae and its attitude, relative to the local surface terrain. this paper also gives an overview of the implications of philae results for future engineering comet models, required particularly for the design of in-situ (landing) or sample return missions. rosetta is an esa mission with contributions from its member states and nasa. rosetta's philae lander is provided by a consortium led by dlr, mps, cnes and asi with additional contributions from hungary, uk, finland, ireland and austria. | rosetta lander - philae: operations on comet 67p/churyumov-gerasimenko, analysis of wake-up activities and final state |
the d/h ratio is a clue to the origin and evolution of hydrogen-bearing chemical species in solar system materials. d/h has been observed in the coma of many comets, but most such measurements have been for gaseous water. we present the first in situ measurements of the d/h ratios in the organic refractory component of cometary dust particles collected at very low impact speeds in the coma of comet 67p/churyumov-gerasimenko (hereafter 67p) by the cosima instrument onboard rosetta. the values measured by cosima are spatial averages over an approximately 35 × 50 µm2 area. the average d/h ratio for the 25 measured particles is (1.57 ± 0.54) × 10-3, about an order of magnitude higher than the vienna standard mean ocean water (vsmow), but more than an order of magnitude lower than the values measured in gas-phase organics in solar-like protostellar regions and hot cores. this relatively high averaged value suggests that refractory carbonaceous matter in comet 67p is less processed than the most primitive insoluble organic matter (iom) in meteorites, which has a d/h ratio in the range of about 1 to 7 × 10-4. the cometary particles measured in situ also have a higher h/c ratio than the iom. we deduce that the measured d/h in cometary refractory organics is an inheritance from the presolar molecular cloud from which the solar system formed. the high d/h ratios observed in the cometary particles challenges models in which high d/h ratios result solely from processes that operated in the protosolar disc. | d/h in the refractory organics of comet 67p/churyumov-gerasimenko measured by rosetta/cosima |
the magnetic field at a comet is significantly influenced by the solar wind on one side and the outgassing rate on the other. there are no simple spatial models for the magnetic field direction, neither at a comet with low outgassing rates (∼1025 s-1) where ion gyroradius effects are non-negligible, nor at high outgassing rates (∼1027 s-1) where plasma boundaries form. however, the long duration of the european space agency rosetta mission has made it possible to track the evolution of the magnetic field while comet 67p/churyumov-gerasimenko approaches the sun. herein we present a simple model that fits the data quite well, depending on input parameters. the study also includes the influence of the comet's gas production rate and the solar wind conditions, which both have complex effects on the magnetic field, but are clearly recognizable. the evolution of the magnetic field direction related to draping is more complex than previously suggested. classical draping only exists at the comet for high outgassing rates, for lower rates, the magnetic field roughly follows the parker angle. it is shown that the interaction of the solar wind with the comet can be roughly divided into three main classes. | evolution of the magnetic field at comet 67p/churyumov-gerasimenko |
the plasma of comet 67p/churyumov-gerasimenko is analyzed based on the rpc-mip mutual impedance probe data of the rosetta mission. numerical simulations of the rpc-mip instrumental response considering two populations of electrons were fit on experimental responses acquired from january to september 2016 to extract the electron densities and temperatures. a time-tracking of the plasma parameters was performed, leading to the identification of a cold and a warm population of electrons during the period of interest. the respective densities and temperatures lie in the ranges [100; 1000] cm-3 and [0.05; 0.3] ev for the cold electrons and in the ranges [50; 500] cm-3 and [2; 10] ev for the warm electrons. warm electrons most of the time made up between 10 and 30% of the whole population, while the temperature ratio between warm and cold electrons lay mostly between 30 and 70 during the period we studied. the fluctuation range of the plasma parameters, that is, the electron densities and temperatures, appears to have remained rather constant during the last nine months of the mission. we take the limitations of the instrument that are due to the experimental noise into account in our discussion of the results. | plasma characterization at comet 67p between 2 and 4 au from the sun with the rpc-mip instrument |
the kinetic monte carlo method, used in astrochemistry to investigate suprathermal (hot) particles at the molecular level, is presented. different modifications of this method, aimed at studying the influence of suprathermal particles in the processes occurring in gas and dust envelopes surrounding astrophysical objects — prestellar and protostellar cores of molecular clouds, planets, their moons, and comets in the solar and extrasolar planetary systems — are considered. the important role of the fraction of suprathermal particles in astrochemical applications of this approach is demonstrated. the presence of these particles leads to local changes in the chemical composition; causes non-thermal emissions in gas and dust envelopes; enhances the chemical exchange between the gas and dust fractions of envelope; leads to the formation of extended hot coronae of planets; increases non-thermal atmospheric losses, thus determining the evolution of planetary atmosphere on astronomical time scales; and facilitates the formation of complex molecules in gas and dust envelopes of astrophysical objects. | suprathermal particles in astrochemistry |
measurements of the fractional momentum loss (sloss≡δ pt/pt ) of high-transverse-momentum-identified hadrons in heavy-ion collisions are presented. using π0 in au +au and cu +cu collisions at √{snn}=62.4 and 200 gev measured by the phenix experiment at the relativistic heavy ion collider and and charged hadrons in pb +pb collisions measured by the alice experiment at the large hadron collider, we studied the scaling properties of sloss as a function of a number of variables: the number of participants, npart, the number of quark participants, nqp, the charged-particle density, d nch/d η , and the bjorken energy density times the equilibration time, ɛbjτ0 . we find that the pt, where sloss has its maximum, varies both with centrality and collision energy. above the maximum, sloss tends to follow a power-law function with all four scaling variables. the data at √{snn}=200 gev and 2.76 tev, for sufficiently high particle densities, have a common scaling of sloss with d nch/d η and ɛbjτ0 , lending insight into the physics of parton energy loss. | scaling properties of fractional momentum loss of high-pt hadrons in nucleus-nucleus collisions at √{sn n} from 62.4 gev to 2.76 tev |
aims: we aim at analytically modelling the solar wind proton trajectories during their interaction with a partially ionised cometary atmosphere, not in terms of bulk properties of the flow but in terms of single particle dynamics.methods: we first derive a generalised gyromotion, in which the electric field is reduced to its motional component. steady-state is assumed, and simplified models of the cometary density and of the electron fluid are used to express the force experienced by individual solar wind protons during the interaction.results: a three-dimensional (3d) analytical expression of the gyration of two interacting plasma beams is obtained. applying it to a comet case, the force on protons is always perpendicular to their velocity and has an amplitude proportional to 1/r2. the solar wind deflection is obtained at any point in space. the resulting picture presents a caustic of intersecting trajectories, and a circular region is found that is completely free of particles. the particles do not lose any kinetic energy and this absence of deceleration, together with the solar wind deflection pattern and the presence of a solar wind ion cavity, is in good agreement with the general results of the rosetta mission.conclusions: the qualitative match between the model and the in situ data highlights how dominant the motional electric field is throughout most of the interaction region for the solar wind proton dynamics. the model provides a simple general kinetic description of how momentum is transferred between these two collisionless plasmas. it also shows the potential of this semi-analytical model for a systematic quantitative comparison to the data. | solar wind dynamics around a comet. a 2d semi-analytical kinetic model |
context. the electron temperature of the plasma is one important aspect of the environment. electrons created by photoionization or impact ionization of atmospheric gas have energies 10 ev. in an active comet coma, the gas density is high enough for rapid cooling of the electron gas to the neutral gas temperature (a few hundred kelvin). how cooling evolves in less active comets has not been studied before.aims: we aim to investigate how electron cooling varied as comet 67p/churyumov-gerasimenko changed its activity by three orders of magnitude during the rosetta mission.methods: we used in situ data from the rosetta plasma and neutral gas sensors. by combining langmuir probe bias voltage sweeps and mutual impedance probe measurements, we determined at which time cold electrons formed at least 25% of the total electron density. we compared the results to what is expected from simple models of electron cooling, using the observed neutral gas density as input.results: we demonstrate that the slope of the langmuir probe sweep can be used as a proxy for the presence of cold electrons. we show statistics of cold electron observations over the two-year mission period. we find cold electrons at lower activity than expected by a simple model based on free radial expansion and continuous loss of electron energy. cold electrons are seen mainly when the gas density indicates that an exobase may have formed.conclusions: collisional cooling of electrons following a radial outward path is not sufficient to explain the observations. we suggest that the ambipolar electric field keeps electrons in the inner coma for a much longer time, giving them time to dissipate energy by collisions with the neutrals. we conclude that better models are required to describe the plasma environment of comets. they need to include at least two populations of electrons and the ambipolar field. | cold electrons at comet 67p/churyumov-gerasimenko |
understanding the history and evolution of small bodies, such as dust grains and comets, in planet-forming disks is very important to reveal the architectural laws responsible for the creation of planetary systems. these small bodies in cold regions of the disks are typically considered to be mixtures of dust particles with molecular ices, where ices cover the surface of a dust core or are actually physically mixed with dust. while the first case, ice-on-dust, has been intensively studied in the laboratory in recent decades, the second case, ice-mixed-with-dust, presents uncharted territory. this work is the first laboratory study of the temperature-programmed desorption of water ice mixed with amorphous carbon and silicate grains. we show that the kinetics of desorption of h2o ice depends strongly on the dust/ice mass ratio, probably due to the desorption of water molecules from a large surface of fractal clusters composed of carbon or silicate grains. in addition, it is shown that water ice molecules are differently bound to silicate grains in contrast to carbon. the results provide a link between the structure and morphology of small cosmic bodies and the kinetics of desorption of water ice included in them. | temperature programmed desorption of water ice from the surface of amorphous carbon and silicate grains as related to planet-forming disks |
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. 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. originally foreseen till the end of 2015, the mission was extended for another nine months to follow the comet on its outbound arc of the orbit. in view of the acquired experience and of the approaching end of mission the spacecraft was flown at much closer distances from the nucleus so that the scientific instruments had the chance to perform unique measurements. following this phase of very close orbits, on the 30th of september 2016 rosetta was set on a collision course trajectory with the comet to terminate the mission with a controlled impact. this paper describes the details of the extended mission phase and the issues encountered during these months. it also includes the changes implemented on the spacecraft and in the operations concept to optimise the remaining mission time. the paper also includes the lessons learned from this unique and complex mission phase. | the final year of the rosetta mission |
winds from asymptotic giant branch (agb) stars not only provide mass and energy return, but also produce dust grains in massive elliptical galaxies. due to the fast stellar velocity, the wind is thought to form a comet-like tail, similar to mira in the local bubble. many massive elliptical galaxies and cluster centrals host extended dusty cold filaments. we carry out both analytical and numerical studies of the interaction between an agb wind and the surrounding hot gas. we find that the cooling time of the tail is inversely proportional to the ambient pressure. in the absence of cooling, or in low-pressure environments (e.g., the outskirts of elliptical galaxies), agb winds are quickly mixed into the hot gas, and all the agb winds have a similar appearance and head-to-tail ratio. in high-pressure environments, such as the local bubble and the central regions of massive elliptical galaxies, some of the gas in the mixing layer between the stellar wind and the surrounding hot gas can cool efficiently and cause the tail to become longer. our simulated tail of mira itself has a similar length and velocity to that observed, and appears similar to the simulated agb tail in the central regions of massive galaxies. while confirmation awaits future studies, we speculate that instead of thermal instability, the induced condensation at the mixing layer of agb winds may be the origin of cold filaments in massive galaxies and galaxy clusters. this naturally explains the existence of dust and polycyclic aromatic hydrocarbon in the filaments. | the fate of asymptotic giant branch winds in massive galaxies and the intracluster medium |
we consider a space-time continuous directed polymer in random environment. the path is brownian and the medium is poissonian. we review many results obtained in the last decade, and also we present new ones. in this fundamental setup, we can make use of fine formulas and strong tools from stochastic analysis for gaussian or poisson measure, together with martingale techniques. these notes cover the matter of a course presented during the jean-morlet chair 2017 of cirm "random structures in statistical mechanics and mathematical physics" in marseille. | brownian polymers in poissonian environment: a survey |
this paper presents the past evolution of the orbital elements of comet 67p/churyumov-gerasimenko, target of the rosetta spacecraft. the gravitational orbit of the comet is affected by the sublimation of ice from the nucleus that triggers non-gravitational forces. the comet also experienced several close encounters with jupiter, especially notable were those in 1959 and 1923 (less than 1 au). these perturbations cause the chaoticity of the comet orbit at short time scale. the goal of this paper is to have a precise idea of the comet's recent dynamical history. this is done by studying the mean trends of the orbital elements of the comet and also by characterising its chaotic motion with the fast lyapunov indicator. to compute these mean trends, two sets (considering or not the non-gravitational forces) of 1000 clone orbits of the comet were considered. this paper shows that the last close encounter with jupiter on february 4, 1959, drastically modified the orbital elements of the comet (especially the perihelion distance: from more than 2.7 au before the encounter to 1.3 au after the encounter). the motion of the comet is also shown to be chaotic before the close encounter with jupiter on october 2, 1923. the mean trends of the orbital elements of the comet are also presented in this paper (for two time scales: 270 years in the past and 10 000 years in the past). | the recent dynamical history of comet 67p/churyumov-gerasimenko |
we examine the massive colliding cluster el gordo, one of the most massive clusters at high redshift. we use a free-form lensing reconstruction method that avoids making assumptions about the mass distribution. we use data from the relics program and identify new multiply lensed system candidates. the new set of constraints and free-form method provide a new independent mass estimate of this intriguing colliding cluster. our results are found to be consistent with earlier parametric models, indirectly confirming the assumptions made in earlier work. by fitting a double gnfw profile to the lens model and extrapolating to the virial radius, we infer a total mass for the cluster of ${m}_{200c}=({1.08}_{-0.12}^{+0.65})\times {10}^{15}$ m ${}_{\odot }$ . we estimate the uncertainty in the mass due to errors in the photometric redshifts and discuss the uncertainty in the inferred virial mass due to the extrapolation from the lens model. we also find in our lens map a mass overdensity corresponding to the large cometary tail of hot gas, reinforcing its interpretation as a large tidal feature predicted by hydrodynamical simulations that mimic el gordo. finally, we discuss the observed relation between the plasma and the mass map, finding that the peak in the projected mass map may be associated with a large concentration of colder gas exhibiting possible star formation. el gordo is one of the first clusters that will be observed with jwst, which is expected to unveil new high-redshift lensed galaxies around this interesting cluster and provide a more accurate estimation of its mass. | free-form lens model and mass estimation of the high-redshift galaxy cluster act-cl j0102-4915, "el gordo" |
we present visible and mid-infrared imagery and photometry of temporary jovian co-orbital comet p/2019 ld2 taken with hubble space telescope/wide field camera 3 (hst/wfc3), spitzer space telescope/infrared array camera (spitzer/irac), and the growth telescope network, visible spectroscopy from keck/low-resolution imaging spectrometer (lris), and archival zwicky transient facility observations taken between 2019 april and 2020 august. our observations indicate that the nucleus of ld2 has a radius between 0.2 and 1.8 km assuming a 0.08 albedo and a coma dominated by ∼100 μm-scale dust ejected at ∼1 m s-1 speeds with a ∼1' jet pointing in the southwest direction. ld2 experienced a total dust mass loss of ∼108 kg at a loss rate of ∼6 kg s-1 with afρ/cross section varying between ∼85 cm/125 km2 and ∼200 cm/310 km2 from 2019 april 9 to 2019 november 8. if the increase in afρ/cross section remained constant, it implies ld2's activity began ∼2018 november when within 4.8 au of the sun, implying the onset of h2o sublimation. we measure co/co2 gas production of ≲1027 mol s-1/≲1026 mol s-1 from our 4.5 μm spitzer observations; g-r = 0.59 ± 0.03, r-i = 0.18 ± 0.05, and i-z = 0.01 ± 0.07 from growth observations; and h2o gas production of ≲80 kg s-1 scaling from our estimated c2 production of ${q}_{{c}_{2}}\lesssim 7.5\times {10}^{24}$ mol s-1 from keck/lris spectroscopy. we determine that the long-term orbit of ld2 is similar to jupiter-family comets having close encounters with jupiter within ∼0.5 hill radius in the last ∼3 y and within 0.8 hill radius in ∼9 y. additionally, 78.8% of our orbital clones are ejected from the solar system within 1 × 106 yr, having a dynamical half-life of 3.4 × 105 yr. | initial characterization of active transitioning centaur, p/2019 ld2 (atlas), using hubble, spitzer, ztf, keck, apache point observatory, and growth visible and infrared imaging and spectroscopy |
the neutral and plasma environment is critical in understanding the interaction of the solar wind and comet 67p/churyumov-gerasimenko (cg), the target of the european space agency's rosetta mission. to serve this need and support the rosetta mission, we have developed a 3-d four-fluid model, which is based on bats-r-us (block-adaptive tree solarwind roe-type upwind scheme) within swmf (space weather modeling framework) that solves the governing multifluid mhd equations and the euler equations for the neutral gas fluid. these equations describe the behavior and interactions of the cometary heavy ions, the solar wind protons, the electrons, and the neutrals. this model incorporates different mass loading processes, including photoionization and electron impact ionization, charge exchange, dissociative ion-electron recombination, and collisional interactions between different fluids. we simulated the plasma and neutral gas environment near perihelion in three different cases: an idealized comet with a spherical body and uniform neutral gas outflow, an idealized comet with a spherical body and illumination-driven neutral gas outflow, and comet cg with a realistic shape model and illumination-driven neutral gas outflow. we compared the results of the three cases and showed that the simulations with illumination-driven neutral gas outflow have magnetic reconnection, a magnetic pileup region and nucleus directed plasma flow inside the nightside reconnection region, which have not been reported in the literature. | four-fluid mhd simulations of the plasma and neutral gas environment of comet 67p/churyumov-gerasimenko near perihelion |
context. after the landing of philae, consert probed the nucleus of 67p/churyumov-gerasimenko (67p) and observed no heterogeneities at metric scale within the probed part of the small lobe of 67p. further studies have then quantified the observed homogeneity in terms of maximum permittivity contrast versus the typical size of heterogeneities.aims: the aim of this article is to interpret the sensitivity limits of consert measurements in terms of composition, and to provide constraints on the maximum variability in composition, porosity, and local dust-to-ice ratio.methods: the sensitivity of consert measurements to local variations in density, dust-to-ice ratio, and composition was analyzed using permittivity modeling of mixtures.results: we interpret the maximum detectable heterogeneity size and contrast in terms of composition and porosity of the nucleus. the sensitivity to porosity is ±10 percent points for heterogeneities with a characteristic length scale of a few meters; the sensitivity to local variations in the composition is limited.conclusions: in terms of accretion, our results are compatible only with scenarios generating porosity heterogeneities at scales lower than one meter, or with porosity variations smaller than ±10 percent points. this is clearly compatible with an accretion model of a gentle gravitational collapse of a pebble cloud. | homogeneity of 67p/churyumov-gerasimenko as seen by consert: implication on composition and formation |
pockmarks consists of depressions on the seafloor and are often considered to originate from fluid escape activities. in this study, we report 125 pockmarks near the andu seamount in the southern margin of the south china sea (scs) and preliminarily investigate their genesis based on multi-beam bathymetry and multichannel seismic reflection data. most of them are giant and mega pockmarks and display circular, elliptical, elongated, crescent, comet and irregular shapes on a plan-view. the mapped pockmarks cover a total surface of 291.354 km2. most of them occur in strings or clusters, while some are isolated. the filled volume of pockmark is calculated for the first time in this region, which may have significant implications for sedimentary process. beneath most of the pockmarks, high-angle faults, high-amplitude reflections, filled-up structures and other fluid escape features are imaged on seismic sections. the structural settings, especially the fault systems, may control the genesis and development of pockmarks in this area. we also speculate that seabed gradients may contribute to generation of crescent pockmarks. according to seismic sections, sediment types and morphological analysis, we finally proposed that the genesis of these pockmarks may mainly be triggered by gas escape activities in the study area. this work may have important implications for the gas exploration in the nansha region, southern south china sea. | a preliminary study on morphology and genesis of giant and mega pockmarks near andu seamount, nansha region (south china sea) |
carbon-based nanomaterials are being increasingly used, demanding strong information to support their safety in terms of human health. as ingestion is one of the most important exposure routes in humans, we have determined their potential risk by using an in vitro model simulating the human intestinal barrier and evaluated the effects of both graphene oxide (go) and graphene nanoplatelets (gnps). a coculture of differentiated caco-2/ht29 cells presenting inherent intestinal epithelium characteristics (i.e. mucus secretion, brush border, tight junctions, etc.) were treated with go or gnps for 24 h. different endpoints such as viability, membrane integrity, nps localization, cytokines secretion, and genotoxic damage were evaluated to have a wide view of their potentially harmful effects. no cytotoxic effects were observed in the cells that constitute the barrier model. in the same way, no adverse effects were detected neither in the integrity of the barrier (teer) nor in its permeability (ly). nevertheless, a different bio-adhesion and biodistribution behavior was observed for go and gnps by confocal microscopy analysis, with a more relevant uptake of gnps. no oxidative damage induction was detected, either by the dcfh-da assay or the fpg enzyme in the comet assay. conversely, both go and gnps were able to induce dna breaks, as observed in the comet assay. finally, low levels of anti-inflammatory cytokines were detected, suggesting a weak anti-inflammatory response. our results show the moderate/severe risk posed by go/gnps exposures, given the observed genotoxic effects, suggesting that more extensive genotoxic evaluations must be done to properly assess the genotoxic hazard of these nanomaterials. | interactions of graphene oxide and graphene nanoplatelets with the in vitro caco-2/ht29 model of intestinal barrier |
because of the diurnal thermal cycle and the irregular shape of the nucleus, gas outflow of comet 67p/churyumov-gerasimenko could be highly anisotropic as indicated by the colliminated dust jet structures on the sunlit side. based on the osiris imaging observations of the outgassing effect, a simple model of surface sublimation can be constructed by taking into account the dependence on the solar insolation. with preliminary information on the time variability of the global gas production rate, a sequence of gas coma models can be generated at different epochs before and after perihelion. we also investigate different patterns of dust particle dynamics under the influences of nuclear rotation and gas drag. from these considerations, a consistent picture of the spatial distribution of dusty materials across the surface of comet 67p as it moves around the perihelion can be developed. it is found that because of the redeposition of the ejected dust from the southern hemisphere to the northern hemisphere during the southern summer season the hapi region could gain up to 0.4 m while the wosret region would lose up to 1.8 m of dust mantle per orbit. | gas outflow and dust transport of comet 67p/churyumov-gerasimenko |
we report on the detection and characterization of more than 6300 polygons on the surface of the nucleus of comet 67p/churyumov-gerasimenko, using images acquired by the osiris camera onboard rosetta between august 2014 and march 2015. they are found in consolidated terrains and grouped in localized networks. they are present at all latitudes (from north to south) and longitudes (head, neck, and body), sometimes on pit walls or following lineaments. about 1.5% of the observed surface is covered by polygons. polygons have an homogeneous size across the nucleus, with 90% of them in the size range 1 - 5 m and a mean size of 3.0 ± 1.4 m. they show different morphologies, depending on the width and depth of their trough. they are found in networks with 3- or 4-crack intersection nodes. the polygons observed on 67p are consistent with thermal contraction crack polygons formed by the diurnal or seasonal temperature variations in a hard (mpa) and consolidated sintered layer of water ice, located a few centimeters below the surface. our thermal analysis shows an evolution of thermal contraction crack polygons according to the local thermal environment, with more evolved polygons (i.e. deeper and larger troughs) where the temperature and the diurnal and seasonal temperature range are the highest. thermal contraction crack polygons are young surface morphologies that probably formed after the injection of 67p in the inner solar system, typically 100,000 years ago, and could be as young as a few orbital periods, following the decreasing of its perihelion distance in 1959 from 2.7 to 1.3 a.u. meter scale thermal contraction crack polygons should be common features on the nucleus of jupiter family comets. | meter-scale thermal contraction crack polygons on the nucleus of comet 67p/churyumov-gerasimenko |
infrared (ir) band strengths are needed to extract accurate molecular abundances from astronomical observations of interstellar and solar system ices. however, laboratory measurements of such intensities often have required multiple assumptions about ice composition and thickness. here we describe a method that circumvents most of the common assumptions and uncertainties in ir band-strength determinations. we have applied the method to measure ir band strengths of ocs, h2s, and so2 in the absence and presence of h2o ice at 10 k, the first measurements of their type. our results show for the first time that the presence of h2o makes little difference in ir intensities for these three sulfur-containing molecules' strongest ir features. the immediate application will be to laboratory studies of low-temperature chemistry of interstellar and cometary ices. | a new method for measuring infrared band strengths in h2o ices: first results for ocs, h2s, and so2 |
the high obliquity (~50°) of comet 67p/churyumov-gerasimenko (67p) is responsible for a long-lasting winter polar night in the southern regions of the nucleus. we report observations made with the submillimeter and millimeter continuum channels of the microwave instrument onboard the rosetta orbiter (miro) of the thermal emission from these regions during the period august-october 2014. before these observations, the southern polar regions had been in darkness for approximately five years. subsurface temperatures in the range 25-50 k are measured. thermal model calculations of the nucleus near-surface temperatures carried out over the orbit of 67p, coupled with radiative transfer calculations of the miro channels brightness temperatures, suggest that these regions have a thermal inertia within the range 10-60 j m-2 k-1 s-0.5. such low thermal inertia values are consistent with a highly porous, loose, regolith-like surface. these values are similar to those derived elsewhere on the nucleus. a large difference in the brightness temperatures measured by the two miro continuum channels is tentatively attributed to dielectric properties that differ significantly from the sunlit side, within the first few tens of centimeters. this is suggestive of the presence of ice(s) within the miro depths of investigation in the southern polar regions. these regions started to receive sunlight in may of 2015, and refinements of the shape model in these regions, as well as continuing miro observations of 67p, will allow refining these results and reveal the thermal properties and potential ice content of the southern regions in more detail. | dark side of comet 67p/churyumov-gerasimenko in aug.-oct. 2014. miro/rosetta continuum observations of polar night in the southern regions |
we present a complete census of the 14n/15n isotopic ratio in the most abundant n-bearing molecules toward the cold envelope of the protocluster omc-2 fir4, the best known sun progenitor. to this scope, we analyzed the unbiased spectral survey obtained with the iram 30 m telescope at 3, 2, and 1 mm. we detected several lines of cn, hcn, hnc, hc3n, n2h+, and their respective 13c and 15n isotopologues. the lines’ relative fluxes are compatible with lte conditions, and moderate line opacities have been corrected via a population diagram method or theoretical relative intensity ratios of the hyperfine structures. the five species lead to very similar 14n/15n isotopic ratios, without any systematic difference between amine- and nitrile-bearing species as previously found in other protostellar sources. the weighted average of the 14n/15n isotopic ratio is 270 ± 30. this 14n/15n value is remarkably consistent with the [250-350] range measured for the local galactic ratio but significantly differs from the ratio measured in comets (around 140). high-angular resolution observations are needed to examine whether this discrepancy is maintained at smaller scales. in addition, using the cn, hcn, and hc3n lines, we derived a 12c/13c isotopic ratio of 50 ± 5. | first measurement of the 14n/15n ratio in the analog of the sun progenitor omc-2 fir4 |
we present the ion composition in the coma of comet 67p with newly detected ion species over the 28-37 u mass range, probed by rosetta orbiter spectrometer for ion and neutral analysis (rosina)/double focusing mass spectrometer (dfms). in summer 2015, the nucleus reached its highest outgassing rate and ion-neutral reactions started to take place at low cometocentric distances. minor neutrals can efficiently capture protons from the ion population, making the protonated version of these neutrals a major ion species. so far, only nh_4^+ has been reported at comet 67p. however, there are additional neutral species with proton affinities higher than that of water (besides nh3) that have been detected in the coma of comet 67p: ch3oh, hcn, h2co and h2s. their protonated versions have all been detected. statistics showing the number of detections with respect to the number of scans are presented. the effect of the negative spacecraft potential probed by the rosetta plasma consortium/langmuir probe on ion detection is assessed. an ionospheric model has been developed to assess the different ion density profiles and compare them to the rosina/dfms measurements. it is also used to interpret the rosina/dfms observations when different ion species have similar masses, and their respective densities are not high enough to disentangle them using the rosina/dfms high-resolution mode. the different ion species that have been reported in the coma of 67p are summarized and compared with the ions detected at comet 1p/halley during the giotto mission. | ion composition at comet 67p near perihelion: rosetta observations and model-based interpretation |
here we try to find the origin of 1i/2017 u1 'oumuamua, the first interstellar object recorded inside the solar system. to this aim, we searched for close encounters between 'oumuamua and all nearby stars with known kinematic data during their past motion. we had checked over 200 thousand stars and found just a handful of candidates. if we limit our investigation to within a 60 pc sphere surrounding the sun, then the most probable candidate for the 'oumuamua parent stellar habitat is the star ucac4 535-065571. however gj 876 is also a favourable candidate. however, the origin of 'oumuamua from a much more distant source is still an open question. additionally, we found that the quality of the original orbit of 'oumuamua is accurate enough for such a study and that none of the checked stars had perturbed its motion significantly. all numerical results of this research are available in the appendix. | investigating the dynamical history of the interstellar object 'oumuamua |
the isotopic ratio of nitrogen in nearby protoplanetary disks, recently measured in cn and hcn, indicates that a fractionated reservoir of volatile nitrogen is available at the earliest stage of comet formation. this reservoir also presents a 3:1 enrichment in 15n relative to the elemental ratio of 330, identical to that between the solar system comets and the protosun, suggesting that similar processes are responsible for the fractionation in the protosolar nebula (psn) and in these psn analogs. however, where, when, and how the fractionation of nitrogen takes place is an open question. previously obtained hcn/hc15n abundance ratios suggest that hcn may already be enriched in 15n in prestellar cores, although doubts remain on these measurements, which rely on the double-isotopologue method. here we present direct measurements of the hcn/h13cn and hcn/hc15n abundance ratios in the l1498 prestellar core based on spatially resolved spectra of hcn(1-0), (3-2), h13cn(1-0), and hc15n(1-0) rotational lines. we use state-of-the-art radiative transfer calculations using alico, a 1d radiative transfer code capable of treating hyperfine overlaps. from a multiwavelength analysis of dust emission maps of l1498, we derive a new physical structure of the l1498 cloud. we also use new, high-accuracy hcn-h2 hyperfine collisional rates, which enable us to quantitatively reproduce all the features seen in the line profiles of hcn(1-0) and hcn(3-2), especially the anomalous hyperfine line ratios. special attention is devoted to derive meaningful uncertainties on the abundance ratios. the obtained values, hcn/h13cn = 45 ± 3 and hcn/hc15n = 338 ± 28, indicate that carbon is heavily fractionated in hcn, but nitrogen is not. for the h13cn/hc15n abundance ratio, our detailed study validates to some extent analyses based on the single excitation temperature assumption. comparisons with other measurements from the literature suggest significant core-to-core variability. furthermore, the heavy 13c enrichment we found in hcn could explain the superfractionation of nitrogen measured in solar system chondrites. | abundance of hcn and its c and n isotopologues in l1498 |
evidence for long-lived sources of cryovolcanism on the nucleus of the comet 29p/schwassmann-wachmann has been found from a study of its times of outburst (t0) and the morphological development of inner coma structures. analysis of data from the minor planet center observations archive spanning 2002-2014 and other observations have yielded 64 outburst times of mainly well-observed events with a median timing uncertainty of 0.40 d. outbursts comprise largely (i) isolated explosive events; or (ii) multiple outbursts occurring typically within 5-15 d of each other. on rare occasions, a form of continuous or gradually increasing activity is manifest, appearing to be the result of a series of mini-outbursts. quasi-periodicity in t0 is manifested as an excess of outbursts every 52-60 d, along with a paucity of events every ∼30 d and ∼90 d. seasonal changes in activity are evident from the temporal analysis of the outburst data. an unambiguous periodicity of 57.6 ± 0.4 d has been found in the times of 26 outbursts during 2010-2014, with all active sources at that time localised within a longitude span of ∼135-150°. cluster analysis of t0 data for 2002-2010 and 2010-2014, and hst imaging from 1996 confirm and refine the apparent periodicity, indicating that outbursts appear to be grouped in longitude centred on at least 6 circumferential locations. sources of activity generally persist for at least 10-20 yr, and some appear discrete in nature, able to re-outburst after a single day-night cycle. given that outbursts are triggered by solar heating, the analysis yields a value for the mean solar day of 57.71 ± 0.06 d, equivalent to a sidereal rotation period of 57.09 ± 0.06 d, assuming the more probable prograde direction of spin. a novel outburst mechanism is outlined in which some cometary ices, principally solid ch4, confined under pressure (>12 kpa) beneath a stabilisation crust, begin to melt and absorb supervolatile gases, mainly co and n2. these gases liberate considerable heat (5-7 kj mol-1) via their enthalpy of solution inducing further melting deep within the nucleus where direct insolation heating is absent. this gas-solute process is most active near the solid-liquid interface, where the solvent temperature is lowest and gas solubility is highest. an outburst occurs when insolation heating of the crust above a gas-laden subsurface reservoir softens paraffinic hydrocarbons and causes a crustal plate to dislodge under the accumulated gas pressure, the sudden release of which provokes the explosive ex-solution of dissolved gases, principally co, propelling entrained dust and debris into space. fissures reseal as the plate sinks back under the gravitational influence of the large nucleus and the adhesive, waxy hydrocarbon fraction solidifies, permitting a new outburst cycle to begin. a detailed account of the gas ex-solution mechanism is the subject of a partner paper (miles, r. [2015]. icarus). | discrete sources of cryovolcanism on the nucleus of comet 29p/schwassmann-wachmann and their origin |
the nonlinear propagation of dust acoustic (da) waves (daws) in an unmagnetized electron depleted dusty plasma (containing opposite polarity warm dust grains and non-extensive positive ions) has been theoretically investigated. the nonlinear schrödinger equation (nlse) is derived by employing the reductive perturbation method. it is observed that the dusty plasma system under consideration supports two branches of modes, namely, fast and slow da modes, and that both of these two modes can be modulationally stable or unstable depending on the sign of ratio of the dispersive and nonlinear coefficients of the nlse. the conditions for the formation of bright and dark envelope solitons as well as the first- and second-order rogue waves have been examined. the numerical analysis has shown that the basic features (viz., stability/instability, amplitude and width of the envelope solitons and rogue structures, etc.) of the daws associated with the fast and slow da modes are significantly modified by dust masses, dust charges, non-extensivity of ions, and other various plasma parameters. the implications of the results to space plasma research (viz., magnetosphere of jupiter, upper mesosphere, saturn's f-ring, cometary tail, etc.) are briefly discussed. | dust-acoustic envelope solitons and rogue waves in an electron depleted plasma |
context. comets are primitive objects, remnants of the volatile-rich planetesimals from which the solar system condensed. knowing their structure and composition is thus crucial for the understanding of our origins. after the successful landing of philae on the nucleus of 67p/churyumov-gerasimenko in november 2014, for the first time, the rosetta mission provided the opportunity to measure the low frequency electrical properties of a cometary mantle with the permittivity probe sesame-pp (surface electric sounding and acoustic monitoring experiment-permittivity probe).aims: in this paper, we conduct an in-depth analysis of the data from active measurements collected by sesame-pp at abydos, which is the final landing site of philae, to constrain the porosity and, to a lesser extent, the composition of the surface material down to a depth of about 1 m.methods: sesame-pp observations on the surface are then analyzed by comparison with data acquired during the descent toward the nucleus and with numerical simulations that explore different possible attitudes and environments of philae at abydos using a method called the capacity-influence matrix method.results: reasonably assuming that the two receiving electrode channels have not drifted with respect to each other during the ten-year journey of the rosetta probe to the comet, we constrain the dielectric constant of the first meter below the surface at abydos to be >2.45 ± 0.20, which is consistent with a porosity <50% if the dust phase is analogous to carbonaceous chondrites and <75% in the case of less primitive ordinary chondrites. this indicates that the near surface of the nucleus of 67p/churyumov-gerasimenko is more compacted than its interior and suggests that it could consist of a sintered dust-ice layer. | electrical properties and porosity of the first meter of the nucleus of 67p/churyumov-gerasimenko. as constrained by the permittivity probe sesame-pp/philae/rosetta |
sublimating gas molecules scatter off of the surface of an icy body in the same manner as photons (lambertian scattering). this means that for every photon-driven body force, there should be a sublimation-driven analog that affects icy bodies. thermal photons emitted from the surfaces of asymmetrically shaped bodies in the solar system generate net torques that change the spin rates of these bodies over time. the long-term averaging of this torque is called the yorp effect. here we propose a sublimation-driven analog to the yorp effect (sublimation-yorp or syorp), in which sublimating gas molecules emitted from the surfaces of icy bodies in the solar system also generate net torques on the bodies. however, sublimating gas molecules carry ∼104-105 times more momentum away from the body than thermal photons, resulting in much greater body torques. previous studies of sublimative torques focused on emissions from highly localized sources on the surfaces of jupiter family comet nuclei, and have therefore required extensive empirical observations to predict the resulting behavior of the body. by contrast, syorp applies to non-localized emissions across the entire body, which likely dominates sublimation-drive torques on small icy chunks and dynamically young comets outside the jupiter family, and can therefore be applied without high-resolution spacecraft observations of their surfaces. instead, we repurpose the well-tested mathematical machinery of the yorp effect to account for sublimation-driven torques. we show how an syorp-driven mechanism best matches observations of the rarely observed, sun-oriented linear features (striae) in the tails of comets, whose formation mechanism has remained enigmatic for decades. the syorp effect naturally explains why striae tend to be observed between near-perihelion and ∼1 au from the sun for comets with perihelia less than 0.6 au, and solves longstanding problems with moving enough material into the cometary tail to form visible striae. we show that the syorp mechanism can form striae that match the striae of comet west, estimate the sizes of the stria-forming chunks, and produce a power-law fit to these parent chunks with a power law index of -1.4-0.6+0.3 . lastly, we predict potential observables of this syorp mechanism, which may appear as clouds or material that appear immediately prior to stria formation, or as a faint, wispy dust feature within the dust tail, between the nucleus and the striae. | the formation of striae within cometary dust tails by a sublimation-driven yorp-like effect |
we present a model of cometary dust capable of simulating the dynamics within the first few tens of km of the comet surface. recent measurements by the grain impact analyser and dust accumulator and cometary secondary ion mass analyser instruments on rosetta show that the nucleus emits fluffy dust particles with porosities above 50 per cent and sizes up to at least mm (fulle et al. 2015b; rotundi et al. 2015; schulz et al. 2015). retrieval of the physical properties of these particles requires a model of the effective forces governing their dynamics. here, we present a model capable of simulating realistic, large and porous particles using hierarchical aggregates, which shows previous extrapolations to be inadequate. the main strengths of our approach are that we can simulate very large (mm-scale) non-spherical agglomerates and can accurately determine their (1) effective cross-section and ratio of cross-section to mass, (2) gas drag coefficient, and (3) light scattering properties. in practical terms, we find that a more detailed treatment of the dust structure results in three to five times higher velocities for large dust particles in the inner coma than previously estimated using spherical particles of the same mass. we apply our model to the dynamics of dust in the vicinity of the nucleus of comet 67p and successfully reproduce the dust speeds reported early on when the comet was roughly 3.5 au from the sun. at this stage, we employ a simple spherical comet nucleus, we model activity as constant velocity gas expansion from a uniformly active surface, and use mie scattering. we discuss pathways to improve on these simplifications in the future. | acceleration of cometary dust near the nucleus: application to 67p/churyumov-gerasimenko |
to explore the hypothesis that kic 8462852's aperiodic dimming is caused by artificial megastructures in orbit, rather than a natural cause such as cometary fragments in a highly elliptical orbit, we searched for electromagnetic signals from kic 8462852 indicative of extraterrestrial intelligence. the primary observations were in the visible optical regime using the boquete optical seti observatory in panama. in addition, as a recommended preparatory exercise for the possible future detection of a candidate signal, three of six observing runs simultaneously searched radio frequencies at the allen telescope array in california. no periodic optical signals greater than 67 photons m-2 within a time frame of 25 ns were seen. if, for example, any inhabitants of kic 8462852 were targeting our solar system with 5 mj laser pulses, locally illuminating an approximately 3 au diameter disk, the signal could have been detected at the boquete observatory. the limits on narrowband radio signals were 180-300 jy hz at 1 and 8 ghz, respectively. while the power requirement for a detectable, isotropic narrowband radio transmission from kic 8462852 is quite high, even modest targeting on the part of the putative extraterrestrials can lower this power substantially. | optical seti observations of the anomalous star kic 8462852 |
we obtained broad- and narrowband images of the hyperactive comet 46p/wirtanen on 33 nights during its 2018/2019 apparition, when the comet made a historic close approach to the earth. with our extensive coverage, we investigated the temporal behavior of the comet on both seasonal and rotational timescales. we used cn observations to explore the coma morphology, revealing that there are two primary active areas that produce spiral structures. the direction of rotation of these structures changes from pre- to postperihelion, indicating that the earth crossed the comet's equatorial plane sometime around perihelion. we also used the cn images to create photometric light curves that consistently show two peaks in the activity, confirming the two source regions. we measured the nucleus's apparent rotation period at a number of epochs using both the morphology and the light curves. these results all show that the rotation period is continuously changing throughout our observation window, increasing from 8.98 hr in early november to 9.14 hr around perihelion and then decreasing again to 8.94 hr in february. although the geometry changes rapidly around perihelion, the period changes cannot be primarily due to synodic effects. the repetition of structures in the coma, both within a night and from night to night, strongly suggests that the nucleus is in a near-simple rotation state. we also detected two outbursts, one on december 12 and the other on january 28. using the apparent velocities of the ejecta in these events, 68 ± 5 and 162 ± 15 m s-1, respectively, we derived start times of 2018 december 12 at 00:13 ut ± 7 minutes and 2019 january 27 at 20:01 ut ± 30 minutes. | narrowband observations of comet 46p/wirtanen during its exceptional apparition of 2018/19. i. apparent rotation period and outbursts |
multi-band photometric monitoring of comet 29p/schwassmann-wachmann 1 was conducted with the standard johnson-cousins filter set. observations extended from 2018 july to 2021 december. the comet was detected to show at least 12 relatively large outbursts, during which its brightness increased by 1.5 to 5 mag as measured through a 5″ aperture. the outbursts resulted in a clear variation of the cometary brightness profile. the derived slopes of the surface brightness profiles showed a significant variation with time from a shallower slope to a steeper one at the beginning of each outburst and then slowly returning to pre-outburst values. there did not seem to be any obvious change in the color indices as the outbursts occurred. however, for the quadruple outbursts in late september of 2021, we could confidently spot a change in the color of the comet, indicating that the color indices seem to be less than the mean values, especially in the b - v term. dust production derived by using the dust production rate parameter, afρ, from the r-band photometry measurements shows the outburst to be accompanied by a large increasing trend. using a simple model and the derived outflow velocity of 0.11 km s-1 from the expanding shell features, an estimated lower limit of 1.0 × 108 kg up to 2.7 × 109 kg of dust was released during the quadruple outbursts by using a specific dust size of 1 μm. | long-term monitoring of comet 29p/schwassmann-wachmann from the lulin observatory |
we report observations of colour in the inner coma of comet c/2013 uq4 (catalina) with the broad-band b and r filters. we find significant temporal variations of the colour slope, ranging from -12.67 ± 8.16% per 0.1 μm to 35.09 ± 11.70% per 0.1 μm. it is significant that the comet changes colour from red to blue over a period of only 2 d. such dispersion cannot be characterized with an average colour slope. we also observe comet c/2013 uq4 (catalina) in infrared using spitzer and find no significant co/co2 gaseous species in its coma. therefore, we classify comet c/2013 uq4 (catalina) as a dust-rich comet and attribute the measured colour slope to its dust. we analyse the colour slope using the model of agglomerated debris particles and conclude that the c/2013 uq4 coma was chemically heterogeneous, consisting of at least two components. the first component producing the bluest colour is consistent with mg-rich silicates. there are three different options for the second component producing the reddest colour. this colour is consistent with either mg-fe silicates, kerogen type ii or organic matter processed with a low dose of ultraviolet radiation. | colour variations of comet c/2013 uq4 (catalina) |
the jupiter-family comet (jfc) 46p/wirtanen passed the earth at a distance of 0.077 au on 2018 december 16 ut, presenting a rare opportunity to study the chemical structure of its coma. with the james clerk maxwell telescope we achieved a resolution of 800 km, which is smaller than the scale lengths of some distributed cometary molecules at the comet's heliocentric distance of 1 au. spectroscopic observations of the j = 4 - 3 transition of hcn showed generally uniform levels of outgassing activity during the observing period, 2018 december 14-20. gas expansion velocities were ∼0.6 km s-1, and the derived average hcn production rate was 7.4 × 1024 mol s-1. hcn and ch3oh emissions were detected at least 30″ (1600 km) from the nucleus, and the abundances of these species were typical for a jfc. the radial distribution of ch3oh is consistent with an extended source of sublimation such as a population of icy grains—cometary halo ice primaries, or chips—as has been invoked previously to explain hyperactivity in comets. the abundance of h2co is normal if it is a daughter species. hnc and co were not detected, but a sensitive nondetection of cs implies an unusually low cs:h2o ratio of <0.02%. the peak brightness of the 850 μm continuum emissions from icy coma dust particles of size ∼1 mm fell from (52 ± 6) to (40 ± 3) mjy beam-1 through the week, while the size of the dust coma remained essentially constant, with dust extending to ∼1000 km from the nucleus. the total mass of those particles was ∼2 × 108 kg. | james clerk maxwell telescope spectral and continuum imaging of hyperactive comet 46p/wirtanen |
magnesium stable isotope ratios and minor element abundances of five olivine particles from comet 81p/wild 2 were examined by secondary ion mass spectrometry (sims). wild 2 olivine particles exhibit only small variations in δ25mg values from -1.0 +0.4/-0.5‰ to 0.6 +0.5/-0.6‰ (2σ). this variation can be simply explained by mass-dependent fractionation from mg isotopic compositions of the earth and bulk meteorites, suggesting that wild 2 olivine particles formed in the chondritic reservoir with respect to mg isotope compositions. we also determined minor element abundances, and o and mg isotope ratios of olivine grains in amoeboid olivine aggregates (aoas) from kaba (cv3.1) and dom 08006 (co3.01) carbonaceous chondrites. our new sims minor element data reveal uniform, low feo contents of ∼0.05 wt% among aoa olivines from dom 08006, suggesting that aoas formed at more reducing environments in the solar nebula than previously thought. furthermore, the sims-derived feo contents of the aoa olivines are consistently lower than those obtained by electron microprobe analyses (∼1 wt% feo), indicating possible fluorescence from surrounding matrix materials and/or fe,ni-metals in aoas during electron microprobe analyses. for mg isotopes, aoa olivines show more negative mass-dependent fractionation (-3.8 ± 0.5‰ ≤ δ25mg ≤ -0.2 ± 0.3‰; 2σ) relative to wild 2 olivines. further, these mg isotope variations are correlated with their host aoa textures. large negative mg isotope fractionations in olivine are often observed in pore-rich aoas, while those in compact aoas tend to have near-chondritic mg isotopic compositions. these observations indicate that pore-rich aoas preserved their gas-solid condensation histories, while compact aoas experienced thermal processing in the solar nebula after their condensation and aggregation. importantly, one 16o-rich wild 2 lime olivine particle (t77/f50) shows negative mg isotope fractionation (δ25mg = -0.8 ± 0.4‰, δ26mg = -1.4 ± 0.9‰; 2σ) relative to bulk chondrites. minor element abundances of t77/f50 are in excellent agreement with those of olivines from pore-rich aoas in dom 08006. the observed similarity in o and mg isotopes, and minor element abundances suggest that t77/f50 formed in an environment similar to aoas, probably near the proto-sun, and then was transported to the kuiper belt, where comet 81p/wild 2 likely accreted. | correlated isotopic and chemical evidence for condensation origins of olivine in comet 81p/wild 2 and in aoas from cv and co chondrites |
many efforts have been made in the last 30 years to develop more relevant in vitro models to study genotoxic responses of drugs and environmental contaminants. while 2d heparg cells are one of the most promising models for liver toxicology, a switch to 3d cultures that integrate both in vivo architecture and cell-cell interactions has occurred to achieve even more predictive models. preliminary studies have indicated that 3d heparg cells are suitable for liver toxicity screening. our study aimed to evaluate the response of heparg spheroids exposed to various genotoxic compounds using the single cell gel electrophoresis assay. heparg spheroids were used at 10 days after seeding and exposed for 24 and 48 hours to certain selected chemical compounds (methylmethansulfonate (mms), etoposide, benzo[a]pyrene (b[a]p), cyclophosphamide (cpa), 7,12-dimethylbenz[a]anthracene (dmba), 2-acetylaminofluorene (2-aaf), 4-nitroquinoline (4-nqo), 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (phip), 2-amino-3-methylimidazo[4,5-f]quinolone (iq), acrylamide, and 2-4-diaminotoluene (2,4-dat)). after treatment, the comet assay was performed on single cell suspensions and cytotoxicity was determined by the atp assay. comet formation was observed for all compounds except iq, etoposide and 2,4-dat. treatment of spheroids with rifampicin increased cyp3a4 activity, demonstrating the metabolic capacity of heparg spheroids. these data on genotoxicity in 3d heparg spheroids are promising, but further experiments are required to prove that this model can improve the predictivity of in vitro models to detect human carcinogens. | three-dimensional heparg spheroids as a liver model to study human genotoxicity in vitro with the single cell gel electrophoresis assay |
after 3/2013 in sky over kiev we registered falling >12 of cometary nuclei fragments, 5 large and a few dozen of small meteoroid fragments. | observations of twilight bolide with using of ``churyumov fireball network'' |
the universe is molecularly rich, comprising from the simplest molecule (h2) to complex organic molecules (e.g., ch3cho and nh2cho), some of which of biological relevance (e.g., amino acids). this chemical richness is intimately linked to the different physical phases forming solar-like planetary systems, in which at each phase, molecules of increasing complexity form. interestingly, synthesis of some of these compounds only takes place in the presence of interstellar (is) grains, i.e., solid-state sub-micron sized particles consisting of naked dust of silicates or carbonaceous materials that can be covered by water-dominated ice mantles. surfaces of is grains exhibit particular characteristics that allow the occurrence of pivotal chemical reactions, such as the presence of binding/catalytic sites and the capability to dissipate energy excesses through the grain phonons. the present know-how on the physicochemical features of is grains has been obtained by the fruitful synergy of astronomical observational with astrochemical modelling and laboratory experiments. however, current limitations of these disciplines prevent us from having a full understanding of the is grain surface chemistry as they cannot provide fundamental atomic-scale of grain surface elementary steps (i.e., adsorption, diffusion, reaction and desorption). this essential information can be obtained by means of simulations based on computational chemistry methods. one capability of these simulations deals with the construction of atom-based structural models mimicking the surfaces of is grains, the very first step to investigate on the grain surface chemistry. this perspective aims to present the current state-of-the-art methods, techniques and strategies available in computational chemistry to model (i.e., construct and simulate) surfaces present in is grains. although we focus on water ice mantles and olivinic silicates as is test case materials to exemplify the modelling procedures, a final discussion on the applicability of these approaches to simulate surfaces of other cosmic grain materials (e.g., cometary and meteoritic) is given. | computational surface modelling of ices and minerals of interstellar interest—insights and perspectives |
sublimation, the direct transition from solid to gas phase, is a process responsible for shaping and changing the reflectance properties of many solar system surfaces. in this study, we have characterized the evolution of the structure/texture and of the visible and near-infrared (vis-nir) spectral reflectance of surfaces made of water ice mixed with analogues of complex extraterrestrial organic matter, named tholins, under low temperature (<-70 °c) and pressure (10-5 mbar) conditions. the experiments were carried out in the sciteas simulation setup recently built as part of the laboratory for outflow studies of sublimating materials (lossy) at the university of bern (pommerol, a. et al. [2015a]. planet. space sci. 109-110, 106-122). as the water ice sublimated, we observed in situ the formation of a sublimation lag deposit made of a water-free porous (>90% porosity) network of organic filaments on top of the ice. the temporal evolution of the tholins and water ice spectral features (reflectance at the absorption bands wavelengths, red slope, from 0.40 to 1.90 μm) are analyzed throughout the sublimation of the samples. we studied how different mixtures of tholins with water (0.1 wt.% tholins as coating or inclusions within the water particles), and different ice particle sizes (4.5 ± 2.5 or 67 ± 31 μm) influence the morphological and spectral evolutions of the samples. the sublimation of the ice below the mantle produces a gas flow responsible for the ejection of mm to cm-sized fragments of the deposit in outbursts-like events. the results show remarkable differences between these samples in term of mantle structure, speed of mantle building, rates and surface area of mantle ejections. these data provide useful references for interpreting remote-sensing observations of icy solar system surfaces, in particular the activity of comet nuclei where sublimation of organic-rich ices and deposition of organic-dust particles likely play a major role. consequently, the data presented here could be of high interest for the interpretation of rosetta, and also new horizons, observations. | sublimation of ice-tholins mixtures: a morphological and spectro-photometric study |
we processed images taken with the hubble space telescope(hst) to investigate any morphological features in the inner coma suggestive of a peculiar activity on the nucleus of the interstellar comet 2i/borisov. the coma shows an evident elongation, in the position angle (pa) ∼0-180○ direction, which appears related to the presence of a jet originating from a single active source on the nucleus. a counterpart of this jet directed towards pa ∼10○ was detected through analysis of the changes of the inner coma morphology on hst images taken on different dates and processed with different filters. these findings indicate that the nucleus is probably rotating with a spin axis projected near the plane of the sky and oriented at pa ∼100-280○ , and that the active source is lying in a near-equatorial position. subsequent observations of hst allowed us to determine the direction of the spin axis at right ascension (ra) = 17h20m ± 15○ and declination (dec.) = $-35^\circ \, \pm 10^\circ$ . photometry of the nucleus on hst images of 2019 october 12 only spans ∼7 h, insufficient to reveal a rotational period. the morphology exhibited by the interstellar comet 2i/borisov is very similar to that of comet c/2014 b1 suggesting that the activation processes are the same as those observed in the solar system native comets. | interstellar comet 2i/borisov exhibits a structure similar to native solar system comets |
we characterized the 67p/churyumov-gerasimenko's dust activity, by analysing individual dust particle velocity and momentum measurements of grain impact analyser and dust accumulator (giada), the dust detector onboard the esa/rosetta spacecraft, collecting dust from tens to hundreds of kilometres from the nucleus. specifically, we developed a procedure to trace back the motion of dust particles down to the nucleus, identifying the surface's region ejecting each dust particle. this procedure has been developed and validated for the first part of the mission by longobardo et al. and was extended to the entire giada data set in this work. the results based on this technique allowed us to investigate the link between the dust porosity (fluffy/compact) and the morphology of the ejecting surface (rough/smooth). we found that fluffy and compact particles, despite the lack of correlation in their coma spatial distribution (at large nucleocentric distances) induced by their different velocities, have common ejection regions. in particular, the correlation between the distributions of fluffy and compact particles is maintained up to an altitude of about 10 km. fluffy particles are more abundant in rough terrains. this could be the result of past cometary activity that resurfaced the smooth terrains and/or of the comet formation process that stored the fluffy particles inside the voids between the pebbles. the variation of fluffy particle concentration between rough and smooth terrains agrees with predictions of comet formation models. finally, no correlation between dust distribution on the nucleus and surface thermal properties was found. | 67p/churyumov-gerasimenko's dust activity from pre- to post-perihelion as detected by rosetta/giada |
ion-neutral chemical reactions are important in several areas of chemistry, including in some regions of the interstellar medium, planetary atmospheres, and comets. reactions of ccl+ with c2h2 are measured, and the main products include c3h2+ and c3h+, both relevant in extraterrestrial environments. accurate branching ratios are obtained, which favor the formation of c3h2+ over c3h+ by a factor of four. the measured rate constants are on the order of langevin, and complementary electronic structure calculations are used to aid in the interpretation of experimental results. | reactions of translationally cold trapped ccl+ with acetylene (c2h2) |
we report on photometry and imaging of the jupiter family comets 41p/tuttle-giacobini-kresak and 45p/honda-mrkos-pajdusakova with the trappist-north (transiting planets and planetesimals small telescope) telescope. we observed 41p on 34 nights from february 16 to july 27, 2017, pre- and post-perihelion (rh = 1.04 au), and collected data for comet 45p after perihelion (rh = 0.53 au) from february 10 to march 30, 2017. we computed the production rates of the daughter species oh, nh, cn, c3 and c2 and we measured the dust proxy, afρ, for both comets. the peak of water-production rate of 41p was (3.46 ± 0.20) × 1027 molecules s-1 on april 3, 2017, when the comet was at 1.05 au from the sun. we have shown that the activity of 41p is decreasing by about 30-40% from one apparition to the next. we measured a mean water-production rate for 45p of (1.43 ± 0.62) × 1027 molecules s-1 during a month after perihelion. our results show that these jupiter family comets had low gas and dust activity and no outburst was detected. relative abundances, expressed as ratios of production rates and the afρ parameter with respect to oh and to cn, were compared to those measured in other comets. we found that 41p and 45p have a typical composition in terms of carbon-bearing species. the study of coma features exhibited by the cn gas species allowed the measurement of the rotation period of 41p, showing a surprisingly large increase of the rotation period from (30 ± 5) h at the end of march to (50 ± 10) h at the end of april, 2017, in agreement with recent observations by other teams. | monitoring of the activity and composition of comets 41p/tuttle-giacobini-kresak and 45p/honda-mrkos-pajdusakova |
context. the rosetta spacecraft escorted comet 67p/churyumov-gerasimenko from 2014 to september 2016. the mission provided in situ observations of the cometary plasma during different phases of the cometary activity, which enabled us to better understand its evolution as a function of heliocentric distance.aims: in this study, different electron populations, called warm and hot, observed by the ion and electron sensor (ies) of the rosetta plasma consortium (rpc) are investigated near the comet during the escorting phase of the rosetta mission.methods: the estimates for the suprathermal electron densities and temperatures were extracted using ies electron data by fitting a double-kappa function to the measured velocity distributions. the fitting results were validated using observations from other rpc instruments. we give upgraded estimates for the warm and hot population densities compared to values previously shown in literature.results: the fitted density and temperature estimates for both electron populations seen by ies are expressed as a function of heliocentric distance to study their evolution with the cometary activity. in addition, we studied the dependence between the electron properties and cometocentric distance.conclusions: we observed that when the neutral outgassing rate of the nucleus is high (i.e., near perihelion) the suprathermal electrons are well characterized by a double-kappa distribution. in addition, warm and hot populations show a significant dependence with the heliocentric distance. the populations become clearly denser near perihelion while their temperatures are observed to remain almost constant. moreover, the warm electron population density is shown to be strongly dependent on the radial distance from the comet. finally, based on our results we reject the hypothesis that hot electron population seen by ies consists of solely suprathermal (halo) solar wind electrons, while we suggest that the hot electron population mainly consists of solar wind thermal electrons that have undergone acceleration near the comet. | plasma properties of suprathermal electrons near comet 67p/churyumov-gerasimenko with rosetta |
the gateway centaur and jupiter co-orbital p/2019 ld2 (atlas) provides the first opportunity to observe the migration of a solar system small body from a centaur orbit to a jupiter family comet (jfc) four decades from now. the gateway transition region is beyond where water ice can power cometary activity, and coma production there is as poorly understood as in all centaurs. we present contemporaneous multiwavelength observations of ld2 from 2020 july 2-4: gemini north visible imaging, nasa irtf near-infrared spectroscopy, and aro submillimeter telescope millimeter-wavelength spectroscopy. precovery decam images limit the nucleus's effective radius to ≤1.2 km and no large outbursts were seen in archival catalina sky survey observations. ld2's coma has $g^{\prime} -r^{\prime} =0.70\pm 0.07$ , $r^{\prime} -i^{\prime} =0.26\pm 0.07$ , a dust-production rate of ∼10-20 kg s-1, and an outflow velocity between v ∼ 0.6-3.3 m s-1. we did not detect co toward ld2 on 2020 july 2-3, with a 3σ upper limit of q(co) < 4.4 × 1027 mol s-1 (⪅ 200 kg s-1). near-infrared spectra show evidence for water ice at the 1%-10% level depending on grain size. spatial profiles and archival data are consistent with sustained activity. the evidence supports the hypothesis that ld2 is a typical small centaur that will become a typical jfc, and thus, it is critical to understanding the transition between these two populations. finally, we discuss potential strategies for a community-wide, long-baseline monitoring effort. | contemporaneous multiwavelength and precovery observations of the active centaur p/2019 ld2 (atlas) |
we present initial time-resolved observations of the split comet 332p/ikeya-murakami taken using the hubble space telescope. our images reveal a dust-bathed cluster of fragments receding from their parent nucleus at projected speeds in the range 0.06-3.5 m s-1 from which we estimate ejection times from 2015 october to december. the number of fragments with effective radii ≳ 20 m follows a differential power law with index γ = -3.6 ± 0.6, while smaller fragments are less abundant than expected from an extrapolation of this power law. we argue that, in addition to losses due to observational selection, torques from anisotropic outgassing are capable of destroying the small fragments by driving them quickly to rotational instability. specifically, the spin-up times of fragments ≲ 20 m in radius are shorter than the time elapsed since ejection from the parent nucleus. the effective radius of the parent nucleus is {r}e ≤slant 275 m (geometric albedo 0.04 assumed). this is about seven times smaller than previous estimates and results in a nucleus mass at least 300 times smaller than previously thought. the mass in solid pieces, 2× {10}9 {kg}, is about 4% of the mass of the parent nucleus. as a result of its small size, the parent nucleus also has a short spin-up time. brightness variations in time-resolved nucleus photometry are consistent with rotational instability playing a role in the release of fragments. | fragmentation kinematics in comet 332p/ikeya-murakami |
we review the reservoirs of methane clathrates that may exist in the different bodies of the solar system. methane was formed in the interstellar medium prior to having been embedded in the protosolar nebula gas phase. this molecule was subsequently trapped in clathrates that formed from crystalline water ice during the cooling of the disk and incorporated in this form in the building blocks of comets, icy bodies, and giant planets. methane clathrates may play an important role in the evolution of planetary atmospheres. on earth, the production of methane in clathrates is essentially biological, and these compounds are mostly found in permafrost regions or in the sediments of continental shelves. on mars, methane would more likely derive from hydrothermal reactions with olivine-rich material. if they do exist, martian methane clathrates would be stable only at depth in the cryosphere and sporadically release some methane into the atmosphere via mechanisms that remain to be determined. | methane clathrates in the solar system |
although the debate regarding the origin of the cyano (cn) radical in comets has been ongoing for many decades, it has yielded no definitive answer to date. cn could previously only be studied remotely, strongly hampering efforts to constrain its origin because of very limited spatial information. thanks to the european space agency's rosetta spacecraft, which orbited comet 67p/churyumov-gerasimenko for 2 yr, we can investigate, for the first time, cn around a comet at high spatial and temporal resolution. on board rosetta's orbiter module, the high-resolution double-focusing mass spectrometer dfms, part of the rosina instrument suite, analysed the neutral volatiles (including hcn and the cn radical) in the inner coma of the comet throughout that whole 2-yr phase and at variable cometocentric distances. from a thorough analysis of the full-mission data, the abundance of cn radicals in the cometary coma has been derived. data from a close flyby event in 2015 february indicate a distributed origin for the cn radical in comet 67p/churyumov-gerasimenko. | first in situ detection of the cn radical in comets and evidence for a distributed source |
in the paper the migration of dust and ice grains over the surface of the comet nucleus is considered and analysed. this phenomenon is a consequence of interplay between the components of three forces tangential to the surface of comet nucleus and acting on the comet's grain. these forces are the gravity of the comet's nucleus, the frictional force coming from the comet's nucleus surface, and the centrifugal force resulting from the rotation of the comet nucleus. as a result, the grains move from the intermediate geographic latitudes towards the `equator of the comet', while the grains located closer to the poles of the comet's nucleus and in the comet's equatorial belt remain immovable. we have shown that the grains traveling to the equator of the comet can be thrown out into space. the analysis was performed for three different assumed shapes of comet nuclei, a sphere, a flattened rotational ellipsoid, and an elongated rotational ellipsoid. it was shown that due to dust migration over the surface of comet nucleus, regardless of its assumed shape, three types of areas can be distinguished: polar caps, midlatitude belts, and equatorial belts, whose sizes may vary significantly. the sizes however depend on the shape of the comet nucleus and on the friction of cometary particles. the time-scale of cometary particles migration was also estimated. the authors suggest that the migration of dust and small fragments of cometary matter studied here may speak in favour of clarification recently discovered redistribution of these particles on the surface of 67p/churyumov-gerasimenko comet and the redistribution of cometary particles across the nucleus. we also suggest that the analysed phenomenon may have a significant impact on comet activity. | on migration of dust and ice grains on the surface of the rotating comet nucleus |
context. the recently announced oort-cloud comet c/2014 un271 (bernardinelli-bernstein) is remarkable in at least three respects: (i) it was discovered inbound as far as ∼29 au from the sun (with prediscovery images up to ∼34 au); (ii) it already showed cometary activity at almost 24 au; and (iii) its nuclear magnitude (hr ∼ 8.0) indicates an exceptionally large object. detection of gases is expected in the upcoming years as the comet heads toward a perihelion of ∼11 au in 2031.aims: the goal is to determine the object's diameter and albedo from thermal measurements.methods: we used alma in extended configuration (resolution ∼0.064″) to measure the 1287 μm (233 ghz) continuum flux of the comet. observations were performed on august 8, 2021, at a 20.0 au distance from the sun. the high spatial resolution was chosen in order to filter out any dust contribution. we also used a recently published afρ value to estimate the dust production rate and the expected dust thermal signal for various assumptions on particle size distribution.results: we detected the thermal emission of the object at ∼10σ, with a flux of 0.128 ± 0.012 mjy. based on observational constraints and our theoretical estimates of the dust contribution, the entirety of the measured flux can be attributed to the nucleus. from neatm modeling combined with the hr magnitude, we determine a surface-equivalent diameter of 137 ± 17 km and a red geometric albedo of 5.3 ± 1.2%. this confirms that c/2014 un271 is by far the largest oort-cloud object ever found (almost twice as large as comet c/1995 o1 hale-bopp) and, except for the centaur 95p/chiron, which shows outburst-like activity, the largest known comet in the solar system. on the other hand, the c/2014 un271 albedo is typical of comets, adding credence to a "universal" comet nucleus albedo.conclusions: with its distant perihelion and uniquely large size, c/2014 un271 (bernardinelli-bernstein) is the prominent archetype of distant comets whose activity is driven by hypervolatiles. monitoring of dust and gas emission as the comet approaches and passes perihelion will permit its activity time pattern to be studied and compared to the distant (outbound) activity of hale-bopp. post-perihelion thermal measurements will permit the study of possible albedo changes, such as a surface brightening compared to pre-perihelion, as was observed for hale-bopp. | size and albedo of the largest detected oort-cloud object: comet c/2014 un271 (bernardinelli-bernstein) |
in 2015 august, comet 67p/churyumov-gerasimenko, the target comet of the esa rosetta mission, reached its perihelion at ∼1.24 au. here, we estimate for a three-day period near perihelion, effective ion speeds at distances ∼200-250 km from the nucleus. we utilize two different methods combining measurements from the rosetta plasma consortium (rpc)/mutual impedance probe with measurements either from the rpc/langmuir probe or from the rosetta orbiter spectrometer for ion and neutral analysis (rosina)/comet pressure sensor (cops) (the latter method can only be applied to estimate the effective ion drift speed). the obtained ion speeds, typically in the range 2-8 km s-1, are markedly higher than the expected neutral outflow velocity of ∼1 km s-1. this indicates that the ions were de-coupled from the neutrals before reaching the spacecraft location and that they had undergone acceleration along electric fields, not necessarily limited to acceleration along ambipolar electric fields in the radial direction. for the limited time period studied, we see indications that at increasing distances from the nucleus, the fraction of the ions' kinetic energy associated with radial drift motion is decreasing. | effective ion speeds at ∼200-250 km from comet 67p/churyumov-gerasimenko near perihelion |
a hierarchical model of microstructure of cometary dust seems to accurately capture the morphological complexity of these particles as observed by the rosetta mission to the 67p/churyumov-gerasimenko. the main aim of this work is to investigate how uncertainties in the knowledge of the microstructure of the surface layer affect our estimates of gas production. new models that incorporate scale of inhomogenieties in a sound theoretical framework are used for hierarchical dust layers. the studied layers are constructed in two steps: first we design ballistic aggregates as the building units and then, using these porous blocks, assemble the layers constrained by the known porosity range. the mean pore size and permeability are studied. modelling is performed for various values of porosity, grain size, and layer thickness. the simulation results are embedded in the thermal model, explicitly including a radiation thermal conductivity and a resistance of the dust layer for the gas flow. it is shown that the average pore size is satisfactorily approximated by a linear function of the ratio of the effective porosity of the layer to the degree of filling. simple fittings were obtained for the layer permeability. our results indicate that in the expected range of nucleus porosity, the gas production rate is weakly dependent on the detailed layer microstructure, and appropriate effective values of homogeneous dust layers can be used to evaluate the gas activity. we also note that adding complex elements into the model yields unavoidable statistical uncertainties within several tens of per cent. | the effect of hierarchical structure of the surface dust layer on the modelling of comet gas production |
the cometary secondary ion mass analyzer (cosima) collects dust particles in the coma of 67p/churyumov-gerasimenko, images them with a resolution of 14 μm × 14 μm, and measures their composition via time-of-flight secondary ion mass spectrometry. the particles are collected on targets exposed to the cometary flux for periods ranging from several hours to a week. images are acquired with the internal camera, the cosiscope, before and after each exposure period. this paper focuses on the evolution of the dust flux and of the size distribution of the particles derived from the cosiscope images during the two years of the mission. the dust flux reaches its maximum at perihelion. we suggest that the delay of 20 d between the activity measured by cosima and the gas activity measured by the other instruments on rosetta is caused by the presence of a volatile-poor dust layer on the nucleus that is removed around perihelion, uncovering volatile-rich layers that then become active. the difference in morphology between the northern and southern hemispheres observed by osiris, the south being more sintered, is also recorded in the cosima data by a change in the size distribution during the southern summer, as the large porous aggregates disappear from the cosima collection. the properties of the particles collected during an outburst in early september 2016 indicate that these particles were ejected by a violent event and might originate from regions of low tensile strength. | evolution of the physical properties of dust and cometary dust activity from 67p/churyumov-gerasimenko measured in situ by rosetta/cosima |
proper droplet diameter and kinetic energy can effectively reduce the risk of soil erosion during low‑pressure sprinkler irrigation. in this study, we comprehensively evaluated the radial distributions of droplet diameter, velocity, kinetic energy, kinetic energy per unit droplet volume, and specific power for three common low‑pressure sprinklers, i.e. nelson d3000, r3000 and komet kpt, with two operating pressures (103 and 138 kpa) and three nozzle outlet diameters (3.97, 5.95 and 7.94 mm). additionally, the relationships between these droplet characteristic parameters were analysed. overall, the maximum values of five characteristic parameters were observed at the end of the jet under various treatments. although r3000 had a larger droplet diameter (0.10–6.42 mm) and kinetic energy (1.08 × 10−10–8.59 × 10−3 j) distribution range than d3000 and kpt, the d3000 specific power (mean value of 0.4274 w m−2) was the highest of the three sprinklers. therefore, kpt was suggested more in low‑pressure sprinkler irrigation than in the other two sprinklers because its application rates were reduced along the radial direction. in addition to selecting a suitable sprinkler type for minimizing specific power, determining a reasonable sprinkler spacing was also significant (p < 0.01). furthermore, a universal specific power model with an appropriate accuracy (rmse = 0.011 w m−2, nrmse = 29%) was proposed. this study provides a key basis for the selection and combination of low‑pressure sprinklers.résuméun diamètre de gouttelettes et une énergie cinétique appropriés peuvent réduire efficacement le risque dérosion des sols lors de l'irrigation par aspersion à basse pression. dans cette étude, nous avons évalué de façon complète les distributions radiales du diamètre des gouttelettes, de la vitesse, de l'énergie cinétique, de l'énergie cinétique par unité de volume des gouttelettes, et de la puissance spécifique pour trois sprinklers courants à basse pression, à savoir nelson d3000, r3000 et komet kpt, avec deux pressions de fonctionnement (103 et 138 kpa) et trois diamètres de sortie de la buse (3,97, 5.95 et 7.94 mm). de plus, les relations entre ces paramètres caractéristiques des gouttelettes ont été analysées. au total, les valeurs maximales de cinq paramètres caractéristiques ont été observées en fin de jet sous différents traitements. bien que la r3000 ait un plus grand diamètre de gouttelettes (0.10–6.42 mm) et une plus grande énergie cinétique (1,08 × 10−10–8,59 × 10−3 j) que la d3000 et le kpt, la puissance spécifique d3000 (valeur moyenne de 0,4274 w m−2) a été la plus élevée parmi les trois sprinklers. par conséquent, le kpt a été suggéré davantage dans le cas de l'irrigation par arrosage à basse pression que dans celui des deux autres arroseurs, parce que ses taux d'application étaient réduits le long de la direction radiale. outre le choix d'un type d'arrosage approprié pour minimiser la puissance spécifique, la détermination dun espacement raisonnable de larrosage a également été importante (p < 0.01). en outre, un modèle de puissance spécifique universel avec une précision appropriée (rmse = 0.011 w m−2, nrmse = 29%) a été proposé. cette étude constitue une base essentielle pour le choix et la combinaison des arroseurs à basse pression. | comprehensively evaluating and modelling droplet diameters and kinetic energies of low‑pressure sprinklers |
the rosetta spacecraft of the european space agency made ground-breaking observations of comet 67p/churyumov-gerasimenko and of its cometary environment. we search for magnetic holes in that environment, i.e., significant depressions in the magnetic field strength, measured by the rosetta fluxgate magnetometer (mag) in april and may 2015. in that time frame of two months, we identified 23 magnetic holes. the cometary activity was intermediate and increasing because rosetta was on the inbound leg toward the sun. while in april solar wind protons were still observed by rosetta near the comet, in may these protons were already mostly replaced by heavy cometary ions. magnetic holes have frequently been observed in the solar wind. we find, for the first time, that magnetic holes exist in the cometary environment even when solar wind protons are almost absent. some of the properties of the magnetic holes are comparable to those of solar wind holes; they are associated with density enhancements, sometimes associated with co-located current sheets and fast solar wind streams, and are of similar scales. however, particularly in may, the magnetic holes near the comet appear to be more processed, featuring shifted density enhancements and, sometimes, bipolar signatures in magnetic field strength rather than simple depressions. the magnetic holes are of global size with respect to the coma. however, at the comet, they are compressed owing to magnetic field pile-up and draping so that they change in shape. there, the magnetic holes become of comparable size to heavy cometary ion gyroradii, potentially enabling kinetic interactions. | first observations of magnetic holes deep within the coma of a comet |
a comet is a highly dynamic object, undergoing a permanent state of change. these changes have to be carefully classified and considered according to their intrinsic temporal and spatial scales. the rosetta mission has, through its contiguous in-situ and remote sensing coverage of comet 67p/churyumov-gerasimenko (hereafter 67p) over the time span of august 2014 to september 2016, monitored the emergence, culmination, and winding down of the gas and dust comae. this provided an unprecedented data set and has spurred a large effort to connect in-situ and remote sensing measurements to the surface. in this review, we address our current understanding of cometary activity and the challenges involved when linking comae data to the surface. we give the current state of research by describing what we know about the physical processes involved from the surface to a few tens of kilometres above it with respect to the gas and dust emission from cometary nuclei. further, we describe how complex multidimensional cometary gas and dust models have developed from the halley encounter of 1986 to today. this includes the study of inhomogeneous outgassing and determination of the gas and dust production rates. additionally, the different approaches used and results obtained to link coma data to the surface will be discussed. we discuss forward and inversion models and we describe the limitations of the respective approaches. the current literature suggests that there does not seem to be a single uniform process behind cometary activity. rather, activity seems to be the consequence of a variety of erosion processes, including the sublimation of both water ice and more volatile material, but possibly also more exotic processes such as fracture and cliff erosion under thermal and mechanical stress, sub-surface heat storage, and a complex interplay of these processes. seasons and the nucleus shape are key factors for the distribution and temporal evolution of activity and imply that the heliocentric evolution of activity can be highly individual for every comet, and generalisations can be misleading. | cometary comae-surface links |
the provenance of gems (glass with embedded metal and sulfides) in cometary type interplanetary dust particles is investigated using analytical scanning transmission electron microscopy and secondary ion mass spectrometry. we review the current state of knowledge and closely examine the densities, elemental compositions and distributions, iron oxidation states, and isotopic compositions of a subset of gems in chondritic porous interplanetary dust. we find that gems are underdense with estimated densities that are 35-65% of compositionally equivalent crystalline aggregates. gems low densities result in a lower contribution to the bulk compositions of idps than has been assumed based on their volume fraction. we also find that element/si ratios, assumed to be primary (indigenous), are instead perturbed by contamination and secondary alteration, including pulse heating during atmospheric entry. fe in pyrrhotite inclusions was oxidized and mg, s, ca, and fe were depleted relative to lithophile al and si, resulting in reduction in element/si ratios. because they trap outgassing elements, fe-rich oxide rims that formed on the surface of gems are serendipitous "witness plates" to the changes in composition that accompany atmospheric entry. as a result of alteration, gems elemental compositions cannot reliably inform about their provenance. except for highly anomalous oxygen isotope ratios measured in some large gems grains that indicate a contribution from circumstellar dust, oxygen isotope compositions are generally poor indicators of provenance. prior work indicates that most gems fall close to the terrestrial oxygen isotope composition, which, however, does not exclude a presolar interstellar origin. nitrogen isotopic compositions are more diagnostic. elevated 15n/14n ratios indicate that gems accreted in conjunction with formation of organic matter by ion-molecule reactions in a cold (<50 k) presolar environment like the extreme outer nebula or interstellar medium. considering all observations, we conclude that gems are most likely processed interstellar silicates. | on the provenance of gems, a quarter century post discovery |
mercury’s surface is darker than that of the moon. iron-bearing minerals and submicroscopic metallic iron produced by space weathering are the primary known darkening materials on airless bodies. yet mercury’s iron abundance at the surface is lower than the moon’s; another material is therefore likely to be responsible for mercury’s dark surface. enhanced darkening by submicroscopic metallic iron particles under intense space weathering at mercury’s surface is insufficient to reconcile the planet’s low reflectance with its low iron abundance. here we show that the delivery of cometary carbon by micrometeorites provides a mechanism to darken mercury’s surface without violating observational constraints on iron content. we calculate the micrometeorite flux at mercury and numerically simulate the fraction of carbonaceous material retained by the planet following micrometeorite impacts. we estimate that 50 times as many carbon-rich micrometeorites per unit surface area are delivered to mercury, compared with the moon, resulting in approximately 3-6 wt% carbon at mercury’s surface (in graphite, amorphous, or nanodiamond form). spectroscopic analysis of products of hypervelocity impact experiments demonstrates that the incorporation of carbon effectively darkens and weakens spectral features, consistent with remote observations of mercury. carbon delivery by micrometeorites provides an explanation for mercury’s globally low reflectance and may contribute to the darkening of planetary surfaces elsewhere. | darkening of mercury's surface by cometary carbon |
alice is a far-ultraviolet imaging spectrograph on board rosetta that, among multiple objectives, is designed to observe emissions from various atomic and molecular species from within the coma of comet 67p/churyumov-gerasimenko. the initial observations, made following orbit insertion in 2014 august, showed emissions of atomic hydrogen and oxygen spatially localized close to the nucleus and attributed to photoelectron impact dissociation of h2o vapor. weaker emissions from atomic carbon were subsequently detected and also attributed to electron impact dissociation, of co2, the relative h i and c i line intensities reflecting the variation of co2 to h2o column abundance along the line of sight through the coma. beginning in 2015 mid-april, alice sporadically observed a number of outbursts above the sunward limb characterized by sudden increases in the atomic emissions, particularly the semi-forbidden o i λ1356 multiplet, over a period of 10-30 minutes, without a corresponding enhancement in long-wavelength solar reflected light characteristic of dust production. a large increase in the brightness ratio o i λ1356/o i λ1304 suggests o2 as the principal source of the additional gas. these outbursts do not correlate with any of the visible images of outbursts taken with either osiris or the navigation camera. beginning in 2015 june the nature of the alice spectrum changed considerably with co fourth positive band emission observed continuously, varying with pointing but otherwise fairly constant in time. however, co does not appear to be a major driver of any of the observed outbursts. | the nature and frequency of the gas outbursts in comet 67p/churyumov-gerasimenko observed by the alice far-ultraviolet spectrograph on rosetta |
context. after the successful landing of philae on the nucleus of 67p/churyumov-gerasimenko, the rosetta mission provided the first opportunity of performing measurements with the consert tomographic radar in november 2014. consert data were acquired during this first science sequence. they unambiguously showed that propagation through the smaller lobe of the nucleus was achieved.aims: while the ultimate objective of the consert radar is to perform the tomography of the nucleus, this paper focuses on the local characterization of the shallow subsurface in the area of philae's final landing site, specifically determining the possible presence of a permittivity gradient below the nucleus surface.methods: a number of electromagnetic simulations were made with a ray-tracing code to parametrically study how the gradient of the dielectric constant in the near-subsurface affects the ability of consert to receive signals.results: at the 90 mhz frequency of consert, the dielectric constant is a function of porosity, composition, and temperature. the dielectric constant values considered for the study are based on observations made by the other instruments of the rosetta mission, which indicate a possible near-surface gradient in physical properties and on laboratory measurements made on analog samples.conclusions: the obtained simulated data clearly show that if the dielectric constant were increasing with depth, it would have prevented the reception of signal at the consert location during the first science sequence. we conclude from our simulations that the dielectric constant most probably decreases with depth. | consert suggests a change in local properties of 67p/churyumov-gerasimenko's nucleus at depth |
interstellar ice is believed to be a cradle of complex organic compounds, commonly found within icy comets and interstellar clouds, in association with ultraviolet (uv) irradiation and subsequent warming. we found that uv-irradiated amorphous ices composed of h2o, ch3oh, and nh3 and of pure h2o behave like liquids over the temperature ranges of 65 to 150 kelvin and 50 to 140 kelvin, respectively. this low-viscosity liquid-like ice may enhance the formation of organic compounds including prebiotic molecules and the accretion of icy dust to form icy planetesimals under certain interstellar conditions. | liquid-like behavior of uv-irradiated interstellar ice analog at low temperatures |
context. the esa rosetta mission has been investigating the environment of comet 67p/churyumov-gerasimenko (67p) since august 2014. among the experiments on board the spacecraft, the rosina experiment (rosetta orbiter spectrometer for ion and neutral analysis) includes two mass spectrometers to analyse the composition of neutrals and ions and a comet pressure sensor (cops) to monitor the density and velocity of neutrals in the coma.aims: we study heterogeneities in the coma during three periods starting in october 2014 (summer in the northern hemisphere) and ending in february 2016 (end of winter in the northern hemisphere). we provide a detailed description of the main volatiles dynamics (h2o, co2, co) and their abundance ratios.methods: we analysed and compared the data of the reflectron-type time-of-flight (rtof) mass spectrometer with data from both the double focusing mass spectrometer (dfms) and cops during the comet escort phase. this comparison has demonstrated that the observations performed with each rosina sensor are indeed consistent. furthermore, we used a direct simulation monte carlo (dsmc) model to compare modelled densitites with in situ detections.results: our analysis shows how the active regions of the main volatiles evolve with the seasons with a variability mostly driven by the illumination conditions; this is the case except for an unexpected dichotomy suggesting the presence of a dust layer containing water deposited in the northern hemisphere during previous perihelions hiding the presence of co2. the influence of various parameters is investigated in detail: distance to the comet, heliocentric distance, longitude and latitude of sub-satellite point, local time, and phase angle. | the heterogeneous coma of comet 67p/churyumov-gerasimenko as seen by rosina: h2o, co2, and co from september 2014 to february 2016 |
context. from august 2014 to september 2016, rosetta escorted comet 67p/churyumov-gerasimenko (67p) during its journey around the sun. one of the aims of rosetta was to characterize cometary activity and the consequent formation of dust flux structures in cometary comae.aims: we characterize and quantify the submicrometer- to micrometer-sized dust flux that may be shaped in privileged directions within the coma of 67p inbound to and outbound from perihelion.methods: the in situ dust-measuring instrument giada, part of the rosetta/esa payload, consisted of three subsystems, one of which was the micro balance subsystem (mbs), composed of five quartz crystal microbalances. from may 2014 to september 2016, mbs measured the submicrometer- to micrometer-sized deposited dust mass every 5 min.results: we characterized the submicrometer- to micrometer-sized dust mass flux in the coma of 67p. the anti-sunward and the radial direction are preferred, and the flux is higher in the anti-sunward direction. the measured cumulative dust mass in the anti-sunward direction is 2.38 ± 0.04 × 10-7 kg, and in the radial direction, it is 1.18 ± 0.02 × 10-7 kg. we explain the anti-sunward dust flux as the effect of nonuniform gas emission between the night- and dayside of the nucleus, which acts in combination with the solar radiation pressure. we compared the cumulated dust mass of particles ≤5 μm with particles ≥100 μm. the retrieved ratio of ≈2% implies a differential size distribution index of ≈-3.0, which confirms that particles of size ≥0.1 mm dominate the dust coma cross-section of 67p during the entire orbit.conclusions: submicrometer- to micrometer-sized dust mass flux measurements were made for the first time from the arising of cometary activity until its extinction. they indicate that these particles do not provide a substantial optical scattering in the coma of 67p with respect to the scattering caused by millimeter-sized particles. in addition, mbs data reveal that the measured dust flux is highly anisotropic: anti-sunward plus radial. | giada microbalance measurements on board rosetta: submicrometer- to micrometer-sized dust particle flux in the coma of comet 67p/churyumov-gerasimenko |
knowledge of the magnetization of planetary bodies constrains their origin and evolution, as well as the conditions in the solar nebular at that time. on the basis of magnetic field measurements during the descent and subsequent multiple touchdown of the rosetta lander philae on the comet 67p/churyumov-gerasimenko (67p), we show that no global magnetic field was detected within the limitations of analysis. the rosetta magnetometer and plasma monitor (romap) suite of sensors measured an upper magnetic field magnitude of less than 2 nanotesla at the cometary surface at multiple locations, with the upper specific magnetic moment being <3.1 × 10-5 ampere-square meters per kilogram for meter-size homogeneous magnetized boulders. the maximum dipole moment of 67p is 1.6 × 108 ampere-square meters. we conclude that on the meter scale, magnetic alignment in the preplanetary nebula is of minor importance. | the nonmagnetic nucleus of comet 67p/churyumov-gerasimenko |
abundant molecular oxygen was discovered in the coma of comet 67p/churyumov-gerasimenko. its origin was ascribed to primordial gaseous o2 incorporated into the nucleus during the comet's formation. this thesis was put forward after discounting several o2 production mechanisms in comets, including photolysis and radiolysis of water, solar wind-surface interactions and gas-phase collisions. here we report an original eley-rideal reaction mechanism, which permits direct o2 formation in single collisions of energetic water ions with oxidized cometary surface analogues. the reaction proceeds by h2o+ abstracting a surface o-atom, then forming an excited precursor state, which dissociates to produce o2-. subsequent photo-detachment leads to molecular o2, whose presence in the coma may thus be linked directly to water molecules and their interaction with the solar wind. this abiotic o2 production mechanism is consistent with reported trends in the 67p coma and raises awareness of the role of energetic negative ions in comets. | dynamic molecular oxygen production in cometary comae |
the optical, spectroscopic, and infrared remote imaging system (osiris) onboard the european space agency's rosetta spacecraft acquired images of comet 67p/churyumov-gerasimenko (67p) and its surrounding dust coma starting from may 2014 until september 2016. in this paper we present methods and results from analysis of osiris images regarding the dust outflow in the innermost coma of 67p. the aim is to determine the global dust outflow behaviour and place constraints on physical processes affecting particles in the inner coma. we study the coma region right above the nucleus surface, spanning from the nucleus centre out to a distance of about 50 km comet centric distance (approximately 25 average comet radii). we primarily adopt an approach used by thomas and keller (1990) to study the dust outflow. we present the effects on azimuthally-averaged values of the dust reflectance of non-radial flow and non-point-source geometry, acceleration of dust particles, sublimation of icy dust particles after ejection from the surface, dust particle fragmentation, optical depth effects and the influence of gravitationally bound particles. all of these physical processes could modify the observed distribution of light scattered by the dust coma. in the image analysis, profiles of azimuthally averaged dust brightness as a function of impact parameter b (azimuthal average, "ā-curve") were fitted with a simple function that best fits the shape of our profile curves (f(b ; u , v , w , z) = u /bv + wb + z). the analytical fit parameters (u, v, w, z), which hold the key information about the dust outflow behaviour, were saved in a comprehensive database. through statistical analysis of these information, we show that the spatial distribution of dust follows free-radial outflow behaviour (i.e. force-free radial outflow with constant velocity) beyond distances larger than ∼11.9 km from the comet centre, which corresponds to a relative distance of about 6 average comet radii from the comet centre. hence, we conclude that beyond this distance, and on average, fragmentation and gravitationally bound particles are negligible processes in determining the optically scattered light distribution in the innermost coma. closer to the nucleus we observe dust outflow behaviour that deviates from free-radial outflow. a comparison of our result profiles with numerical models using a direct simulation monte carlo (dsmc) approach with dust particle distributions calculated using a test particle approach has been used to demonstrate the influence of a complex shape and particle acceleration on the azimuthal average profiles. we demonstrate that, while other effects such as fragmentation or sublimation of dust particles cannot be ruled out, acceleration of the dust particles and effects arising from the shape of the irregular nucleus (non-point source geometry) are sufficient to explain the observed dust outflow behaviour from image data analysis. as a by-product of this work, we have calculated "afρ" values for the 1/r regime. we found a peak in the coma activity in terms of afρ (normalised to a phase angle of 90°) of ∼210 cm 20 days after perihelion. furthermore, based on simplified models of particle motion within bound orbits, it is shown that limits on the total cross-sectional area of bound particles might be derived through further analysis. an example is given. | on deviations from free-radial outflow in the inner coma of comet 67p/churyumov-gerasimenko |
people have probably been watching the sky since the beginning of human history. observers in pre-telescopic ages recorded anomalous events, which now provide uniquely valuable information for modern scientists. records that include drawings are particularly useful, since the verbal expressions recorded by pre-telescopic observers, who did not know the physical nature of the phenomena, are often ambiguous. however, drawings concerning specific datable events in the historical documents are far fewer than the verbal records. therefore, in this paper we show the possible earliest drawings of datable auroras and a two-tail comet included in a manuscript of the chronicle of zūqnīn, a syriac chronicle up to 775/776 ce, to interpret their nature. careful perusing the original syriac autograph manuscript, ms vat.sir.162, provide not only historical facts in the realm around amida, but also information concerning low-latitude aurora observations due to extreme space weather events and the existence of sun-grazing comets. | the earliest drawings of datable auroras and a two-tail comet from the syriac chronicle of zūqnīn |
context. this paper describes the modelling of gas and dust data acquired in the period august to october 2014 from the european space agency's rosetta spacecraft when it was in close proximity to the nucleus of comet 67p/churyumov-gerasimenko.aims: with our 3d gas and dust comae models this work attempts to test the hypothesis that cliff activity on comet 67p/churyumov-gerasimenko can solely account for the local gas density data observed by the rosetta orbiter spectrometer for ion and neutral analysis (rosina) and the dust brightnesses seen by the optical, spectroscopic, and infrared remote imaging system (osiris) in the considered time span.methods: the model uses a previously developed shape model of the nucleus. from this, the water sublimation rates and gas temperatures at the surface are computed. the gas expansion is modelled with a 3d direct simulation monte carlo algorithm. a dust drag algorithm is then used to compute dust volume number densities in the coma, which are then converted to brightnesses using mie theory and a line-of-sight integration. furthermore we have studied the impact of topographic re-radiation on the models.results: we show that gas activity from only cliff areas produces a fit to the rosina/cops data that is as statistically good as a purely insolation-driven model. in contrast, pure cliff activity does not reproduce the dust brightness observed by osiris and can thus be ruled out. on the other hand, gas activity from the hapi region in addition to cliff activity produces a statistically better fit to the rosina/cops data than purely insolation-driven outgassing and also fits the osiris observations rather well. we found that topographic re-radiation does not contribute significantly to the sublimation behaviour of h2o but plays an important role in how the gas flux interacts with the irregular shape of the nucleus.conclusions: we demonstrate that fits to the observations are non-unique. we can conclude however that gas and dust activity from cliffs and the hapi region are consistent with the rosina/cops and osiris data sets for the considered time span and are thus a plausible solution. models with activity from low gravitational slopes alone provide a statistically inferior solution. | cliffs versus plains: can rosina/cops and osiris data of comet 67p/churyumov-gerasimenko in autumn 2014 constrain inhomogeneous outgassing? |
sublimative outgassing of comets produces torques that alter the rotation state of their nuclei. recently, parameterized sublimative torque models have been developed to study rotation state changes of individual comet nuclei and populations of cometary bodies. however, these models simplify the interactions between the escaping gas and cometary surface into only a few parameters that hide the details of these complex interactions. here we directly compare the x-parameter model (samarasinha and mueller, 2013) with the syorp model (steckloff and jacobson, 2016) to tease out insights into the details of the gas-surface interactions driving sublimative torques. we find that, for both of these models to accurately model sublimative torques, the number of sublimating molecules that contribute to the net torque is largely independent of the detailed shape and activity of the nucleus, but rather depends primarily on the size of the nucleus and the effective heliocentric distance of the comet. we suggest that cometary activity must be largely restricted to regions of steep gravitational surface slopes (above the angle of repose), where mass wasting can refresh activity by shedding mantles of refractory materials and exposing fresh volatiles. we propose a new classification scheme for comets based on the frequency of this mass-wasting process (relative to the timescale of activity fading): quasi-equilibrium, episodic, quasi-dormant, and extinct. | the sublimative torques of jupiter family comets and mass wasting events on their nuclei |
this paper studies the transition probabilities of electric dipole transitions between 10 low-lying states of the cn radical. these states are x2σ+, a2π, b2σ+, a4σ+, b4π, 14σ-, 24π, 14δ, 16σ+, and 16π. the potential energy curves are calculated using the casscf method, which is followed by the icmrci approach with the davidson correction. the transition dipole moments between different states are calculated. to improve the accuracy of potential energy curves, core-valence correlation and scalar relativistic corrections, as well as the extrapolation of potential energies to the complete basis set limit are included. the franck-condon factors and einstein coefficients of emissions are calculated. the radiative lifetimes are determined for the vibrational levels of the a2π, b2σ+, b4π, 14σ-, 24π, 14δ, and 16π states. according to the transition probabilities and radiative lifetimes, some guidelines for detecting these states spectroscopically are proposed. the spin-orbit coupling effect on the spectroscopic and vibrational properties is evaluated. the splitting energy in the a2π state is determined to be 50.99 cm-1, which compares well with the experimental ones. the potential energy curves, transition dipole moments, spectroscopic parameters, and transition probabilities reported in this paper can be considered to be very reliable. the results obtained here can be used as guidelines for detecting these transitions, in particular those that have not been measured in previous experiments or have not been observed in the sun, comets, stellar atmospheres, dark interstellar clouds, and diffuse interstellar clouds. | transition dipole moments and transition probabilities of the cn radical |
deep chandra acis observations of the region around the putative pulsar, cxou j061705.3+222127, in the supernova remnant (snr) ic 443 reveal an ∼5″ radius ring-like structure surrounding the pulsar and a jet-like feature oriented roughly north-south across the ring and through the pulsar's location at 06h17m5.ˢ200 + 22°21‧27.″52 (j2000.0 coordinates). the observations further confirm that (1) the spectrum and flux of the central object are consistent with a rotation-powered pulsar, (2) the non-thermal spectrum and morphology of the surrounding nebula are consistent with a pulsar wind, and (3) the spectrum at greater distances is consistent with thermal emission from the snr. the cometary shape of the nebula, suggesting motion toward the southwest, appears to be subsonic: there is no evidence either spectrally or morphologically for a bow shock or contact discontinuity; the nearly circular ring is not distorted by motion through the ambient medium; and the shape near the apex of the nebula is narrow. comparing this observation with previous observations of the same target, we set a 99% confidence upper limit to the proper motion of cxou j061705.3+222127 to be less than 44 mas yr-1 (310 km s-1 for a distance of 1.5 kpc), with the best-fit (but not statistically significant) projected direction toward the west. | high spatial resolution x-ray spectroscopy of the ic 443 pulsar wind nebula and environs |
comet c/2013 us10 (catalina) was a dynamically new oort cloud comet whose apparition presented a favorable geometry for observations near close-earth approach (≃0.93 au) at heliocentric distances ≲2 au when insolation and sublimation of volatiles drive maximum activity. here we present mid-infrared 6.0 ≲ λ(μm) ≲ 40 spectrophotometric observations at two temporal epochs from nasa's stratospheric observatory for infrared astronomy and the nasa infrared telescope facility that yield an inventory of the refractory materials and their physical characteristics through thermal modeling analysis. the grain composition is dominated by dark dust grains (modeled as amorphous carbon) with a silicate-to-carbon ratio ≲0.9, little crystalline stoichiometry (no distinct 11.2 μm feature attributed to mg-rich crystalline olivine), and the submicron grain-size distribution peaking at ≃0.6 μm. the 10 μm silicate feature was weak, ≈12.8% ± 0.1% above the local continuum, and the bolometric grain albedo was low (≲14%). comet c/2013 us10 (catalina) is a carbon-rich object. this material, which is well represented by the optical constants of amorphous carbon, is similar to the material that darkens and reddens the surface of comet 67p/churyumov-gerasimenko. we argue this material is endemic to the nuclei of comets, synthesizing results from the study of stardust samples, interplanetary dust particle investigations, and micrometeoritic analyses. the atomic carbon-to-silicate ratio of comet c/2013 us10 (catalina) and other comets joins a growing body of evidence suggesting the existence of a c/si gradient in the primitive solar system, providing new insight into planetesimal formation and the distribution of isotopic and compositional gradients extant today. | the coma dust of comet c/2013 us10 (catalina): a window into carbon in the solar system |
recent observations show activity in long-period comet c/2017 k2 at heliocentric distances beyond the orbit of uranus. with this as motivation, we constructed a simple model that takes a detailed account of gas transport modes and simulates the time-dependent sublimation of supervolatile ice from beneath a porous mantle on an incoming cometary nucleus. the model reveals a localized increase in carbon monoxide (co) sublimation close to heliocentric distance rh= 150 au (local blackbody temperature ~23 k), followed by a plateau and then a slow increase in activity toward smaller distances. this localized increase occurs as heat transport in the nucleus transitions between two regimes characterized by the rising temperature of the co front at larger distances and nearly isothermal co at smaller distances. as this transition is a general property of sublimation through a porous mantle, we predict that future observations of sufficient sensitivity will show that inbound comets (and interstellar interlopers) will exhibit activity at distances far beyond the planetary region of the solar system. | cometary activity beyond the planets |
between 2014 december 31 and 2015 march 17, the osiris cameras on rosetta documented the growth of a $140\, \mathrm{\hbox{-}m}$ wide and $0.5\, \mathrm{\hbox{-}m}$ deep depression in the hapi region on comet 67p/churyumov-gerasimenko. this shallow pit is one of several that later formed elsewhere on the comet, all in smooth terrain that primarily is the result of airfall of coma particles. we have compiled observations of this region in hapi by the microwave instrument miro on rosetta, acquired during october and november 2014. we use thermophysical and radiative transfer models in order to reproduce the miro observations. this allows us to place constraints on the thermal inertia, diffusivity, chemical composition, stratification, extinction coefficients, and scattering properties of the surface material, and how they evolved during the months prior to pit formation. the results are placed in context through long-term comet nucleus evolution modelling. we propose that (1) miro observes signatures that are consistent with a solid-state greenhouse effect in airfall material; (2) co2 ice is sufficiently close to the surface to have a measurable effect on miro antenna temperatures, and likely is responsible for the pit formation in hapi observed by osiris; (3) the pressure at the co2 sublimation front is sufficiently strong to expel dust and water ice outwards, and to compress comet material inwards, thereby causing the near-surface compaction observed by consert, sesame, and groundbased radar, manifested as the 'consolidated terrain' texture observed by osiris. | co2-driven surface changes in the hapi region on comet 67p/churyumov-gerasimenko |
the solar system formed from interstellar dust and gas in a molecular cloud. astronomical observations show that typical interstellar dust consists of amorphous (a-) silicate and organic carbon. bona fide physical samples for laboratory studies would yield unprecedented insight about solar system formation, but they were largely destroyed. the most likely repositories of surviving presolar dust are the least altered extraterrestrial materials, interplanetary dust particles (idps) with probable cometary origins. cometary idps contain abundant submicron a-silicate grains called gems (glass with embedded metal and sulfides), believed to be carbon-free. some have detectable isotopically anomalous a-silicate components from other stars, proving they are preserved dust inherited from the interstellar medium. however, it is debated whether the majority of gems predate the solar system or formed in the solar nebula by condensation of high-temperature (>1,300 k) gas. here, we map idp compositions with single nanometer-scale resolution and find that gems contain organic carbon. mapping reveals two generations of grain aggregation, the key process in growth from dust grains to planetesimals, mediated by carbon. gems grains, some with a-silicate subgrains mantled by organic carbon, comprise the earliest generation of aggregates. these aggregates (and other grains) are encapsulated in lower-density organic carbon matrix, indicating a second generation of aggregation. since this organic carbon thermally decomposes above ∼450 k, gems cannot have accreted in the hot solar nebula, and formed, instead, in the cold presolar molecular cloud and/or outer protoplanetary disk. we suggest that gems are consistent with surviving interstellar dust, condensed in situ, and cycled through multiple molecular clouds. | multiple generations of grain aggregation in different environments preceded solar system body formation |
organic compounds have been delivered over time to mars via meteorites, comets and interplanetary dust particles. the fate of organic material on the surface of mars must be affected by the martian environment, in particular by ultraviolet (uv) and other ionizing radiation. penetration depth of uv radiation into soils is in the sub-millimetre to millimetre range and depends on the properties of the soil. the aim of this research is to study the possible protective role of martian analogue minerals and soils for survivability of biomolecules against uv radiation and to compare their decomposition rates within a 1 mm-thick portion of the surface. results demonstrated that minerals offer significant protection to biomolecules purine, pyrimidine and uracil against uv photolysis. in the absence of these minerals, organic compounds are completely degraded when subjected directly to uv photolysis equivalent to only 5 martian day's exposure. however, similar uv exposure of organics dried from solution onto powdered calcium carbonate (calcite; caco3), calcium sulphate (anhydrite; caso4), clay-bearing atacama dessert soil and 7 å clay mineral kaolinite [al2si2o5(oh)4] results in only 1–2% loss of organics. mixtures of purine and uracil with calcium carbonate exposed to gamma radiation of 3 gy (3 gray), which corresponds to approximately 15 000 days on mars, results in up to 10% loss of organics. by contrast, these organic compounds completely decomposed upon mixing with iron oxide (fe2o3) before uv irradiation. as the search for extinct or extant life on mars has been identified as a goal of top priority in nasa's mars exploration program and continues with several missions planned to the red planet by both nasa and the european space agency (esa) in the next few decades, our findings may play a useful role in identifying optimal target sites on the martian surface for future missions. | shielding biomolecules from effects of radiation by mars analogue minerals and soils |
two major questions regarding comets have been up to now far from any solution. (i) how is it possible that water-ice sublimation from the nucleus surface does not lead to an insulating crust, stopping every gas and dust ejection within a few days? (ii) how is it possible that the gas flow crossing the refractory surface crust ejects dust particles bonded by tensile strengths larger than tens of pa when the perihelion gas pressure at the nucleus-coma interface is less than one pa? we have developed a simple but rigorous analytical model, assuming that the cometary nucleus consists of agglomerates of ice and dust (“clusters”). as soon as the clusters become exposed to sunlight, gas diffusion from their inside leads to their dehydration. we find that (i) the gas diffusing from the interior to the surface of a sunlit cluster has a steep density gradient at the cluster surface, which blasts the cluster into particles of sizes larger than or equal to those actually observed by rosetta dust instruments; (ii) the heat-conduction and diffusion timescales are much shorter than the dehydration timescale, ensuring that the described process prevents any dumping of the nucleus activity driven by water-ice sublimation from 4 au inbound to 4 au outbound; and (iii) the clusters are in fact cm-sized pebbles, so that a cometary nucleus made of pebbles is confirmed to be the only one consistent with cometary gas and dust activity, a process unexplained until now. | how comets work |
in 2017 april, we acquired comprehensive high-resolution spectra of newly discovered comet c/2017 e4 (lovejoy) as it approached perihelion, and before its disintegration. we detected many cometary emission lines in the range (2.8-5.3) μm, in four customized instrument settings (l1-c, l3, lp1-c, and m1) of ishell—the new near-ir high-resolution immersion echelle spectrograph at nasa/irtf (maunakea, hawaii). we identified 12 molecular species: nine primary volatiles (h2o, hcn, nh3, co, c2h2, c2h6, ch4, ch3oh, h2co) and three product species (cn, nh2, oh). we detected 85 h2o emission lines from 12 water vibrational bands across l1-c and m1 settings. the many detected water emission lines enabled retrieval of accurate measures for ortho- and para-h2o independently, thereby reducing systematic uncertainty in the derived ortho-para ratio and nuclear spin temperature. excitation analyses and emission profile analyses were performed for all species, and molecular abundance ratios relative to water are compared with values found for other oort cloud comets in our infrared database. abundance ratios are consistent for most species, with the exception of underabundant methanol and overabundant ammonia in e4. | the volatile composition of comet c/2017 e4 (lovejoy) before its disruption, as revealed by high-resolution infrared spectroscopy with ishell at the nasa/irtf |
cometary outbursts, i.e. a sudden increase in brightness, have aroused astronomers' curiosity since the 1920s (hughes). in 1927, comet 29p/schwassmann-wachmann 1 was discovered, probably at the maximum of one of its outbursts. many authors have tried to explain the source of cometary outbursts. they formed various models and hypotheses, but none of them have been definitely confirmed and generally accepted. in the present article, we propose a new model for the outburst activity of comets. this model is based on an idea of ipatov & a'hearn. this idea is a conclusion from the analysis of time variations in velocity of particles ejected from comet 9p/tempel after collision with the deep impact impactor. according to this, there are large cavities a few metres below the surface of comet nuclei. these cavities contain material under high gas pressure. in suitable conditions, the surface layers over the cavities can be thrown away and the deeper layers of the nucleus exposed. finally, an outburst of the comet may be observed. the possible scenario in terms of outbursts is given. the amplitude of the jump in comet luminosity is also calculated. numerical simulations were carried out for a wide range of cometary physical parameters, which are suitable for comets belonging to the class of jupiter-family comets. they provide values of jumps in comet brightness that are consistent with those observed during the outbursts of many comets. | a model of cometary outbursts: a new simple approach to the classical question |
context. on 12 march 2015 the osiris wac camera onboard the esa rosetta spacecraft orbiting comet 67p/churyumov-gerasimenko observed a small outburst originating from the imhotep region at the foot of the big lobe of the comet. these measurements are unique since it was the first time that the initial phase of a transient outburst event could be directly observed.aims: we investigate the evolution of the dust jet in order to derive clues about the outburst source mechanism and the ejected dust particles, in particular the dust mass, dust-to-gas ratio and the particle size distribution.methods: analysis of the images and of the observation geometry using comet shape models in combination with gasdynamic modeling of the transient dust jet were the main tools used in this study. synthetic images were computed for comparison with the observations.results: analysis of the geometry revealed that the source region was not illuminated until 1.5 h after the event implying true nightside activity was observed. the outburst lasted for less than one hour and the average dust production rate during the initial four minutes was of the order of 1 kg/s. during this time the outburst dust production rate was approximately constant, no sign for an initial explosion could be detected. for dust grains between 0.01-1 mm a power law size distribution characterized by an index of about 2.6 provides the best fit to the observed radiance profiles. the dust-to-gas ratio of the outburst jet is in the range 0.6-1.8. | a mini outburst from the nightside of comet 67p/churyumov-gerasimenko observed by the osiris camera on rosetta |
we report and analyze updated molecular abundances in 20 comets obtained by employing modern data reduction procedures and molecular models. using box and scatter plots, we examine how the different molecular species are distributed among the comet population, while by means of pie charts, we investigate the relative proportions of these molecular species in each comet. we compare these results with the orbital parameters of the selected comets to identify trends related to their dynamical history. with these analyses, we tentatively identify at least three chemical classes based mainly on relative abundances of co, ch3oh, ch4, c2h6, hcn, and nh3. the combination of relative abundances and orbital parameters is then compared with recent chemical models of planetary system formation. this approach may help in investigating the origins and evolution of the material in cometary nuclei. among other aspects, we underline the need to increase our sample size, especially for hypervolatiles (i.e., ch4 and co) in jupiter family comets. | investigation of the origins of comets as revealed through infrared high-resolution spectroscopy i. molecular abundances |
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