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several sulphur-bearing species have already been observed in different families of comets. however, the knowledge on the minor sulphur species is still limited. the comet's sulphur inventory is closely linked to the pre-solar cloud and holds important clues to the degree of reprocessing of the material in the solar nebula and during comet accretion. sulphur in pre-solar clouds is highly depleted, which is quite puzzling as the s/o ratio in the diffuse interstellar medium is cosmic. this work focuses on the abundance of the previously known species h2s, ocs, so, s2, so2 and cs2 in the coma of comet 67p/churyumov-gerasimenko measured by rosetta orbiter spectrometer for ion and neutral analysis/double focusing mass spectrometer between equinox and perihelion 2015. furthermore, we present the first detection of s3, s4, ch3sh and c2h6s in a comet, and we determine the elemental abundance of s/o in the bulk ice of (1.47 ± 0.05) × 10-2. we show that so is present in the coma originating from the nucleus, but not cs in the case of 67p, and for the first time establish that s2 is present in a volatile and a refractory phase. the derived total elemental sulphur abundance of 67p is in agreement with solar photospheric elemental abundances and shows no sulphur depletion as reported for dense interstellar clouds. also the presence of s2 at heliocentric distances larger than 3 au indicates that sulphur-bearing species have been processed by radiolysis in the pre-solar cloud and that at least some of the ice from this cloud has survived in comets up the present. | sulphur-bearing species in the coma of comet 67p/churyumov-gerasimenko |
aims: the osiris camera onboard the rosetta spacecraft has been acquiring images of the comet 67p/churyumov-gerasimenko (67p)'s nucleus at spatial resolutions down to ~0.17 m/px ever since aug. 2014. these images have yielded unprecedented insight into the morphological diversity of the comet's surface. this paper presents an overview of the regional morphology of comet 67p.methods: we used the images that were acquired at orbits ~20-30 km from the center of the comet to distinguish different regions on the surface and introduce the basic regional nomenclature adopted by all papers in this rosetta special feature that address the comet's morphology and surface processes. we used anaglyphs to detect subtle regional and topographical boundaries and images from close orbit (~10 km from the comet's center) to investigate the fine texture of the surface.results: nineteen regions have currently been defined on the nucleus based on morphological and/or structural boundaries, and they can be grouped into distinctive region types. consolidated, fractured regions are the most common region type. some of these regions enclose smooth units that appear to settle in gravitational sinks or topographically low areas. both comet lobes have a significant portion of their surface covered by a dusty coating that appears to be recently placed and shows signs of mobilization by aeolian-like processes. the dusty coatings cover most of the regions on the surface but are notably absent from a couple of irregular large depressions that show sharp contacts with their surroundings and talus-like deposits in their interiors, which suggests that short-term explosive activity may play a significant role in shaping the comet's surface in addition to long-term sublimation loss. finally, the presence of layered brittle units showing signs of mechanical failure predominantly in one of the comet's lobes can indicate a compositional heterogeneity between the two lobes. tables 2 and 3 and appendix a are available in electronic form at http://www.aanda.org | regional surface morphology of comet 67p/churyumov-gerasimenko from rosetta/osiris images |
the radio science experiment rsi on-board rosetta determined the mass of the nucleus of comet 67p/churyumov-gerasimenko at the start of the prime mission from 2014 august to november (gm = 666.2 ± 0.2 m3 s-2 or 9982 ± 3 × 1012 kg) and shortly before the end of the mission from 2016 july to september (gm = 665.5 ± 0.1 m3 s-2 or 9971.5 ± 1.5 × 1012 kg). the mass-loss is δm = 10.5 ± 3.4.109 kg, about 0.1 per cent of the nucleus mass. almost 50 per cent of the mass-loss occurred during the 32 d before and 62 d after perihelion. the nucleus mass combined with the new very precise nucleus volume of 18.56 ± 0.02 km3 yields a bulk density of 537.8 ± 0.6 kg m-3. this low bulk density suggests that the nucleus is highly porous. the porosity is constrained by the observed bulk density, the density of ices, mostly water ice, and the density of compacted nucleus dust material. for a range of compacted dust material density from 2000 to 3500 kg m-3, the porosity varies between 65-79 per cent when the dust-to-ice mass ratio fnucleus for the nucleus body lies in the range 3 ≤ fnucleus ≤ 7. the nucleus is thus a highly porous very dusty body with very little ice. the total mass-loss δm puts hard constraints on the models of interpretation of the observations from other instruments on rosetta. the loss from gas, based on rosina and miro observations, suggests that fcoma, the dust-to-gas mass ratio of cometary matter in the coma beyond the hill sphere of the nucleus is of the order of fcoma ≤ 0.5 and therefore at least six times, and perhaps as much as 14 times, smaller than fnucleus. it seems that the lost gas mass was overestimated by the instruments. the lost gas mass shall be 5.25 × 109 kg for fcoma = 1. for any other gas mass values > 5.25 × 109 kg fcoma is < 1. most of the lifted dust mass will not leave the cometary hill sphere but falls back to the nucleus. for 3 ≤ fnucleus ≤ 7 and fcoma = 0.5, the fallback mass is between 1.8.δm and 4.3.δm. | the nucleus of comet 67p/churyumov-gerasimenko - part i: the global view - nucleus mass, mass-loss, porosity, and implications |
the factors shaping cometary nuclei are still largely unknown, but could be the result of concurrent effects of evolutionary and primordial processes. the peculiar bilobed shape of comet 67p/churyumov-gerasimenko may be the result of the fusion of two objects that were once separate or the result of a localized excavation by outgassing at the interface between the two lobes. here we report that the comet's major lobe is enveloped by a nearly continuous set of strata, up to 650 metres thick, which are independent of an analogous stratified envelope on the minor lobe. gravity vectors computed for the two lobes separately are closer to perpendicular to the strata than those calculated for the entire nucleus and adjacent to the neck separating the two lobes. therefore comet 67p/churyumov-gerasimenko is an accreted body of two distinct objects with `onion-like' stratification, which formed before they merged. we conclude that gentle, low-velocity collisions occurred between two fully formed kilometre-sized cometesimals in the early stages of the solar system. the notable structural similarities between the two lobes of comet 67p/churyumov-gerasimenko indicate that the early-forming cometesimals experienced similar primordial stratified accretion, even though they formed independently. | two independent and primitive envelopes of the bilobate nucleus of comet 67p |
complex organic molecules (coms) detected in the gas phase are thought to be mostly formed on icy grains, but no unambiguous detection of icy coms larger than ch3oh has been reported so far. exploring this matter in more detail has become possible with the jwst the critical 5-10 $\mu$m range. in the joys+ program, more than 30 protostars are being observed with the miri/mrs. this study explores the coms ice signatures in the low and high-mass protostar, iras 2a and iras 23385, respectively. we fit continuum and silicate subtracted observational data with ir laboratory ice spectra. we use the eniigma fitting tool to find the best fit between the lab data and the observations and to performs statistical analysis of the solutions. we report the best fits for the spectral ranges between 6.8 and 8.6 $\mu$m in iras 2a and iras 23385, originating from simple molecules, coms, and negative ions. the strongest feature in this range (7.7 $\mu$m) is dominated by ch4 and has contributions of so2 and ocn-. our results indicate that the 7.2 and 7.4 $\mu$m bands are mostly dominated by hcoo-. we find statistically robust detections of coms based on multiple bands, most notably ch3cho, ch3ch2oh, and ch3ocho. the likely detection of ch3cooh is also reported. the ice column density ratios between ch3ch2oh and ch3cho of iras 2a and iras 23385, suggests that these coms are formed on icy grains. finally, the derived ice abundances for iras 2a correlate well with those in comet 67p/gc within a factor of 5. based on the miri/mrs data, we conclude that coms are present in interstellar ices, thus providing additional proof for a solid-state origin of these species in star-forming regions. the good correlation between the ice abundances in comet 67p and iras 2a is in line with the idea that cometary coms can be inherited from the early protostellar phases. | jwst observations of young protostars (joys+): detection of icy complex organic molecules and ions. i. ch$_4$, so$_2$, hcoo$^-$, ocn$^-$, h$_2$co, hcooh, ch$_3$ch$_2$oh, ch$_3$cho, ch$_3$ocho, ch$_3$cooh |
the grain impact analyzer and dust accumulator (giada) instrument on board esa’s rosetta mission is constraining the origin of the dust particles detected within the coma of comet 67 p/churyumov-gerasimenko (67p). the collected particles belong to two families: (i) compact particles (ranging in size from 0.03 to 1 mm), witnessing the presence of materials that underwent processing within the solar nebula and (ii) fluffy aggregates (ranging in size from 0.2 to 2.5 mm) of sub-micron grains that may be a record of a primitive component, probably linked to interstellar dust. the dynamics of the fluffy aggregates constrain their equivalent bulk density to \lt 1 kg m-3. these aggregates are charged, fragmented, and decelerated by the spacecraft negative potential and enter giada in showers of fragments at speeds \lt 1 m s-1. the density of such optically thick aggregates is consistent with the low bulk density of the nucleus. the mass contribution of the fluffy aggregates to the refractory component of the nucleus is negligible and their coma brightness contribution is less than 15%. | density and charge of pristine fluffy particles from comet 67p/churyumov-gerasimenko |
a systematic analysis of the mixing ratios with respect to h2o for eight species (ch3oh, hcn, nh3, h2co, c2h2, c2h6, ch4, and co) measured with high-resolution infrared spectroscopy in thirty comets between 1997 and 2013 is presented. some trends are beginning to emerge when mixing ratios in individual comets are compared to average mixing ratios obtained for all species within the population. the variation in mixing ratios for all measured species is at least an order of magnitude. overall, jupiter-family comets are depleted in volatile species with respect to h2o compared to long-period oort cloud comets, with the most volatile species showing the greatest relative depletion. there is a high positive correlation between the mixing ratios of hcn, c2h6, and ch4, whereas nh3, h2co, and c2h2 are moderately correlated with each other but generally uncorrelated or show only weak correlation with other species. co is generally uncorrelated with the other measured species possibly because it has the highest volatility and is therefore more susceptible to thermal evolutionary effects. most of these correlations appear to be independent of dynamical class with a few possible exceptions. molecular mixing ratios for ch3oh, hcn, c2h6, and ch4 show an expected behavior with heliocentric distance suggesting a dominant ice source, whereas there is emerging evidence that the mixing ratios of nh3, h2co, c2h2, nh2, and cn may increase at small heliocentric distances, suggesting the possibility of additional sources related to the thermal decomposition of organic dust. although this provides information on the composition of the most volatile grains in comets, it presents an additional difficulty in classifying comet chemistry because most comets within this dataset were only observed over a limited range of heliocentric distance. although there is remarkable compositional diversity resulting in a unique chemical fingerprint for each comet, a hierarchical tree cluster analysis is used to determine a taxonomic classification system containing four groups and eleven subgroups. optical and infrared comparisons indicate that mixing ratios of daughter species and potential parents from cometary ices are sometimes but not always consistent with one another. this suggests that in many comets there are significant sources of c2 and/or cn from grains, and that the importance of these sources is variable within the comet population. | emerging trends and a comet taxonomy based on the volatile chemistry measured in thirty comets with high-resolution infrared spectroscopy between 1997 and 2013 |
the cosima mass spectrometer on board the rosetta orbiter has collected dust in the near coma of comet 67p/churyumov-gerasimenko since august 11, 2014. the collected dust particles are identified by taking images with a microscope (cosiscope) under grazing incidence illumination before and after exposure of the target to cometary dust. more than 10,000 dust particles >14 μm in size collected from august 11, 2014 to april 3, 2015 have been detected on three distinct target assemblies, including ∼500 dust particles with sizes ranging from 50 to more than 500 μm, that can be resolved by cosiscope (pixel size 14 μm). during this period, the heliocentric distance decreased from 3.5 au to less than 2 au. the collection efficiency on targets covered with "metal black" has been very high, due to the low relative velocity of incoming dust. therefore, the cosiscope observations provide the first optical characterization of an unbiased sample of particles collected in the inner coma of a comet. the typology of particles >100 μm in size is dominated by clusters with a wide range of structure and strength, most originating from the disruption of large aggregates (>1 mm in size) shortly before collection. a generic relationship between these clusters and idps/antarctic meteorites is likely in the framework of accretion models. about 15% of particles larger than 100 μm are compact particles with two likely contributions, one being linked to clusters and another leaving the cometary nucleus as single compact particles. | typology of dust particles collected by the cosima mass spectrometer in the inner coma of 67p/churyumov gerasimenko |
although water vapour is the main species observed in the coma of comet 67p/churyumov-gerasimenko and water is the major constituent of cometary nuclei, limited evidence for exposed water-ice regions on the surface of the nucleus has been found so far. the absence of large regions of exposed water ice seems a common finding on the surfaces of many of the comets observed so far. the nucleus of 67p/churyumov-gerasimenko appears to be fairly uniformly coated with dark, dehydrated, refractory and organic-rich material. here we report the identification at infrared wavelengths of water ice on two debris falls in the imhotep region of the nucleus. the ice has been exposed on the walls of elevated structures and at the base of the walls. a quantitative derivation of the abundance of ice in these regions indicates the presence of millimetre-sized pure water-ice grains, considerably larger than in all previous observations. although micrometre-sized water-ice grains are the usual result of vapour recondensation in ice-free layers, the occurrence of millimetre-sized grains of pure ice as observed in the imhotep debris falls is best explained by grain growth by vapour diffusion in ice-rich layers, or by sintering. as a consequence of these processes, the nucleus can develop an extended and complex coating in which the outer dehydrated crust is superimposed on layers enriched in water ice. the stratigraphy observed on 67p/churyumov-gerasimenko is therefore the result of evolutionary processes affecting the uppermost metres of the nucleus and does not necessarily require a global layering to have occurred at the time of the comet’s formation. | exposed water ice on the nucleus of comet 67p/churyumov-gerasimenko |
in 2016, unambiguous evidence for the presence of the amino acid glycine, an important prebiotic molecule, was deduced based on in situ mass-spectral studies of the coma surrounding cometary ice. this finding is significant because comets are thought to have preserved the icy grains originally found in the interstellar medium prior to solar system formation. energetic processing of cosmic ices via photochemistry and radiation chemistry is thought to be the dominant mechanism for the extraterrestrial synthesis of prebiotic molecules. radiation chemistry is defined as the "study of the chemical changes produced by the absorption of radiation of sufficiently high energy to produce ionization." ionizing radiation in cosmic chemistry includes high-energy particles (e.g., cosmic rays) and high-energy photons (e.g., extreme-uv). in contrast, photochemistry is defined as chemical processes initiated by photon-induced electronic excitation not involving ionization. vacuum-uv (6.2-12.4 ev) light may, in addition to photochemistry, initiate radiation chemistry because the threshold for producing secondary electrons is lower in the condensed phase than in the gas phase. unique to radiation chemistry are four phenomena: (1) production of a cascade of low-energy (<20 ev) secondary electrons which are thought to be the dominant driving force for radiation chemistry, (2) reactions initiated by cations, (3) non-uniform distribution of reaction intermediates, and (4) non-selective chemistry leading to the production of multiple reaction products. the production of low-energy secondary electrons during radiation chemistry may also lead to new reaction pathways not available to photochemistry. in addition, low-energy electron-induced radiation chemistry may predominate over photochemistry because of the sheer number of low-energy secondary electrons. moreover, reaction cross-sections can be several orders of magnitude larger for electrons than for photons. discerning the role of photochemistry vs. radiation chemistry in as trochemistry is challenging because astrophysical photon-induced chemistry studies have almost exclusively used light sources that produce >10 ev photons. because a primary objective of chemistry is to provide molecular-level mechanistic explanations for macroscopic phenomena, our ultimate goal in this review paper is to critically evaluate our current understanding of cosmic ice energetic processing which likely leads to the synthesis of extraterrestrial prebiotic molecules. | extraterrestrial prebiotic molecules: photochemistry vs. radiation chemistry of interstellar ices |
we summarize the collective knowledge of physical and surface properties of comet nuclei, focusing on those that are obtained from remote observations. we now have measurements or constraints on effective radius for over 200 comets, rotation periods for over 60, axial ratios and color indices for over 50, geometric albedos for over 25, and nucleus phase coefficients for over 20. the sample has approximately tripled since the publication of comets ii, with ir surveys using spitzer and neowise responsible for the bulk of the increase in effective radii measurements. advances in coma morphology studies and long-term studies of a few prominent comets have resulted in meaningful constraints on rotation period changes in nearly a dozen comets, allowing this to be added to the range of nucleus properties studied. the first delay-doppler radar and visible light polarimetric measurements of comet nuclei have been made since comets ii and are considered alongside the traditional methods of studying nuclei remotely. we use the results from recent in situ missions, notably rosetta, to put the collective properties obtained by remote observations into context, emphasizing the insights gained into surface properties and the prevalence of highly elongated and/or bilobate shapes. we also explore how nucleus properties evolve, focusing on fragmentation and the likely related phenomena of outbursts and disintegration. knowledge of these behaviors has been shaped in recent years by diverse sources: high resolution images of nucleus fragmentation and disruption events, the detection of thousands of small comets near the sun, regular photometric monitoring of large numbers of comets throughout the solar system, and detailed imaging of the surfaces of mission targets. finally, we explore what advances in the knowledge of the bulk nucleus properties may be enabled in coming years. | physical and surface properties of comet nuclei from remote observations |
debris belts on the periphery of planetary systems, encompassing the region occupied by planetary orbits, are massive analogues of the solar system's kuiper belt. they are detected by thermal emission of dust released in collisions amongst directly unobservable larger bodies that carry most of the debris disc mass. we estimate the total mass of the discs by extrapolating up the mass of emitting dust with the help of collisional cascade models. the resulting mass of bright debris discs appears to be unrealistically large, exceeding the mass of solids available in the systems at the preceding protoplanetary stage. we discuss this 'mass problem' in detail and investigate possible solutions to it. these include uncertainties in the dust opacity and planetesimal strength, variation of the bulk density with size, steepening of the size distribution by damping processes, the role of the unknown 'collisional age' of the discs, and dust production in recent giant impacts. while we cannot rule out the possibility that a combination of these might help, we argue that the easiest solution would be to assume that planetesimals in systems with bright debris discs were 'born small', with sizes in the kilometre range, especially at large distances from the stars. this conclusion would necessitate revisions to the existing planetesimal formation models, and may have a range of implications for planet formation. we also discuss potential tests to constrain the largest planetesimal sizes and debris disc masses. | solution to the debris disc mass problem: planetesimals are born small? |
the presence of solid carbonaceous matter in cometary dust was established by the detection of elements such as carbon, hydrogen, oxygen and nitrogen in particles from comet 1p/halley. such matter is generally thought to have originated in the interstellar medium, but it might have formed in the solar nebula—the cloud of gas and dust that was left over after the sun formed. this solid carbonaceous material cannot be observed from earth, so it has eluded unambiguous characterization. many gaseous organic molecules, however, have been observed; they come mostly from the sublimation of ices at the surface or in the subsurface of cometary nuclei. these ices could have been formed from material inherited from the interstellar medium that suffered little processing in the solar nebula. here we report the in situ detection of solid organic matter in the dust particles emitted by comet 67p/churyumov-gerasimenko the carbon in this organic material is bound in very large macromolecular compounds, analogous to the insoluble organic matter found in the carbonaceous chondrite meteorites. the organic matter in meteorites might have formed in the interstellar medium and/or the solar nebula, but was almost certainly modified in the meteorites’ parent bodies. we conclude that the observed cometary carbonaceous solid matter could have the same origin as the meteoritic insoluble organic matter, but suffered less modification before and/or after being incorporated into the comet. | high-molecular-weight organic matter in the particles of comet 67p/churyumov-gerasimenko |
the detection of interstellar objects passing through the solar system offers the promise of constraining the physical and chemical processes involved in planetary formation in other extrasolar systems. while the effect of outgassing by 1i/2017 u1 (’oumuamua) was dynamically observed, no direct detection of the ejected material was made. the discovery of the active interstellar comet 2i/borisov means spectroscopic investigations of the sublimated ices is possible for this object. we report the first detection of gas emitted by an interstellar comet via the near-uv emission of cn from 2i/borisov at a heliocentric distance of r = 2.7 au on 2019 september 20. the production rate was found to be q(cn) = (3.7 ± 0.4) × 1024 s-1, using a simple haser model with an outflow velocity of 0.5 km s-1. no other emission was detected, with an upper limit to the production rate of c2 of 4 × 1024 s-1. the spectral reflectance slope of the dust coma over 3900 å < λ < 6000 å is steeper than at longer wavelengths, as found for other comets. broadband rcphotometry on 2019 september 19 gave a dust production rate of afρ = 143 ± 10 cm. modeling of the observed gas and dust production rates constrains the nuclear radius to 0.7-3.3 km assuming reasonable nuclear properties. overall, we find the gas, dust, and nuclear properties for the first active interstellar object are similar to normal solar system comets. | detection of cn gas in interstellar object 2i/borisov |
the m dwarf gj 436 hosts a transiting warm neptune known to experience atmospheric escape. previous observations revealed the presence of a giant hydrogen exosphere transiting the star for more than 5 h, and absorbing up to 56% of the flux in the blue wing of the stellar lyman-α line of neutral hydrogen (h i lyα). the unexpected size of this comet-like exosphere prevented observing the full transit of its tail. in this letter, we present new lyα observations of gj 436 obtained with the space telescope imaging spectrograph (stis) instrument onboard the hubble space telescope. the stability of the lyα line over six years allowed us to combine these new observations with archival data sets, substantially expanding the coverage of the exospheric transit. hydrogen atoms in the tail of the exospheric cloud keep occulting the star for 10-25 h after the transit of the planet, remarkably confirming a previous prediction based on 3d numerical simulations with the evaporating exoplanet code (eve). this result strengthens the interpretation that the exosphere of gj 436b is shaped by both radiative braking and charge exchanges with the stellar wind. we further report flux decreases of 15 ± 2% and 47 ± 10% in the red wing of the lyα line and in the line of ionised silicon (si iii). despite some temporal variability possibly linked with stellar activity, these two signals occur during the exospheric transit and could be of planetary origin. follow-up observations will be required to assess the possibility that the redshifted lyα and si iii absorption signatures arise from interactions between the exospheric flow and the magnetic field of the star. | the long egress of gj 436b's giant exosphere |
this chapter reviews the estimates of the dust-to-gas and refractory-to-ice mass ratios derived from rosetta measurements in the lost materials and the nucleus of 67p/churyumov-gerasimenko, respectively. first, the measurements by rosetta instruments are described, as well as relevant characteristics of 67p. the complex picture of the activity of 67p, with its extreme north-south seasonal asymmetry, is presented. individual estimates of the dust-to-gas and refractory-to-ice mass ratios are then presented and compared, showing wide ranges of plausible values. rosetta's wealth of information suggests that estimates of the dust-to-gas mass ratio made in cometary comae at a single point in time may not be fully representative of the refractory-to-ice mass ratio within the cometary nuclei being observed. | dust-to-gas and refractory-to-ice mass ratios of comet 67p/churyumov-gerasimenko from rosetta observations |
comets are composed of dust and frozen gases. the ices are mixed with the refractory material either as an icy conglomerate, or as an aggregate of pre-solar grains (grains that existed prior to the formation of the solar system), mantled by an ice layer. the presence of water-ice grains in periodic comets is now well established. modelling of infrared spectra obtained about ten kilometres from the nucleus of comet hartley 2 suggests that larger dust particles are being physically decoupled from fine-grained water-ice particles that may be aggregates, which supports the icy-conglomerate model. it is known that comets build up crusts of dust that are subsequently shed as they approach perihelion. micrometre-sized interplanetary dust particles collected in the earth's stratosphere and certain micrometeorites are assumed to be of cometary origin. here we report that grains collected from the jupiter-family comet 67p/churyumov-gerasimenko come from a dusty crust that quenches the material outflow activity at the comet surface. the larger grains (exceeding 50 micrometres across) are fluffy (with porosity over 50 per cent), and many shattered when collected on the target plate, suggesting that they are agglomerates of entities in the size range of interplanetary dust particles. their surfaces are generally rich in sodium, which explains the high sodium abundance in cometary meteoroids. the particles collected to date therefore probably represent parent material of interplanetary dust particles. this argues against comet dust being composed of a silicate core mantled by organic refractory material and then by a mixture of water-dominated ices. at its previous recurrence (orbital period 6.5 years), the comet's dust production doubled when it was between 2.7 and 2.5 astronomical units from the sun, indicating that this was when the nucleus shed its mantle. once the mantle is shed, unprocessed material starts to supply the developing coma, radically changing its dust component, which then also contains icy grains, as detected during encounters with other comets closer to the sun. | comet 67p/churyumov-gerasimenko sheds dust coat accumulated over the past four years |
we present the discovery of a transiting exoplanet candidate in the k2 field-1 with an orbital period of 9.1457 hr: k2-22b. the highly variable transit depths, ranging from ∼0% to 1.3%, are suggestive of a planet that is disintegrating via the emission of dusty effluents. we characterize the host star as an m-dwarf with teff ≃ 3800 k. we have obtained ground-based transit measurements with several 1-m class telescopes and with the gtc. these observations (1) improve the transit ephemeris; (2) confirm the variable nature of the transit depths; (3) indicate variations in the transit shapes; and (4) demonstrate clearly that at least on one occasion the transit depths were significantly wavelength dependent. the latter three effects tend to indicate extinction of starlight by dust rather than by any combination of solid bodies. the k2 observations yield a folded light curve with lower time resolution but with substantially better statistical precision compared with the ground-based observations. we detect a significant “bump” just after the transit egress, and a less significant bump just prior to transit ingress. we interpret these bumps in the context of a planet that is not only likely streaming a dust tail behind it, but also has a more prominent leading dust trail that precedes it. this effect is modeled in terms of dust grains that can escape to beyond the planet's hill sphere and effectively undergo “roche lobe overflow,” even though the planet's surface is likely underfilling its roche lobe by a factor of 2. | the k2-esprint project i: discovery of the disintegrating rocky planet k2-22b with a cometary head and leading tail |
the chemical composition of cometary ices provides clues for the conditions of formation and evolution of the early solar system. a large number of molecules have been identified in cometary atmospheres, from both ground-based observations and space, including in situ investigations. this includes large organic molecules, which are also observed in star-forming regions. this paper presents a review of molecular abundances measured in cometary atmospheres from remote sensing observations with ground-based and space-based telescopes. the diversity of composition observed in comet populations is presented and discussed. this article is part of the themed issue 'cometary science after rosetta'. | the composition of cometary ices |
the nonlinear propagation of heavy-ion-acoustic (hia) waves (hiaws) in a four-component multi-ion plasma (containing inertial heavy negative ions and light positive ions, as well as inertialess nonextensive electrons and positrons) has been theoretically investigated. the nonlinear schrödinger (nls) equation is derived by employing the reductive perturbation method. it is found that the nls equation leads to the modulational instability (mi) of hiaws, and to the formation of hia rogue waves (hiarws), which are due to the effects of nonlinearity and dispersion in the propagation of hiaws. the conditions for the mi of hiaws and the basic properties of the generated hiarws are identified. it is observed that the striking features (viz., instability criteria, growth rate of mi, amplitude and width of hiarws, etc.) of the hiaws are significantly modified by the effects of nonextensivity of electrons and positrons, the ratio of light positive ion mass to heavy negative ion mass, the ratio of electron number density to light positive ion number density, the ratio of electron temperature to positron temperature, etc. the relevancy of our present investigation to the observations in space (viz., cometary comae and earth's ionosphere) and laboratory (viz., solid-high intense laser plasma interaction experiments) plasmas is pointed out. | heavy ion-acoustic rogue waves in electron-positron multi-ion plasmas |
the origin of the interstellar object 1i/`oumuamua has defied explanation. in a companion paper (jackson & desch, 2021), we show that a body of n2 ice with axes 45 m × 44 m × 7.5 m at the time of observation would be consistent with its albedo, nongravitational acceleration, and lack of observed co or co2 or dust. here we demonstrate that impacts on the surfaces of pluto like kuiper belt objects (kbos) would have generated and ejected ∼1014 collisional fragments—roughly half of them h2o ice fragments and half of them n2 ice fragments—due to the dynamical instability that depleted the primordial kuiper belt. we show consistency between these numbers and the frequency with which we would observe interstellar objects like 1i/`oumuamua, and more comet like objects like 2i/borisov, if other stellar systems eject such objects with efficiency like that of the sun; we infer that differentiated kbos and dynamical instabilities that eject impact generated fragments may be near universal among extrasolar systems. galactic cosmic rays would erode such fragments over 4.5 gyr, so that fragments are a small fraction (∼0.1%) of long period oort comets, but c/2016 r2 may be an example. we estimate `oumuamua was ejected about 0.4-0.5 gyr ago, from a young (∼108 yr) stellar system, which we speculate was in the perseus arm. objects like `oumuamua may directly probe the surface compositions of a hitherto unobserved type of exoplanet: "exo plutos." `oumuamua may be the first sample of an exoplanet brought to us. | 1i/`oumuamua as an n2 ice fragment of an exo pluto surface ii: generation of n2 ice fragments and the origin of `oumuamua |
the philae lander, part of the rosetta mission to investigate comet 67p/churyumov-gerasimenko, was delivered to the cometary surface in november 2014. here we report the precise circumstances of the multiple landings of philae, including the bouncing trajectory and rebound parameters, based on engineering data in conjunction with operational instrument data. these data also provide information on the mechanical properties (strength and layering) of the comet surface. the first touchdown site, agilkia, appears to have a granular soft surface (with a compressive strength of 1 kilopascal) at least ~20 cm thick, possibly on top of a more rigid layer. the final landing site, abydos, has a hard surface. | the landing(s) of philae and inferences about comet surface mechanical properties |
during its 2 yr mission around comet 67p/churyumov-gerasimenko, esa's rosetta spacecraft had the unique opportunity to follow closely a comet in the most active part of its orbit. many studies have presented the typical features associated with the activity of the nucleus, such as localized dust and gas jets. here, we report on series of more energetic transient events observed during the 3 months surrounding the comet's perihelion passage in 2015 august. we detected and characterized 34 outbursts with the rosetta cameras, one every 2.4 nucleus rotations. we identified three main dust plume morphologies associated with these events: a narrow jet, a broad fan, and more complex plumes featuring both previous types together. these plumes are comparable in scale and temporal variation to what has been observed on other comets. we present a map of the outbursts' source locations, and discuss the associated topography. we find that the spatial distribution sources on the nucleus correlate well with morphological region boundaries, especially in areas marked by steep scarps or cliffs. outbursts occur either in the early morning or shortly after the local noon, indicating two potential processes: morning outbursts may be triggered by thermal stresses linked to the rapid change of temperature; afternoon events are most likely related to the diurnal or seasonal heat wave reaching volatiles buried under the first surface layer. in addition, we propose that some events can be the result of a completely different mechanism, in which most of the dust is released upon the collapse of a cliff. | summer fireworks on comet 67p |
this review presents our understanding of cometary dust at the end of 2017. for decades, insight about the dust ejected by nuclei of comets had stemmed from remote observations from earth or earth's orbit, and from flybys, including the samples of dust returned to earth for laboratory studies by the stardust return capsule. the long-duration rosetta mission has recently provided a huge and unique amount of data, obtained using numerous instruments, including innovative dust instruments, over a wide range of distances from the sun and from the nucleus. the diverse approaches available to study dust in comets, together with the related theoretical and experimental studies, provide evidence of the composition and physical properties of dust particles, e.g., the presence of a large fraction of carbon in macromolecules, and of aggregates on a wide range of scales. the results have opened vivid discussions on the variety of dust-release processes and on the diversity of dust properties in comets, as well as on the formation of cometary dust, and on its presence in the near-earth interplanetary medium. these discussions stress the significance of future explorations as a way to decipher the formation and evolution of our solar system. | cometary dust |
we use the c/n ratio as a monitor of the delivery of key ingredients of life to nascent terrestrial worlds. total elemental c and n contents, and their ratio, are examined for the interstellar medium, comets, chondritic meteorites and terrestrial planets; we include an updated estimate for the bulk silicate earth (c/n = 49.0 +/- 9.3). using a kinetic model of disk chemistry, and the sublimation/condensation temperatures of primitive molecules, we suggest that organic ices and macro-molecular (refractory or carbonaceous dust) organic material are the likely initial c and n carriers. chemical reactions in the disk can produce nebular c/n ratios of ~1-12, comparable to those of comets and the low end estimated for planetesimals. an increase of the c/n ratio is traced between volatile-rich pristine bodies and larger volatile-depleted objects subjected to thermal/accretional metamorphism. the c/n ratios of the dominant materials accreted to terrestrial planets should therefore be higher than those seen in carbonaceous chondrites or comets. during planetary formation, we explore scenarios leading to further volatile loss and associated c/n variations owing to core formation and atmospheric escape. key processes include relative enrichment of nitrogen in the atmosphere and preferential sequestration of carbon by the core. the high c/n bse ratio therefore is best satisfied by accretion of thermally processed objects followed by large-scale atmospheric loss. these two effects must be more profound if volatile sequestration in the core is effective. the stochastic nature of these processes hints that the surface/atmospheric abundances of biosphere-essential materials will likely be variable. | tracing the ingredients for a habitable earth from interstellar space through planet formation |
the philae lander provides a unique opportunity to investigate the internal structure of a comet nucleus, providing information about its formation and evolution in the early solar system. we present comet nucleus sounding experiment by radiowave transmission (consert) measurements of the interior of comet 67p/churyumov-gerasimenko. from the propagation time and form of the signals, the upper part of the “head” of 67p is fairly homogeneous on a spatial scale of tens of meters. consert also reduced the size of the uncertainty of philae’s final landing site down to approximately 21 by 34 square meters. the average permittivity is about 1.27, suggesting that this region has a volumetric dust/ice ratio of 0.4 to 2.6 and a porosity of 75 to 85%. the dust component may be comparable to that of carbonaceous chondrites. | properties of the 67p/churyumov-gerasimenko interior revealed by consert radar |
the physical properties of cometary nuclei observed today relate to their complex history and help to constrain their formation and evolution. in this article, we review some of the main physical properties of cometary nuclei and focus in particular on the thermal, mechanical, structural and dielectric properties, emphasising the progress made during the rosetta mission. comets have a low density of 480 ± 220 kg m^{-3} and a low permittivity of 1.9-2.0, consistent with a high porosity of 70-80%, are weak with a very low global tensile strength <100 pa, and have a low bulk thermal inertia of 0-60 j k^{-1} m^{-2} s^{-1/2} that allowed them to preserve highly volatiles species (e.g. co, co2, ch4, n2) into their interior since their formation. as revealed by 67p/churyumov-gerasimenko, the above physical properties vary across the nucleus, spatially at its surface but also with depth. the broad picture is that the bulk of the nucleus consists of a weakly bonded, rather homogeneous material that preserved primordial properties under a thin shell of processed material, and possibly covered by a granular material; this cover might in places reach a thickness of several meters. the properties of the top layer (the first meter) are not representative of that of the bulk nucleus. more globally, strong nucleus heterogeneities at a scale of a few meters are ruled out on 67p's small lobe. | the thermal, mechanical, structural, and dielectric properties of cometary nuclei after rosetta |
we design and implement a scalable hard particle monte carlo simulation toolkit (hpmc), and release it open source as part of hoomd-blue. hpmc runs in parallel on many cpus and many gpus using domain decomposition. we employ bvh trees instead of cell lists on the cpu for fast performance, especially with large particle size disparity, and optimize inner loops with simd vector intrinsics on the cpu. our gpu kernel proposes many trial moves in parallel on a checkerboard and uses a block-level queue to redistribute work among threads and avoid divergence. hpmc supports a wide variety of shape classes, including spheres/disks, unions of spheres, convex polygons, convex spheropolygons, concave polygons, ellipsoids/ellipses, convex polyhedra, convex spheropolyhedra, spheres cut by planes, and concave polyhedra. nvt and npt ensembles can be run in 2d or 3d triclinic boxes. additional integration schemes permit frenkel-ladd free energy computations and implicit depletant simulations. in a benchmark system of a fluid of 4096 pentagons, hpmc performs 10 million sweeps in 10 min on 96 cpu cores on xsede comet. the same simulation would take 7.6 h in serial. hpmc also scales to large system sizes, and the same benchmark with 16.8 million particles runs in 1.4 h on 2048 gpus on olcf titan. | scalable metropolis monte carlo for simulation of hard shapes |
in this study, heavy metal pollution in the pazarsuyu stream of giresun province and the protective role of lycopene against the toxicity caused by this pollution were investigated using the allium test. germination percentage, root length and weight gain as physiological markers of toxicity; mitotic index (mi), micronucleus (mn) and chromosomal aberrations (cas) as genetic markers of toxicity; malondialdehyde (mda) level, superoxide dismutase (sod) and catalase (cat) activities as biochemical markers of toxicity, and meristematic cell damages were used as anatomical markers. for this aim allium cepa l. bulbs were divided into six groups and germinated for 72 h with 215 mg/l and 430 mg/l doses of lycopene, tap water and stream water. heavy metals pollution was analyzed with icp-ms and fe > sr > ba > be > mo > li were determined according to the rate of presence in the water samples of pazarsuyu. as a result, germination-related parameters and meristematic cell proliferation of bulbs germinated with pazarsuyu water samples decreased significantly. germination percentage, root length and weight gain of the group treated with pazarsuyu water samples were decreased 50%, 73% and 68%, respectively compared to control. in addition, mn and cas frequencies, indicating the genotoxic effects, were increased and significant abnormalities were detected in mda, sod and cat levels, which indicate the deterioration of antioxidant/oxidant balance. ca observed with high frequency was also confirmed by dna fragmentation determined by the comet test. stream water application promoted anatomical damages such as epidermis and cortex cell damage, accumulation of some substances in cortex cells, flattened cell nucleus and non-apparent appearance of conduction tissue in root tip meristem cells. all these abnormalities observed in a. cepa root tip cells were associated with the presence of heavy metals in the water samples. simultaneous application of lycopene with stream water reduced the effects of heavy metals and resulted in a dose-dependent improvement in all parameters studied. lycopene application showed a protective role by providing an increase in germination parameters and mi, decrease in mn and cas frequencies, and improvements in mda, sod and cat activities. as a result, heavy metals detected in the water samples of pazarsuyu stream caused multiple toxicities in the bio-indicator plant, and lycopene reduced this toxicity and recorded a protective role. | comprehensive toxicity screening of pazarsuyu stream water containing heavy metals and protective role of lycopene |
we reconstruct the temporal evolution of the source distribution for the four major gas species h2o, co2, co, and o2 on the surface of comet 67p/churyumov-gerasimenko during its 2015 apparition. the analysis applies an inverse coma model and fits to data between 2014 august 6 and 2016 september 5 measured with the double focusing mass spectrometer (dfms) of the rosetta orbiter spectrometer for ion and neutral analysis (rosina) and the comet pressure sensor (cops). the spatial distribution of gas sources with their temporal variation allows one to construct surface maps for gas emissions and to evaluate integrated production rates. for all species peak production rates and integrated production rates per orbit are evaluated separately for the northern and southern hemisphere. the nine most active emitting areas on the comet's surface are defined and their correlation to emissions for each of the species is discussed. | surface localization of gas sources on comet 67p/churyumov-gerasimenko based on dfms/cops data |
the d/h ratio in cometary water has been shown to vary between 1 and 3 times the earth's oceans value, in both oort cloud comets and jupiter-family comets originating from the kuiper belt. this has been taken as evidence that comets contributed a relatively small fraction of the terrestrial water. we present new sensitive spectroscopic observations of water isotopologues in the jupiter-family comet 46p/wirtanen carried out using the great spectrometer aboard the stratospheric observatory for infrared astronomy (sofia). the derived d/h ratio of (1.61 ± 0.65)×10-4 is the same as in the earth's oceans. although the statistics are limited, we show that interesting trends are already becoming apparent in the existing data. a clear anti-correlation is seen between the d/h ratio and the active fraction, defined as the ratio of the active surface area to the total nucleus surface. comets with an active fraction above 0.5 typically have d/h ratios in water consistent with the terrestrial value. these hyperactive comets, such as 46p/wirtanen, require an additional source of water vapor in their coma, explained by the presence of subliming icy grains expelled from the nucleus. the observed correlation may suggest that hyperactive comets belong to a population of ice-rich objects that formed just outside the snow line, or in the outermost regions of the solar nebula, from water thermally reprocessed in the inner disk that was transported outward during the early disk evolution. the observed anti-correlation between the active fraction and the nucleus size seems to argue against the first interpretation, as planetesimals near the snow line are expected to undergo rapid growth. alternatively, isotopic properties of water outgassed from the nucleus and icy grains may be different due to fractionation effects at sublimation. in this case, all comets may share the same earth-like d/h ratio in water, with profound implications for the early solar system and the origin of earth's oceans. | terrestrial deuterium-to-hydrogen ratio in water in hyperactive comets |
context. the properties of the smallest subunits of cometary dust contain information on their origin and clues to the formation of planetesimals and planets. compared to interplanetary dust particles or particles collected during the stardust mission, dust collected in the coma of comet 67p/churyumov-gerasimenko (67p) during the rosetta mission provides a resource of minimally altered material with known origin whose structural properties can be used to further the investigation of the early solar system.aims: the cometary dust particle morphologies found at comet 67p on the micrometer scale are classified, and their structural analysis is extended to the nanometer scale.methods: we present a novel method for achieving the highest spatial resolution of imaging obtained with the midas atomic force microscope on board rosetta. 3d topographic images with resolutions down to 8 nm were analyzed to determine the subunit sizes of particles on the nanometer scale.results: three morphological classes can be determined: (i) fragile agglomerate particles of sizes larger than about 10 μm comprised of micrometer-sized subunits that may themselves be aggregates and show a moderate packing density on the surface of the particles. (ii) a fragile agglomerate with a size of about a few tens of micrometers comprised of micrometer-sized subunits that are suggested to be aggregates themselves and are arranged in a structure with a fractal dimension lower than two. (iii) small micrometer-sized particles comprised of subunits in the size range of hundreds of nanometers that show surface features that are again suggested to represent subunits. their differential size distributions follow a log-normal distribution with means of about 100 nm and standard deviations between 20 and 35 nm.conclusions: the properties of the dust particles found by midas represent an extension of the dust results of rosetta to the micro- and nanometer scale. all micrometer-sized particles are hierarchical dust agglomerates of smaller subunits. the arrangement, appearance, and size distribution of the smallest determined surface features are reminiscent of those found in chondritic porous interplanetary dust particles. they represent the smallest directly detected subunits of comet 67p. | dust of comet 67p/churyumov-gerasimenko collected by rosetta/midas: classification and extension to the nanometer scale |
in this chapter we review our knowledge of our galaxy's cometary population outside our oort cloud - exocomets and interstellar objects (isos). we start with a brief overview of planetary system formation, viewed as a general process around stars. we then take a more detailed look at the creation and structure of exocometary belts, as revealed by the unprecedented combination of theoretical and observational advances in recent years. the existence and characteristics of individual exocomets orbiting other stars is summarized, before looking at the mechanisms by which they may be ejected into interstellar space. the discovery of the first two isos is then described, along with the surprising differences in their observed characteristics. we end by looking ahead to what advances may take place in the next decade. | interstellar objects and exocomets |
we present the first study of the three-dimensional (3d) dynamics of the gas in the entire southern orion cloud complex. we used the parallaxes and proper motions of young stellar objects (ysos) from gaia dr2 as a proxy for gas distance and proper motion, and the gas radial velocities from archival co data, to compute the space motions of the different star-forming clouds in the complex, including subregions in orion a, orion b, and two outlying cometary clouds. from the analysis of the clouds' orbits in space and time, we find that they were closest about 6 myr ago and are moving radially away from roughly the same region in space. this coherent 100-pc scale radial motion supports a scenario where the entire complex is reacting to a major feedback event, which we name the orion-bb (big blast) event. this event, which we tentatively associate with the recently discovered orion x stellar population, shaped the distribution and kinematics of the gas we observe today, although it is unlikely to have been the sole major feedback event in the region. we argue that the dynamics of most of the ysos carry the memory of the feedback-driven star formation history in orion and that the majority of the young stars in this complex are a product of large-scale triggering, which can raise the star formation rate by at least an order of magnitude, as for the head of orion a (the integral shape filament). our results imply that a feedback, compression, and triggering process lies at the genesis of the orion nebula cluster and ngc 2023/2024 in orion b, thus confirming broadly the classical feedback-driven scenario proposed in elmegreen & lada (1977, apj, 214, 725). the space motions of the well-known young compact clusters, σ orionis and ngc 1977, are consistent with this scenario. a momentum estimate suggests that the energy of a few to several supernovae is needed to power the coherent 3d gas motion we measure in this paper. 3d version of fig. 10 and movies associated to figs. 7-9 and b.1 are available at https://www.aanda.org | 3d dynamics of the orion cloud complex. discovery of coherent radial gas motions at the 100-pc scale |
quantifying the composition of the material in protoplanetary disks is essential to determining the potential for exoplanetary systems to produce and support habitable environments. when considering potential habitability, complex organic molecules are relevant, key among which is methanol (ch3oh). methanol primarily forms at low temperatures via the hydrogenation of co ice on the surface of icy dust grains and is a necessary basis for the formation of more complex species such as amino acids and proteins. we report the detection of ch3oh in a disk around a young, luminous a-type star, hd 100546. this disk is warm and therefore does not host an abundant reservoir of co ice. we argue that the ch3oh cannot form in situ, and hence that this disk has probably inherited complex-organic-molecule-rich ice from an earlier cold dark cloud phase. this is strong evidence that at least some interstellar organic material survives the disk-formation process and can then be incorporated into forming planets, moons and comets. therefore, crucial pre-biotic chemical evolution already takes place in dark star-forming clouds. | an inherited complex organic molecule reservoir in a warm planet-hosting disk |
the kuiper belt of our solar system is a source of short-period comets that may have delivered water and other volatiles to earth and the other terrestrial planets. however, the distribution of water and other volatiles in extrasolar planetary systems is largely unknown. we report the discovery of an accretion of a kuiper-belt-object analog onto the atmosphere of the white dwarf wd 1425+540. the heavy elements c, n, o, mg, si, s, ca, fe, and ni are detected, with nitrogen observed for the first time in extrasolar planetary debris. the nitrogen mass fraction is ∼2%, comparable to that in comet halley and higher than in any other known solar system object. the lower limit to the accreted mass is ∼1022 g, which is about one hundred thousand times the typical mass of a short-period comet. in addition, wd 1425+540 has a wide binary companion, which could facilitate perturbing a kuiper-belt-object analog into the white dwarf’s tidal radius. this finding shows that analogs to objects in our kuiper belt exist around other stars and could be responsible for the delivery of volatiles to terrestrial planets beyond the solar system. part of the data presented herein were obtained at the w.m. keck observatory, which is operated as a scientific partnership among caltech, the university of california and nasa. the observatory was made possible by the generous financial support of the w.m. keck foundation. | the chemical composition of an extrasolar kuiper-belt-object |
we report on magnetic field measurements made in the innermost coma of 67p/churyumov-gerasimenko in its low-activity state. quasi-coherent, large-amplitude (δ b/b ~ 1), compressional magnetic field oscillations at ~ 40 mhz dominate the immediate plasma environment of the nucleus. this differs from previously studied cometary interaction regions where waves at the cometary ion gyro-frequencies are the main feature. thus classical pickup-ion-driven instabilities are unable to explain the observations. we propose a cross-field current instability associated with newborn cometary ion currents as a possible source mechanism. | observation of a new type of low-frequency waves at comet 67p/churyumov-gerasimenko |
large language models (llms) present strong general capabilities, and a current compelling challenge is stimulating their specialized capabilities, such as machine translation, through low-cost instruction tuning. the standard instruction-following data is sequentially organized as the concatenation of an instruction, an input, and a response. as the attention mechanism of llms has limitations on local focus, llms tend to focus more on the words or sentences nearby at each position. this leads to a high risk of instruction forgetting during decoding. to alleviate the above issues, we propose swie (segment-weighted instruction embedding) and an instruction-following dataset overmiss. swie improves the model instruction understanding by adding a global instruction representation on the following input and response representations. overmiss improves model faithfulness by comparing over-translation and miss-translation results with the correct translation. we apply our methods to two main-stream open-source llms, bloom and llama. the experimental results demonstrate significant improvements in translation performance with swie based on bloomz-3b, particularly in zero-shot and long text translations due to reduced instruction forgetting risk. additionally, overmiss outperforms the baseline in translation performance (e.g. an increase in bleu scores from 0.69 to 3.12 and an average improvement of 0.48 percentage comet scores for llama-7b) with further enhancements seen in models combining overmiss and swie (e.g. the blue scores increase up to 0.56 from english to german across three different backbones), and both exhibit improvements in the faithfulness metric based on word alignment. | improving translation faithfulness of large language models via augmenting instructions |
it has recently been shown by egal et al. that some types of existing meteor in-atmosphere trajectory estimation methods may be less accurate than others, particularly when applied to high-precision optical measurements. the comparative performance of trajectory solution methods has previously only been examined for a small number of cases. besides the radiant, orbital accuracy depends on the estimation of pre-atmosphere velocities, which have both random and systematic biases. thus, it is critical to understand the uncertainty in velocity measurement inherent to each trajectory estimation method. in this first of a series of two papers, we introduce a novel meteor trajectory estimation method that uses the observed dynamics of meteors across stations as a global optimization function and that does not require either a theoretical or an empirical flight model to solve for velocity. we also develop a 3d observational meteor trajectory simulator that uses a meteor ablation model to replicate the dynamics of meteoroid flight, as a means to validate different trajectory solvers. we both test this new method and compare it to other methods, using synthetic meteors from three major showers spanning a wide range of velocities and geometries (draconids, geminids, and perseids). we determine which meteor trajectory solving algorithm performs better for all-sky, moderate field-of-view, and high-precision narrow-field optical meteor detection systems. the results are presented in the second paper in this series. finally, we give detailed equations for estimating meteor trajectories and analytically computing meteoroid orbits, and provide the python code of the methodology as open-source software. | estimating trajectories of meteors: an observational monte carlo approach - i. theory |
recent atacama large millimeter/submillimeter array observations present mounting evidence for the presence of exocometary gas released within kuiper belt analogs around nearby main-sequence stars. this represents a unique opportunity to study their ice reservoir at the younger ages when volatile delivery to planets is most likely to occur. we here present the detection of co j = 2-1 emission colocated with dust emission from the cometary belt in the 440 myr old fomalhaut system. through spectrospatial filtering, we achieve a 5.4σ detection and determine that the ring's sky-projected rotation axis matches that of the star. the co mass derived ((0.65{--}42)× {10}-7 {m}\oplus ) is the lowest of any circumstellar disk detected to date and must be of exocometary origin. using a steady-state model, we estimate the co+co2 mass fraction of exocomets around fomalhaut to be between 4.6% and 76%, consistent with solar system comets and the two other belts known to host exocometary gas. this is the first indication of a similarity in cometary compositions across planetary systems that may be linked to their formation scenario and is consistent with direct interstellar medium inheritance. in addition, we find tentative evidence that (49+/- 27)% of the detected flux originates from a region near the eccentric belt's pericenter. if confirmed, the latter may be explained through a recent impact event or co pericenter glow due to exocometary release within a steady-state collisional cascade. in the latter scenario, we show how the azimuthal dependence of the co release rate leads to asymmetries in gas observations of eccentric exocometary belts. | detection of exocometary co within the 440 myr old fomalhaut belt: a similar co+co2 ice abundance in exocomets and solar system comets |
the chemical composition of planets is inherited from that of the natal protoplanetary disk at the time of planet formation. increasing observational evidence suggests that planet formation occurs in less than 1-2 myr. this motivates the need for spatially resolved spectral observations of young class i disks, as carried out by the alma chemical survey of disk-outflow sources in taurus (alma-dot). in the context of alma-dot, we observe the edge-on disk around the class i source iras 04302+2247 (the butterfly star) in the 1.3 mm continuum and five molecular lines. we report the first tentative detection of methanol (ch3oh) in a class i disk and resolve, for the first time, the vertical structure of a disk with multiple molecular tracers. the bulk of the emission in the co 2-1, cs 5-4, and o-h2co 31, 2 - 21, 1 lines originates from the warm molecular layer, with the line intensity peaking at increasing disk heights, z, for increasing radial distances, r. molecular emission is vertically stratified, with co observed at larger disk heights (aperture z/r ∼ 0.41-0.45) compared to both cs and h2co, which are nearly cospatial (z/r ∼ 0.21-0.28). in the outer midplane, the line emission decreases due to molecular freeze-out onto dust grains (freeze-out layer) by a factor of > 100 (co) and 15 (cs). the h2co emission decreases by a factor of only about 2, which is possibly due to h2co formation on icy grains, followed by a nonthermal release into the gas phase. the inferred [ch3oh]/[h2co] abundance ratio is 0.5-0.6, which is 1-2 orders of magnitude lower than for class 0 hot corinos, and a factor ∼2.5 lower than the only other value inferred for a protoplanetary disk (in tw hya, 1.3-1.7). additionally, it is at the lower edge but still consistent with the values in comets. this may indicate that some chemical reprocessing occurs in disks before the formation of planets and comets. the reduced images and datacubes are 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/cat/j/a+a/642/l7 | alma chemical survey of disk-outflow sources in taurus (alma-dot). ii. vertical stratification of co, cs, cn, h2co, and ch3oh in a class i disk |
we present the first census of the interstellar complex organic molecules (icoms) in the low-mass class i protostar svs13-a, obtained by analysing data from the iram-30 m large project asai (astrochemical surveys at iram). they consist of a high-sensitivity unbiased spectral survey at the 1mm, 2mm, and 3mm iram bands. we detected five icoms: acetaldehyde (ch3cho), methyl formate (hcooch3), dimethyl ether (ch3och3), ethanol (ch3ch2oh), and formamide (nh2cho). in addition, we searched for other icoms and ketene (h2cco), formic acid (hcooh) and methoxy (ch3o), whose only ketene was detected. the numerous detected lines, from 5 to 37 depending on the species, cover a large upper level energy range, between 15 and 254 k. this allowed us to carry out a rotational diagram analysis and derive rotational temperatures between 35 and 110 k, and column densities between 3 × 1015 and 1 × 1017 cm-2 on the 0.3 arcsec size previously determined by interferometric observations of glycolaldehyde. these new observations clearly demonstrate the presence of a rich chemistry in the hot corino towards svs13-a. the measured icoms abundances were compared to other class 0 and i hot corinos, as well as comets, previously published in the literature. we find evidence that (i) svs13-a is as chemically rich as younger class 0 protostars, and (ii) the icoms relative abundances do not substantially evolve during the protostellar phase. | the census of interstellar complex organic molecules in the class i hot corino of svs13-a |
we discuss observations of the journey throughout the solar system of a large interplanetary coronal mass ejection (icme) that was ejected at the sun on 14 october 2014. the icme hit mars on 17 october, as observed by the mars express, mars atmosphere and volatile evolution mission (maven), mars odyssey, and mars science laboratory (msl) missions, 44 h before the encounter of the planet with the siding-spring comet, for which the space weather context is provided. it reached comet 67p/churyumov-gerasimenko, which was perfectly aligned with the sun and mars at 3.1 au, as observed by rosetta on 22 october. the icme was also detected by stereo-a on 16 october at 1 au, and by cassini in the solar wind around saturn on the 12 november at 9.9 au. fortuitously, the new horizons spacecraft was also aligned with the direction of the icme at 31.6 au. we investigate whether this icme has a nonambiguous signature at new horizons. a potential detection of this icme by voyager 2 at 110-111 au is also discussed. the multispacecraft observations allow the derivation of certain properties of the icme, such as its large angular extension of at least 116°, its speed as a function of distance, and its magnetic field structure at four locations from 1 to 10 au. observations of the speed data allow two different solar wind propagation models to be validated. finally, we compare the forbush decreases (transient decreases followed by gradual recoveries in the galactic cosmic ray intensity) due to the passage of this icme at mars, comet 67p, and saturn. | interplanetary coronal mass ejection observed at stereo-a, mars, comet 67p/churyumov-gerasimenko, saturn, and new horizons en route to pluto: comparison of its forbush decreases at 1.4, 3.1, and 9.9 au |
context. close-in exoplanets interact with their host stars gravitationally as well as via their magnetized plasma outflows. the rich dynamics that arises may result in distinct observable features.aims: our objective is to study and classify the morphology of the different types of interaction that can take place between a giant close-in planet (a hot jupiter) and its host star, based on the physical parameters that characterize the system.methods: we perform 3d magnetohydrodynamic numerical simulations to model the star-planet interaction, incorporating a star, a hot jupiter, and realistic stellar and planetary outflows. we explore a wide range of parameters and analyze the flow structures and magnetic topologies that develop.results: our study suggests the classification of star-planet interactions into four general types, based on the relative magnitudes of three characteristic length scales that quantify the effects of the planetary magnetic field, the planetary outflow, and the stellar gravitational field in the interaction region. we describe the dynamics of these interactions and the flow structures that they give rise to, which include bow shocks, cometary-type tails, and inspiraling accretion streams. we point out the distinguishing features of each of the classified cases and discuss some of their observationally relevant properties.conclusions: the magnetized interactions of star-planet systems can be categorized, and their general morphologies predicted, based on a set of basic stellar, planetary, and orbital parameters. | classification of magnetized star-planet interactions: bow shocks, tails, and inspiraling flows |
if a component of the dark matter has dissipative interactions, it could collapse to form a thin dark disk in our galaxy that is coplanar with the baryonic disk. it has been suggested that dark disks could explain a variety of observed phenomena, including periodic comet impacts. using the first data release from the gaia space observatory, we search for a dark disk via its effect on stellar kinematics in the milky way. our new limits disfavor the presence of a thin dark matter disk, and we present updated measurements on the total matter density in the solar neighborhood. | constraining a thin dark matter disk with g a i a |
we report the clear detection of c$_2$ and of abundant nh$_2$ in the first prominently active interstellar comet, 2i/borisov. we observed 2i on three nights in november 2019 at optical wavelengths 4800--9300 åwith the multi-unit spectroscopic explorer (muse) integral-field spectrograph on the eso/very large telescope. these data, together with observations close in time from both 0.6-m trappist telescopes, provide constraints on the production rates of species of gas in 2i's coma. from the muse detection on all epochs of several bands of the optical emission of the c$_2$ swan system, a rich emission spectrum of nh$_2$ with many highly visible bands, and the red (1-0) bandhead of cn, together with violet cn detections by trappist, we infer production rates of $q$(c$_2$) = $1.1\times10^{24}$ mol s$^{-1}$, $q$(nh$_2$) = $4.8\times10^{24}$ mol s$^{-1}$ and $q$(cn) = $(1.8\pm0.2)\times 10^{24}$ mol s$^{-1}$. in late november at 2.03~au, 2i had a production ratio of c$_2$/cn$=0.61$, only barely carbon-chain depleted, in contrast to earlier reports measured further from the sun of strong carbon-chain depletion. thus, 2i has shown evolution in its c$_2$ production rate: a parent molecule reservoir has started sublimating. at $q$(nh$_2$)/$q$(cn) = 2.7, this second interstellar object is enriched in nh$_2$, relative to the known solar system sample. | interstellar comet 2i/borisov as seen by muse: c$_2$, nh$_2$ and red cn detections |
comet c/2016 r2 (panstarrs) has a peculiar volatile composition, with co being the dominant volatile, as opposed to h2o, and one of the largest n2/co ratios ever observed in a comet. using observations obtained with the spitzer space telescope, nasa’s infrared telescope facility, the 3.5 m astrophysical research consortium telescope at apache point observatory, the discovery channel telescope at lowell observatory, and the arizona radio observatory 10 m submillimeter telescope, we quantified the abundances of 12 different species in the coma of r2 panstarrs: co, co2, h2o, ch4, c2h6, hcn, ch3oh, h2co, ocs, c2h2, nh3, and n2. we confirm the high abundances of co and n2 and heavy depletions of h2o, hcn, ch3oh, and h2co compared to co reported by previous studies. we provide the first measurements (or most sensitive measurements/constraints) on h2o, co2, ch4, c2h6, ocs, c2h2, and nh3, all of which are depleted relative to co by at least 1-2 orders of magnitude compared to values commonly observed in comets. the observed species also show strong enhancements relative to h2o, and, even when compared to other species like ch4 or ch3oh, most species show deviations from typical comets by at least a factor of 2-3. the only mixing ratios found to be close to typical are ch3oh/co2 and ch3oh/ch4. the co2/co ratio is within a factor of 2 of those observed for c/1995 o1 (hale-bopp) and c/2006 w3 (christensen) at a similar heliocentric distance, though it is at least an order of magnitude lower than many other comets observed with akari. while r2 panstarrs was located at a heliocentric distance of 2.8 au at the time of our observations in 2018 january/february, we argue, using sublimation models and comparison to other comets observed at similar heliocentric distance, that this alone cannot account for the peculiar observed composition of this comet and therefore must reflect its intrinsic composition. we discuss possible implications for this clear outlier in compositional studies of comets obtained to date and encourage future dynamical and chemical modeling in order to better understand what the composition of r2 panstarrs tells us about the early solar system. | the peculiar volatile composition of co-dominated comet c/2016 r2 (panstarrs) |
isotopic ratios in comets provide keys for the understanding of the origin of cometary material, and the physical and chemical conditions in the early solar nebula. we review here measurements acquired on the d/h, 14n/15n, 16o/18o, 12c/13c, and 32s/34s ratios in cometary grains and gases, and discuss their cosmogonic implications. the review includes analyses of potential cometary material available in collections on earth, recent measurements achieved with the herschel space observatory, large optical telescopes, and rosetta, as well as recent results obtained from models of chemical-dynamical deuterium fractionation in the early solar nebula. prospects for future measurements are presented. | cometary isotopic measurements |
ls i +61° 303 is one of the rare gamma-ray binaries1 that emit most of their luminosity in photons with energies beyond 100 mev (ref. 2). it is well characterized—the ~26.5 day orbital period is clearly detected at many wavelengths2-4—and other aspects of its multifrequency behaviour make it the most interesting example of its class. the morphology of high-resolution radio images changes with orbital phase, displaying a cometary tail pointing away from the high-mass star component5 and ls i +61° 303 also shows superorbital variability3,6-9. a couple of energetic (~1037 erg s−1), short, magnetar-like bursts have been plausibly ascribed to it10-13. although the phenomenology of ls i +61° 303 has been the subject of theoretical scrutiny for decades, there has been a lack of certainty regarding the nature of the compact object in the binary that has hampered our understanding of the source. here, using observations with the five-hundred-meter aperture spherical radio telescope, we report the existence of transient radio pulsations from the direction of ls i +61° 303 with a period p = 269.15508 ± 0.00016 ms at a significance of >20σ. these pulsations strongly argue for the existence of a rotating neutron star within ls i +61° 303. | radio pulsations from a neutron star within the gamma-ray binary ls i +61° 303 |
experiments were conducted to investigate the health of fish channa punctatus inhabiting heavy metal-loaded waste water. heavy metals in the order of fe > mn > zn > co > ni > cu = cr were present in the waste water. gills had high metal load followed by liver and then kidney. albumin, albumin to globulin (a:g) ratio, triglyceride, high density lipoprotein (hdl) and very low density lipoprotein (vldl) were found to be lower but phospholipid, low density lipoprotein (ldl), total protein, lipid and cholesterol were higher as compared to the reference. oxidative stress markers such as superoxide dismutase (sod), catalase (cat), glutathione s transferase (gst) and lipid peroxidation (lpo) were significantly higher in all tissues, whereas reduced glutathione (gsh) levels were comparatively low. damage to dna was observed with significantly higher mean tail length of comets in the exposed fish gill cells (30.9 µm) followed by liver (24.3 µm) and kidney (20.6 µm) as compared to reference fish (5.2, 4.8 and 5.9 µm respectively). histopathology in gill, liver and kidney also showed marked damage. integrated biochemical, oxidative stress, genotoxicity and histopathological findings are valuable biomarkers for native fish adaptive patterns, and monitoring of water quality/pollution of freshwater ecosystems. | multiple biomarker responses (serum biochemistry, oxidative stress, genotoxicity and histopathology) in channa punctatus exposed to heavy metal loaded waste water |
the coma of comet 67p/churyumov-gerasimenko has been probed by the rosetta spacecraft and shows a variety of different molecules. the rosina comet pressure sensor and the double focusing mass spectrometer provide in situ densities for many volatile compounds including the 14 gas species h2o, co2, co, h2s, o2, c2h6, ch3oh, h2co, ch4, nh3, hcn, c2h5oh, ocs, and cs2. we fit the observed densities during the entire comet mission between 2014 august and 2016 september to an inverse coma model. we retrieve surface emissions on a cometary shape with 3996 triangular elements for 50 separated time intervals. for each gas, we derive systematic error bounds and report the temporal evolution of the production, peak production, and the time-integrated total production. we discuss the production for the two lobes of the nucleus and for the northern and southern hemispheres. moreover, we provide a comparison of the gas production with the seasonal illumination. | the gas production of 14 species from comet 67p/churyumov-gerasimenko based on dfms/cops data from 2014 to 2016 |
understanding the effects of high-energy radiation and stellar winds on planetary atmospheres is vital for explaining the observed properties of close-in exoplanets. observations of transiting exoplanets in the triplet of metastable helium lines at 10830 å allow extended atmospheres and escape processes to be studied for individual planets. we observed one transit of wasp-107b with nirspec on keck at 10830 å. our observations, for the first time, had significant posttransit phase coverage, and we detected excess absorption for over an hour after fourth contact. the data can be explained by a comet-like tail extending out to ~7 planet radii, which corresponds to roughly twice the roche lobe radius of the planet. planetary tails are expected based on three-dimensional simulations of escaping exoplanet atmospheres, particularly those including the interaction between the escaped material and strong stellar winds, and have been previously observed at 10830 å in at least one other exoplanet. with both the largest midtransit absorption signal and the most extended tail observed at 10830 å, wasp-107b remains a keystone exoplanet for atmospheric escape studies. | the posttransit tail of wasp-107b observed at 10830 å |
images of comet 67p/churyumov-gerasimenko acquired by the osiris (optical, spectroscopic and infrared remote imaging system) imaging system onboard the european space agency’s rosetta spacecraft at scales of better than 0.8 meter per pixel show a wide variety of different structures and textures. the data show the importance of airfall, surface dust transport, mass wasting, and insolation weathering for cometary surface evolution, and they offer some support for subsurface fluidization models and mass loss through the ejection of large chunks of material. | the morphological diversity of comet 67p/churyumov-gerasimenko |
the evolutionary past of our solar system can be pieced together by comparing analogous low-mass protostars with remnants of our protosolar nebula - comets. sulphur-bearing molecules may be unique tracers of the joint evolution of the volatile and refractory components. alma band 7 data from the large unbiased protostellar interferometric line survey are used to search for s-bearing molecules in the outer disc-like structure, ∼60 au from iras 16293-2422 b, and are compared with data on 67p/churyumov-gerasimenko (67p/c-g) stemming from the rosina (rosetta orbiter spectrometer for ion and neutral analysis) instrument aboard rosetta. species such as so2, so, ocs, cs, h2cs, h2s, and ch3sh are detected via at least one of their isotopologues towards iras 16293-2422 b. the search reveals a first-time detection of oc33s towards this source and a tentative first-time detection of c36s towards a low-mass protostar. the data show that iras 16293-2422 b contains much more ocs than h2s in comparison to 67p/c-g; meanwhile, the so/so2 ratio is in close agreement between the two targets. iras 16293-2422 b has a ch3sh/h2cs ratio in range of that of our solar system (differences by a factor of 0.7-5.3). it is suggested that the levels of uv radiation during the initial collapse of the systems may have varied and have potentially been higher for iras 16293-2422 b due to its binary nature; thereby, converting more h2s into ocs. it remains to be conclusively tested if this also promotes the formation of s-bearing complex organics. elevated uv levels of iras 16293-2422 b and a warmer birth cloud of our solar system may jointly explain the variations between the two low-mass systems. | the alma-pils survey: the sulphur connection between protostars and comets: iras 16293-2422 b and 67p/churyumov-gerasimenko |
the properties of dust in the protoplanetary disc are key to understanding the formation of planets in our solar system. many models of dust growth predict the development of fractal structures which evolve into non-fractal, porous dust pebbles representing the main component for planetesimal accretion. in order to understand comets and their origins, the rosetta orbiter followed comet 67p/churyumov-gerasimenko for over two years and carried a dedicated instrument suite for dust analysis. one of these instruments, the midas (micro-imaging dust analysis system) atomic force microscope, recorded the 3d topography of micro- to nanometre-sized dust. all particles analysed to date have been found to be hierarchical agglomerates. most show compact packing; however, one is extremely porous. this paper contains a structural description of a compact aggregate and the outstanding porous one. both particles are tens of micrometres in size and show rather narrow subunit size distributions with noticeably similar mean values of 1.48^{+0.13}_{-0.59} μm for the porous particle and 1.36^{+0.15}_{-0.59} μm for the compact. the porous particle allows a fractal analysis, where a density-density correlation function yields a fractal dimension of df = 1.70 ± 0.1. giada, another dust analysis instrument on board rosetta, confirms the existence of a dust population with a similar fractal dimension. the fractal particles are interpreted as pristine agglomerates built in the protoplanetary disc and preserved in the comet. the similar subunits of both fractal and compact dust indicate a common origin which is, given the properties of the fractal, dominated by slow agglomeration of equally sized aggregates known as cluster-cluster agglomeration. | fractal cometary dust - a window into the early solar system |
we present high-resolution imaging observations of interstellar comet 2i/borisov (formerly c/2019 q4) obtained using the hubble space telescope. scattering from the comet is dominated by a coma of large particles (characteristic size ∼0.1 mm) ejected anisotropically. convolution modeling of the coma surface brightness profile sets a robust limit to the spherical-equivalent nucleus radius rn ≤ 0.5 km (geometric albedo 0.04 assumed). we obtain an independent constraint based on the nongravitational acceleration of the nucleus, finding rn > 0.2 km (nucleus density ρ = 500 kg m-3 assumed). the profile and the nongravitational constraints cannot be simultaneously satisfied if ρ ≤ 25 kg m-3; the nucleus of comet borisov cannot be a low-density fractal assemblage of the type proposed elsewhere for the nucleus of 1i/'oumuamua. we show that the spin-up timescale to outgassing torques, even at the measured low production rates, is comparable to or shorter than the residence time in the sun's water sublimation zone. the spin angular momentum of the nucleus should be changed significantly during the current solar flyby. lastly, we find that the differential interstellar size distribution in the 0.5 mm to 100 m size range can be represented by power laws with indices <4 and that interstellar bodies of 100 m size scale strike earth every one to two hundred million years. | the nucleus of interstellar comet 2i/borisov |
to understand how phosphorus (p)-bearing molecules are formed in star-forming regions, we have analysed the atacama large millimeter/submillimeter array (alma) observations of pn and po towards the massive star-forming region afgl 5142, combined with a new analysis of the data of the comet 67p/churyumov-gerasimenko taken with the rosetta orbiter spectrometer for ion and neutral analysis (rosina) instrument onboard rosetta. the alma maps show that the emission of pn and po arises from several spots associated with low-velocity gas with narrow linewidths in the cavity walls of a bipolar outflow. po is more abundant than pn in most of the spots, with the po/pn ratio increasing as a function of the distance to the protostar. our data favour a formation scenario in which shocks sputter phosphorus from the surface of dust grains, and gas-phase photochemistry induced by uv photons from the protostar allows efficient formation of the two species in the cavity walls. our analysis of the rosina data has revealed that po is the main carrier of p in the comet, with po/pn > 10. since comets may have delivered a significant amount of prebiotic material to the early earth, this finding suggests that po could contribute significantly to the phosphorus reservoir during the dawn of our planet. there is evidence that po was already in the cometary ices prior to the birth of the sun, so the chemical budget of the comet might be inherited from the natal environment of the solar system, which is thought to be a stellar cluster including also massive stars. | alma and rosina detections of phosphorus-bearing molecules: the interstellar thread between star-forming regions and comets |
the rosetta spacecraft has investigated comet 67p/churyumov-gerasimenko from large heliocentric distances to its perihelion passage and beyond. we trace the seasonal and diurnal evolution of the colors of the 67p nucleus, finding changes driven by sublimation and recondensation of water ice. the whole nucleus became relatively bluer near perihelion, as increasing activity removed the surface dust, implying that water ice is widespread underneath the surface. we identified large (1500 square meters) ice-rich patches appearing and then vanishing in about 10 days, indicating small-scale heterogeneities on the nucleus. thin frosts sublimating in a few minutes are observed close to receding shadows, and rapid variations in color are seen on extended areas close to the terminator. these cyclic processes are widespread and lead to continuously, slightly varying surface properties. | rosetta’s comet 67p/churyumov-gerasimenko sheds its dusty mantle to reveal its icy nature |
context. dust jets (i.e., fuzzy collimated streams of cometary material arising from the nucleus) have been observed in situ on all comets since the giotto mission flew by comet 1p/halley in 1986, and yet their formation mechanism remains unknown. several solutions have been proposed involving either specific properties of the active areas or the local topography to create and focus the gas and dust flows. while the nucleus morphology seems to be responsible for the larger features, high resolution imagery has shown that broad streams are composed of many smaller jets (a few meters wide) that connect directly to the nucleus surface.aims: we monitored these jets at high resolution and over several months to understand what the physical processes are that drive their formation and how this affects the surface.methods: using many images of the same areas with different viewing angles, we performed a 3-dimensional reconstruction of collimated jets and linked them precisely to their sources on the nucleus.results: we show here observational evidence that the northern hemisphere jets of comet 67p/churyumov-gerasimenko arise from areas with sharp topographic changes and describe the physical processes involved. we propose a model in which active cliffs are the main source of jet-like features and therefore of the regions eroding the fastest on comets. we suggest that this is a common mechanism taking place on all comets. | are fractured cliffs the source of cometary dust jets? insights from osiris/rosetta at 67p/churyumov-gerasimenko |
we develop an evolutionary model of the long-period comet (lpc) population, starting from their birthplace in a massive trans-neptunian disk that was dispersed by migrating giant planets. most comets that remain bound to the solar system are stored in the oort cloud. galactic tides and passing stars make some of these bodies evolve into observable comets in the inner solar system. our approach models each step in a full-fledged numerical framework. subsequent analysis consists of applying plausible fading models and computing the original orbits to compare with observations. our results match the observed semimajor axis distribution of lpcs when whipple’s power-law fading scheme with an exponent κ ={0.6}-0.2+0.1 is adopted. the cumulative perihelion (q) distribution is well fit by a linear increase plus a weak quadratic term. beyond q = 15 au, however, the population increases steeply, and the isotropy of lpc orbital planes breaks. we find tentative evidence from the perihelion distribution of lpcs that the returning comets are depleted in supervolatiles and become active due to water ice sublimation for q ≤ 3 au. using an independent calibration of the population of the initial disk, our predicted lpc flux is smaller than observations suggest by a factor of ≃2. current data only characterize comets from the outer oort cloud (semimajor axes ≳104 au). a true boost in understanding the oort cloud’s structure should result from future surveys when they detect lpcs with perihelia beyond 15 au. our results provide observational predictions of what can be expected from these new data. | origin and evolution of long-period comets |
aims: we study the link between gravitational slopes and the surface morphology on the nucleus of comet 67p/churyumov-gerasimenko and provide constraints on the mechanical properties of the cometary material (tensile, shear, and compressive strengths).methods: we computed the gravitational slopes for five regions on the nucleus that are representative of the different morphologies observed on the surface (imhotep, ash, seth, hathor, and agilkia), using two shape models computed from osiris images by the stereo-photoclinometry (spc) and stereo-photogrammetry (spg) techniques. we estimated the tensile, shear, and compressive strengths using different surface morphologies (overhangs, collapsed structures, boulders, cliffs, and philae's footprint) and mechanical considerations.results: the different regions show a similar general pattern in terms of the relation between gravitational slopes and terrain morphology: i) low-slope terrains (0-20°) are covered by a fine material and contain a few large (>10 m) and isolated boulders; ii) intermediate-slope terrains (20-45°) are mainly fallen consolidated materials and debris fields, with numerous intermediate-size boulders from <1 m to 10 m for the majority of them; and iii) high-slope terrains (45-90°) are cliffs that expose a consolidated material and do not show boulders or fine materials. the best range for the tensile strength of overhangs is 3-15 pa (upper limit of 150 pa), 4-30 pa for the shear strength of fine surface materials and boulders, and 30-150 pa for the compressive strength of overhangs (upper limit of 1500 pa). the strength-to-gravity ratio is similar for 67p and weak rocks on earth. as a result of the low compressive strength, the interior of the nucleus may have been compressed sufficiently to initiate diagenesis, which could have contributed to the formation of layers. our value for the tensile strength is comparable to that of dust aggregates formed by gravitational instability and tends to favor a formation of comets by the accrection of pebbles at low velocities. | gravitational slopes, geomorphology, and material strengths of the nucleus of comet 67p/churyumov-gerasimenko from osiris observations |
we present an improved model for interplanetary dust grain fluxes in the outer solar system constrained by in situ dust density observations. a dynamical dust grain tracing code is used to establish relative dust grain densities and three-dimensional velocity distributions in the outer solar system for four main sources of dust grains: jupiter-family comets, halley-type comets, oort-cloud comets, and edgeworth-kuiper belt objects. model densities are constrained by in situ dust measurements by the new horizons student dust counter, the pioneer 10 meteoroid detector, and the galileo dust detection system (dds). the model predicts that jupiter-family comet grains dominate the interplanetary dust grain mass flux inside approximately 10 au, oort-cloud cometary grains may dominate between 10 and 25 au, and edgeworth-kuiper belt grains are dominant outside 25 au. the model also predicts that while the total interplanetary mass flux at jupiter roughly matches that inferred by the analysis of the galileo dds measurements, mass fluxes to saturn, uranus, and neptune are at least one order-of-magnitude lower than that predicted by extrapolations of dust grain flux models from 1 au. finally, we compare the model predictions of interplanetary dust oxygen influx to the giant planet atmospheres with various observational and photochemical constraints and generally find good agreement, with the exception of jupiter, which suggests the possibility of additional chemical pathways for exogenous oxygen in jupiter's atmosphere. | an improved model for interplanetary dust fluxes in the outer solar system |
at present, there exists no consensus in the astronomical community regarding either the bulk composition or the formation mechanism for the interstellar object 1i/2017 u1 ('oumuamua). with the goal of assessing the merits of the various scenarios that have been suggested to explain 'oumuamua's appearance and observed properties, we report a number of new analyses and provide an up-to-date review of the current hypotheses. we consider the interpretations that can reconcile 'oumuamua's observed non-keplerian trajectory with the nondetection of traditional cometary volatiles. we examine the ability of these proposed formation pathways to populate the galaxy with sufficient interstellar objects such that the detection of 'oumuamua by pan-starrs would be statistically favored. we consider two exotic ices, hydrogen and nitrogen, showing that the frigid temperature requirement for the former and the necessary formation efficiency of the latter pose serious difficulties for these interpretations. via order-of-magnitude arguments and hydrodynamical cratering simulations, we show that impacts on extrasolar kuiper belt analogues are not expected to generate n2 ice fragments as large as 'oumuamua. in addition, we discuss observational tests to confirm the presence of these ices in future interstellar objects. next, we examine the explanations that attribute 'oumuamua's properties to other compositions: ultraporous dust aggregates and thin membranes powered by solar radiation pressure, among others. while none of these hypotheses are perfectly satisfactory, we make predictions that will be testable by the vera rubin observatory to resolve the tension introduced by 'oumuamua. | constraints on the occurrence of 'oumuamua-like objects |
outbursts occur commonly on comets1 with different frequencies and scales2,3. despite multiple observations suggesting various triggering processes4,5, the driving mechanism of such outbursts is still poorly understood. landslides have been invoked6 to explain some outbursts on comet 103p/hartley 2, although the process required a pre-existing dust layer on the verge of failure. the rosetta mission observed several outbursts from its target comet 67p/churyumov-gerasimenko, which were attributed to dust generated by the crumbling of materials from collapsing cliffs7,8. however, none of the aforementioned works included definitive evidence that landslides occur on comets. amongst the many features observed by rosetta on the nucleus of the comet, one peculiar fracture, 70 m long and 1 m wide, was identified on images obtained in september 2014 at the edge of a cliff named aswan9. on 10 july 2015, the rosetta navigation camera captured a large plume of dust that could be traced back to an area encompassing the aswan escarpment7. five days later, the osiris camera observed a fresh, sharp and bright edge on the aswan cliff. here we report the first unambiguous link between an outburst and a cliff collapse on a comet. we establish a new dust-plume formation mechanism that does not necessarily require the breakup of pressurized crust or the presence of supervolatile material, as suggested by previous studies7. moreover, the collapse revealed the fresh icy interior of the comet, which is characterized by an albedo >0.4, and provided the opportunity to study how the crumbling wall settled down to form a new talus. | the pristine interior of comet 67p revealed by the combined aswan outburst and cliff collapse |
cometary studies suggest that the organic composition of the early solar nebula was rich in complex nitrile species such ch3cn. recent alma detections in protoplanetary disks suggest that these species may be common during planet and comet formation, but connecting gas-phase measurements to cometary abundances first requires constraints on formation chemistry and distributions of these species. we present here the detection of seven spatially resolved transitions of ch3cn in the protoplanetary disk around the t-tauri star tw hya. using a rotational diagram analysis, we find a disk-averaged column density of {n}t={1.45}-0.15+0.19× {10}12 cm-2 and a rotational temperature of {t}rot}={32.7}-3.4+3.9 k. a radially resolved rotational diagram shows the rotational temperature to be constant across the disk, suggesting that the ch3cn emission originates from a layer at z/r ∼ 0.3. through comparison of the observations with predictions from a disk chemistry model, we find that grain-surface reactions likely dominate ch3cn formation and that in situ disk chemistry is sufficient to explain the observed ch3cn column density profile without invoking inheritance from the protostellar phase. however, the same model fails to reproduce a solar system cometary abundance of ch3cn relative to h2o in the midplane, suggesting that either vigorous vertical mixing or some degree of inheritance from interstellar ices occurred in the solar nebula. | the distribution and excitation of ch3cn in a solar nebula analog |
context. the detection of complex organic molecules related with prebiotic chemistry in star-forming regions allows us to investigate how the basic building blocks of life are formed.aims: ethylene glycol (ch2oh)2 is the simplest sugar alcohol and the reduced alcohol of the simplest sugar glycoladehyde (ch2ohcho). we study the molecular abundance and spatial distribution of (ch2oh)2, ch2ohcho and other chemically related complex organic species (ch3ocho, ch3och3, and c2h5oh) towards the chemically rich massive star-forming region g31.41+0.31.methods: we analyzed multiple single-dish (green bank telescope and iram 30 m) and interferometric (submillimeter array) spectra towards g31.41+0.31, covering a range of frequencies from 45 to 258 ghz. we fitted the observed spectra with a local thermodynamic equilibrium (lte) synthetic spectra, and obtained excitation temperatures and column densities. we compared our findings in g31.41+0.31 with the results found in other environments, including low- and high-mass star-forming regions, quiescent clouds and comets.results: we report for the first time the presence of the agg' conformer of (ch2oh)2 towards g31.41+0.31, detecting more than 30 unblended lines. we also detected multiple transitions of other complex organic molecules such as ch2ohcho, ch3ocho, ch3och3, and c2h5oh. the high angular resolution images show that the (ch2oh)2 emission is very compact, peaking towards the maximum of the 1.3 mm continuum. these observations suggest that low abundance complex organic molecules, like (ch2oh)2 or ch2ohcho, are good probes of the gas located closer to the forming stars. our analysis confirms that (ch2oh)2 is more abundant than ch2ohcho in g31.41+0.31, as previously observed in other interstellar regions. comparing different star-forming regions we find evidence of an increase of the (ch2oh)2/ch2ohcho abundance ratio with the luminosity of the source. the ch3och3/ch3ocho and (ch2oh)2/c2h5oh ratios are nearly constant with luminosity. we also find that the abundance ratios of pairs of isomers (ch2ohcho/ch3ocho and c2h5oh/ch3och3) decrease with the luminosity of the sources.conclusions: the most likely explanation for the behavior of the (ch2oh)2/ch2ohcho ratio is that these molecules are formed by different chemical formation routes not directly linked, although different formation and destruction efficiencies in the gas phase cannot be ruled out. the most likely formation route of (ch2oh)2 is by combination of two ch2oh radicals on dust grains. we also favor that ch2ohcho is formed via the solid-phase dimerization of the formyl radical hco. the interpretation of the observations also suggests a chemical link between ch3ocho and ch3och3, and between (ch2oh)2 and c2h5oh. the behavior of the abundance ratio c2h5oh/ch3och3 with luminosity may be explained by the different warm-up timescales in hot cores and hot corinos. the reduced spectra (ascii files) are only available at the cds via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?j/a+a/598/a59 | formation of ethylene glycol and other complex organic molecules in star-forming regions |
comets contain the best-preserved material from the beginning of our planetary system. their nuclei and comae composition reveal clues about physical and chemical conditions during the early solar system when comets formed. rosina (rosetta orbiter spectrometer for ion and neutral analysis) onboard the rosetta spacecraft has measured the coma composition of comet 67p/churyumov-gerasimenko with well-sampled time resolution per rotation. measurements were made over many comet rotation periods and a wide range of latitudes. these measurements show large fluctuations in composition in a heterogeneous coma that has diurnal and possibly seasonal variations in the major outgassing species: water, carbon monoxide, and carbon dioxide. these results indicate a complex coma-nucleus relationship where seasonal variations may be driven by temperature differences just below the comet surface. | time variability and heterogeneity in the coma of 67p/churyumov-gerasimenko |
aims: the discovery of the first active interstellar object 2i/borisov provides an unprecedented opportunity to study planetary formation processes in another planetary system. in particular, spectroscopic observations of 2i allow us to constrain the composition of its nuclear ices.methods: we obtained optical spectra of 2i with the 4.2 m william herschel and 2.5 m isaac newton telescopes between 2019 september 30 and october 13, when the comet was between 2.5 au and 2.4 au from the sun. we also imaged the comet with broadband filters on 15 nights from september 11 to october 17, as well as with a cn narrow-band filter on october 18 and 20, with the trappist-north telescope.results: broadband imaging confirms that the dust coma colours (b - v = 0.82 ± 0.02, v - r = 0.46 ± 0.03, r - i = 0.44 ± 0.03, b - r = 1.28 ± 0.03) are the same as for solar system comets. we detect cn emission in all spectra and in the trappist narrow-band images with production rates between 1.6 × 1024 and 2.1 × 1024 molec/s. no other species are detected. we determine three-sigma upper limits for c2, c3, and oh production rates of 6 × 1023 molec/s, 2 × 1023 molec/s and 2 × 1027 molec/s, respectively, on october 02. there is no significant increase of the cn production rate or a(0)fρ during our observing period. finally, we place a three-sigma upper limit on the q(c2)/q(cn) ratio of 0.3 (on october 13). from this, we conclude that 2i is highly depleted in c2, and may have a composition similar to solar system carbon-chain depleted comets. | 2i/borisov: a c2-depleted interstellar comet |
the presence of numerous complex organic molecules (coms; defined as those containing six or more atoms) around protostars shows that star formation is accompanied by an increase of molecular complexity. these coms may be part of the material from which planetesimals and, ultimately, planets formed. comets represent some of the oldest and most primitive material in the solar system, including ices, and are thus our best window into the volatile composition of the solar protoplanetary disk. molecules identified to be present in cometary ices include water, simple hydrocarbons, oxygen, sulfur, and nitrogen-bearing species, as well as a few coms, such as ethylene glycol and glycine. we report the detection of 21 molecules in comet c/2014 q2 (lovejoy), including the first identification of ethyl alcohol (ethanol, c2h5oh) and the simplest monosaccharide sugar glycolaldehyde (ch2ohcho) in a comet. the abundances of ethanol and glycolaldehyde, respectively 5 and 0.8% relative to methanol (0.12 and 0.02% relative to water), are somewhat higher than the values measured in solar- type protostars. overall, the high abundance of coms in cometary ices supports the formation through grain-surface reactions in the solar system protoplanetary disk. | ethyl alcohol and sugar in comet c/2014 q2 (lovejoy) |
when comets approach the sun, their surface is heated and the volatile species start to sublimate. due to the increasing gas pressure, dust is ejected off the surface, which can be observed as cometary coma, dust tail, and trail. however, the underlying physical processes are not fully understood. using state-of-the-art results for the transport of heat and gas as well as of the mechanical properties of cometary matter, we intend to describe the activity pattern of comets when they approach the sun. we developed a novel thermophysical model to simulate the dust ejection from comet 67/churyumov-gerasimenko's south-pole region at perihelion. based on the input parameters, this model computes the sub-surface temperature profile, the pressure build-up, and the redistribution of volatiles inside the cometary sub-surface region and provides mass-loss rates of dust and gas as well as typical sizes and ice content of the ejected dust chunks. our thermophysical model allows for continuous gas and dust ejection from the southern hemisphere of comet 67/churyumov-gerasimenko at perihelion. we find that the model output is in general agreement with the observed rosetta data. the sublimation of co2 ice drives the ejection of very large (≳ 10 cm) chunks, which contain 10 {{ per cent}} to 90 {{ per cent}} of the initial water-ice content. in contrast, the outgassing of h2o ice causes the lift-off of small clusters of dust aggregates, which contain no ice. | on the activity of comets: understanding the gas and dust emission from comet 67/churyumov-gerasimenko's south-pole region during perihelion |
solar system formation models predict that the building blocks of planetesimals were mm- to cm-sized pebbles, aggregates of ices and non-volatile materials, consistent with the compact particles ejected by comet 67p/churyumov-gerasimenko (67p hereafter) and detected by giada (grain impact analyzer and dust accumulator) on-board the rosetta spacecraft. planetesimals were formed by the gentle gravitational accretion of pebbles, so that they have an internal macroporosity of 40 per cent. we measure the average dust bulk density ρ _d = 795_{-65}^{+840} kg m-3 that, coupled to the nucleus bulk density, provides the average dust-to-ices mass ratio δ = 8.5. we find that the measured densities of the 67p pebbles are consistent with a mixture of (15 ± 6) per cent of ices, (5 ± 2) per cent of fe-sulphides, (28 ± 5) per cent of silicates, and (52 ± 12) per cent of hydrocarbons, in average volume abundances. this composition matches both the solar and ci-chondritic chemical abundances, thus showing that giada has sampled the typical non-volatile composition of the pebbles that formed all planetesimals. the giada data do not constrain the abundance of amorphous silicates versus crystalline mg, fe-olivines and pyroxenes. we find that the pebbles have a microporosity of (52 ± 8) per cent (internal volume filling factor ϕp = 0.48 ± 0.08), implying an average porosity for the 67p nucleus of (71 ± 8) per cent, lower than previously estimated. | comet 67p/churyumov-gerasimenko preserved the pebbles that formed planetesimals |
during the formation of our solar system, a large number of planetesimals were ejected into interstellar space by gravitational encounters with the planets. debris disks observations and numerical simulations indicate that many other planetary systems, now known to be quite common, would have undergone a similar dynamical clearing process. it is therefore expected that the galaxy should be teeming with expelled planetesimals, largely unaltered since their ejection. this is why astronomers were perplexed that none had been detected passing through the solar system. then, in 2017, the discovery of1i/'oumuamua transformed the situation from puzzlement to bewilderment. its brief visit and limited observations left important questions about its nature and origin unanswered and raised the possibility that 1i/'oumuamua could be a never-seen-before intermediate product of planet formation. if so, this could open a new observational window to study the primordial building blocks of planets, setting unprecedented constraints on planet formation models. two years later 2i/borisov was discovered, with an unquestionable cometary composition, confirming that a population of icy interstellar planetesimals exists. these objects have remained largely unchanged since their ejection, like time capsules of their planetary system most distant past. interstellar planetesimals could potentially be trapped into star and planet formation environments, acting as seeds for planet formation, helping overcome the meter-size barrier that challenges the growth of cm-sized pebbles into km-sized objects. interstellar planetesimals play a pivotal role in our understanding of planetary system formation and evolution and point to the possibility that one day, we will be able to hold a fragment from another world in our hand. | interstellar planetesimals |
atmospheric escape from close-in exoplanets is thought to be crucial in shaping observed planetary populations. recently, significant progress has been made in observing this process in action through excess absorption in-transit spectra and narrowband light curves. we model the escape of initially homogeneous planetary winds interacting with a stellar wind. the ram pressure balance of the two winds governs this interaction. when the impingement of the stellar wind on the planetary outflow is mild or moderate, the planetary outflow expands nearly spherically through its sonic surface before forming a shocked boundary layer. when the confinement is strong, the planetary outflow is redirected into a cometary tail before it expands to its sonic radius. the resultant transmission spectra at the he 1083 nm line are accurately represented by a 1d spherical wind solution in cases of mild to moderate stellar wind interaction. in cases of strong stellar wind interaction, the degree of absorption is enhanced and the cometary tail leads to an extended egress from transit. the crucial features of the wind-wind interaction are, therefore, encapsulated in the light curve of he 1083 nm equivalent width as a function of time. the possibility of extended he 1083 nm absorption well beyond the optical transit carries important implications for planning out-of-transit observations that serve as a baseline for in-transit data. | stellar wind confinement of evaporating exoplanet atmospheres and its signatures in 1083 nm observations |
results are presented from a video-based meteoroid orbit survey conducted in new zealand between sept. 2014 and dec. 2016, which netted 24,906 orbits from +5 to -5 magnitude meteors. 44 new southern hemisphere meteor showers are identified after combining this data with that of other video-based networks. results are compared to showers reported from recent radar-based surveys. we find that video cameras and radar often see different showers and sometimes measure different semi-major axis distributions for the same meteoroid stream. for identifying showers in sparse daily orbit data, a shower look-up table of radiant position and speed as a function of time was created. this can replace the commonly used method of identifying showers from a set of mean orbital elements by using a discriminant criterion, which does not fully describe the distribution of meteor shower radiants over time. | a survey of southern hemisphere meteor showers |
we describe here updates and new elements of the catalogue of cometary orbits and their dynamical evolution (code) that, in its original 2020 version, has been introduced by królikowska & dybczyński (2020). currently, the code catalogue offers rich sets of orbital solutions for almost a complete sample of oort spike comets discovered between 1900 and 2021. we often offer several orbital solutions based on different nongravitational force models or different observational material treatments. an important novelty is that the `previous` (at previous perihelion or at 120,000 au from the sun in the past) and `next` (at next perihelion or 120,000 au after leaving the planetary zone) orbits are given in two variants. one with the dynamical model restricted only to the full galactic tide (with all individual stars omitted) and the second one, where all currently known stellar perturbers are also taken into account. calculations of the previous and next orbits were performed using the up-to-date stepped database of potential stellar perturbers. | news in the code catalogue |
james webb space telescope's nirspec infrared imaging spectrometer observed the outer solar system dwarf planets eris and makemake in reflected sunlight at wavelengths spanning 1 through 5 microns. both objects have high albedo surfaces that are rich in methane ice, with a texture that permits long optical path lengths through the ice for solar photons. there is evidence for n2 ice absorption around 4.2 um on eris, though not on makemake. no co ice absorption is seen at 4.67 um on either body. for the first time, absorption bands of two heavy isotopologues of methane are observed at 2.615 um (13ch4), 4.33 um (12ch3d), and 4.57 um (12ch3d). these bands enable us to measure d/h ratios of (2.5 +/- 0.5) x 10-4 and (2.9 +/- 0.6) x 10-4, along with 13c/12c ratios of 0.012 +/- 0.002 and 0.010 +/- 0.003 in the surface methane ices of eris and makemake, respectively. the measured d/h ratios are much lower than that of presumably primordial methane in comet 67p/churyumov-gerasimenko, but they are similar to d/h ratios in water in many comets and larger outer solar system objects. this similarity suggests that the hydrogen atoms in methane on eris and makemake originated from water, indicative of geochemical processes in past or even ongoing hot environments in their deep interiors. the 13c/12c ratios are consistent with commonly observed solar system values, suggesting no substantial enrichment in 13c as could happen if the methane currently on their surfaces was the residue of a much larger inventory that had mostly been lost to space. possible explanations include geologically recent outgassing from the interiors as well as processes that cycle the surface methane inventory to keep the uppermost surfaces refreshed. | measurement of d/h and 13c/12c ratios in methane ice on eris and makemake: evidence for internal activity |
we study non-hermitian higher-order weyl semimetals (nhhowsms) possessing real spectra and having inversion i (i -nhhowsm) or time-reversal symmetry t (t -nhhowsm). when the reality of bulk spectra is lost, the nhhowsms exhibit various configurations of surface fermi arcs and exceptional fermi rings (efrs), providing a setup to investigate them on an equal footing. the efrs only appear in the region between second-order weyl nodes. we also discover weyl nodes originating from non-hermiticity, called non-hermitian weyl nodes (nhwns). remarkably, we find t -nhhowsms which host only second-order nhwns, having both surface and hinge fermi arcs protected by the quantized biorthogonal chern number and quadrupole moment, respectively. we call this intrinsically non-hermitian phase exceptional howsm. in contrast to ordinary weyl nodes, the nhwns can instantly deform to line nodes, forming a monopole comet. the nhwns also show exceptional tilt rigidity, which is a strong resistance towards titling due to attachment to exceptional structures. this phenomenon can be a promising experimental knob. finally, we reveal the exceptional stability of fermi arcs called exceptional helicity. surface fermi arcs having opposite chirality can live on the same surface without gapping out each other due to the complex nature of the spectrum. our work motivates an immediate experimental realization of nhhowsms. | non-hermitian higher-order weyl semimetals |
ice naturally forms in the disordered or ``amorphous'' state when accreted from vapor at temperatures and pressures found in the interstellar medium and in the frigid, low density outer regions of the sun's protoplanetary disk. it is therefore the expected form of ice in comets and other primitive bodies that have escaped substantial heating since formation. despite expectations, however, the observational evidence for amorphous ice in comets remains largely indirect. this is both because the spectral features of amorphous ice are subtle and because the solar system objects for which we possess high quality data are mostly too close to the sun and too hot for amorphous ice to survive near the surface, where it can be detected. this chapter reviews the properties of amorphous ice, the evidence for its existence and its consequences for the behavior of comets. | amorphous ice in comets: evidence and consequences |
the primordial disc of small icy planetesimals, once located at 15-$30\, \mathrm{au}$ from the sun, was disrupted by giant planet migration in the early solar system. the primordial disc thereby became the source region of objects in the present-day kuiper belt, scattered disc, and oort cloud. i present the thermophysical code 'numerical icy minor body evolution simulator', or nimbus, and use it to study the thermophysical evolution of planetesimals in the primordial disc prior to its disruption. such modelling is mandatory in order to understand the behaviour of dynamically new comets from the oort cloud, as well as the activity of centaurs and short-period comets from the scattered disc, that return pre-processed to the vicinity of the sun. i find that bodies in the midst of the primordial disc with diameters ranging 4-$200\, \mathrm{km}$ lost all their co ice on time-scales of order 0.1-$10\, \mathrm{myr}$ depending on size, through a combination of protosolar and long-lived radionuclide heating. co and other hypervolatiles therefore require a less volatile host for their storage. i consider two possible hosts: amorphous water ice and co2 ice. because of the high luminosity of the protosun, some primordial disc bodies may have sustained significant crystallization, co:co2 segregation, and co2 sublimation in the uppermost few tens of metres. i discuss how this may affect coma abundance ratios and distant activity in dynamically new comets. | thermophysical evolution of planetesimals in the primordial disc |
the recently discovered population of interstellar objects presents us with the opportunity to characterize material from extrasolar planetary and stellar systems up close. the forthcoming vera c. rubin observatory legacy survey of space and time (lsst) will provide an unprecedented increase in sensitivity to these objects compared to the capabilities of currently operational observational facilities. we generate a synthetic population of 'oumuamua-like objects drawn from their galactic kinematics and identify the distribution of impact parameters, eccentricities, hyperbolic velocities, and sky locations of objects detectable with the lsst, assuming no cometary activity. this population is characterized by a clustering of trajectories in the direction of the solar apex and antiapex, centered at orbital inclinations of ~90°. we identify the ecliptic or solar apex as the optimal sky location to search for future interstellar objects as a function of survey limiting magnitude. moreover, we identify the trajectories of detectable objects that will be reachable for in situ rendezvous with a dedicated mission with the capabilities of the forthcoming comet interceptor or proposed bridge concept. by scaling our fractional population statistics with the inferred spatial number density, we estimate that the lsst will detect of order ~15 interstellar objects over the course of its ~10 yr observational campaign. furthermore, we find that there should be ~1-3 and ~0.0007-0.001 reachable targets for missions with propulsion capabilities comparable to bridge and comet interceptor, respectively. these numbers are lower limits and will be readily updateable when the number density and size-frequency distribution of interstellar objects are better constrained. | the population of interstellar objects detectable with the lsst and accessible for in situ rendezvous with various mission designs |
it has recently been suggested that a gravitational transition of the effective newton's constant geff by about 10%, 50–150 myrs ago could lead to the resolution of both the hubble crisis and the growth tension of the standard λcdm model. hints for such an abrupt transition with weaker gravity at times before the transition, have recently been identified in tully–fisher galactic mass-velocity data, and also in cepheid snia calibrator data. here we use monte-carlo simulations to show that such a transition could significantly increase (by a factor of 3 or more) the number of long period comets (lpcs) impacting the solar system from the oort cloud (semi-major axis of orbits ≳104au). this increase is consistent with observational evidence from the terrestrial and lunar cratering rates, indicating that the impact flux of kilometer sized objects increased by at least a factor of 2 over that last 100 myrs compared to the long term average. this increase may also be connected with the chicxulub impactor event that produced the cretaceous–tertiary (k-t) extinction of 75% of life on earth (including dinosaurs) about 66 myrs ago. we use monte-carlo simulations to show that for isotropic oort cloud comet distribution with initially circular orbits, random velocity perturbations (induced e.g., by passing stars and/or galactic tidal effects), lead to a deformation of the orbits that increases significantly when geff increases. a 10% increase in geff leads to an increase in the probability of the comets to enter the loss cone and reach the planetary region (pericenter of less than 10 au) by a factor that ranges from 5% (for velocity perturbation much smaller than the comet initial velocity) to more than 300% (for total velocity perturbations comparable with the initial comet velocity). | is the hubble crisis connected with the extinction of dinosaurs? |
induced magnetospheres form around conductive non-magnetized planetary objects (such as the ionospheres of mars, venus, titan, pluto and comets) in the electrodynamic interaction with a magnetized flowing plasma, such as the solar wind. the resulting induced currents couple the ionosphere and the deflected plasma, thus they provide insight into the solar wind's role in powering the heating, escape and evolution of planetary atmospheres. in contrast to the analogous current systems in intrinsic magnetospheres, which were mapped decades ago at earth, the current systems of induced magnetospheres are largely unexplored. here, we use five years of magnetic field measurements from the mars atmosphere and volatile evolution (maven) orbiter to empirically map the current systems of the martian induced magnetosphere. we find unexpected features, in particular: coupling of the ionosphere and the bow shock, asymmetries between the north-south electric hemispheres and a twist in the near-mars current system. the current flow pattern in the induced magnetosphere of mars indicates a system driven by a magnetospheric convective electric field, powered by the solar wind interaction. | the global current systems of the martian induced magnetosphere |
we analyse the rosetta orbiter spectrometer for ion and neutral analysis (rosina) - the double focusing mass spectrometer data between 2014 august and 2016 february to examine the effect of seasonal variations on the four major species within the coma of 67p/churyumov-gerasimenko (h2o, co2, co, and o2), resulting from the tilt in the orientation of the comet's spin axis. using a numerical data inversion, we derive the non-uniform activity distribution at the surface of the nucleus for these species, suggesting that the activity distribution at the surface of the nucleus has not significantly been changed and that the differences observed in the coma are solely due to the variations in illumination conditions. a three-dimensional direct simulation monte carlo model is applied where the boundary conditions are computed with a coupling of the surface activity distributions and the local illumination. the model is able to reproduce the evolution of the densities observed by rosina including the changes happening at equinox. while o2 stays correlated with h2o as it was before equinox, co2 and co, which had a poor correlation with respect to h2o pre-equinox, also became well correlated with h2o post-equinox. the integration of the densities from the model along the line of sight results in column densities directly comparable to the virtis-h observations. also, the evolution of the volatiles' production rates is derived from the coma model showing a steepening in the production rate curves after equinox. the model/data comparison suggests that the seasonal effects result in the northern hemisphere of 67p's nucleus being more processed with a layered structure while the southern hemisphere constantly exposes new material. | direct simulation monte carlo modelling of the major species in the coma of comet 67p/churyumov-gerasimenko |
context. the rosetta magnetometer rpc-mag has been exploring the plasma environment of comet 67p/churyumov-gerasimenko since august 2014. the first months were dominated by low-frequency waves which evolved into more complex features. however, at the end of july 2015, close to perihelion, the magnetometer detected a region that did not contain any magnetic field at all.aims: these signatures match the appearance of a diamagnetic cavity as was observed at comet 1p/halley in 1986. the cavity here is more extended than previously predicted by models and features unusual magnetic field configurations, which need to be explained.methods: the onboard magnetometer data were analyzed in detail and used to estimate the outgassing rate. a minimum variance analysis was used to determine boundary normals.results: our analysis of the data acquired by the rosetta plasma consortium instrumentation confirms the existence of a diamagnetic cavity. the size is larger than predicted by simulations, however. one possible explanation are instabilities that are propagating along the cavity boundary and possibly a low magnetic pressure in the solar wind. this conclusion is supported by a change in sign of the sun-pointing component of the magnetic field. evidence also indicates that the cavity boundary is moving with variable velocities ranging from 230-500 m/s. | first detection of a diamagnetic cavity at comet 67p/churyumov-gerasimenko |
we develop an activity model based on ice sublimation and gas diffusion inside cm-sized pebbles making-up a cometary nucleus. our model explains cometary activity assuming no free parameters and fixing the nucleus surface temperature ts, its gradient below the nucleus surface at thermal equilibrium, the pressure inside the porous pebbles, and the gas flux from them. we find that (i) the nucleus erosion rate and water vapour flux are independent of the nucleus refractory-to-ice ratio, which affects the dehydration rate only; (ii) water-driven dust ejection occurs in thermal quasi-equilibrium at ts > 205 k; (iii) the smallest and largest ejected dust sizes depend on the nucleus surface temperature and its gradient at depths of few cm; and (iv) the water-driven nucleus erosion rate is independent of the water vapour flux. regarding comet 67p/churyumov-gerasimenko, we find that (i) during the northern and southern polar summers, the nucleus active areas are ≈5 km2; (ii) >95 per cent of the southern pristine nucleus has a refractory-to-water-ice mass ratio >5; and (iii) the different temperature dependences of the dehydration and erosion rates explain the seasonal cycle: at perihelion, dm-sized chunks ejected by the sublimation of co2 ices are rapidly enveloped by an insulating crust, preserving most water ice up to their fallout on the northern dust deposits; the inbound water-driven activity at low temperatures triggers a complete erosion of the fallout if its water-ice mass fraction is >0.1 per cent. | how comets work: nucleus erosion versus dehydration |
jupiter-family comets (jfcs) are the evolutionary products of trans-neptunian objects (tnos) that evolve through the giant planet region as centaurs and into the inner solar system. through numerical orbital evolution calculations following a large number of tno test particles that enter the centaur population, we have identified a short-lived dynamical gateway, a temporary low-eccentricity region exterior to jupiter through which the majority of jfcs pass. we apply an observationally based size distribution function to the known centaur population and obtain an estimated gateway region population. we then apply an empirical fading law to the rate of incoming jfcs implied by the the gateway region residence times. our derived estimates are consistent with observed population numbers for the jfc and gateway populations. currently, the most notable occupant of the gateway region is 29p/schwassmann-wachmann 1 (sw1), a highly active, regularly outbursting centaur. sw1's present-day, very-low-eccentricity orbit was established after a 1975 jupiter conjunction and will persist until a 2038 jupiter conjunction doubles its eccentricity and pushes its semimajor axis out to its current aphelion. subsequent evolution will likely drive sw1's orbit out of the gateway region, perhaps becoming one of the largest jfcs in recorded history. the jfc gateway region coincides with a heliocentric distance range where the activity of observed cometary bodies increases significantly. sw1's activity may be typical of the early evolutionary processing experienced by most jfcs. thus, the gateway region, and its most notable occupant sw1, are critical to both the dynamical and physical transition between centaurs and jfcs. | 29p/schwassmann-wachmann 1, a centaur in the gateway to the jupiter-family comets |
as the sun moves through the surrounding partially ionized medium, neutral hydrogen atoms penetrate the heliosphere, and through charge exchange with the supersonic solar wind, create a population of hot pick-up ions (puis). until recently, the consensus was that the shape of the heliosphere is comet-like. the termination shock crossing by voyager 2 demonstrated that the heliosheath (the region of shocked solar wind) pressure is dominated by puis; however, the impact of the puis on the global structure of the heliosphere has not been explored. here we use a novel magnetohydrodynamic model that treats the puis as a separate fluid from the thermal component of the solar wind. the depletion of puis, due to charge exchange with the neutral hydrogen atoms of the interstellar medium in the heliosheath, cools the heliosphere, `deflating' it and leading to a narrower heliosheath and a smaller and rounder shape, confirming the shape suggested by cassini observations. the new model reproduces both the properties of the puis, based on the new horizons observations, and the solar wind ions, based on the voyager 2 spacecraft observations as well as the solar-like magnetic field data outside the heliosphere at voyager 1 and voyager 2. | a small and round heliosphere suggested by magnetohydrodynamic modelling of pick-up ions |
comets evolve due to sublimation of ices embedded inside porous dust, triggering dust emission (that is, erosion) followed by mass loss, mass redistribution and surface modifications. surface changes were revealed by the deep impact and stardust next missions for comet 9p/tempel 1 (ref. 1), and a full inventory of the processes modifying cometary nuclei was provided by rosetta while it escorted comet 67p/churyumov-gerasimenko for approximately two years2-4. such observations also showed puzzling water-ice-rich spots that stood out as patches optically brighter and spectrally bluer than the average cometary surface5-9. these are up to tens of metres large and indicate macroscopic compositional dishomogeneities apparently in contrast with the structural homogeneity above centimetre scales of pebble-made nuclei10. here we show that the occurrence of blue patches determines the seasonal variability of the nucleus colour4,11,12 and gives insight into the internal structure of comets. we define a new model that links the centimetre-sized pebbles composing the nucleus10 and driving cometary activity13,14 to metre-sized water-ice-enriched blocks embedded in a drier matrix. the emergence of blue patches is due to the matrix erosion driven by co2-ice sublimation that exposes the water-ice-enriched blocks, which in turn are eroded by water-ice sublimation when exposed to sunlight. our model explains the observed seasonal evolution of the nucleus and reconciles the available data at micro (sub-centimetre) and macro (metre) scales. | macro and micro structures of pebble-made cometary nuclei reconciled by seasonal evolution |
aims: we derive for the first time the size-frequency distribution of boulders on a comet, 67p/churyumov-gerasimenko (67p), computed from the images taken by the rosetta/osiris imaging system. we highlight the possible physical processes that lead to these boulder size distributions.methods: we used images acquired by the osiris narrow angle camera, nac, on 5 and 6 august 2014. the scale of these images (2.44-2.03 m/px) is such that boulders ≥7 m can be identified and manually extracted from the datasets with the software arcgis. we derived both global and localized size-frequency distributions. the three-pixel sampling detection, coupled with the favorable shadowing of the surface (observation phase angle ranging from 48° to 53°), enables unequivocally detecting boulders scattered all over the illuminated side of 67p.results: we identify 3546 boulders larger than 7 m on the imaged surface (36.4 km2), with a global number density of nearly 100/km2 and a cumulative size-frequency distribution represented by a power-law with index of -3.6 +0.2/-0.3. the two lobes of 67p appear to have slightly different distributions, with an index of -3.5 +0.2/-0.3 for the main lobe (body) and -4.0 +0.3/-0.2 for the small lobe (head). the steeper distribution of the small lobe might be due to a more pervasive fracturing. the difference of the distribution for the connecting region (neck) is much more significant, with an index value of -2.2 +0.2/-0.2. we propose that the boulder field located in the neck area is the result of blocks falling from the contiguous hathor cliff. the lower slope of the size-frequency distribution we see today in the neck area might be due to the concurrent processes acting on the smallest boulders, such as i) disintegration or fragmentation and vanishing through sublimation; ii) uplifting by gas drag and consequent redistribution; and iii) burial beneath a debris blanket. we also derived the cumulative size-frequency distribution per km2 of localized areas on 67p. by comparing the cumulative size-frequency distributions of similar geomorphological settings, we derived similar power-law index values. this suggests that despite the selected locations on different and often opposite sides of the comet, similar sublimation or activity processes, pit formation or collapses, as well as thermal stresses or fracturing events occurred on multiple areas of the comet, shaping its surface into the appearance we see today. | size-frequency distribution of boulders ≥7 m on comet 67p/churyumov-gerasimenko |
organohalogens, a class of molecules that contain at least one halogen atom bonded to carbon, are abundant on the earth where they are mainly produced through industrial and biological processes1. consequently, they have been proposed as biomarkers in the search for life on exoplanets2. simple halogen hydrides have been detected in interstellar sources and in comets, but the presence and possible incorporation of more complex halogen-containing molecules such as organohalogens into planet-forming regions is uncertain3,4. here we report the interstellar detection of two isotopologues of the organohalogen ch3cl and put some constraints on ch3f in the gas surrounding the low-mass protostar iras 16293-2422, using the atacama large millimeter/submillimeter array (alma). we also find ch3cl in the coma of comet 67p/churyumov-gerasimenko (67p/c-g) by using the rosetta orbiter spectrometer for ion and neutral analysis (rosina) instrument. the detections reveal an efficient pre-planetary formation pathway of organohalogens. cometary impacts may deliver these species to young planets and should thus be included as a potential abiotical production source when interpreting future organohalogen detections in atmospheres of rocky planets. | protostellar and cometary detections of organohalogens |
primitive objects like comets hold important information on the material that formed our solar system. several comets have been visited by spacecraft and many more have been observed through earth- and space-based telescopes. still our understanding remains limited. molecular abundances in comets have been shown to be similar to interstellar ices and thus indicate that common processes and conditions were involved in their formation. the samples returned by the stardust mission to comet wild 2 showed that the bulk refractory material was processed by high temperatures in the vicinity of the early sun. the recent rosetta mission acquired a wealth of new data on the composition of comet 67p/churyumov-gerasimenko (hereafter 67p/c-g) and complemented earlier observations of other comets. the isotopic, elemental, and molecular abundances of the volatile, semi-volatile, and refractory phases brought many new insights into the origin and processing of the incorporated material. the emerging picture after rosetta is that at least part of the volatile material was formed before the solar system and that cometary nuclei agglomerated over a wide range of heliocentric distances, different from where they are found today. deviations from bulk solar system abundances indicate that the material was not fully homogenized at the location of comet formation, despite the radial mixing implied by the stardust results. post-formation evolution of the material might play an important role, which further complicates the picture. this paper discusses these major findings of the rosetta mission with respect to the origin of the material and puts them in the context of what we know from other comets and solar system objects. | on the origin and evolution of the material in 67p/churyumov-gerasimenko |
on 12 november 2014, the philae lander descended towards comet 67p/churyumov-gerasimenko, bounced twice off the surface, then arrived under an overhanging cliff in the abydos region. the landing process provided insights into the properties of a cometary nucleus1-3. here we report an investigation of the previously undiscovered site of the second touchdown, where philae spent almost two minutes of its cross-comet journey, producing four distinct surface contacts on two adjoining cometary boulders. it exposed primitive water ice—that is, water ice from the time of the comet's formation 4.5 billion years ago—in their interiors while travelling through a crevice between the boulders. our multi-instrument observations made 19 months later found that this water ice, mixed with ubiquitous dark organic-rich material, has a local dust/ice mass ratio of 2.3-0.16+0.2:1 ?, matching values previously observed in freshly exposed water ice from outbursts4 and water ice in shadow5,6. at the end of the crevice, philae made a 0.25-metre-deep impression in the boulder ice, providing in situ measurements confirming that primitive ice has a very low compressive strength (less than 12 pascals, softer than freshly fallen light snow) and allowing a key estimation to be made of the porosity (75 ± 7 per cent) of the boulders' icy interiors. our results provide constraints for cometary landers seeking access to a volatile-rich ice sample. | the philae lander reveals low-strength primitive ice inside cometary boulders |
the rosetta mission provides an unprecedented possibility to study the interaction of comets with the solar wind. as the spacecraft accompanies comet 67p/churyumov-gerasimenko from its very low-activity stage through its perihelion phase, the physics of mass loading is witnessed for various activity levels of the nucleus. while observations at other comets provided snapshots of the interaction region and its various plasma boundaries, rosetta observations allow a detailed study of the temporal evolution of the innermost cometary magnetosphere. owing to the short passage time of the solar wind through the interaction region, plasma instabilities such as ring-beam and non-gyrotropic instabilities are of less importance during the early life of the magnetosphere. large-amplitude ultra-low-frequency (ulf) waves, the `singing' of the comet, is probably due to a modified ion weibel instability. this instability drives a cross-field current of implanted cometary ions unstable. the initial pick-up of these ions causes a major deflection of the solar wind protons. proton deflection, cross-field current and the instability induce a threefold structure of the innermost interaction region with the characteristic mach cone and whistler wings as stationary interaction signatures as well as the ulf waves representing the dynamic aspect of the interaction. this article is part of the themed issue 'cometary science after rosetta'. | interaction of the solar wind with comets: a rosetta perspective |
recent alma observations unveiled the structure of co gas in the 23 myr old β pictoris planetary system, a component that has been discovered in many similarly young debris discs. we here present alma co j = 2-1 observations, at an improved spectro-spatial resolution and sensitivity compared to previous co j = 3-2 observations. we find that (1) the co clump is radially broad, favouring the resonant migration over the giant impact scenario for its dynamical origin, (2) the co disc is vertically tilted compared to the main dust disc, at an angle consistent with the scattered light warp. we then use position-velocity diagrams to trace keplerian radii in the orbital plane of the disc. assuming a perfectly edge-on geometry, this shows a co scaleheight increasing with radius as r0.75, and an electron density [derived from co line ratios through non-local thermodynamic equilibrium (nlte) analysis] in agreement with thermodynamical models. furthermore, we show how observations of optically thin line ratios can solve the primordial versus secondary origin dichotomy in gas-bearing debris discs. as shown for β pictoris, subthermal (nlte) co excitation is symptomatic of h2 densities that are insufficient to shield co from photodissociation over the system's lifetime. this means that replenishment from exocometary volatiles must be taking place, proving the secondary origin of the disc. in this scenario, assuming steady state production/destruction of co gas, we derive the co+co2 ice abundance by mass in β pic's exocomets to be at most ∼6 per cent, consistent with comets in our own solar system and in the coeval hd181327 system. | exocometary gas structure, origin and physical properties around β pictoris through alma co multitransition observations |
we review the complex relationship between the dust-to-gas mass ratio usually estimated in the material lost by comets, and the refractory-to-ice mass ratio inside the nucleus, which constrains the origin of comets. such a relationship is dominated by the mass transfer from the perihelion erosion to fallout over most of the nucleus surface. this makes the refractory-to-ice mass ratio inside the nucleus up to 10 times larger than the dust-to-gas mass ratio in the lost material, because the lost material is missing most of the refractories which were inside the pristine nucleus before the erosion. we review the refractory-to-ice mass ratios available for the comet nuclei visited by space missions, and for the kuiper belt objects with well-defined bulk density, finding the 1-σ lower limit of 3. therefore, comets and kbos may have less water than ci-chondrites, as predicted by models of comet formation by the gravitational collapse of cm-sized pebbles driven by streaming instabilities in the protoplanetary disc. | the refractory-to-ice mass ratio in comets |
comet 67p/churyumov-gerasimenko (67p hereinafter) is characterized by a dust transfer from the southern hemi-nucleus to the night-side northern dust deposits, which constrains the dust-to-ices mass ratio inside the nucleus to values a factor of 2 larger than that provided by the lost mass of gas and non-volatiles. this applies to all comets because the gas density in all night comae cannot prevent the dust fallback. taking into account grain impact analyser and dust accumulator (giada) data collected during the entire rosetta mission, we update the average dust bulk density to ρ {}{}_d = 785_{-115}^{+520} kg m-3 that, coupled to the 67p nucleus bulk density, confirms an average dust-to-ices mass ratio δ = 7.5 inside 67p. the improved dust densities are consistent with a mixture of (20 ± 8) per cent of ices, (4 ± 1) per cent of fe sulphides, (22 ± 2) per cent of silicates and (54 ± 5) per cent of hydrocarbons, on average volume abundances. these values correspond to solar chemical abundances, as suggested by the elemental c/fe ratio observed in 67p. the ice content in 67p matches that inferred in kuiper belt objects, (20 ± 12) per cent on average volume abundance and suggests a water content in all trans-neptunian objects lower than in ci chondrites. the 67p icy pebbles and the dust collected by giada have a microporosity of (49 ± 5) and (59 ± 8) per cent, respectively. | the dust-to-ices ratio in comets and kuiper belt objects |
several comets observed at close range have bilobate shapes1, including comet 67p/churyumov-gerasimenko (67p/c-g), which was imaged by the european space agency's rosetta mission2,3. bilobate comets are thought to be primordial because they are rich in supervolatiles (for example, n2 and co) and have a low bulk density, which implies that their formation requires a very low-speed accretion of two bodies. however, slow accretion does not only occur during the primordial phase of the solar system; it can also occur at later epochs as part of the reaccumulation process resulting from the collisional disruption of a larger body4, so this cannot directly constrain the age of bilobate comets. here, we show by numerical simulation that 67p/c-g and other elongated or bilobate comets can be formed in the wake of catastrophic collisional disruptions of larger bodies while maintaining their volatiles and low density throughout the process. since this process can occur at any epoch of our solar system's history, from early on through to the present day5, there is no need for these objects to be formed primordially. these findings indicate that observed prominent geological features, such as pits and stratified surface layers4,5, may not be primordial. | catastrophic disruptions as the origin of bilobate comets |
methyl isocyanate (ch3nco) belongs to a select group of interstellar molecules considered to be relevant precursors in the formation of larger organic compounds, including those with peptide bonds. the molecule has only been detected in a couple of high-mass protostars and potentially on comets. a formation route on icy grains has been postulated for this molecule but experimental evidence is lacking. here we extend the range of environments where methyl isocyanate is found and unambiguously identify ch3nco through the detection of 43 unblended transitions in the alma protostellar interferometric line survey (pils) of the low-mass solar-type protostellar binary iras 16293-2422. the molecule is detected towards both components of the binary with a ratio hnco/ch3nco ∼ 4-12. the isomers ch3cno and ch3ocn are not identified, resulting in upper abundance ratios of ch3nco/ch3cno > 100 and ch3nco/ch3ocn > 10. the resulting abundance ratios compare well with those found for related n-containing species towards high-mass protostars. to constrain its formation, a set of cryogenic uhv experiments is performed. vuv irradiation of ch4:hnco mixtures at 20 k strongly indicate that methyl isocyanate can be formed in the solid state through ch3 and (h)nco recombinations. combined with gas-grain models that include this reaction, the solid-state route is found to be a plausible scenario to explain the methyl isocyanate abundances found in iras 16293-2422. | the alma-pils survey: detection of ch3nco towards the low-mass protostar iras 16293-2422 and laboratory constraints on its formation |
context. since its rendezvous with comet 67p/churyumov-gerasimenko (67p), the rosetta spacecraft has provided invaluable information contributing to our understanding of the cometary environment. on board, the virtis and rosina instruments can both measure gas parameters in the rarefied cometary atmosphere, the so-called coma, and provide complementary results with remote sensing and in situ measurement techniques, respectively. the data from both rosina and virtis instruments suggest that the source regions of h2o and co2 are not uniformly distributed over the surface of the nucleus even after accounting for the changing solar illumination of the irregularly shaped rotating nucleus. the source regions of h2o and co2 are also relatively different from one another.aims: the use of a combination of a formal numerical data inversion method with a fully kinetic coma model is a way to correlate and interpret the information provided by these two instruments to fully understand the volatile environment and activity of comet 67p.methods: in this work, the nonuniformity of the outgassing activity at the surface of the nucleus is described by spherical harmonics and constrained by rosina-dfms data. this activity distribution is coupled with the local illumination to describe the inner boundary conditions of a 3d direct simulation monte-carlo (dsmc) approach using the adaptive mesh particle simulator (amps) code applied to the h2o and co2 coma of comet 67p.results: we obtain activity distribution of h2o and co2 showing a dominant source of h2o in the hapi region, while more co2 is produced in the southern hemisphere. the resulting model outputs are analyzed and compared with virtis-m/-h and rosina-dfms measurements, showing much better agreement between model and data than a simpler model assuming a uniform surface activity. the evolution of the h2o and co2 production rates with heliocentric distance are derived accurately from the coma model showing agreement between the observations from the different instruments and ground-based observations.conclusions: we derive the activity distributions for h2o and co2 at the surface of the nucleus described in spherical harmonics, which we couple to the local solar illumination to constitute the boundary conditions of our coma model. the model presented reproduces the coma observations made by the rosina and virtis instruments on board the rosetta spacecraft showing our understanding of the physics of 67p's coma. this model can be used for further data analyses, such as dust modeling, in a future work. | three-dimensional direct simulation monte-carlo modeling of the coma of comet 67p/churyumov-gerasimenko observed by the virtis and rosina instruments on board rosetta |
since osiris started acquiring high-resolution observations of the surface of the nucleus of comet 67p/churyumov-gerasimenko, over one hundred meter-sized bright spots have been identified in numerous types of geomorphologic regions, but mostly located in areas receiving low insolation. the bright spots are either clustered, in debris fields close to decameter-high cliffs, or isolated without structural relation to the surrounding terrain. they can be up to ten times brighter than the average surface of the comet at visible wavelengths and display a significantly bluer spectrum. they do not exhibit significant changes over a period of a few weeks. all these observations are consistent with exposure of water ice at the surface of boulders produced by dislocation of the weakly consolidated layers that cover large areas of the nucleus. laboratory experiments show that under simulated comet surface conditions, analog samples acquire a vertical stratification with an uppermost porous mantle of refractory dust overlaying a layer of hard ice formed by recondensation or sintering under the insulating dust mantle. the evolution of the visible spectrophotometric properties of samples during sublimation is consistent with the contrasts of brightness and color seen at the surface of the nucleus. clustered bright spots are formed by the collapse of overhangs that is triggered by mass wasting of deeper layers. isolated spots might be the result of the emission of boulders at low velocity that are redepositioned in other regions. | osiris observations of meter-sized exposures of h2o ice at the surface of 67p/churyumov-gerasimenko and interpretation using laboratory experiments |
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