Presolar Cloud Research Articles

Molecular distribution and 13C isotope composition of volatile organic compounds in the Murchison and Sutter's Mill carbonaceous chondrites

AbstractVolatile organic compounds (VOCs) are carbon‐containing chemicals that may evaporate rapidly at room temperature and standard pressure. Such organic compounds can be preserved inside carbonaceous chondrite matrices. However, unlike meteoritic soluble organic matter (SOM) and insoluble organic matter (IOM), VOCs are typically lost (at least in part) during sample processing (meteorite crushing) and exposure to terrestrial atmosphere and/or solvents. Like SOM and IOM, VOCs can provide valuable insights into the chemical inventory of the meteorite parent body and even the presolar cloud from which our solar system formed, as well as the composition and processes that occurred during the early formation of our solar system and the asteroidal stage. Thus, in this work, we designed and built an instrument that allowed us to access the VOCs present in samples of the carbonaceous chondrites Murchison and Sutter's Mill after mineral disaggregation by means of freeze–thaw cycling. We simultaneously evaluated the abundances and compound‐specific 13C‐distributions of the volatiles evolving after meteorite powdering at ~20, 60, and 100°C. Carbon monoxide (CO) and methane (CH4) were released from these meteorites as the most abundant VOCs. They were combusted together for analysis and showed positive δ13C values, indicative of their extraterrestrial origins. Carbon dioxide (CO2) was also an abundant VOC in both meteorites, and its isotopic values suggest that it was mainly formed from dissolved carbonates in the samples. We also detected aldehydes, ketones, and aromatic compounds in low amounts. Contrary to Murchison, which mostly yielded VOCs with positive δ13C values, Sutter's Mill yielded VOCs with negative δ13C values. The less enriched 13C isotope composition of the VOCs detected in Sutter's Mill suggest that they are either terrestrial contaminants, such as VOCs in compressed gas dusters and common laboratory solvents, or compounds disconnected from interstellar sources and/or formed through parent body processing. Understanding the relative abundances and determining the molecular distributions and isotopic compositions of free meteoritic VOCs are key in assessing their extraterrestrial origins and those of chondritic SOM and IOM. Our newly developed technique will be valuable in the study of the samples brought to the Earth from carbonaceous asteroid Bennu by NASA's OSIRIS‐REx mission.

The EXCITING Experiment Exploring the Behavior of Nitrogen and Noble Gases in Interstellar Ice Analogs

Comets represent some of the most pristine bodies in our solar system and can provide a unique insight into the chemical makeup of the early solar system. Due to their icy volatile-rich nature, they may have played an important role in delivering volatile elements and organic material to the early Earth. Understanding how comets form can therefore provide a wealth of information on how the composition of volatile elements evolved in the solar system from the presolar molecular cloud up until the formation of the terrestrial planets. Because noble gases are chemically inert and have distinct condensation temperatures, they can be used to infer the temperatures of formation and thermal history of cometary ices. In this work, we present a new experimental setup called EXCITING to investigate the origin and formation conditions of cometary ices. By trapping nitrogen and noble gases in amorphous water ice, our experiment is designed to study the elemental and isotopic behavior of volatile elements in cometary ice analogs. We report new results of noble gas and nitrogen enrichment in cometary ice analogs and discuss the limitations of the experimental conditions in light of those supposed for comets. We show that forming ice analogs at ∼70 K best reproduce the noble gas and N2 abundances of comet 67P/Churyumov–Gerasimenko, considering a solar-like starting composition. This formation temperature is higher than previous estimates for cometary ices and suggests that the formation of cometary building blocks may have occurred in the protosolar nebula rather than in the colder molecular cloud.

Possible Implications of Relatively High Levels of Initial 60Fe in Iron Meteorites for the Noncarbonaceous–Carbonaceous Meteorite Dichotomy and Solar Nebula Formation

Cook et al. found that iron meteorites have an initial abundance ratio of the short-lived isotope 60Fe to the stable isotope 56Fe of 60Fe/56Fe ∼ (6.4 ± 2.0) × 10−7. This appears to require the injection of live 60Fe from a Type II supernova (SN II) into the presolar molecular cloud core, as the observed ratio is over a factor of 10 times higher than would be expected to be found in the ambient interstellar medium (ISM) as a result of galactic chemical evolution. The supernova triggering and injection scenario offers a ready explanation for an elevated initial 60Fe level, and in addition provides a physical mechanism for explaining the noncarbonaceous–carbonaceous (NC–CC) dichotomy of meteorites. The NC–CC scenario hypothesizes the solar nebula first accreted material that was enriched in supernova-derived nuclides, and then later accreted material depleted in supernova-derived nuclides. While the NC–CC dichotomy refers to stable nuclides, not short-lived isotopes like 60Fe, the SN II triggering hypothesis provides an explanation for the otherwise unexplained change in nuclides being accreted by the solar nebula. Three-dimensional hydrodynamical models of SN II shock-triggered collapse show that after triggering collapse of the presolar cloud core, the shock front sweeps away the local ISM while accelerating the resulting protostar/disk to a speed of several kilometers per second, sufficient for the protostar/disk system to encounter within ∼1 Myr the more distant regions of a giant molecular cloud complex that might be expected to have a depleted inventory of supernova-derived nuclides.

High D/H ratios in water and alkanes in comet 67P/Churyumov-Gerasimenko measured with Rosetta/ROSINA DFMS

Context. Isotopic abundances in comets are key to understanding and reconstructing the history and origin of material in the Solar System. Data for deuterium-to-hydrogen (D/H) ratios in water are available for several comets. However, no long-term studies of the D/H ratio in water of a comet during its passage around the Sun have been reported thus far. Linear alkanes are important organic molecules that have been found on several Solar System bodies, including comets. To date, the processes of their deuteration are still poorly understood, only the upper limits of isotopic ratios for D/H and 13C/12C in linear alkanes are currently available. Aims. The aim of this work is to carry out a detailed analysis of the D/H ratio in water as a function of cometary activity and spacecraft location above the nucleus. In addition, a first determination of the D/H and 13C/12C ratios in the first four linear alkanes, namely, methane (CH4), ethane (C2H6), propane (C3H8), and butane (C4H10) in the coma of 67P/Churyumov-Gerasimenko is provided. Methods. We analysed in situ measurements from the Rosetta/ROSINA Double Focusing Mass Spectrometer (DFMS). Results. The D/H ratio from HDO/H2O and the 16O/17O ratio from H216O/H217O did not change during 67P’s passage around the Sun between 2014 and 2016. All D/H ratio measurements were compatible within 1σ, with the mean value of 5.01 × 10−4 and its relative variation of 2.0%. This suggests that the D/H ratio in 67P’s coma is independent of heliocentric distance, level of cometary activity, or spacecraft location with respect to the nucleus. Additionally, the 16O/17O ratio could be determined with a higher accuracy than previously possible, yielding a value of 2347 with a relative variation of 2.3%. For the alkanes, the D/H ratio is between 4.1 and 4.8 times higher than in H2O, while the 13C/12C ratio is compatible, within the uncertainties, with the available data for other Solar System objects. The relatively high D/H ratio in alkanes is in line with results for other cometary organic molecules and it suggests that these organics may be inherited from the presolar molecular cloud from which the Solar System formed.

Isotopic Compositions of Ruthenium Predicted from the NuGrid Project

Abstract The isotopic compositions of ruthenium (Ru) are measured from presolar silicon carbide (SiC) grains. In a popular scenario, the presolar SiC grains formed in the outskirt of an asymptotic giant branch (AGB) star, left the star as a stellar wind, and joined the presolar molecular cloud from which the solar system formed. The Ru isotopes formed inside the star, moved to the stellar surface during the AGB phase, and were locked into the SiC grains. Following this scenario, we analyze the Nucleosynthesis Grid (NuGrid) data, which provide the abundances of the Ru isotopes in the stellar wind for a set of stars in a wide range of initial masses and metallicities. We apply the C > O (carbon abundance larger than the oxygen abundance) condition, which is commonly adopted for the condition of the SiC formation in the stellar wind. The NuGrid data confirm that SiC grains do not form in the winds of massive stars. The isotopic compositions of Ru in the winds of low-mass stars can explain the measurements. We find that lower-mass stars (1.65 M ☉ and 2 M ☉) with low metallicity (Z = 0.0001) can explain most of the measured isotopic compositions of Ru. We confirm that the abundance of 99 Ru inside the presolar grain includes the contribution from the in situ decay of 99 Tc. We also verify our conclusion by comparing the isotopic compositions of Ru integrated over all the pulses with those calculated at individual pulses.

D/H in the refractory organics of comet 67P/Churyumov-Gerasimenko measured by Rosetta/COSIMA

ABSTRACT The D/H ratio is a clue to the origin and evolution of hydrogen-bearing chemical species in Solar system materials. D/H has been observed in the coma of many comets, but most such measurements have been for gaseous water. We present the first in situ measurements of the D/H ratios in the organic refractory component of cometary dust particles collected at very low impact speeds in the coma of comet 67P/Churyumov-Gerasimenko (hereafter 67P) by the COSIMA instrument onboard Rosetta. The values measured by COSIMA are spatial averages over an approximately 35 × 50 µm2 area. The average D/H ratio for the 25 measured particles is (1.57 ± 0.54) × 10−3, about an order of magnitude higher than the Vienna Standard Mean Ocean Water (VSMOW), but more than an order of magnitude lower than the values measured in gas-phase organics in solar-like protostellar regions and hot cores. This relatively high averaged value suggests that refractory carbonaceous matter in comet 67P is less processed than the most primitive insoluble organic matter (IOM) in meteorites, which has a D/H ratio in the range of about 1 to 7 × 10−4. The cometary particles measured in situ also have a higher H/C ratio than the IOM. We deduce that the measured D/H in cometary refractory organics is an inheritance from the presolar molecular cloud from which the Solar system formed. The high D/H ratios observed in the cometary particles challenges models in which high D/H ratios result solely from processes that operated in the protosolar disc.

Mass-independent fractionation of titanium isotopes and its cosmochemical implications

Isotopes of heavy elements are produced in various amounts by nuclear processes in stars(1,2). Consequently, the presence of isotopic anomalies in the Solar System is considered to reflect the presence of presolar grains condensed in previous generations of stars(3) and not a (proto-) Solar System process. However, for oxygen, the major rock-forming element, it has been shown that physico-chemical reactions applicable to the presolar cloud or the protoplanetary disk were a possible source of isotopic variations due to mass-independent isotopic fractionation (MIF)(4,5). Here we show that MIF effects are not restricted to oxygen, but can also be produced for titanium. Titanium-rich grains experimentally condensed from a TiCl4(g)/C5H12(g) plasma exhibit MIF effects from -25% to +120% for all Ti isotopic ratios. These large Ti isotopic variations follow the model developed for oxygen MIF6 and mimic the Ti isotopic anomalies observed in some presolar grains. This effect is ascribed to the reactions between chemically indistinguishable isotopes(6) and could contribute to the complexity of isotopic anomalies observed in Solar System materials(1,7-14).

Heterogeneous evolution of the galaxy and the origin of the short-lived nuclides in the early solar system

Abstract We present galactic chemical evolution (GCE) models of the short-lived radionuclides (SLRs), 26Al, 36Cl, 41Ca, 53Mn and 60Fe, across the entire Milky Way galaxy. The objective is to understand the spatial and temporal distribution of the SLRs in the galaxy. The gamma-ray observations infer widespread distribution of 26Al and 60Fe across the galaxy. The signatures of the SLRs in the early solar system (ESS) are found in meteorites. We present homogeneous GCE simulation models for SLRs across the galaxy. We also develop a set of heterogeneous GCE models to understand the evolution of the galaxy within independent spatial grids of area, 0.1–1 kpc2. These grids evolve distinctly in terms of nucleosynthetic contributions of massive stars. We succeeded in simulating the formation and evolution of generations of stellar clusters/association. Based on the formulation, we provide a novel method to amalgamate the origin of the solar system with the gradual evolution of the galaxy along with a self-consistent origin of SLRs. We explore the possibility of the birth of the solar system in an environment where one of the stellar clusters formed ≥ 25 Million years earlier. The decaying 53Mn and 60Fe remnants from the evolved massive stars from the cluster probably contaminated the local medium associated with the presolar molecular cloud. A Wolf-Rayet wind from a distant massive star, belonging to a distinct cluster, probably contributed, 26Al (and 41Ca) to the presolar cloud. The irradiation production of 7,10Be and 36Cl occurred later in ESS.

Complex Organic Molecules in Comets from Remote-Sensing Observations at Millimeter Wavelengths

Remote observations of comets, especially using high spectral resolution millimeter spectroscopy, have enabled the detection of over 25 molecules in comets for the last twenty years. Among the molecules identified at radio wavelengths, complex organic molecules (COMs) such as acetaldehyde, ethylene-glycol, formamide, methyl-formate or ethanol have been observed in several comets and their abundances relative to water and methanol precisely determined. Significant upper limits on the abundance of several other COMs have been determined and put constraints on the dominant isomer for three of them. The abundances measured in comets are generally of comparable order of magnitude as those measured in star-forming regions, suggesting that comets contain preserved material from the presolar cloud from which the solar system was born.

Triggering Collapse of the Presolar Dense Cloud Core and Injecting Short-lived Radioisotopes with a Shock Wave. VI. Protostar and Protoplanetary Disk Formation

Abstract Cosmochemical evaluations of the initial meteoritical abundance of the short-lived radioisotope (SLRI) 26Al have remained fairly constant since 1976, while estimates for the initial abundance of the SLRI 60Fe have varied widely recently. At the high end of this range, 60Fe initial abundances have seemed to require 60Fe nucleosynthesis in a core-collapse supernova, followed by incorporation into primitive meteoritical components within ∼1 Myr. This paper continues the detailed exploration of this classical scenario, using models of the self-gravitational collapse of molecular cloud cores that have been struck by suitable shock fronts, leading to the injection of shock front gas into the collapsing cloud through Rayleigh–Taylor fingers formed at the shock–cloud interface. As before, these models are calculated using the FLASH three-dimensional, adaptive mesh refinement, gravitational hydrodynamical code. While the previous models used FLASH 2.5, the new models employ FLASH 4.3, which allows sink particles to be introduced to represent the newly formed protostellar object. Sink particles permit the models to be pushed forward farther in time to the phase where a ∼1 M ⊙ protostar has formed, orbited by a rotating protoplanetary disk. These models are thus able to define what type of target cloud core is necessary for the supernova triggering scenario to produce a plausible scheme for the injection of SLRIs into the presolar cloud core: a ∼3 M ⊙ cloud core rotating at a rate of ∼3 × 10−14 rad s−1 or higher.

Noble Gas Abundance Ratios Indicate the Agglomeration of 67P/Churyumov–Gerasimenko from Warmed-up Ice

Abstract The origin of cometary volatiles remains a major open question in planetary science. Comets may have either agglomerated from crystalline ices condensed in the protosolar nebula (PSN) or from amorphous ice originating from the molecular cloud and interstellar medium. Here, based on the recent argon, krypton, and xenon measurements performed by the ROSINA mass spectrometer on board the European Space Agency’s Rosetta spacecraft in the coma of 67P/Churyumov–Gerasimenko, we show that these noble gas relative abundances can be explained if the comet’s building blocks formed from a mixture of gas and H2O grains resulting from the annealing of pristine amorphous ice (i.e., originating from the presolar cloud) in the PSN. In this scenario, the different volatiles released during the amorphous-to-crystalline ice phase transition would have been subsequently trapped at lower temperatures in stoichiometric hydrate or clathrate hydrate forms by the crystalline water ice generated by the transition. Once crystalline water was completely consumed by clathration in the ∼25–80 K temperature range, the volatile species remaining in the gas phase would have formed pure condensates at lower temperatures. The formation of clathrates hydrates and pure condensates to explain the noble gas relative abundances is consistent with a proposed interstellar origin of molecular oxygen detected in 67P/Churyumov–Gerasimenko, and with the measured molecular nitrogen depletion in comets.

On the Origin of Early Solar System Radioactivities: Problems with the Asymptotic Giant Branch and Massive Star Scenarios

Abstract Recent improvements in stellar models for intermediate-mass stars and massive stars (MSs) are recalled, together with their expectations for the synthesis of radioactive nuclei of lifetimes τ ≲ 25 Myr, in order to re-examine the origins of now extinct radioactivities that were alive in the solar nebula. The Galactic inheritance broadly explains most of them, especially if r-process nuclei are produced by neutron star merging, according to recent models. Instead, 26Al, 41Ca, 135Cs, and possibly 60Fe require nucleosynthetic events close to the solar formation. We outline the persisting difficulties to account for these nuclei by intermediate-mass stars (2 ≲ M/M ⊙ ≲ 7–8). Models of their final stages now predict the ubiquitous formation of a 13C reservoir as a neutron capture source; hence, even in the presence of 26Al production from deep mixing or hot bottom burning, the ratio 26Al/107Pd remains incompatible with measured data, with a large excess in 107Pd. This is shown for two recent approaches to deep mixing. Even a late contamination by an MS encounters problems. In fact, the inhomogeneous addition of supernova debris predicts nonmeasured excesses on stable isotopes. Revisions invoking specific low-mass supernovae and/or the sequential contamination of the presolar molecular cloud might be affected by similar problems, although our conclusions here are weakened by our schematic approach to the addition of SN ejecta. The limited parameter space that remains to be explored for solving this puzzle is discussed.

Dust cleansing of star-forming gas

Aims. We explore the possibility that solar chemical composition, as well as the similar composition of the rich open cluster M 67, have been affected by dust cleansing of the presolar or precluster cloud due to the radiative forces from bright early-type stars in its neighbourhood. Methods. We estimate possible cleansing effects using semi-analytical methods, which are essentially based on momentum conservation. Results. Our calculations indicate that the amounts of cleansed neutral gas are limited to a relatively thin shell surrounding the H II region around the early-type stars. Conclusions. It seems possible that the proposed mechanism acting in individual giant molecular clouds may produce significant abundance effects for masses corresponding to single stars or small groups of stars. The effects of cleansing are, however, severely constrained by the thinness of the cleansed shell of gas and by turbulence in the cloud. This is why the mechanism can hardly be important in cleansing masses corresponding to rich clusters, such as the mass of the original M 67.

Multiple generations of grain aggregation in different environments preceded solar system body formation

The solar system formed from interstellar dust and gas in a molecular cloud. Astronomical observations show that typical interstellar dust consists of amorphous (a-) silicate and organic carbon. Bona fide physical samples for laboratory studies would yield unprecedented insight about solar system formation, but they were largely destroyed. The most likely repositories of surviving presolar dust are the least altered extraterrestrial materials, interplanetary dust particles (IDPs) with probable cometary origins. Cometary IDPs contain abundant submicron a-silicate grains called GEMS (glass with embedded metal and sulfides), believed to be carbon-free. Some have detectable isotopically anomalous a-silicate components from other stars, proving they are preserved dust inherited from the interstellar medium. However, it is debated whether the majority of GEMS predate the solar system or formed in the solar nebula by condensation of high-temperature (>1,300 K) gas. Here, we map IDP compositions with single nanometer-scale resolution and find that GEMS contain organic carbon. Mapping reveals two generations of grain aggregation, the key process in growth from dust grains to planetesimals, mediated by carbon. GEMS grains, some with a-silicate subgrains mantled by organic carbon, comprise the earliest generation of aggregates. These aggregates (and other grains) are encapsulated in lower-density organic carbon matrix, indicating a second generation of aggregation. Since this organic carbon thermally decomposes above ∼450 K, GEMS cannot have accreted in the hot solar nebula, and formed, instead, in the cold presolar molecular cloud and/or outer protoplanetary disk. We suggest that GEMS are consistent with surviving interstellar dust, condensed in situ, and cycled through multiple molecular clouds.

Synthesis of Molecular Oxygen via Irradiation of Ice Grains in the Protosolar Nebula

Abstract Molecular oxygen has been detected in the coma of comet 67P/Churyumov–Gerasimenko with a mean abundance of 3.80 ± 0.85% by the ROSINA mass spectrometer on board the Rosetta spacecraft. To account for the presence of this species in comet 67P/Churyumov–Gerasimenko, it has been shown that the radiolysis of ice grain precursors of comets is a viable mechanism in low-density environments, such as molecular clouds. Here, we investigate the alternative possibility that the icy grains present in the midplane of the protosolar nebula were irradiated during their vertical transport between the midplane and the upper layers over a large number of cycles, as a result of turbulent mixing. Consequently, these grains spent a non-negligible fraction of their lifetime in the disk’s upper regions, where the irradiation by cosmic rays was strong. To do so, we used a coupled disk-transport-irradiation model to calculate the time evolution of the molecular oxygen abundance radiolytically produced in ice grains. Our computations show that, even if a significant fraction of the icy particles has followed a back and forth cycle toward the upper layers of the disk over tens of millions of years, a timespan far exceeding the formation timescale of comet 67P/Churyumov–Gerasimenko, the amount of produced molecular oxygen is at least two orders of magnitude lower than the Rosetta observations. We conclude that the most likely scenario remains the formation of molecular oxygen in low-density environments, such as the presolar cloud, prior to the genesis of the protosolar nebula.

Isotopic composition of CO2in the coma of 67P/Churyumov-Gerasimenko measured with ROSINA/DFMS

Measurements of isotopic abundances in cometary ices are key to understanding and reconstructing the history and origin of material in the solar system. Comets are considered the most pristine material in the solar system. Isotopic fractionation (enrichment of an isotope in a molecule compared to the initial abundance) is sensitive to environmental conditions at the time of comet formation. Therefore, measurements of cometary isotope ratios can provide information on the composition, density, temperature, and radiation during formation of the molecules, during the chemical evolution from the presolar cloud to the protosolar nebula, and the protoplanetary disk before accretion in solid bodies. Most isotopic abundances of 12 C/ 13 C and 16 O/ 18 O in comets to date are in agreement with terrestrial abundances. Prior to the Rosetta mission, measurements of 12 C/ 13 C in comets were only available for HCN, CN, and C 2 and for 16 O/ 18 O in H 2 O. Measurements of 12 C/ 13 C in comets were only available from ground based observations and remote sensing, while 16 O/ 18 O in H 2 O had also been measured in-situ. To date, no measurements of the CO 2 isotopologues in comets were available. This paper presents the first measurements of the CO 2 isotopologues in the coma of 67P/Churyumov-Gerasimenko (67P). Methods. We analyzed measurements taken by the Double Focusing Mass Spectrometer (DFMS) of the ROSINA experiment on board the ESA spacecraft Rosetta in the coma of 67P. The CO 2 isotopologues results for 67P are: 12 C/ 13 C = 84 ± 4, 16 O/ 18 O = 494 ± 8, and 13 C 16 O 2 / 12 C 18 O 16 O = 5.87 ± 0.07. The oxygen isotopic ratio is within error bars compatible with terrestrial abundances but not with solar wind measurements. Conclusions. The carbon isotopic ratio and the combined carbon and oxygen isotopic ratio are slightly (14%) enriched in 13 C, within 1σ uncertainty, compared to solar wind abundances and solar abundances. The small fractionation of 12 C/ 13 C in CO 2 is probably compatible with an origin of the material in comets from the native cloud.

D2O and HDS in the coma of 67P/Churyumov-Gerasimenko.

The European Rosetta mission has been following comet 67P/Churyumov-Gerasimenko for 2 years, studying the nucleus and coma in great detail. For most of these 2 years the Rosetta Orbiter Sensor for Ion and Neutral Analysis (ROSINA) has analysed the volatile part of the coma. With its high mass resolution and sensitivity it was able to not only detect deuterated water HDO, but also doubly deuterated water, D2O and deuterated hydrogen sulfide HDS. The ratios for [HDO]/[H2O], [D2O]/[HDO] and [HDS]/[H2S] derived from our measurements are (1.05 ± 0.14) × 10-3, (1.80 ± 0.9) × 10-2 and (1.2 ± 0.3) × 10-3, respectively. These results yield a very high ratio of 17 for [D2O]/[HDO] relative to [HDO]/[H2O]. Statistically one would expect just 1/4. Such a high value can be explained by cometary water coming unprocessed from the presolar cloud, where water is formed on grains, leading to high deuterium fractionation. The high [HDS]/[H2S] ratio is compatible with upper limits determined in low-mass star-forming regions and also points to a direct correlation of cometary H2S with presolar grain surface chemistry.This article is part of the themed issue 'Cometary science after Rosetta'.

Variations in cometary dust composition fromGiottotoRosetta, clues to their formation mechanisms

This paper reviews the current knowledge on the composition of cometary dust (ice, minerals and organics) in order to constrain their origin and formation mechanisms. Comets have been investigated by astronomical observations, space missions (Giotto to Rosetta), and by the analysis of cometary dust particles collected on Earth, chondritic porous interplanetary dust particles (CP-IDPs) and ultracarbonaceous Antarctic micrometeorites (UCAMMs). Most ices detected in the dense phases of the interstellar medium (ISM) have been identified in cometary volatiles. However, differences also suggest that cometary ices cannot be completely inherited from the ISM. Cometary minerals are dominated by crystalline Mg-rich silicates, Fe sulphides and glassy phases including GEMS (glass with embedded metals and sulphides). The crystalline nature and refractory composition of a significant fraction of the minerals in comets imply a high temperature formation/processing close to the proto-Sun, resetting a possible presolar signature of these phases. These minerals were further transported up to the external regions of the disc and incorporated in comet nuclei. Cometary matter contains a low abundance of isotopically anomalous minerals directly inherited from the presolar cloud. At least two different kinds of organic matter are found in dust of cometary origin, with low or high nitrogen content. N-poor organic matter is also observed in primitive interplanetary materials (like carbonaceous chondrites) and its origin is debated. The N-rich organic matter is only observed in CP-IDPs and UCAMMs and can be formed by Galactic cosmic ray irradiation of N2- and CH4-rich icy surface at large heliocentric distance beyond a ‘nitrogen snow line’.

Sulphur-bearing species in the coma of comet 67P/Churyumov–Gerasimenko

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 H 2 S, OCS, SO, S 2 , SO 2 and CS 2 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 S 3 , S 4 , CH 3 SH and C 2 H 6 S 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 S 2 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 S 2 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.

Chemical enrichment of the pre-solar cloud by supernova dust grains

The presence of short-lived radioisotopes (SLRs) in solar system meteorites has been interpreted as evidence that the solar system was exposed to a supernova shortly before or during its formation. Yet results from hydrodynamical models of SLR injection into the proto-solar cloud or disc suggest that gas-phase mixing may not be efficient enough to reproduce the observed abundances. As an alternative, we explore the injection of SLRs via dust grains as a way to overcome the mixing barrier. We numerically model the interaction of a supernova remnant containing SLR-rich dust grains with a nearby molecular cloud. The dust grains are subject to drag forces and both thermal and non-thermal sputtering. We confirm that the expanding gas shell stalls upon impact with the dense cloud and that gas-phase SLR injection occurs slowly due to hydrodynamical instabilities at the cloud surface. In contrast, dust grains of sufficient size (> 1 micron) decouple from the gas and penetrate into the cloud within 0.1 Myr. Once inside the cloud, the dust grains are destroyed by sputtering, releasing SLRs and rapidly enriching the dense (potentially star-forming) regions. Our results suggest that SLR transport on dust grains is a viable mechanism to explain SLR enrichment.