C. Engrand

Comet 67P/Churyumov-Gerasimenko sheds dust coat accumulated over the past four years

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 Hartleyxa02 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 6.5xa0years), 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.

Extreme Deuterium Excesses in Ultracarbonaceous Micrometeorites from Central Antarctic Snow

Dust to Dust Interplanetary dust particles are thought to sample the most primitive materials in the solar system. Because of their large deuterium enrichments, they are thought to have formed in interstellar molecular clouds—the birthplaces of stars—and to predate the solar system. Duprat et al. (p. 742; see Perspective by Nittler) describe two large interplanetary dust particles collected from Antarctic snow. The particles contain large zones of organic matter with deuterium excesses 10 to 30 times the terrestrial value. Because the organic matter is associated with crystalline silicates similar to those formed within the solar accretion disk, it is expected that the particles themselves formed in the Suns protoplanetary disk, contradicting the idea that all organics with deuterium excesses are of interstellar origin. Interplanetary dust particles recovered from Antarctic snow may provide a sample of the early solar system. Primitive interplanetary dust is expected to contain the earliest solar system components, including minerals and organic matter. We have recovered, from central Antarctic snow, ultracarbonaceous micrometeorites whose organic matter contains extreme deuterium (D) excesses (10 to 30 times terrestrial values), extending over hundreds of square micrometers. We identified crystalline minerals embedded in the micrometeorite organic matter, which suggests that this organic matter reservoir could have formed within the solar system itself rather than having direct interstellar heritage. The high D/H ratios, the high organic matter content, and the associated minerals favor an origin from the cold regions of the protoplanetary disk. The masses of the particles range from a few tenths of a microgram to a few micrograms, exceeding by more than an order of magnitude those of the dust fragments from comet 81P/Wild 2 returned by the Stardust mission.

Clues to the origin of interplanetary dust particles from the isotopic study of their hydrogen-bearing phases

Abstract Ion microprobe quantitative imaging was performed for H, D, 12,13 C, 16,18 O, 27 Al, and 28,29,30 Si in five stratospheric particles with a 1.5 × 1.5 μm spatial resolution to determine the carriers of high D/H ratios and to give new clues about the parent bodies of interplanetary dust particles (IDPs). Among these particles, four appear to be of extraterrestrial origin. Using imaging, the large variations of D/H ratios can be correlated at the micrometer scale with chemical composition so that endmembers can be identified. From the systematics of variation of C/H and D/H ratios, water present as hydroxyls in phyllosilicates (at C/H = 0) and three different types of organic matter (OM1, 2, and 3) were identified. Water exhibits D/H ratios lying in the chondritic domain (D/H ∼150 × 10 −6 ), whereas OMs are enriched in deuterium. OM1 is similar to the macromolecular organic matter of carbonaceous chondrites (D/H = 250 × 10 −6 and C/H = 1.5). OM2 (D/H = 1500 × 10 −6 and C/H = 1.0) is close to cometary HCN. OM3 (D/H = 2000 × 10 −6 and C/H = 3.0) is a highly condensed carbonaceous phase with no counterpart in known extraterrestrial objects. The C, O, and Si isotopic compositions are solar within ± 10%. The identification of these phases allows a better understanding of the origin of D/H variations inferred for the protosolar nebula. The only mechanism that can explain such high D/H ratios is interstellar chemistry. However, D/H ratios in interplanetary dust organic matter remain lower than those measured in molecules from cold molecular clouds. This difference can be accounted for by an isotopic exchange with liquid water in IDP parent bodies. In addition, the close association of the chondritic component OM1 with the likely cometary component OM2 is evidence for a link between carbonaceous chondrites and comets. Although IDPs contain cometary organic matter, the water-D/H ratio is lower than that measured in comets (310 × 10 −6 ). IDPs seem thus constituted of various materials formed over a large range of heliocentric distances.

Comet 67P/Churyumov–Gerasimenko: Close-up on Dust Particle Fragments

The COmetary Secondary Ion Mass Analyser instrument on board ESAs Rosetta mission has collected dust particles in the coma of comet 67P/Churyumov-Gerasimenko. During the early-orbit phase of the Rosetta mission, particles and particle agglomerates have been imaged and analyzed in the inner coma at distances between 100 km and 10 km off the cometary nucleus and at more than 3 AU from the Sun. We identified 585 particles of more than 14 μm in size. The particles are collected at low impact speeds and constitute a sample of the dust particles in the inner coma impacting and fragmenting on the targets. The sizes of the particles range from 14 μm up to sub-millimeter sizes and the differential dust flux size distribution is fitted with a power law exponent of -3.1. After impact, the larger particles tend to stick together, spread out or consist of single or a group of clumps, and the flocculent morphology of the fragmented particles is revealed. The elemental composition of the dust particles is heterogeneous and the particles could contain typical silicates like olivine and pyroxenes, as well as iron sulfides. The sodium to iron elemental ratio is enriched with regard to abundances in CI carbonaceous chondrites by a factor from ˜1.5 to ˜15. No clear evidence for organic matter has been identified. The composition and morphology of the collected dust particles appear to be similar to that of interplanetary dust particles.

High-molecular-weight organic matter in the particles of comet 67P/Churyumov–Gerasimenko

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.

A unique basaltic micrometeorite expands the inventory of solar system planetary crusts

Micrometeorites with diameter ≈100–200 μm dominate the flux of extraterrestrial matter on Earth. The vast majority of micrometeorites are chemically, mineralogically, and isotopically related to carbonaceous chondrites, which amount to only 2.5% of meteorite falls. Here, we report the discovery of the first basaltic micrometeorite (MM40). This micrometeorite is unlike any other basalt known in the solar system as revealed by isotopic data, mineral chemistry, and trace element abundances. The discovery of a new basaltic asteroidal surface expands the solar system inventory of planetary crusts and underlines the importance of micrometeorites for sampling the asteroids surfaces in a way complementary to meteorites, mainly because they do not suffer dynamical biases as meteorites do. The parent asteroid of MM40 has undergone extensive metamorphism, which ended no earlier than 7.9 Myr after solar system formation. Numerical simulations of dust transport dynamics suggest that MM40 might originate from one of the recently discovered basaltic asteroids that are not members of the Vesta family. The ability to retrieve such a wealth of information from this tiny (a few micrograms) sample is auspicious some years before the launch of a Mars sample return mission.

Mechanical and electrostatic experiments with dust particles collected in the inner coma of comet 67P by COSIMA onboard Rosetta

The in situ cometary dust particle instrument COSIMA (COmetary Secondary Ion Mass Analyser) onboard ESAs Rosetta mission has collected about 31u2009000 dust particles in the inner coma of comet 67P/Churyumov–Gerasimenko since August 2014. The particles are identified by optical microscope imaging and analysed by time-of-flight secondary ion mass spectrometry. After dust particle collection by low speed impact on metal targets, the collected particle morphology points towards four families of cometary dust particles. COSIMA is an in situ laboratory that operates remotely controlled next to the comet nucleus. The particles can be further manipulated within the instrument by mechanical and electrostatic means after their collection by impact. The particles are stored above 0°C in the instrument and the experiments are carried out on the refractory, ice-free matter of the captured cometary dust particles. An interesting particle morphology class, the compact particles, is not fragmented on impact. One of these particles was mechanically pressed and thereby crushed into large fragments. The particles are good electrical insulators and transform into rubble pile agglomerates by the application of an energetic indium ion beam during the secondary ion mass spectrometry analysis. This article is part of the themed issue ‘Cometary science after Rosetta’.

Evidence for depletion of heavy silicon isotopes at comet 67P/Churyumov-Gerasimenko

Context. The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) was designed to measure the composition of the gas in the coma of comet 67P/Churyumov-Gerasimenko, the target of the European Space Agencys Rosetta mission. In addition to the volatiles, ROSINA measured refractories sputtered off the comet by the interaction of solar wind protons with the surface of the comet. nAims. The origin of different solar system materials is still heavily debated. Isotopic ratios can be used to distinguish between different reservoirs and investigate processes occurring during the formation of the solar system. nMethods. ROSINA consisted of two mass spectrometers and a pressure sensor. In the ROSINA Double Focusing Mass Spectrometer (DFMS), the neutral gas of cometary origin was ionized and then deflected in an electric and a magnetic field that separated the ions based on their mass-to-charge ratio. The DFMS had a high mass resolution, dynamic range, and sensitivity that allowed detection of rare species and the known major volatiles. nResults. We measured the relative abundance of all three stable silicon isotopes with the ROSINA instrument on board the Rosetta spacecraft. Furthermore, we measured

Dome C ultracarbonaceous Antarctic micrometeorites: Infrared and Raman fingerprints⋆

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Interstellar and interplanetary solids in the laboratory

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