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Lunar minerals and impact glasses, convert the polyatomic beam of solar wind (SW) ions into a flux of small molecules ( e.g. , H 2 , N 2 , H 2 O, CO, CO 2 , CH 4 , C 2 H 4 , C 2 H 6 , HCN, metal carbides and deuterides, etc.). They thus behave as “Solid State Molecular Reactors”. Moreover, ~100–200 μ m size micrometeoroids ( μ Ms) have also been exposed to the SW in the zodiacal cloud, before being captured by the Earth and recovered as Antarctic micrometeorites. They are mostly composed of a PAH-rich hydrous-carbonaceous material, which amplifies their power as molecular reactors. In particular, during the first ~200 Myr of the post-lunar period, about 75% of the μ Ms have been melted and/or volatilized upon atmospheric entry. The release of their volatile species triggered a cosmic volcanism around the mesopause that ruled the formation of the early Earth’s atmosphere and climate. Furthermore, a fraction of the μ Ms that survive unmelted upon atmospheric entry did settle on the proto-oceans floors. Upon further burial in sediments their constituent PAH-rich kerogen was cracked into abiotic oil, which generated giant oil slicks that fed prebiotic chemistry. Many stars, of all ages and types, are embedded into a secondary debris-disk loaded with ion implanted μ Ms. Some of them are expelled to the interstellar medium (ISM) where they behave first as “dormant-invisible” molecular reactors, until they became reactivated by various processes to synthesize interstellar molecules. This short paper only focus on some highlights of this research dealing with the synthesis of important interstellar molecules, including the most abundant ones (H 2 and CO) and H 2 O, HCN and PAHs, all involved in prebiotic chemistry.

— Depending on their velocity, entry angle and mass, micrometeorites suffer different degrees of heating during their deceleration in the Earth's atmosphere, leading, in most cases, to significant textural, mineralogical and chemical modifications. One of these modifications is the formation of a magnetite shell around most micrometeorites, which until now could not be reproduced, neither theoretically nor experimentally. The present study was designed to better understand the entry heating effects on micrometeorites and especially the formation of the magnetite shell. Fragments of the Murchison and Orgueil meteorites were used as analogue material in flash-heating experiments performed in a high-temperature furnace; effects of temperature, heating duration, and oxygen fugacity were investigated. These experiments were able to reproduce most of the micrometeorites textures, from the vesicular fine-grained micrometeorites to the totally melted cosmic spherules. For the first time, the formation of a magnetite shell could be observed on micrometeorite analogues. We suggest that the most plausible mechanism for the formation of this shell is a peripheral partial melting with subsequent magnetite crystallization at the surface of the micrometeorite. Furthermore, with this study, it is possible to estimate the atmospheric entry conditions of micrometeorites, such as the peak temperature and the duration of flash-heating.

— The D/H ratios and water contents were measured by ion microprobe analysis in 52 individual Antarctic micrometeorites (AMMs) and 10 Antarctic cosmic spherules (ACSs) containing nuggets of iron hydroxide (COPS phase). In AMMs, δD values vary from −366 to +249%‰ and water contents lie between 0.4-3.7 wt%. The COPS nuggets in cosmic spherules have high water contents (2 to 8 wt%) and exhibit δD values from −144 to +167%‰, which is indicative of an extraterrestrial origin of their constituent water. The silicate portion of ACSs also contain extraterrestrial H equivalent to ∼0.l to 1.2 wt% water. Deuterium-exchange experiments were performed with isotopically spiked water. These experiments demonstrate that water in mineral phases of AMMs and ACSs is indigenous and does not result from contamination during residence in Antarctic ice. The frequency distribution of D/H ratios in AMMs allows us to further narrow the relationship between AMMs and carbonaceous chondrites to CM and CI chondrites but contrasts with that of stratospheric interplanetary dust particles (IDPs) of similar sizes (from −10 to 50 μm). The relatively narrow range of D/H ratios measured in AMMs as well as in ACSs (which are more resistant and thus less susceptible to collection biases) suggests that D-rich IDP-like particles are very rare in our AMMs collections. This indicates that these D-rich grains might constitute a minor fraction of the micrometeorite flux in the interplanetary medium and that possible collection biases in Antarctica would not be responsible for their strong depletion in the AMMs collections.

Over 100 000 large interplanetary dust particles in the 50-500 micrometers size range have been recovered in clean conditions from approximately 600 tons of Antarctic melt ice water as both unmelted and partially melted/dehydrated micrometeorites and cosmic spherules. Flux measurements in both the Greenland and Antarctica ice sheets indicate that the micrometeorites deliver to the Earth's surface approximately 2000x more extraterrestrial material than brought by meteorites. Mineralogical and chemical studies of Antarctic micrometeorites indicate that they are only related to the relatively rare CM and CR carbonaceous chondrite groups, being mostly chondritic carbonaceous objects composed of highly unequilibrated assemblages of anhydrous and hydrous minerals. However, there are also marked differences between these two families of solar system objects, including higher C/O ratios and a very marked depletion of chondrules in micrometeorite matter; hence, they are "chondrites-without-chondrules." Thus, the parent meteoroids of micrometeorites represent a dominant and new population of solar system objects, probably formed in the outer solar system and delivered to the inner solar system by the most appropriate vehicles, comets. One of the major purposes of this paper is to discuss applications of micrometeorite studies that have been previously presented to exobiologists but deal with the synthesis of prebiotic molecules on the early Earth, and more recently, with the early history of the solar system.

The petrology and geochemistry of twentythree chondritic dust particles with masses of 1–47 μg (sizes 100–400 μm) were recovered from blue ice near Cap Prudhomme, Antarctica, and studied by INAA, ASEM, EMPA, and optical microscopy. Sample selection criteria were irregular shape and (for a subsample) black color, with the aim of studying as many unmelted micrometeorites (MMs) as possible. Of thirteen unmelted MMs, six were phyllosilicate-dominated MMs, and seven were coarsegrained crystalline MMs consisting mainly of olivine and pyroxene. The remaining ten particles were largely melted and consisted of a foamy melt with variable amounts of relic phases (scoriaceous MMs). Thus, of the black particles selected, an astonishing portion, 40% (by number), consisted of largely unmelted MMs. Although unmelted, most phyllosilicate MMs have been thermally metamorphosed to a degree that most of the phyllosilicates were destroyed, but not melted. The original preterrestrial mineralogy is occasionally preserved and consists of serpentine-like phyllosilicates with variable amounts of cronstedtite, tochilinite-like oxides, olivine, and pyroxene. The crystalline MMs consist of olivine, low-Ca pyroxene, tochilinite-like oxides, and occasional Ni-poor metal. Relics in scoriaceous MMs consist of the same phases. Mineral compositions and the coexistence of phyllosilicates with anhydrous phases are typical of CM and CR-type carbonaceous chondrites. However, the olivine/pyroxene ratio (~ 1) and the lack of carbonates, sulfates, and of very Fe-poor, refractory element-rich olivines and pyroxenes sets the MMs apart from CM and CR chondrites. The bulk chemistry of the phyllosilicate MMs is similar to that of CM chondrites. However, several elements are either depleted (Ca, Ni, S, less commonly Na, Mg, and Mn) or enriched (K, Fe, As, Br, Rb, Sb, and Au) in MMs as compared to CM chondrites. Similar depletions and enrichments are also found in the scoriaceous MMs. We suggest that the depletions are probably due to terrestrial leaching of sulfates and carbonates from unmelted MMs. The overabundance of some elements may also be due to processes acting during atmospheric passage such as the recondensation of meteoric vapors in the high atmosphere. Most MMs are coated by magnetite of platy or octahedral habit, which is rich in Mg, Al, Si, Mn, and Ni. We interpret the magnetites to be products of recondensation processes in the high (>90 km) atmosphere, which are, therefore, probably the first refractory aerominerals identified.

About 5000 unmelted and well preserved “giant” chondritic micrometeorites with sizes ≈50–200 μ have been extracted from ≈100 tons of antarctica blue ice. They have been unexpectedly well shielded against both terrestrial weathering and frictional heating in the atmosphere. Mineralogical studies indicate that they are all related to primitive “unequilibrated” meteorites (mostly carbonaceous chondrites). About 50% of them are made of friable and porous aggregates of submicron-sized grains, that represent a highly desequilibrated assemblage of minerals, including hydrous silicates and anhydrous clasts, metal oxides and sulfides, and some carbonaceous material related to the broad family of “hydrogenated refractory carbon.” Each carbon-rich micrometeorite might have behaved as a “minicenter” of prebiotic synthesis on the early Earth, through the “in-situ” catalyzed hydrolysis of this carbonaceous material.

The International Rosetta Mission is set for a rendezvous with Comet 67 P/Churyumov-Gerasimenko in 2014. On its 10 year journey to the comet, the spacecraft will also perform a fly-by of the two asteroids Stein and Lutetia in 2008 and 2010, respectively. The mission goal is to study the origin of comets, the relationship between cometary and interstellar material and its implications with regard to the origin of the Solar System. Measurements will be performed that shed light into the development of cometary activity and the processes in the surface layer of the nucleus and the inner coma. The Micro-Imaging Dust Analysis System (MIDAS) instrument is an essential element of Rosetta's scientific payload. It will provide 3D images and statistical parameters of pristine cometary particles in the nm-μm range from Comet 67P/Churyumov-Gerasimenko. According to cometary dust models and experience gained from the Giotto and Vega missions to 1P/Halley, there appears to be an abundance of particles in this size range, which also covers the building blocks of pristine interplanetary dust particles. The dust collector of MIDAS will point at the comet and collect particles drifting outwards from the nucleus surface. MIDAS is based on an Atomic Force Microscope (AFM), a type of scanning microprobe able to image small structures in 3D. AFM images provide morphological and statistical information on the dust population, including texture, shape, size and flux. Although the AFM uses proven laboratory technology, MIDAS is its first such application in space. This paper describes the scientific objectives and background, the technical implementation and the capabilities of MIDAS as they stand after the commissioning of the flight instrument, and the implications for cometary measurements.

Large interplanetary dust particles (micrometeorites) with sizes of 100– 200 μm , recovered from the Greenland and Antarctica ice sheets, represent by far the dominant source of primitive extraterrestrial material accreted by the Earth today. Comparisons of mineralogical, chemical and isotopic analyses of micrometeorites and meteorites indicate that micrometeorites are mostly related to the relatively rare group (2% of the meteorite falls) of the primitive hydrous-carbonaceous meteorites, and not to the most abundant classes of the ordinary chondrites and differentiated meteorites. But there are differences between these two classes of extraterrestrial objects, such as a high pyroxene to olivine ratio, a strong depletion in chondrules, a much smaller size of the most refractory components, and a much higher AIB (α-isobutyric amino acid) to isovaline ratio in micrometeorites as compared to meteorites. They indicate that micrometeorites represent a new population of solar system objects, not represented as yet in the meteorite collections. The major objective of this work is to predict various effects of the accretion of early micrometeorites on the Earth during the period of heavy bombardment suffered by the Earth–Moon system ⩾3.9 Ga ago. The application of a simple arithmetics of accretion to a selection of measurements (average contents of neon, carbon, nitrogen and water in micrometeorites, and isotopic composition of their Ne and H), shows that during the peak of this cataclysmic epoch (sterilization period) which occurred just after the formation of the young Earth (4.45 Ga ago), the accretion of early micrometeorites did play a major role in the formation of the terrestrial atmosphere and oceans. Later on, during the early life period (around 4 Ga ago), when liquid water and organics could condense and/or survive, micrometeorites were possibly functioning as tiny chemical reactors to synthesize the prebiotic molecules required for the origin of life. Efforts were made to start reducing the number of major speculations in this “early-micrometeorite-accretion” scenario (EMMAC), which is finally extended with some confidence to Mars, where the survival of micrometeorites upon atmospheric entry looks even more favorable than on the Earth.

Micrometeorites (MMs) represent the most common interplanetary dust particles (50-500 μm). They are similar to carbonaceous chondrites (CCs) but do not match, mineralogically and chemically, known CC types.

Since the Apollo missions, we have emphasized the following points, which are based on theoretical calculations and on laboratory studies of the properties of evaporated silicate deposits and of lunar samples. The mass of vapor generated by impacts on the lunar surface is comparable in magnitude to the mass of impact melt glasses; the physics of impact into a porous regolith requires that much of this vapor be retained in the soil rather than lost to space (as is widely believed); experimental coatings made from vaporized or sputtered lunar basalt contain abundant inclusions of submicroscopic, super paramagnetic metallic Fe; and this Fe may explain the magnetic signature, low albedo, reddened spectrum, and subdued absorption bands of lunar regolith. Our conclusions have been generally rejected by the lunar geochemical community for two reasons: there seemed to be no direct evidence for vapor deposits in Apollo samples, and it seemed that the lunar optical properties could be explained by the presence of impact melt glasses alone. However, advances in our understanding of the optical properties of glasses and of light scattering by planetary regoliths, and now the direct detection of vapor deposits, show that these objections are not valid. Vapor phase transport is a major process on the lunar surface, and unless its effects are taken into account, the chemical, magnetic, and optical properties of the regolith cannot be understood.

"Interplanetary Dust Particles" with sizes approximately 10 micrometers collected in the stratosphere (IDPs), as well as much larger "giant" micrometeorites retrieved from Antarctic ice melt water (AMMs), are mostly composed of unequilibrated assemblages of minerals, thus being related to primitive unequilibrated meteorites. Two independent evaluations of the mass flux of micrometeorites measuring approximately 50 micrometers to approximately 200 micrometers, recovered from either the Greenland or the Antarctic ice sheets have been reported (approximately 20,000 tons/a). A comparison with recent evaluation of the flux of meteorites reaching the Earth's surface (up to masses of 10,000 tons), indicates that micrometeorites represent about 99.5% of the extraterrestrial material falling on the Earth's surface each year. As they show carbon concentrations exceeding that of the most C-rich meteorite (Orgueil), they are the major contributors of extraterrestrial C-rich matter accreting to the Earth today. Moreover they are complex microstructured aggregates of grains. They contain not only a variety of C-rich matter, such as a new "dirty" magnetite phase enriched in P, S, and minor elements, but also a diversity of potential catalysts (hydrous silicates, oxides, sulfides and metal grains of Fe/Ni composition, etc.). They could have individually functioned on the early Earth, as "micro-chondritic-reactors" for the processing of prebiotic organic molecules in liquid water. Future progress requires the challenging development of meaningful laboratory simulation experiments, and a better understanding of the partial reprocessing of micrometeorites in the atmosphere.

Cosmogenic Be-10 and Al-26 were measured in a suite of stony cosmic spherules derived from deep-sea sediments and the Greenland ice cap. These spherules show clear evidence of exposure to galactic cosmic ray and solar cosmic ray bombardment on time scales from a few times 100,000 years up to as much as 10 to the 7th years. The exposure took place in the inner solar system, not in highly eccentric orbits. When they reached the earth, the particles were not much larger than their present size, but it is not excluded that most of their cosmic ray exposure took place very close to the surface of an asteroidal body.

The work reported here confirms the existence of (largely) unmelted micrometeorites in sediments from seasonal lakes on the Greenland ice sheet. Approximately1/3 of the dark and irregularly shaped particles selected from disaggregated sediments had sufficient contents of Ne to permit measurement of isotopic compositions. The sediments consisted of a mixture of biologically derived contaminants and minor amounts of terrestrial minerals and extraterrestrial fragments. Although terrestrial mineral contamination varied by a factor of 20 for the two different sediments, a yield of ∼ 1000 cosmic particles/kg of wet cryoconite was found for both samples. The isotopic compositions of the Ne in both rounded and irregular (25% of total) particles that contained enough gas to measure were non-terrestrial, plotting in a field defined by solar wind (SW), solar energetic particle (SEP) and spallation. Exposure ages ranged from

This article also appears in: Proceedings of the Seventeenth Lunar and Planetary Science Conference; International audience; Instrumental neutron activation analysis was used to determine the concentrations of Ir, Fe, Ni, Co, Cr, and Sc in 16 "iron" and 8 "chondritic" magnetic deep sea spherules with sizes of about 200$\mu$m. With the exception of two iron spheres, high iridium concentrations were found in each sphere, thus confirming their extraterrestrial origin. Moreover, the "depth profile" of the residual mass of iridium was monitored in individual spheres by comparing the variations of the intensities of the $\gamma$ ray lines of iridium, cobalt, and iron during a very gradual sequential polishing. Each sharp but partial discontinuity, observed only in the Ir profiles, reflects the intersection of a polished section with a tiny Ir-rich nugget. These profiles show that such nuggets are much more common in both the iron and the chondritic spheres than previously thought. We found new types of disequilibrated clusters of micrometer-sized nuggets of platinum metals. All types of nuggets, which were characterized with a scanning electron microscope and an energy dispersive X ray spectrometer, seem to be associated with magnetite. The compositions of the nuggets, as well as the fractionation patterns for Ir, Fe, Ni, Co, and Sc in the spheres, suggest that most of the Ir-rich iron spheres originate from iron-rich bodies.

To help assess possible water reserves on the moon derived from the solar wind that could supply a lunar base, oligoclase, ilmenite, and simulated lunar glass have been irradiated with a beam of 2.5 keV/amu deuterium ions to simulate solar wind proton bombardment of lunar materials. For ilmenite and simulated lunar glass the areal density of deuterium D(ϕ) (D/cm2) increases with the incident ion fluence ϕ until a critical reemission fluence is reached (ϕ = 3.3–3.5×1017 D/cm2). At this point deuterium begins to be lost from both targets until D(ϕ) reaches a saturation value Ds = 2–4×1017 D/cm2 at ϕ8 = 1–2 × 1018 D/cm2. Oligoclase begins losing deuterium when the beam is turned on; D(ϕ) never reaches a saturation value up to the limit of our experiment (≈ 3×103 D/cm2). There is a small yield of ∼10−4 D2O+/Dincident during the implantation. Thermal release patterns for D2 in our targets do not correspond to those observed for H2 in lunar soils, but the differences may be explainable after further experiments. Although ilmenite stores about twice as much deuterium as the other target materials, it is unknown whether this small enrichment factor will be sufficient to make it a potential source of lunar water.

The paper discusses several discoveries made in the study of lunar material. In the examination of the effects of solar wind implantations the topics covered include (1) solar wind radiation damage parameters and their aging characteristics, (2) the theory of the ancient solar wind, (3) the solar wind sputtering erosion rate, (4) the physicochemical properties of amorphous coatings, (5) maturity indexes and the macroscopic properties of the lunar regolith, (6) solar wind gas bubbles, (7) the composition of very heavy nuclei in the contemporary solar wind, and (8) track aging processes. Conclusions are drawn from the results about other extraterrestrial features such as the parent bodies of the meteorites, early solar nebulas, and interstellar clouds.

A new microprobing procedure is used to characterize both the physical and chemical surface properties of individual grains on a microscale. In this procedure the same sub-micrometre sized area of a given grain is successively analysed with a high voltage electron microscope, an Auger microprobe and a field emission scanning electron microscope. This analytical technique has been applied to study lunar weathering effects and to tentatively infer the irradiation history of a Nerich fraction of the Orgueil meteorite.

Natural crystals of feldspars, pyroxenes, quartz and apatite have been irradiated in the Berkeley HILAC with neon and argon ions of up to 10.2 MeV/amu with maximum doses of 1.6 × 1013 argon ions/cm2 and 4.0 × 1013 neon ions/cm2 respectively. The samples were thinned to thicknesses near 30 microns, stacked to form the target and then bathed completely in the ion beams. X-ray and optical observations revealed: (a) a crystal distortion and curvature with the argon irradiation similar to that produced in mica; (b) a lesser effect from neon bombardment; (c) a fracturing of the crystal samples which depended on the total dose and energy of the incident ion; (d) polygonized structures produced when either neon or argon ions (with sufficiently high concentration) were trapped within the crystals. The interpretation of the measurements, their correlation with the mechanism of track formation and implications concerning the alteration of the lunar surface due to solar particles and the exposure of lunar an...

The microscopic structure of latent tracks in silicates is analyzed using small-angle x-ray scattering methods. Latent tracks are constituted of extended defects, separated by gap zones loaded with point defects. The variation of the linear density of extended defects along the path of the incident ions cannot be scaled with functions previously used, such as the primary rate of ionization. Upon a thermal annealing, the extended defects are much more stable than point defects. These latent tracks are chemically etched and their etching rates are inferred from optical and scanning electron-microscope observations. From these combined studies of latent and etched tracks, a model for the registration of etchable tracks in silicates is developed. In this model, the extended defects dominate the chemical etching and thermal annealing behavior of etchable tracks. The marked differences observed in the sensitivity of various silicates are no longer attributed to a radiation damage mechanism, which would operate much more efficiently in specific silicates, but to the etching behavior of each mineral. Indeed, for a given incident ion, the linear density of extended defects in a given part of the ion residual range appears to be similar in all silicates. This model is used in the framework of a Monte Carlo statistical code to predict etched track-length distributions in silicates, including those relevant to partially annealed tracks. The striking agreement between these predictions and the corresponding observations strongly support our model. It enables us to discuss the most important concepts previously proposed to account for the registration of etchable tracks in silicates, and to suggest a few preliminary guidelines for improving the use of solid-state track detectors.

The weathering of rocks and minerals may involve physical, chemical or biological processes. In particular, as the work of Ehrlich1,2, Aristovska3, Webbley4 and their collaborators shows, biogenic weathering in cold and dry environments has generally been attributed to organic acid excretions from living microorganisms5,6. Here we report measurements of the etch rate of microfractures in amorphous and crystalline silicates exposed to either sterilized hot solutions or microcolonies of siderofungi. The biogenic corrosion of the fungi is much faster than that expected from organic-acid excretions alone, and is probably related to the synthesis of very efficient 'siderophores' (ferric-ion-specific ligands7) in the cell membranes, as have already been documented by cell physiologists8–11 in interpreting the fast transport of iron in cells. This process may act in the natural environment, to produce the differential weathering observed in cosmic spherules made of microscopic 'bars' of olivine and glass, found in terrestrial sediments. These results are relevant to problems ranging from the evolution of soils to the disposal of nuclear wastes.

Explosive volcanic eruptions can follow long cycles of activity1, and the material that they inject into the atmosphere may affect the climate2. We report here the discovery of ultrathin micrometre-sized glass shards from such eruptions, extracted from both ancient Antarctic ices and recent snow samples from Greenland, in which they have been well preserved by ‘deep freezing’. When such shards have been preselected using a high-voltage electron microscope, microanalysis of their eight major constituent elements gives new clues about the origin of major volcanic acid fallout previously detected in the cores (including important characteristics of their parent eruptions), and reveals complex volcanic ash layers that could reflect periods of world-wide enhanced volcanicity. Moreover, the very small Reynolds numbers of the micrometre-sized shards make them promising tracers of dust transport processes in the ancient atmosphere.

Some habit and texture features of the finest and uncrushed grains in the lunar dust samples 10084 and 12070, were studied with the 1 MeV electron microscope of the Institut d'Optique Electronique du C.N.R.S., Toulouse, France. These features were then compared with those observed in broken fragments extracted either from lunar rocks 10047 and 12063 or from different types of meteorites (Orgueil; Pesyanoe, Shergotty). The finest fraction of the Apollo 11 lunar soil samples is constituted of about 80% of well ordered but rounded crystals which are frequently overlaid by a thin skin of amorphous material. In the Apollo 12 soil the grains are coarser, the fraction of crystalline material is higher (∼90%) and the proportion of crystals showing an amorphous skin is smaller, but both lunar soil samples are very different from meteoritic or lunar rock matter. Some implications of the present results concerning the fabric of the finest fraction of the lunar regolith, the ancient solar wind and meteoritic research are briefly outlined.

UItrathin amorphous coatings have been observed by high-voltage electron microscopy on micrometer-sized dust grains from the Apollo 11, Apollo 12, Apollo 14, and Luna 16 missions. Calibration experiments show that these coatings result from an "ancient" implantation of solar wind ions in the grains. This phenomenon has interdisciplinary applications concerning the past activity of the sun, the lunar albedo, the ancient lunar atmosphere and magnetic field, the carbon content of lunar soils, and lunar dynamic processes.

Fossil nuclear particle tracks in lunar and meteoritic materials have been searched for with the 1 MeV electron microscope of the Institut d'Optique Electronique du C.N.R.S., Toulouse, both before and after a chemical etching of the samples. Evidences for very high densities of non etchable tracks of two different kinds have been observed only in the finest fraction (

We have studied the thermal annealing of fossil tracks stored in hypersthene crystals of the Johnstown meteorite. We have observed that the annealing of the tracks depend on their initial length and therefore of the charge of the incident ions. The shortest tracks are annealed at a lower temperature than the longer ones. These long tracks in turn have been found to be less thermally stable on the average than fission fragment tracks. After first pointing out Some difficulties in the study of the annealing of heavy ion tracks in solids, we discuss the possibility of using thermal annealing experiment to identify the origins of the tracks stored in naturally or artificially irradiated silicate minerals.

A concentration process occurring in the melt zone of the Greenland ice cap has produced the richest known deposit of cosmic dust on the surface of the earth. Extraterrestrial particles collected from this region are well preserved and are collectable in large quantities. The collected particles are generally identical to cosmic spheres found on the ocean floor, but a pure glass type was discovered that has not been seen in deep-sea samples. Iron-rich spheres are conspicuously rare in the collected material.

International audience; Extraterrestrial grains with sizes ≳50 µm can now be extracted in large numbers from terrestrial sediments but their origins are still uncertain. At least 99% of these grains have been thought to be spherules1,2 resulting from the melting of their parent bodies in the atmosphere, which destroyed their mineralogical composition. Here we present analyses of extraterrestrial grains extracted from six samples of cryoconite3 (black dust) collected from the melt zone of the Greenland ice cap. In addition to families of grains never reported before, we have found a surprisingly high abundance of unmelted chondritic fragments, in which the nonvolatile component of the parent bodies has been well preserved. The mass distribution of the grains is very similar to that of the micrometeorite flux at 1 AU (ref. 4), indicating that most of the grains are micrometeorites, and not ablation products of larger meteorites. These results yield new constraints on the ablation of such Greenland micrometeorites, as well as guidelines for using them as glaciological and geological markers.