A. Toppani

Accretion of neon, organics, CO2, nitrogen and water from large interplanetary dust particles on the early Earth

Abstract 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.

Factors controlling compositions of cosmic spinels: application to atmospheric entry conditions of meteoritic materials

Abstract During their deceleration through the Earths atmosphere, meteoritic materials, i.e., interplanetary dust particles, micrometeorites and meteorites, experience thermal shocks which may alter their pristine mineralogy, texture or chemical characteristics. Among these changes, one of the most ubiquitous is the formation of spinels resulting from partial melting and subsequent crystallization of the meteoritic material. These “cosmic spinels” differ from terrestrial spinels by their high Ni and Fe3+ contents and show large variations in composition. In order to better understand the factors controlling their chemistry, pulse-heating experiments simulating atmospheric entry of extraterrestrial objects were carried out using Orgueil samples as proxies of meteoritic material. Covering a large range of experimental conditions (temperature 500°C We also show that, due to their fast crystallization kinetics, cosmic spinels can record through their composition, i.e., Al2O3 contents and FeO/Fe2O3 ratio, the diverse conditions of the atmosphere crossed by the extraterrestrial object during its fall towards the Earths surface. Chemistry of cosmic spinels is thus a powerful tool for constraining the entry conditions in the Earths atmosphere of any extraterrestrial object, including altitude of deceleration, entry angle and incident velocity. These in turn, may provide valuable information on the origin of the extraterrestrial material.

A 'dry' condensation origin for circumstellar carbonates.

The signature of carbonate minerals has long been suspected in the mid-infrared spectra of various astrophysical environments such as protostars. Abiogenic carbonates are considered as indicators of aqueous mineral alteration in the presence of CO2-rich liquid water. The recent claimed detection of calcite associated with amorphous silicates in two planetary nebulae and protostars devoid of planetary bodies questions the relevance of this indicator; but in the absence of an alternative mode of formation under circumstellar conditions, this detection remains controversial. The main dust component observed in circumstellar envelopes is amorphous silicates, which are thought to have formed by non-equilibrium condensation. Here we report experiments demonstrating that carbonates can be formed with amorphous silicates during the non-equilibrium condensation of a silicate gas in a H2O-CO2-rich vapour. We propose that the observed astrophysical carbonates have condensed in H2O(g)-CO2(g)-rich, high-temperature and high-density regions such as evolved stellar winds, or those induced by grain sputtering upon shocks in protostellar outflows.