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Évolution des glaces et des composés organiques interstellaires et cométaires : étude expérimentale et analyse des données VIRTIS/ROSETTA

Abstract : Comets are remnants of the "solar nebula", i.e. the protoplanetary disk from which our solar system formed. They are composed of ice and dust containing minerals and organic materials and molecules. The comets' composition reflects at the first order that of interstellar ices. However, this relationship has not been proven. During the pre-accretional phase, the comet material has potentially undergone the action of many physical processes (heat, UV and particule radiation) which have undoubtedly altered its structure and composition.This thesis objective is to better understand, through laboratory experiments, the effect of heating and ion irradiation processes on cometary matter analogues (ice and carbonaceous materials). Some of these experiments focuses on the evolution of the organic molecules deuteration during ice heating. They were conducted at the IPAG (Institute of Planetology and Astrophysics of Grenoble) and the PIIM (Physics Laboratory of Ionic and Molecular Interaction) in Marseille. They demonstrate that isotopic equilibration takes place during crystallization, on a timescale of an hour at temperatures above 120 K, for the chemical groups capable of forming hydrogen bonds with water (OH and NH2 chemical groups). Thus, this equilibration erases the initial fractionation of certain molecules or certain chemical groups, during the post-sublimation phase. This result explains in particular the selective deuteration of isotopologues methanol in protostars hot cores.A second part of the thesis focused on the study of the formation of refractory organic matter found in asteroids and comets. Heating and irradiation experiments were conducted at the IPAG, at the GANIL (Grand National Accelerator heavy ion) in Caen, and the CSNSM (Nuclear Sciences Centre and the Material Sciences) in Orsay. We show that heating simple precursors above 400° C allows the formation of disordered polyaromatic carbons whose structure is close to that observed in the primitive objetcs of the solar system. Ion irradiation effects are more difficult to assess because they depend on the stopping power of precursors target and projectile ions. Nevertheless, constraints on maximum nuclear doses received by primitive matter are deducted (D>14 eV/atom). Ultimately, these simulations demonstrate that heating processes appear to be much more likely than radiolytic processes, but a combination of the two can not be ruled out.
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Mathilde Faure. Évolution des glaces et des composés organiques interstellaires et cométaires : étude expérimentale et analyse des données VIRTIS/ROSETTA. Astrophysique [astro-ph]. Université Grenoble Alpes, 2016. Français. ⟨NNT : 2016GREAY084⟩. ⟨tel-01693407⟩



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