Caractérisation de la matière organique de la comète 67P/Churyumov-Gerasimenko

Abstract : Between August 2014 and September 2016, the time-of-flight secondary ion mass spectrometer COSIMA (COmetary Secondary Ion Mass Analyser) on board the Rosetta orbiter (Rosetta space mission), performed in situ analyses of the dust particles ejected from the comet 67P/Churyumov-Gerasimenko (67P). First results have confirmed the presence in the cometary dust of a high-molecular-weight organic matter that could show similarities with the IOM (Insoluble Organic Matter) extracted from meteorites. The aim of this thesis is to describe even more accurately the organic matter of the 67P cometary particles. To achieve this goal, I have analyzed numerous organic samples using the two ground models of COSIMA. The comparison between the mass spectra acquired on these calibrations samples and those obtained in situ on the cometary particles gave new insights on the cometary organic matter. From analyses performed with the ground models on a large set of carbonaceous terrestrial and extra-terrestrial analogs (respectively sedimentary rocks and IOMs), I deduced the H/C elemental ratio of 33 individual cometary particles. I found that the H/C elemental ratio of the 67P cometary particles was on average equal to 1.04±0.16, which is generally higher than the H/C measured in IOMs extracted from carbonaceous chondrites. This result suggests that the organic matter of 67P cometary particles is more hydrogenated than the meteoritic IOMs. Moreover, further studies involving semi-quantitative analyses have shown that the cometary organic matter would be an immature material, resembling either the IOMs extracted from the most unaltered chondrites or the unprocessed sedimentary rocks. Based on the interstellar ice model, the cometary organic component could be the result of irradiation and heating of ice mixtures usually observed in the interstellar medium. For this reason, I synthesized complex organic material using an experimental setup that can carry out ultraviolet photolysis and heating on ice mixtures. Once produced, these organic residues were analyzed using the ground models of COSIMA. The photolysis at low temperature produces a refractory residue whose the spectrum contains the spectral signature of the HMT (HexaMethyleneTetramine) which is a molecule that has not been detected in the 67P cometary particle mass spectra. The photolysis at both low and room temperatures would produce an organic residue depleted in HMT compared to hydrocarbonaceous compounds. However, because of the presence of an organofluorine contaminant, this last result must be confirmed by synthesizing new and non-contaminated residues. Thus, the data treatment of spectra acquired on the cometary particles of 67P, and, the spectral comparison between these cometary particles and several natural and synthetic carbonaceous samples, have deepened our understanding of the nature of the cometary organic matter, its chemical formation and evolution.
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Robin Isnard. Caractérisation de la matière organique de la comète 67P/Churyumov-Gerasimenko. Chimie. Université Paris Est, 2019. Français. ⟨tel-02395920⟩

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