Analyse moléculaire in situ d'un noyau cométaire : Développement et évaluation des performances d'un système de chromatographie en phase gazeuse pour la mission Rosetta

摘要

The cometary chemical composition is of primary interest from the prebiotic chemistry and the solar system history points of view. This thesis presents the first step of a long term program aiming at determining the chemical composition of the cometary nucleus.Among the different approaches used up today, in situ analysis is the only one able to provide direct information about the cometary nucleus. That is the reason why we chose to use this approach by participating to the development of the COmetary Sampling And Composition experiment (COSAC). This is one among the experiments aboard the Rosetta probe which is dedicated to the analysis of a comet and of its environment. Taking into account the numerous compounds probably present in the cometary nucleus, and the robustness of this analytical technique, gas chromatography (GC) appears as the most suitable one for the identification and for the quantification of the species present in the cometary nucleus. Therefore, we developed a chromatographic sub-system that will be part of the COSAC experiment.The main work consisted in making the separative part of this system, composed of several columns connected in parallel. This part was developed in sight of the separation and the identification of a wide range of species of cometary interest initially identified, from the lightest compounds (noble gases) to high molecular weight organic species (PAH). With this aim, an experimental work was led to compare the analytical properties (selectivity, efficiency) of numerous candidate chromatographic columns, within the in situ temperature conditions (isothermal between 30°C and 60°C). This study, associated to the operating constraints (presence of water, low carrier gas consumption, columns robustness...) allowed to select and optimize the characteristics of 5 different chromatographic columns. This combination corresponds to the minimal number of columns to be used to reach the objectives of the chromatographic system.Once the columns were selected, they were submitted to the different constraints associated to the space instrumentation (vibrations, temperature cycles) and to the space environment (reduced pressure, radiation). This study enabled to show that the analytical properties of the system are not significantly modified compared with those observed at atmospheric outlet pressure, and that the gain of speed of analysis induced by the reduced outlet pressure is advantageous to decrease the time of analysis. Moreover, the optimal inlet column pressure can have been fixed (150 kPa). We also showed that the chromatographic system should allow to directly identify about 75% of the compounds initially targeted.Finally, the sensitivity of the system (columns+detector) was evaluated by measuring the minimal detectable amount of a compound present in the future analyzed samples. It was estimated to be in the range 10-11 mol and 10-12 mol, corresponding to a 1 ppm volume ratio in the most favorable case, when there is enough gaseous sample to completely fill the sampling loop. This last result enables to conclude that the system developed allow to analyze trace compounds present in the cometary nucleus, and that its sensitivity is greater than the one of the spectroscopic observations of the coma (about 0.1%).Beyond the experimental work performed for the chromatographic system development, we also showed that the coupling of a column with the preparative sampling techniques included in COSAC should enable to identify chemical species which cannot be analyzed by direct GC (amino acids), or to point out refractory compounds from their thermal degradation products (poly-HCN). Finally, we present the first steps of experimental and theoretical works developed in sight of the interpretation of the future data collected by the space experiment.