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Communication Dans Un Congrès Année : 2021

Pyrolysis of organic molecules in the resence of chlorides: implications for measurements performed with the SAM experiment in Gale crater, Mars

Cyril Szopa
Rafael Navarro-González
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Michel Cabane
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Daniel P. Glavin
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Caroline Freissinet
Paul R. Mahaffy
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Résumé

Results from Chemcam, SAM, and APXS instruments onboard the Curiosity rover indi-cate that chlorides are present on Mars at Gale Crater, [1,2,3]. Even if no evaporitic layer of chlorides was found, these chemical species could represent a non-negligible fraction of chlorine bearing inorganic mole-cules present at the near surface of the crater floor. La-boratory analog studies have demonstrated that ox-ychlorine phases in Gale Crater material could serve as an O2 and HCl source that causes the oxidation and chlorination of organic molecules present in samples analyzed by pyrolysis. SAM analysis detected chlo-rine bearing organic molecules in the Cumberland sample pyrolyzed at low temperatures (<400°C) [4,5]. This demonstrated that salts can interact with organic molecules present in a sample analyzed by pyrolysis. Total amount of chlorine calculated from APXS analy-sis is always greater than SAM detected total Cl amount of chlorine from oxychlorine phases indicating that chlorides likely dominate chlorine in the Gale Crater materials. The goal of this work was to evaluate Mars analog chloride reactions with organic molecules to determine if the SAM detected chlorinated organics would form. Moreover, other studies showed that other salts, as sulfates, can play a role of protection against the effect of oxychlorines during the pyrolysis [5,6,7]. Indeed, those salts start decomposing at higher temperatures than oxychlorines. Thus, organic molecules embedded in the salt matrix can be isolated from the gases re-leased at low temperatures, preventing their reaction and alteration. Then, their protective role toward the effect of oxychlorine can be a key for detecting organic molecules indigenous to Mars. The understanding of those reaction could also be a key to understand the origin of the organic molecules detected by SAM. For these reasons, our team performed a systematic study in the laboratory of the possible influence of different chlorides on the analysis and detection of organic molecules. Then we performed pyrolysis gas chromatography mass spectrometry analysis, in order to assess their possible role in the analyses performed with the SAM experiment in Gale crater, and potential-ly future in situ experiments that would use such an analytical technique. NaCl, CaCl2 and MgCl2 were used as the reference chlorides. Their thermal be-havior was studied using evolved gas analysis (EGA). More particularly their thermal decomposition was characterized as a function of the temperature up to about 900°C. Both chloride was then mixed with dif-ferent organic molecules of interest for study of the Mars surface, i.e. polyaromatic hydrocarbons, amino acids, and carboxylic acids. Each mixture was submit-ted to two different pyrolytic treatments : i. a SAM like pyrolysis heating the sample from the ambient temperature up to 850°C at a 35°C.min-1 rate; ii. a flash pyrolysis at 850°C. The products of pyrolysis were analyzed using gas chromatography mass spec-trometry (GCMS) and compared. Implications: The products of pyrolysis identified are compared to those detected with the SAM experi-ment. In particular, an emphasis will be done on the chlorinated organic molecules identified with SAM when analyzing the Cumberland sample collected in the Yellow Knife bay area early in the mission. Con-clusions about the possible influence of chlorides on GCMS analyses of Mars surface samples will be done. Acknowledgments: NASA Mars Exploration Program supported for the development and operations of SAM. The French National Space Agency, the Centre National d'Etudes Spatiales (CNES), supported the development and operations of the SAM gas chromatograph
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insu-04466956 , version 1 (19-02-2024)

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Cyril Szopa, Rafael Navarro-González, Arnaud Buch, Michel Cabane, Daniel P. Glavin, et al.. Pyrolysis of organic molecules in the resence of chlorides: implications for measurements performed with the SAM experiment in Gale crater, Mars. 43rd COSPAR Scientific Assembly, Jan 2021, Sydney, Australia. ⟨insu-04466956⟩
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