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Communication Dans Un Congrès Année : 2021

Organic matter on Mars - Inventory and implications

Résumé

One of the biggest challenge for the detection of organics on extraterrestrial environments is the preservation potential of the molecules at the surface and subsurface given the exposure to harsh radiation conditions and oxidants. The Sample Analysis at Mars (SAM) investigation on NASA's Curiosity rover and the Mars Organic Molecule Analyzer (MOMA) instrument on the upcoming ESA/Roscosmos ExoMars "Rosalind Franklin" rover are both analytical laboratories that seek to detect organic molecules and potential biomarkers in the martian near-surface. SAM has been analyzing martian samples since 2012 from 21 scooped or drilled locations. Molecules were detected at various locations in Gale crater in the form of, 1) Chlorinated and sulfurized organic compounds from pyrolyzed samples rich in clays and sulfates [1-3]; 2) Long-chain hydrocarbons (decane and dodecane) with the use of an "opportunistic derivatization" protocol during which interferences with perchlorates were minimized [4]; and 3) Detection of co-evolving CO and CO2 at temperatures compatible with oxidation or decarboxylation of organics [5]. The origins of the molecules detected on Mars so far remain unclear. Organic compounds in the ancient sedimentary rocks could have formed on Mars from igneous, hydrothermal, atmospheric, or biological processes or have been delivered directly to Mars via meteorites and comets. Likewise, the exact nature of the precursor molecules is unclear; laboratory experiments are crucial. Results obtained on a laboratory analog of the Cumberland (CB) sample in Gale crater suggest benzoic acid as the preferred precursor for the martian chlorobenzene detected. Other laboratory investigations suggest that fatty acids are a likely precursor of the long-chain martian hydrocarbons [4]. Those carboxylic acids will be targeted in an upcoming first SAM thermochemolysis experiment using tetramethylammonium hydroxide (TMAH) [6]. The detection of up to 300 ppb of chlorobenzene in the CB sample, composed of 20 % of smectite [7], supports the assertion that clays are good candidates for accumulating and preserving organics over geological timescales. The CB sample was found to have a young surface exposure age of 78 +/- 30 million years [8], which indicates that recently-exposed Mars' surface samples can contain organic matter. The ExoMars rover is designed to enhance the potential detection, abundance and diversity of organic molecules on Mars. The rover's sampling drill will reach as deep as two meters, accessing increasingly preserved materials to be analyzed by MOMA and other instruments. In addition, ExoMars can investigate the unknown gradient of oxychlorine vs. depth within the regolith at the planned landing site in Oxia Planum. MOMA's complementary combination of laser desorption mass spectrometry (LD-MS) and gas chromatography mass spectrometry (GC-MS) techniques [9] seek to maximize the scientific return of this sampling strategy via with both detection of high molecular weight, non-volatile organics and patterns with the LD-MS, and the identification of volatile and semi-volatile lower molecular weight molecules with the GC-MS. The ability to detect organic compounds in martian sedimentary rocks is a function of their initial abundance and entrainment as the rock formed as well as the extent of subsequent degradation during diagenesis, exhumation and exposure to the surface and near-surface radiation. Their actual detection depends on the performance, flexibility, and robustness of instruments operating in the martian environment. In situ investigations with SAM and MOMA provide significant progress towards understanding the presence and diversity of molecular biosignatures in Mars samples, and towards mapping out taphonomic windows of preservation for chemically reduced organic compounds. Lessons learned will feed forward to planning of future experiments, increase our ability to select future landing sites in the search for traces of life, and select the best samples to return to Earth for more thorough analyses.
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insu-04471712 , version 1 (21-02-2024)

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Caroline Freissinet, Arnaud Buch, Daniel Glavin, Paul Mahaffy, Brad Sutter, et al.. Organic matter on Mars - Inventory and implications. 43rd COSPAR Scientific Assembly, Jan 2021, Sydney, Australia. pp.id.374. ⟨insu-04471712⟩
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