Organic Matter Preserved in 3-Billion-Year-Old Lacustrine Mudstones at Gale Crater, Mars
Résumé
In situ analyses of lacustrine mudstones (Murray formation) at Pahrump Hills, Gale crater, Mars show the presence of alkyl, aromatic, and sulfur-bearing organic volatiles upon heating to 500-860°C, as detected by the Sample Analysis at Mars (SAM) instrument suite onboard the Mars Science Laboratory's Curiosity rover. Organic-sulfur compounds, including thiophenes, dimethylsulfide and thiols were identified by gas chromatography-mass spectrometry (GCMS) and indicated in evolved gas analyses (EGA). Aromatic and alkyl hydrocarbon components were indicated by EGA. The molecular distributions observed are specific to samples and cannot be explained by SAM's instrument background. The EGA observations are at temperatures consistent with the pyrolytic cleavage of organic fragments from large molecules (macromolecules) indigenous to the mudstones, and some portion could be derived from the thermal release of organics entrained within minerals Given that the release of organic-sulfur compounds occurs at high temperatures in the SAM oven, these compounds are most likely derived from sulfurized organics in the sediments and are not an artifact of sulfurization reactions in the oven. On Earth, sulfurization is a key process that aids organic matter preservation over geological time-scales by converting organic molecules into recalcitrant macromolecules. Organic matter of the lowermost Murray formation most likely underwent sulfurization either prior to deposition or during early diagenesis. Multiple factors likely attribute to organic preservation in the Gale crater mudstones, however, sulfurization may be largely responsible for the greater abundance and the molecular diversity observed in the lower Murray formation.