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Hydrocarbons on Mars – Subtle Signals at Yellowknife Bay


There has been much interest in the potential for Mars sedimentary rocks to host organic matter that, in turn, might hold clues to processes concerned with its formation, transport and preservation. Like the search for organics in ancient sediments on Earth, these endeavors are technically challenging because of the inherent instability of carbon compounds under conditions that exist in the regolith of both planets. Exposure to ionizing radiation and the heat associated with burial and tectonism are the main obstacles to organic matter preservation in early Earth sediments. On Mars, radiation and chemical oxidation are seen as the prime threats. Also, there could be diagenetic processes peculiar to Mars, for which no terrestrial analogs are presently known. Further, the presence of oxychlorine compounds that release O2 during pyrolysis could lead to combustion of organics during thermal desorption analyses. Fluvio-lacustrine sediments of Yellowknife Bay in Gale Crater, when analyzed by SAM using its pyrolysis with evolved gas analysis (EGA) and gas chromatography mass spectrometry (GC-MS) capability, have afforded suites of chlorohydrocarbons including chloromethane, dichloromethane, trichloromethane, chloromethylpropene, and chlorobenzene1, 2. It can be argued that known SAM instrument background, formed from organic materials within the chromatographic columns, hydrocarbon traps and wet chemistry capability of SAM, can react with gases released from hydrated perchlorates present in the sediments to yield the observed mixtures. However, two compounds, chloromethane and dichloromethane, were also identified by the GC-MS instruments on the 1976 Viking missions3 suggesting that some proportion of the observed organics could be derived from precursors present in the Mars regolith as supported from terrestrial analog studies4. To address these alternatives, the SAM science team is devising and conducting experiments on Mars that involve varying how sediment samples are transferred to, and analyzed by, SAM. Simultaneously, we are conducting laboratory analog experiments to discriminate between the various possible sources of the chlorohydrocarbons identified so far. 1. Leshin, L. A. et al. (2013), Science, in press. 2. Glavin D.P. et al. (2013) JGR Planets 118, doi: 10.1002/jgre.20144. 3. Biemann et al. (1977) JGR 82, 4641–4658. 4. Navarro-González et al. (2010) JGR 115, E12010.