Indigenous and exogenous organics and surface-atmosphere cycling inferred from carbon and oxygen isotopes at Gale crater

The pyrolysis experiments of the SAM instrument on board the Curiosity rover reconstruct the origin of organics at Gale crater. Some of them come from meteorites, but others have been formed in situ, with widespread past formation of carbonates via cryogenesis. More than 0.5 bar of CO2 might have precipitated from the atmosphere. Since landing at Gale crater, Mars, in August 2012, the Curiosity rover has searched for evidence of past habitability, such as organic compounds, which have proved elusive to previous missions. We report results from pyrolysis experiments by Curiosity's Sample Analysis at Mars (SAM) instrument, focusing on the isotopic compositions of evolved CO2 and O-2, which provide clues to the identities and origins of carbon- and oxygen-bearing phases in surface materials. We find that O-2 is enriched in O-18 (delta O-18 about 40 parts per thousand). Its behaviour reflects the presence of oxychlorine compounds at the Martian surface, common to aeolian and sedimentary deposits. Peak temperatures and isotope ratios (delta O-18 from -61 +/- 4 parts per thousand to 64 +/- 7 parts per thousand; delta C-13 from -25 +/- 20 parts per thousand to 56 +/- 11 parts per thousand) of evolved CO2 indicate the presence of carbon in multiple phases. We suggest that some organic compounds reflect exogenous input from meteorites and interplanetary dust, while others could derive from in situ formation processes on Mars, such as abiotic photosynthesis or electrochemical reduction of CO2. The observed carbonate abundances could reflect a sink for about 425-640 millibar of atmospheric CO2, while an additional 100-170 millibar could be stored in oxalates formed at the surface. In addition, oxygen isotope ratios of putative carbonates suggest the possibility of widespread cryogenic carbonate formation during a previous era.