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

Abstract

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 O2, which provide clues to the identities and origins of carbon- and oxygen-bearing phases in surface materials. We find that O2 is enriched in 18O (δ18O about 40‰). Its behaviour reflects the presence of oxychlorine compounds at the Martian surface, common to aeolian and sedimentary deposits. Peak temperatures and isotope ratios (δ18O from −61 ± 4‰ to 64 ± 7‰; δ13C from –25 ± 20‰ to 56 ± 11‰) 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.

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Fig. 1: CO2 isotopic composition displayed in stratigraphic context.
Fig. 2: CO2 isotopic composition (δ18O and δ13C) versus associated evolved gas analysis (EGA) sample peak temperature.
Fig. 3: Sketch displaying major processes and environments that affected the isotopic composition of carbon and oxygen in Gale crater samples.

Data availability

All SAM data are available at NASA’s Planetary Data System.

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Acknowledgements

This work was funded by NASA’s Mars Exploration Program. We thank T. B. Griswold for figure production, R. H. Becker for discussion, and the technical team at the NASA Goddard Space Flight Center Planetary Environments Laboratory for laboratory support.

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Authors

Contributions

H.B.F. developed QMS analytical methods, calculated QMS isotope ratios, interpreted results, performed calibration experiments, and wrote the manuscript and the Supplementary Information. C.R.W. and G.L.F. developed TLS analytical methods and calculated TLS isotope ratios. E.R., C.F. and M.M. assisted with QMS data analysis. H.B.F., A.C.McA., C.A.K., P.D.A. and J.M.T.L. performed supporting laboratory experiments. J.C.S. performed ground-truth isotopic analyses of calibrants. All authors participated in discussion of results or editing of the manuscript.

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Correspondence to H. B. Franz.

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Peer review information Nature Astronomy thanks Alberto Fairen and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 CO2 and O2 evolved from Gale crater samples.

CO2 (isotopologue at m/z 45) evolved from aeolian (a), mudstone (except Vera Rubin Ridge) (b), sandstone (c) and Vera Rubin Ridge samples (d). e, O2 evolved from RN and CB samples. Data for samples in panels ac have been normalized to a single portion aliquot.

Extended Data Fig. 2 Laboratory CO2 data.

CO2 profile (isotopologue at m/z 44 or 45) from EGA analyses with laboratory test stands: carbonates (a); oxalates (b); acetates (c); benzoic and mellitic acids (d). The peak temperatures of CO2 evolved from Martian samples by SAM are compared with those from laboratory runs such as these to help identify the mineral phases present. The CaCO3 was the same synthetic material used for SAM flight model calibration.

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Supplementary Information

Supplementary Discussion, Supplementary Figures 1–4, and Supplementary Tables 1–6.

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Franz, H.B., Mahaffy, P.R., Webster, C.R. et al. Indigenous and exogenous organics and surface–atmosphere cycling inferred from carbon and oxygen isotopes at Gale crater. Nat Astron 4, 526–532 (2020). https://doi.org/10.1038/s41550-019-0990-x

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