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


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.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

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.


  1. 1.

    Freissinet, C. et al. Organic molecules in the Sheepbed mudstone, Gale Crater, Mars. J. Geophys. Res. Planets 120, 495–514 (2015).

  2. 2.

    Eigenbrode, J. L. et al. Organic matter preserved in 3-billion-year-old mudstones at Gale crater, Mars. Science 360, 1096–1101 (2018).

  3. 3.

    Benner, S. A., Devine, K. G., Matveeva, L. N. & Powell, D. H. The missing organic molecules on Mars. Proc. Natl Acad. Sci. USA 97, 2425–2430 (2000).

  4. 4.

    Applin, D. M., Izawa, M. R. M., Cloutis, E. A., Goltz, D. & Johnson, J. R. Oxalate minerals on Mars? Earth Planet. Sci. Lett. 420, 127–139 (2015).

  5. 5.

    Sutter, B. et al. Evolved gas analyses of sedimentary rocks and eolian sediment in Gale Crater, Mars: results of the Curiosity rover’s Sample Analysis at Mars instrument from Yellowknife Bay to the Namib Dune. J. Geophys. Res. Planets 122, 2574–2609 (2017).

  6. 6.

    Pollack, J. B., Kasting, J. F., Richardson, S. M. & Poliakoff, K. The case for a warm, wet climate on early Mars. Icarus 71, 203–224 (1987).

  7. 7.

    Mahaffy, P. R. et al. Abundance and isotopic composition of gases in the martian atmosphere from the Curiosity rover. Science 341, 263–266 (2013).

  8. 8.

    Steele, A. et al. A reduced organic carbon component in martian basalts. Science 337, 212–215 (2012).

  9. 9.

    Jakosky, B. M. & Jones, J. H. The history of Martian volatiles. Rev. Geophys. 35, 1–16 (1997).

  10. 10.

    Webster, C. R. et al. Isotope ratios of H, C, and O in CO2 and H2O of the martian atmosphere. Science 341, 260–263 (2013).

  11. 11.

    Jakosky, B. M. et al. Loss of the Martian atmosphere to space: present-day loss rates determined from MAVEN observations and integrated loss through time. Icarus 315, 146–157 (2018).

  12. 12.

    Atreya, S. K. et al. Primordial argon isotope fractionation in the atmosphere of Mars measured by the SAM instrument on Curiosity and implications for atmospheric loss. Geophys. Res. Lett. 40, 1–5 (2013).

  13. 13.

    Wray, J. J. et al. Orbital evidence for more widespread carbonate-bearing rocks on Mars. J. Geophys. Res. Planets 121, 652–677 (2016).

  14. 14.

    Ehlmann, B. L. et al. Orbital identification of carbonate-bearing rocks on Mars. Science 322, 1828–1832 (2008).

  15. 15.

    Murchie, S. L. et al. A synthesis of Martian aqueous mineralogy after 1 Mars year of observations from the Mars Reconnaissance Orbiter. J. Geophys. Res. Planets 114, E00D06 (2009).

  16. 16.

    Bibring, J. P. et al. Mars surface diversity as revealed by the OMEGA/Mars Express observations. Science 307, 1576–1581 (2005).

  17. 17.

    Christensen, P. R. et al. Mars global surveyor thermal emission spectrometer experiment: investigation description and surface science results. J. Geophys. Res. 106, 23823–23871 (2001).

  18. 18.

    Pepin, R. O. Evolution of the martian atmosphere. Icarus 111, 289–304 (1994).

  19. 19.

    Jakosky, B. M., Pepin, R. O., Johnson, R. E. & Fox, J. L. Mars atmospheric loss and isotopic fractionation by solar-wind-induced sputtering and photochemical escape. Icarus 111, 271–288 (1994).

  20. 20.

    Pepin, R. O. On the origin and early evolution of terrestrial planet atmospheres and meteoritic values. Icarus 92, 2–79 (1991).

  21. 21.

    Jakosky, B. Mars volatile evolution: evidence from stable isotopes. Icarus 94, 14–31 (1991).

  22. 22.

    Luhmann, J. G., Johnson, R. E. & Zhang, M. H. G. Evolutionary impact of sputtering of the martian atmosphere by O+ pickup ions. Geophys. Res. Lett. 19, 2151–2154 (1992).

  23. 23.

    Lundin, R., Lammer, H. & Ribas, I. Planetary magnetic fields and solar forcing: implications for atmospheric evolution. Space Sci. Rev. 129, 245–278 (2007).

  24. 24.

    Bristow, T. F. et al. Low Hesperian P CO2 constrained from in situ mineralogical analysis at Gale Crater, Mars. Proc. Natl Acad. Sci. USA 114, 2166–2170 (2017).

  25. 25.

    Bultel, B., Viennet, J.-C., Poulet, F., Carter, J. & Werner, S. C. Detection of carbonates in Martian weathering profiles. J. Geophys. Res. Planets 124, 989–1007 (2019).

  26. 26.

    Wright, I. P., Grady, M. M. & Pillinger, C. T. Carbon, oxygen and nitrogen isotopic compositions of possible Martian weathering products in EETA 79001. Geochim. Cosmochim. Acta 52, 917–924 (1988).

  27. 27.

    Halevy, I., Fischer, W. W. & Eiler, J. M. Carbonates in the Martian meteorite Allan Hills 84001 formed at 18 +/– 4 degrees C in a near-surface aqueous environment. Proc. Natl Acad. Sci. USA 108, 16895–16899 (2011).

  28. 28.

    Deines, P. Carbon isotope effects in carbonate systems. Geochim. Cosmochim. Acta 68, 2659–2679 (2004).

  29. 29.

    Craddock, R. A. & Greeley, R. Minimum estimates of the amount and timing of gases released into the martian atmosphere from volcanic eruptions. Icarus 204, 512–526 (2009).

  30. 30.

    Valley, J. W. et al. Low-temperature carbonate concretions in the Martian meteorite ALH84001: evidence from stable isotopes and mineralogy. Science 275, 1633–1638 (1997).

  31. 31.

    Chacko, T. & Deines, P. Theoretical calculation of oxygen isotope fractionation factors in carbonate systems. Geochim. Cosmochim. Acta 72, 3642–3660 (2008).

  32. 32.

    Toyota, T. et al. Oxygen isotope fractionation during the freezing of sea water. J. Glaciol. 59, 697–710 (2013).

  33. 33.

    Staudigel, P. T. et al. Cryogenic brines as diagenetic fluids: reconstructing the diagenetic history of the Victoria Land Basin using clumped isotopes. Geochim. Cosmochim. Acta 224, 154–170 (2018).

  34. 34.

    Fairen, A. G. et al. A cold hydrological system in Gale crater, Mars. Planet. Space Sci. 93-94, 101–118 (2014).

  35. 35.

    Friedman, I. & O’Neil, J. R. in Data of Geochemistry 6th edn (ed. M. Fleischer) Ch. KK (USGS, 1977).

  36. 36.

    Elsila, J. E., Charnley, S. B., Burton, A. S., Glavin, D. P. & Dworkin, J. P. Compound-specific carbon, nitrogen, and hydrogen isotopic ratios for amino acids in CM and CR chondrites and their use in evaluating potential formation pathways. Meteorit. Planet. Sci. 47, 1517–1536 (2012).

  37. 37.

    Stern, J. C. et al. The nitrate/(per)chlorate relationship on Mars. Geophys. Res. Lett. 44, 2643–2651 (2017).

  38. 38.

    Lasne, J. et al. Oxidants at the surface of Mars: a review in light of recent exploration results. Astrobiology 16, 977–996 (2016).

  39. 39.

    Davidson, J. A. et al. Carbon kinetic isotope effect in the reaction of CH4 with HO. J. Geophys. Res. Atmos. 92, 2195–2199 (1987).

  40. 40.

    Rudolph, J., Czuba, E. & Huang, L. The stable carbon isotopic fractionation for reactions of selected hydrocarbons with OH-radicals and its relevance for atmospheric chemistry. J. Geophys. Res. 105, 29329–29346 (2000).

  41. 41.

    Atreya, S. K. & Gu, Z. G. Photochemistry and stability of the atmosphere of Mars. Adv. Space Res. 16, 657–658 (1995).

  42. 42.

    Hori, Y., Murata, A. & Yoshinami, Y. Adsorption of CO, intermediately formed in electrochemical reduction of CO2, at a copper electrode. J. Chem. Soc. Faraday Trans. 87, 125 (1991).

  43. 43.

    Rahn, T. & Eiler, J. M. Experimental constraints on the fractionation of 13C/12C and 18O/16O ratios due to adsorption of CO2 on mineral substrates at conditions relevant to the surface of Mars. Geochim. Cosmochim. Acta 65, 839–846 (2001).

  44. 44.

    Bridges, J. C., Hicks, L. J. & Treiman, A. H. in Volatiles in the Martian Crust (eds J. Filiberto & S. P. Schwenzer) 89–118 (Elsevier, 2019).

  45. 45.

    Franz, H. B. et al. Large sulfur isotope fractionations in Martian sediments at Gale crater. Nature Geosci. 10, 658–662 (2017).

  46. 46.

    Webster, C. R. et al. Background levels of methane in Mars’ atmosphere show strong seasonal variations. Science 360, 1093–1096 (2018).

  47. 47.

    Franz, H. B. et al. Analytical techniques for retrieval of atmospheric composition with the quadrupole mass spectrometer of the Sample Analysis at Mars instrument suite on Mars Science Laboratory. Planet. Space Sci. 96, 99–113 (2014).

  48. 48.

    Franz, H. B. et al. Reevaluated martian atmospheric mixing ratios from the mass spectrometer on the Curiosity rover. Planet. Space Sci. 109-110, 154–158 (2015).

  49. 49.

    Franz, H. B. et al. Initial SAM calibration gas experiments on Mars: quadrupole mass spectrometer results and implications. Planet. Space Sci. 138, 44–54 (2017).

  50. 50.

    Franchi, I. A., Wright, I. P., Sexton, A. S. & Pillinger, C. T. The oxygen-isotopic composition of Earth and Mars. Meteorit. Planet. Sci. 34, 657–661 (1999).

  51. 51.

    Coplen, T. B. New IUPAC guidelines for the reporting of stable hydrogen, carbon, and oxygen isotope-ratio data. J. Res. Natl Inst. Stand. Technol. 100, 285 (1995).

  52. 52.

    Rothman, L. S. et al. The HITRAN2012 molecular spectroscopic database. J. Quant. Spectrosc. Radiat. Trans. 130, 4–50 (2013).

  53. 53.

    Mahaffy, P. R. et al. The imprint of atmospheric evolution in the D/H of Hesperian clay minerals on Mars. Science 347, 412–414 (2015).

Download references


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.

Author information




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.

Corresponding author

Correspondence to H. B. Franz.

Ethics declarations

Competing interests

The authors declare no competing interests

Additional information

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

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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.

Supplementary information

Supplementary Information

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

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

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).

Download citation

Further reading