Variability in the sulfur isotopic composition in sediments can reflect atmospheric, geologic and biological processes. Evidence for ancient fluvio-lacustrine environments at Gale crater on Mars and a lack of efficient crustal recycling mechanisms on the planet suggests a surface environment that was once warm enough to allow the presence of liquid water, at least for discrete periods of time, and implies a greenhouse effect that may have been influenced by sulfur-bearing volcanic gases. Here we report in situ analyses of the sulfur isotopic compositions of SO2 volatilized from ten sediment samples acquired by NASA’s Curiosity rover along a 13 km traverse of Gale crater. We find large variations in sulfur isotopic composition that exceed those measured for Martian meteorites and show both depletion and enrichment in 34S. Measured values of δ34S range from −47 ± 14‰ to 28 ± 7‰, similar to the range typical of terrestrial environments. Although limited geochronological constraints on the stratigraphy traversed by Curiosity are available, we propose that the observed sulfur isotopic signatures at Gale crater can be explained by equilibrium fractionation between sulfate and sulfide in an impact-driven hydrothermal system and atmospheric processing of sulfur-bearing gases during transient warm periods.

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This work was funded by NASA’s Mars Exploration Program. The authors thank T. B. Griswold for assistance with figure preparation, B. Franz for editorial support, J. Farquhar for manuscript review, J. Farquhar and A. J. Kaufman for facilitating isotopic analyses of calibrants, and the technical team at the NASA GSFC Planetary Environments Laboratory for laboratory support.

Author information


  1. NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA

    • H. B. Franz
    • , A. C. McAdam
    • , C. Freissinet
    • , P. R. Mahaffy
    • , P. G. Conrad
    • , J. L. Eigenbrode
    • , D. P. Glavin
    • , C. A. Knudson
    • , A. A. Pavlov
    •  & J. C. Stern
  2. NASA Johnson Space Center, Houston, Texas 77058, USA

    • D. W. Ming
    • , P. D. Archer Jr
    • , R. V. Morris
    • , E. B. Rampe
    •  & B. Sutter
  3. Center for Space Science and Technology, University of Maryland Baltimore County, Baltimore, Maryland 21250, USA

    • C. Freissinet
  4. Department of Geology, University of Maryland, College Park, Maryland 20742, USA

    • D. L. Eldridge
    • , J. W. Dottin III
    •  & R. Plummer
  5. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA

    • W. W. Fischer
    • , J. P. Grotzinger
    •  & K. A. Farley
  6. Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA

    • C. H. House
  7. Department of Geosciences, Stony Brook University, Stony Brook, New York 11794, USA

    • J. A. Hurowitz
    •  & S. M. McLennan
  8. Department of Environment, Earth and Ecosystems, The Open University, Milton Keynes MK7 6AA, UK

    • S. P. Schwenzer
  9. Planetary Science Institute, Tucson, Arizona 85719, USA

    • D. T. Vaniman
  10. Jacobs Technology, Houston, Texas 77058, USA

    • P. D. Archer Jr
    •  & B. Sutter
  11. Department of Climate and Space Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA

    • S. K. Atreya
  12. Department of Biology/STIA, Georgetown University, Washington DC 20057, USA

    • S. S. Johnson
  13. Department of Astronomy, University of Maryland, College Park, Maryland 20742, USA

    • C. A. Knudson
  14. Instituto de Ciencias Nucleares, Universidad Nacional Autonoma de Mexico, Cuidad Universitaria, Mexico City 04510, Mexico

    • R. Navarro-González
  15. Aerodyne Industries, Houston, Texas 77058, USA

    • E. B. Rampe
  16. Geophysical Laboratory, Carnegie Institute of Washington, Washington DC 20015, USA

    • A. Steele
  17. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

    • R. E. Summons


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H.B.F. developed analytical methods, calculated and interpreted sulfur isotope ratios, performed calibration experiments, and wrote the manuscript and most of the Supplementary Information. A.C.M. wrote the mineralogy section of the Supplementary Information. H.B.F., A.C.M. and C.A.K. performed supporting laboratory EGA studies. C.F. contributed to analysis of calibration data. D.L.E. calculated theoretical equilibrium fractionation factors for relevant sulfur-bearing species. H.B.F., J.W.D. and R.P. performed ground-truth isotopic analyses of calibrants. All authors participated in discussion of results and/or editing of the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to H. B. Franz.

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