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Origin of acidic surface waters and the evolution of atmospheric chemistry on early Mars


Observations from in situ experiments and planetary orbiters have shown that the sedimentary rocks found at Meridiani Planum, Mars were formed in the presence of acidic surface waters1,2,3,4. The water was thought to be brought to the surface by groundwater upwelling5,6, and may represent the last vestiges of the widespread occurrence of liquid water on Mars. However, it is unclear why the surface waters were acidic. Here we use geochemical calculations, constrained by chemical and mineralogical data from the Mars Exploration Rover Opportunity7,8,9,10, to show that Fe oxidation and the precipitation of oxidized iron (Fe3+) minerals generate excess acid with respect to the amount of base anions available in the rocks present in outcrop. We suggest that subsurface waters of near-neutral pH and rich in Fe2+ were rapidly acidified as iron was oxidized on exposure to O2 or photo-oxidized by ultraviolet radiation at the martian surface. Temporal variation in surface acidity would have been controlled by the availability of liquid water, and as such, low-pH fluids could be a natural consequence of the aridification of the martian surface. Finally, because iron oxidation at Meridiani would have generated large amounts of gaseous H2, ultimately derived from the reduction of H2O, we conclude that surface geochemical processes would have affected the redox state of the early martian atmosphere.

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Figure 1: Groundwater emergence at Meridiani Planum.
Figure 2: Oxidation timelines at Meridiani Planum.
Figure 3: Hydrogen-escape rates for Fe oxidation at Meridiani Planum.


  1. Grotzinger, J. P. et al. Stratigraphy and sedimentology of a dry to wet eolian depositional system, Burns formation, Meridiani Planum, Mars. Earth Planet. Sci. Lett. 240, 11–72 (2005).

    Article  Google Scholar 

  2. Squyres, S. W. et al. Exploration of Victoria Crater by the Mars rover Opportunity. Science 324, 1058–1061 (2009).

    Article  Google Scholar 

  3. McLennan, S. M. et al. Provenance and diagenesis of the evaporite-bearing Burns formation, Meridiani Planum, Mars. Earth Planet. Sci. Lett. 240, 95–121 (2005).

    Article  Google Scholar 

  4. Tosca, N. J. et al. Geochemical modelling of evaporation processes on Mars: Insight from the sedimentary record at Meridiani Planum. Earth Planet. Sci. Lett. 240, 122–148 (2005).

    Article  Google Scholar 

  5. Andrews-Hanna, J. C., Phillips, R. J. & Zuber, M. T. Meridiani Planum and the global hydrology of Mars. Nature 446, 163–166 (2007).

    Article  Google Scholar 

  6. Andrews-Hanna, J. C., Zuber, M. T., Arvidson, R. E. & Wiseman, S. J. Early Mars Hydrology: 1. The Meridiani playa deposits and the sedimentary record of Arabia Terra. J. Geophys. Res. 10.1029/2009JE003485 (in the press).

  7. Gellert, R. & Rieder, R. MER APXS Oxide Abundance Archive, NASA Planetary Data System, MER1/MER2-M-APXS-5-OXIDE-SCI-V1.0 (MER APXS Oxide Abundance Archive, NASA Planetary Data System, MER1/MER2-M-APXS-5-OXIDE-SCI-V1.0, 2006).

  8. Rieder, R. et al. Chemistry of rocks and soils at Meridiani Planum from the alpha particle X-ray spectrometer. Science 306, 1746–1749 (2004).

    Article  Google Scholar 

  9. Klingelhofer, G. et al. Jarosite and hematite at Meridiani Planum from Opportunity’s Mossbauer spectrometer. Science 306, 1740–1745 (2004).

    Article  Google Scholar 

  10. Morris, R. V. et al. Mossbauer mineralogy of rock, soil, and dust at Meridiani Planum, Mars: Opportunity’s journey across sulphate-rich outcrop, basaltic sand and dust, and hematite lag deposits. J. Geophys. Res. 111, E12S15 (2006).

    Article  Google Scholar 

  11. Burns, R. G. & Fisher, D. S. Iron–sulfur mineralogy of Mars–magmatic evolution and chemical-weathering products. J. Geophys. Res. 95, 14415–14421 (1990).

    Article  Google Scholar 

  12. Bigham, J. M. & Nordstrom, D. K. in Iron and Aluminum Hydroxysulphates from Acid Sulphate Waters in Sulphate Minerals Vol. 40 (eds Alpers, C. N., Jambor, J. L. & Nordstrom, D. K.) 351–403 (Mineralogical Society of America, 2000).

    Google Scholar 

  13. Tosca, N. J. & McLennan, S. M. Experimental constraints on the evaporation of partially oxidized acid-sulphate waters at the martian surface. Geochim. Cosmochim. Acta 73, 1205–1222 (2009).

    Article  Google Scholar 

  14. Burns, R. G. Rates and mechanisms of chemical weathering of ferromagnesian silicate minerals on Mars. Geochim. Cosmochim. Acta 57, 4555–4574 (1993).

    Article  Google Scholar 

  15. Burns, R. G. & Fisher, D. S. Rates of oxidative weathering on the surface of Mars. J. Geophys. Res. 98, 3365–3372 (1993).

    Article  Google Scholar 

  16. Halevy, I., Zuber, M. T. & Schrag, D. P. A sulfur dioxide climate feedback on early Mars. Science 318, 1903–1907 (2007).

    Article  Google Scholar 

  17. Knauth, L. P., Burt, D. M. & Wohletz, K. H. Impact origin of sediments at the Opportunity landing site on Mars. Nature 438, 1123–1128 (2005).

    Article  Google Scholar 

  18. McCollom, T. M. & Hynek, B. M. A volcanic environment for bedrock diagenesis at Meridiani Planum on Mars. Nature 438, 1129–1131 (2005).

    Article  Google Scholar 

  19. Niles, P. B. & Michalski, J. Meridiani Planum sediments on Mars formed through weathering in massive ice deposits. Nature Geosci. 2, 215–220 (2009).

    Article  Google Scholar 

  20. Jortner, J. & Stein, G. Photochemical evolution of hydrogen from aqueous solutions of ferrous ions.1. Reaction mechanism at low pH. J. Phys. Chem. 66, 1258–1264 (1962).

    Article  Google Scholar 

  21. Singer, P. C. & Stumm, W. Acidic mine drainage: The rate determining step. Science 167, 1121–1123 (1970).

    Article  Google Scholar 

  22. Catling, D. C., Zahnle, K. J. & McKay, C. Biogenic methane, hydrogen escape, and the irreversible oxidation of early Earth. Science 293, 839–843 (2001).

    Article  Google Scholar 

  23. Yung, Y. L. et al. HDO in the martian atmosphere—implications for the abundance of crustal water. Icarus 76, 146–159 (1988).

    Article  Google Scholar 

  24. Tian, F., Kasting, J. F. & Solomon, S. C. Thermal escape of carbon from the early martian atmosphere. Geophys. Res. Lett. 36, L02205 (2009).

    Article  Google Scholar 

  25. Jakosky, B. M. & Phillips, R. J. Mars’ volatile and climate history. Nature 412, 237–244 (2001).

    Article  Google Scholar 

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

    Article  Google Scholar 

  27. Lammer, H. et al. Atmospheric escape and evolution of terrestrial planets and satellites. Space Sci. Rev. 139, 399–436 (2008).

    Article  Google Scholar 

  28. Bibring, J. P. et al. Global mineralogical and aqueous mars history derived from OMEGA/Mars express data. Science 312, 400–404 (2006).

    Article  Google Scholar 

  29. Clark, B. C. et al. Chemistry and mineralogy of outcrops at Meridiani Planum. Earth Planet. Sci. Lett. 240, 73–94 (2005).

    Article  Google Scholar 

  30. Glotch, T. D. et al. Mineralogy of the light-toned outcrop at Meridiani Planum as seen by the Miniature Thermal Emission Spectrometer and implications for its formation. J. Geophys. Res. 111, E12S03 (2006).

    Google Scholar 

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Research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (J.A.H., R.E.M.). This work was also supported by the California Institute of Technology (W.W.F.) and by an Origins Initiative Postdoctoral Fellowship (N.J.T.). The authors thank Y. Yung, N. Heavens and J. Wilson for constructive comments.

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J.A.H. carried out data analysis, modelling and wrote the paper, W.W.F. and J.A.H. conceived the study, N.J.T. and R.E.M. contributed to modelling and W.W.F., N.J.T. and R.E.M. contributed to writing.

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Correspondence to Joel A. Hurowitz or Nicholas J. Tosca.

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The authors declare no competing financial interests.

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Hurowitz, J., Fischer, W., Tosca, N. et al. Origin of acidic surface waters and the evolution of atmospheric chemistry on early Mars. Nature Geosci 3, 323–326 (2010).

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