1. Centro de Biología Molecular, CSIC–Universidad Autónoma de Madrid, 28049-Cantoblanco, Madrid, Spain
2. Centro de Astrobiología (CSIC–INTA), 28850-Torrejón de Ardoz, Madrid, Spain
3. Department of Hydrology and Water Resources, University of Arizona, Tucson, Arizona 85721, USA
4. Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721, USA

Correspondence to: Alberto G. Fairén¹ Email: agfairen@cbm.uam.es

Abstract

Several lines of evidence have recently reinforced the hypothesis that an ocean existed on early Mars^{1, 2, 3, 4, 5, 6, 7}. Carbonates are accordingly expected to have formed from oceanic sedimentation of carbon dioxide from the ancient martian atmosphere^{7, 8}. But spectral imaging of the martian surface has revealed the presence of only a small amount of carbonate, widely distributed in the martian dust⁹. Here we examine the feasibility of carbonate synthesis in ancient martian oceans using aqueous equilibrium calculations. We show that partial pressures of atmospheric carbon dioxide in the range 0.8–4 bar, in the presence of up to 13.5 mM sulphate and 0.8 mM iron in sea water⁸, result in an acidic oceanic environment with a pH of less than 6.2. This precludes the formation of siderite, usually expected to be the first major carbonate mineral to precipitate⁸. We conclude that extensive interaction between an atmosphere dominated by carbon dioxide and a lasting sulphate- and iron-enriched acidic ocean on early Mars is a plausible explanation for the observed absence of carbonates.

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