Images of geomorphological features that seem to have been produced by the action of liquid water have been considered evidence for wet surface conditions on early Mars1. Moreover, the recent identification of large deposits of phyllosilicates, associated with the ancient Noachian terrains2,3 suggests long-timescale weathering4 of the primary basaltic crust by liquid water2,5. It has been proposed that a greenhouse effect resulting from a carbon-dioxide-rich atmosphere sustained the temperate climate required to maintain liquid water on the martian surface during the Noachian6,7. The apparent absence of carbonates and the low escape rates of carbon dioxide8, however, are indicative of an early martian atmosphere with low levels of carbon dioxide. Here we investigate the geochemical conditions prevailing on the surface of Mars during the Noachian period using calculations of the aqueous equilibria of phyllosilicates. Our results show that Fe3+-rich phyllosilicates probably precipitated under weakly acidic to alkaline pH, an environment different from that of the following period, which was dominated by strongly acid weathering9 that led to the sulphate deposits identified on Mars10,11,12. Thermodynamic calculations demonstrate that the oxidation state of the martian surface was already high, supporting early escape of hydrogen. Finally, equilibrium with carbonates implies that phyllosilicate precipitation occurs preferentially at a very low partial pressure of carbon dioxide. We suggest that the possible absence of Noachian carbonates more probably resulted from low levels of atmospheric carbon dioxide, rather than primary acidic conditions13. Other greenhouse gases may therefore have played a part in sustaining a warm and wet climate on the early Mars.
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This work was supported by a grant from the Arkansas Space Center and the Arkansas Space Grant Consortium.
Author Contributions V.C. made the calculations. V.C., F.P. and J.-P.B. wrote the manuscript.
Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.
This file contains the Supplementary Table 1, which summarizes all the thermodynamic values used in the calculations of the equilibriums, as well as the corresponding references. The sections A, B, C and D describe all the thermodynamic equilibrium equations used to calculate the stability diagrams presented in the figures 1 to 4. (PDF 401 kb)
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Chevrier, V., Poulet, F. & Bibring, J. Early geochemical environment of Mars as determined from thermodynamics of phyllosilicates. Nature 448, 60–63 (2007). https://doi.org/10.1038/nature05961
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