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Science 30 May 2008:
Vol. 320. no. 5880, pp. 1204 - 1207
DOI: 10.1126/science.1155432
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Reports

Water Activity and the Challenge for Life on Early Mars

Nicholas J. Tosca,1* Andrew H. Knoll,1 Scott M. McLennan2

In situ and orbital exploration of the martian surface has shown that acidic, saline liquid water was intermittently available on ancient Mars. The habitability of these waters depends critically on water activity (Formula), a thermodynamic measure of salinity, which, for terrestrial organisms, has sharply defined limits. Using constraints on fluid chemistry and saline mineralogy based on martian data, we calculated the maximum Formula for Meridiani Planum and other environments where salts precipitated from martian brines. Our calculations indicate that the salinity of well-documented surface waters often exceeded levels tolerated by known terrestrial organisms.

1 Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA.
2 Department of Geosciences, State University of New York, Stony Brook, NY 11794, USA.

* To whom correspondence should be addressed. E-mail: ntosca{at}fas.harvard.edu

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Science. ISSN 0036-8075 (print), 1095-9203 (online)