Letter

Nature 459, 401-404 (21 May 2009) | doi:10.1038/nature07978; Received 19 August 2008; Accepted 16 March 2009

Stability against freezing of aqueous solutions on early Mars

Alberto G. Fairén1, Alfonso F. Davila1, Luis Gago-Duport2, Ricardo Amils3,4 & Christopher P. McKay1

  1. Space Science and Astrobiology Division, NASA Ames Research Center, Moffett Field, California 94035, USA
  2. Departamento de Geociencias Marinas, Universidad de Vigo, Lagoas Marcosende, Vigo 36200, Spain
  3. Centro de Astrobiología, CSIC-INTA, Torrejón de Ardoz 28850, Madrid, Spain
  4. Centro de Biología Molecular Severo Ochoa, CSIC-UAM, Cantoblanco 28049, Madrid, Spain

Correspondence to: Alberto G. Fairén1 Correspondence and requests for materials should be addressed to A.G.F. (Email: alberto.g.fairen@nasa.gov).

Many features of the Martian landscape are thought to have been formed by liquid water flow1, 2 and water-related mineralogies on the surface of Mars are widespread and abundant3. Several lines of evidence, however, suggest that Mars has been cold with mean global temperatures well below the freezing point of pure water4. Martian climate modellers5, 6 considering a combination of greenhouse gases at a range of partial pressures find it challenging to simulate global mean Martian surface temperatures above 273 K, and local thermal sources7, 8 cannot account for the widespread distribution of hydrated and evaporitic minerals throughout the Martian landscape3. Solutes could depress the melting point of water9, 10 in a frozen Martian environment, providing a plausible solution to the early Mars climate paradox. Here we model the freezing and evaporation processes of Martian fluids with a composition resulting from the weathering of basalts, as reflected in the chemical compositions at Mars landing sites. Our results show that a significant fraction of weathering fluids loaded with Si, Fe, S, Mg, Ca, Cl, Na, K and Al remain in the liquid state at temperatures well below 273 K. We tested our model by analysing the mineralogies yielded by the evolution of the solutions: the resulting mineral assemblages are analogous to those actually identified on the Martian surface. This stability against freezing of Martian fluids can explain saline liquid water activity on the surface of Mars at mean global temperatures well below 273 K.

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