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Transient liquid water and water activity at Gale crater on Mars


Water is a requirement for life as we know it1. Indirect evidence of transient liquid water has been observed from orbiter on equatorial Mars2, in contrast with expectations from large-scale climate models. The presence of perchlorate salts, which have been detected at Gale crater on equatorial Mars by the Curiosity rover3,4, lowers the freezing temperature of water5. Moreover, perchlorates can form stable hydrated compounds and liquid solutions by absorbing atmospheric water vapour through deliquescence6,7. Here we analyse relative humidity, air temperature and ground temperature data from the Curiosity rover at Gale crater and find that the observations support the formation of night-time transient liquid brines in the uppermost 5 cm of the subsurface that then evaporate after sunrise. We also find that changes in the hydration state of salts within the uppermost 15 cm of the subsurface, as measured by Curiosity, are consistent with an active exchange of water at the atmosphere–soil interface. However, the water activity and temperature are probably too low to support terrestrial organisms8. Perchlorates are widespread on the surface of Mars9 and we expect that liquid brines are abundant beyond equatorial regions where atmospheric humidity is higher and temperatures are lower.

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Figure 1: Ground relative humidity, water activity and temperature at Gale crater and aqueous Ca(ClO4)2 stability conditions.
Figure 2: Surface and subsurface temperatures, night-time H2O VMR and air temperature.
Figure 3: Perennial hydration of subsurface Ca-perchlorate as 4H2O (and 5H2O for Mg-perchlorate, not shown), and moments of liquid surface stability.
Figure 4: Bottom-layer water content in correlation with the RHmax% and top-layer daytime dryness.

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We are grateful to all of the scientists and engineers who spent many years working to make the MSL mission such a success. We also acknowledge the contribution of the COSPAR Special Region Panel, and J-F. Buenestado-Castro for his support in the documentation process. Part of the research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.

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F.J.M-T. and M-P.Z. designed the study, prepared the figures, and led the writing of the paper with contributions from the rest of the authors. P.V-S. processed REMS data. A-M.H., M.G. and O.K. processed RHS/REMS data. E.G.R-V. and V.F.C. provided subsurface model simulations. I.J. and C.H. processed DAN data. A.S.M. provided RSL images. All the authors contributed to the analysis discussion, and to the writing process.

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Correspondence to F. Javier Martín-Torres.

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The authors declare no competing financial interests.

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Martín-Torres, F., Zorzano, MP., Valentín-Serrano, P. et al. Transient liquid water and water activity at Gale crater on Mars. Nature Geosci 8, 357–361 (2015).

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