A cold and wet Mars
Introduction
Early Mars: warm and wet, or cold and dry? This is one of the major uncertainties in martian science. Geomorphological and sedimentological evidence of liquid water flowing and ponding on the surface of the planet covers much of the martian landscape (Parker et al., 1993, Head et al., 1998, Malin and Edgett, 2000a, Malin and Edgett, 2000b, Malin and Edgett, 2003, Clifford and Parker, 2001, Baker, 2001, Fairén et al., 2003, Fairén et al., 2009a, Squyres et al., 2004, Poulet et al., 2005, Bibring et al., 2006, Perron et al., 2007, Mustard et al., 2008), and together with the high D/H of the martian atmosphere pointing to the loss of significant amounts of water to space (Greenwood et al., 2008), collectively indicate that liquid water have been present for long times and in diverse amounts on and/or near the surface in different moments of Mars’ history. To explain the presence of liquid water, two different models have been proposed. The first one argues that the early martian climate was substantially different than that at present, capable of sustaining long-term warmer and wetter periods, if only episodically, in an Earth-like hydrological cycle with large lakes or oceans as the evaporative sources (Sagan and Mullen, 1972, Pollack et al., 1987, Baker et al., 1991, Phillips et al., 2001). In the second model, liquid water-related features on Mars have been explained as a consequence of spatially- and temporally-localized thermal energy sources in a cold and dry planet throughout its entire history (Griffith and Shock, 1997, Cabrol et al., 1997, Segura et al., 2002, Gaidos and Marion, 2003, McEwen et al., 2007), in which not even 5 bar of CO2 would have been enough to rise the temperatures above the freezing point of pure water (Colaprete and Toon, 2003).
The aim of this paper is to suggest an alternative scenario trying to make compatible the widespread geomorphological evidence for long-lasting liquid water on the surface of Mars, with the serious obstacles the climatic models find to simulate planetary conditions in which the mean atmospheric temperatures rise above the freezing point of pure water. To do so, here is considered the hypothesis that aqueous solutions on Mars have been stabilized against freezing by the accumulation of solutes, allowing liquid water to flow in an enduring cold climate.
Section snippets
The stability of liquid solutions in freezing environments on Mars
The freezing point depression of aqueous solutions is a function of chemical composition and pressure. Salts are found in the martian soil throughout the entire surface of the planet, and are the primary control on soil geochemistry, indicating that any primeval hydrosphere was probably more salty than that of early or modern Earth, and consisted of dilute water–salt brines (Fairén et al., 2009b). The role of solutes as a way to depress the melting point and allow the stability in the liquid
Cycling of liquid water on early Mars at sub-zero temperatures
After the geochemical analysis of Fairén et al. (2009b), a global hydrological cycle can be proposed to explain the long-term presence of open bodies of liquid water on a basically cold early Mars. Depending on input variables, climatic models considering the addition of greenhouse gases to the martian atmosphere offer a wide range of mean surface temperatures. Assuming carbon dioxide alone, for 2 bars of CO2 and a solar luminosity 75% that of the current value, temperatures are estimated to be
The Noachian
The hydrological cycle proposed here would have been active only in periods of dense atmosphere, as having a minimum atmospheric pressure is essential for water to form long-term valley networks, and local relatively high temperatures are required to trigger evaporation and snowfall. Following the model by Fairén et al. (2009b), mean surface temperatures in the order of 245–255 K allowed the flow of 15–35% of the planetary water inventory in the liquid form (see Table 2). Local, temporal and
Astrobiological implications of a cold and wet Mars
From a biological perspective, the presence of liquid water is the only constraint for life development on Mars given the presence of different energy sources (similar to those used on Earth, as reactions involving rock and water at very low temperatures provide geochemical energy free to support metabolism for potential martian microorganisms, see Link et al. (2005)) and alternative radiation protection mechanisms (Amils et al., 2007, Gómez et al., 2007). Terrestrial high acidic and salty
Conclusion: the case for a “cold and wet” Mars
Climate models assert that martian surface temperatures have always been maintained below 273 K, and evidences for the flow of liquid water are apparent and abundant throughout the landscape of the planet. Therefore Mars can be considered a “cold and wet” planet for its entire geological history. The hydrogeological model presented here reconciles accumulating evidence for long-lasting liquid water on the surface of Mars in the form of oceans or seas, valleys, channels, gullies, deltas, layered
Acknowledgments
Special thanks are due to L. Gago-Duport, A.F. Davila, T. Oner, C. McKay and B. Mahaney for technical and scientific advice. Comments and suggestions by two anonymous reviewers greatly improved this paper. This work has been supported by ORAU-NPP.
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