On the basis of temperature data from the Viking orbiter mission and year-round pressure readings from the Viking Lander coupled with recent topography data, we have computed the locations and times on Mars that water is stable in liquid form. Water is stable over large areas on the edge of the northern lowlands and in the Isidis, Argyre, and Hellas plains. Moreover, in about 50% of these areas, liquid water may be possible during more than 5% of the martian orbit. These results do not indicate that water is present at these locations, only that, if it were present and heat sources were sufficient to bring the water in thermal equilibrium with the surface, the resulting liquid would be stable against freezing or boiling.
Given the dryness of the martian atmosphere, any liquid water present at the sites we have located is likely to be in the form of thin films that result from the melting of seasonal or nighttime frost deposits. The low water content of the atmosphere implies that even if the liquid is stable, it would evaporate in the atmosphere on a relatively short time scale (e.g., refs. 3 and 4). However, even such transient films of water could have important geochemical implications. The few minutes each year during which a film of liquid water is present at these sites could completely dominate the chemistry of the surface because of the importance of liquid water in mediating chemical weathering. Important reactions that might depend on the presence of liquid water include hematite formation, carbonate formation, and the acid weathering that appears to have occurred on Mars.
Thus, the sites we have identified as locations of liquid water stability may have high soil concentrations of hematite and carbonate. The analysis of samples from these sites could reveal the presence of these interesting minerals and test for the action of liquid water in the chemical weathering of Mars at the present.
Although chemically important, thin films of transient liquid water are not likely to provide suitable sites for life. However, there is clear evidence that in the past, climate conditions on Mars may have been more clement. Sites that are currently just able to support liquid water may have been more favorable in the past and are thus good targets for a search for evidence of past life.
The relation between present sites for liquid water and fluvial activity in the past may explain the favorable correlation between locations that allow for the existence of liquid water today and the distribution of fluvial channels. This correlation may indicate that the formation of fluvial features is controlled by the atmospheric conditions that allow for liquid water stability rather than by the distribution of geothermal sources of ground water. In locations where liquid water is unstable to freezing or boiling, the surface outflows do not propagate, and no fluvial feature is formed.
The fluvial features reported by Malin and Edgett (1) seem to be the youngest such features on the planet, and it is of interest to compare the distribution of these features (figure 1 of ref. 1) with our regions of liquid water stability (Fig. 4). With the exception of the Dao Vallis on the northeastern rim of the Hellas Basin, there is no overlap. Because our stability criteria were for pure water, we suggest that the liquid that formed the recent fluvial features was more stable than liquid water. This would be consistent with saturated brine solutions. For example, NaCl brines could flow for temperatures down to 
20°C and pressures to 1 millibar. Interestingly, the only permafrost springs on Earth that are not driven by volcanic heat sources have salt-rich outflows (15). We would expect, therefore, that large salt accumulations may be associated with these young martian outflows.
Our preliminary analysis has pointed to the role of atmospheric pressure and surface temperature in controlling the stability of liquid water. Our analysis was based entirely on remote sensing data sets. Further analysis combining these data with martian global circulation models could produce more refined estimates of liquid water stability and more confident extrapolations to conditions under a thicker, warmer atmosphere in the past or future.
Acknowledgements
Work for this article was funded by the Fundamental Biology Program, National Aeronautics and Space Administration.
Abbreviations
GIS, Geographical Information System; MOLA, Mars Orbital Laser Altimeter; MGS, Mars Global Surveyor; IRTM, Infrared Thermal Mapper; MAWD, Mars Atmospheric Water Detector.
Footnotes
¶ To whom reprint requests should be addressed. E-mail: cmckay@mail.arc.nasa.gov.
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