On the possibility of liquid water on present-day Mars

Using a validated general circulation model, we determine where and for how long the surface pressure and surface temperature on Mars meet the minimum requirements for the existence of liquid water in the present climate syst em: pressures and temperatures above the triple point of water but below th e boiling point. We find that for pure liquid water, there are five "favora ble" regions where these requirements are satisfied: between 0 degrees and 30 degreesN in the plains of Amazonis, Arabia, and Elysium; and in the Sout hern Hemisphere impact basins of Hellas and Argyre. The combined area of th ese regions represents 29% of the planet's surface area. In the Amazonis re gion these requirements are satisfied for a total integrated time of 37 sol s each Martian year. In the Hellas basin the number of degree days above ze ro is 70, which is well above those experienced in the dry valley lake regi on of Antarctica. These regions are remarkably well correlated with the loc ation of Amazonian paleolakes mapped by Cabrol and Grin [2000] but are poor ly correlated with the seepage gullies found by Malin and Edgett [2000]. In both instances, obliquity variations may play a role. For brine solutions the favorable regions expand and could potentially include most of the plan et for highly concentrated solutions. Whether liquid water ever forms in th ese regions depends on the availability of ice and heat and on the evaporat ion rate. The latter is poorly understood for low-pressure CO2 environments but is likely to be so high that melting occurs rarely, if at all. However , even rare events of liquid water formation would be significant since the y would dominate the chemistry of the soil and would have biological implic ations as well. It is therefore worth reassessing the potential for liquid water formation on present day Mars, particularly in light of recent Mars G lobal Surveyor observations.