Using the phase diagram of liquid water to search for life

Abstract

The correlation between liquid water and life may be our most reliable tool in the search for extraterrestrial life. To help develop this tool, we explore the complex relationship between liquid water, partial pressure, and solute freezing point depression on Earth and Mars and discuss the conditions under which liquid water is metastable on Mars. We establish the physical conditions for the existence of saline aqueous solutions in the pores of the martian near surface substratum. We find that thin films of near subsurface liquid water on Mars at ~-20°C could provide a viable niche for terrestrial psychrophilic halophiles. Since some martian salts can suppress the freezing point of aqueous solutions with minimal suppression of the water activity, some martian liquid water environments with a water activity above ~0.6 may also be able to support terrestrial life at temperatures as low as -30°C, ~10°C lower than the limit of terrestrial life.