Stability of martian water
"Liquid water can be stable against freezing and stable against boiling, but unstable with respect to evaporation. The situation is analogous to Earth's oceans. Liquid water on the surface does not freeze because temperatures are higher than the melting point, and it does not boil because the vapor pressure corresponding to the temperature of the water is less than the surface pressure. Yet the water does evaporate because the atmosphere is not saturated. A similar situation can occur on Mars, though the odds are much more likely for boiling."
"But whether it boils or evaporates, either way the water will cool because of the heat loss and thus it will eventually freeze."