History of water on Mars: a biological perspective.

We divide the history of water on the Martian surface into four epochs based upon the atmospheric temperature and pressure. In Epoch 1, during which a primordial CO2 atmosphere was actively maintained by impact and volcanic recycling, we presume the mean annual temperature to have been above freezing, the pressure to have exceeded one atmosphere, and liquid water to have been widespread. Under such conditions, similar to early Earth, life could have arisen and become abundant. After this initial period of recycling, atmospheric CO2 was irreversibly lost due to carbonate formation and the pressure and temperature declined. In Epoch II, the mean annual temperature fell below freezing but peak temperatures would have exceeded freezing. Ice covered lakes, similar to those in the McMurdo Dry Valleys of Antarctica could have provided a habitat for life. In Epoch III, the mean and peak temperatures were below freezing and there would have been only transient liquid water. Microbial ecosystems living in endolithic rock "greenhouses" could have continued to survive. Finally, in Epoch IV, the pressure dropped to near the triple point pressure of water and liquid water could no longer have existed on the surface and life on the surface would have become extinct.