Ancient Mars. Computer artwork of Mars at least 3 billion years ago, with its moon Phobos (at upper centre). The surface environment of ancient Mars may have allowed life forms to develop. Ancient Mars was warm, with seas covering much of its surface; it also had a thick atmosphere (blue). Water exists on modern Mars only as ice on the surface in polar regions, underground or as vapour or ice crystals in the atmosphere. The atmospheric pressure of modern Mars is only 0.6% that of Earth's, and the average surface temperature is -33 degrees Celsius. This means that it is very unlikely that life still exists on Mars. Phobos was probably an asteroid that Mars captured.