Now cold and dry, apparently void of liquid water on the surface, it is believed that Mars knew more favorable conditions for the emergence of life (including temperature and pressure) in the past. Many clues suggest that Mars possessed water in liquid form. The Viking missions provided images of the planet with valleys, interpreted as ancient river valleys. One can also discern traces of mud or ice. It is assumed that the amount of water on the planet was quite large. Three or four million years ago, Mars was much warmer and the atmospheric pressure was much larger, allowing water to remain liquid at the surface (temperature of evaporation of water is lower when the pressure is low). With such conditions, wide oceans and river systems may have existed, digging huge flooding valleys, ranging from 25 to 100 km wide and 100 to 200 km in length.

One of the main puzzles is to find out where this water is presently. The only form of water ever "seen" on the surface of the planet is located in the polar caps as ice, but in quantities too small compared to the amount of water that we suppose it may have exist in the past. Several hypotheses have been proposed to explain this disappearance. The results from the Mars Odyssey mission seem to favor the hypothesis that the water would be in the sub-surface, in the form of ice, as permafrost but there is also a possibility that it has evaporated.