Mars may have lost to space a substantial amount of its atmosphere, including carbon dioxide, nitrogen, and water vapor. While some of the water may have been irretrievably lost to space, most of it may still reside in various reservoirs on Mars. Among these are a permafrost layer up to 1 km thick, clay minerals and oxidation products, and layered deposits at the poles. Taken together, these reservoirs could hold the equivalent of a layer of water around 0.5 km thick, if spread uniformly over the martian surface. The presence of water fulfills an essential condition for the evolution of life on Mars.
Water is also a key resource for any future manned mission to the Red Planet.