Liquid water cannot exist on the surface of Mars with its present low atmospheric pressure, except at the lowest elevations for short periods
[11][12] but water ice is in no short supply, with two polar ice caps made largely of ice.
[13] In March 2007, NASA announced that the volume of water ice in the south polar ice cap, if melted, would be sufficient to cover the entire planetary surface to a depth of 11 metres.
[14] Additionally, an ice
permafrost mantle stretches down from the pole to latitudes of about 60°.
[13]
http://en.wikipedia.org/wiki/Mars