Water on Mars. Cutaway artwork of a section of the Martian crust showing water ice (blue) trapped in cracks below the surface. Mars is a barren planet of rock and has no liquid water. Water does exist there, but in the form of ice below ground and in the polar regions. It is thought that water once flowed on the surface of Mars, accounting for the erosion features seen. The water was lost to space because of the combined effect of the thin Martian atmosphere and weak Martian gravity. The remnants could be an important source of water for missions to Mars and for efforts to terraform Mars (make it more like Earth).