Model of Martian cryosphere and underlying hydrosphere. The upper surface shows the latitudinally averaged mean elevations. The surface of the basement is 10 km below the ground surface. The thickness of the cryosphere is taken from the atmospheric, seasonal and orbital parameters. The water capacities shown are for a 50 percent surface porosity model. Liquid water might still be trapped beneath the ice at depth.

Source: NASA, Text adpated from Carr, 1966. Water on Mars.