Scientists think at present that the water on Mars is frozen into the ground. The ground is less frozen at low latitudes because it is warmer and water can evaporate. Thus, at low latitudes (near the equator) the frozen ground is 6 km deep, while at high latitudes (corresponding to high altitudes) the frozen ground is only 2.5 km deep.
The figure shows a cross-section of the crust, and the unusual altitude variation of the Martian surface. The figure illustrates the depth of frozen ground at various latitudes, called the cryosphere, as well as the depth to which the aquifer, or layer of liquid water, may exist across the planet. To have liquid water running on the surface of Mars, the aquifer, or liquid region, must be exposed to the surface. This may have happened at various times in the history of Mars as the climate changed.