The drawing 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.
The Martian geography is one of high altitudes at high southern latitudes and low altitudes near the equator. The ground is less frozen at the lower latitudes because it is warmer near the equator and water can evaporate. Thus, the frozen ground is only 2.5 km deep at the Martian equator while it is 6 km deep at the south pole.
To have liquid water running on the surface of Mars, the water region must be exposed to the surface. This may have happened at various times in the history of Mars as the climate changed.