Mars' crust varies in thickness across the planet. In the northern hemisphere, the crust is only about 35 km thick, while in the southern hemisphere, it is about 80 km thick. This is probably caused by a period of uneven cooling that the planet experienced. For unknown reasons, Mars’ Northern Hemisphere cooled more slowly than the Southern Hemisphere, causing it to form a smoother, thinner crust in that area.

This image (right) shows a possible configuration of soil and ice in the first three feet of the surface of Mars.