The planet Mars undergoes dramatic changes in its axial tilt on a timescale of hundreds of thousands of years. This is unlike Earth, whose axial tilt is stabilised by the presence of our Moon. As a result, Mars experiences significant changes in climate. This illustration shows cutaways through the crust of Mars during the Noachian Period (4100-3700 million years ago), when Mars was cold but wet and volcanic. At its warmest (left) water flowed more freely on the surface, whereas at the other extreme (right) substantial amounts of surface water froze. The subsurface aquifers remain present during both extremes but the uppermost ones might have frozen solid during cold extremes.