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 Amazonian Period, which began about 3 billion years ago and continues today. When the axial tilt is the greatest and the climate warmest (right), the Sun heats the poles and drives off volatiles, leading to snow which settles on the ground at mid-latitudes. But at the opposite extreme, the volatiles remain locked up at the poles as the climate cannot support snow. Oddly, therefore, a warmer Mars is a whiter one. Substantial quantities of water are probably locked beneath the surface during both extremes.