Mars is an asymmetrical planet. Its northern hemisphere is relatively smooth and free of craters, with a posed of water. The southern polar cap is smaller and contains carbon dioxide, and the surrounding terrain is heavily cratered (Fig. 1). Hemispheric dichotomies are not unusual in the Solar System. The Moon, for example, has extensive lava plains on the side facing the Earth and is heavily cratered on the other side. Unlike the Moon, however, Mars has an atmosphere that can transport dust and water vapour around the planet. So one might expect that, over the long term, mixing would lead to a balanced distribution of polar ice. But this is not what we see.
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