Researchers may have explained why the north and south poles of Mars are so different. It is because the planet’s atmospheric circulation is affected by the higher terrain in the southern hemisphere. The north polar cap is made mainly of water-ice, while the southern cap appears to be mainly frozen carbon dioxide or “dry ice”. Using a sophisticated computer model of the planet’s atmosphere, researchers report that the thin Martian air (which is mostly carbon dioxide) rises and falls more vigorously in the southern than in the northern hemisphere. They say this creates an overall south-to-north flow of water vapour, which could explain the observed difference in the compositions of the poles.