Mars provides a dictionary definition for the phrase ‘polar differences’. A vast ice cap of water ice and snow dominates the planet’s north pole. Yet around the south pole, a comparatively tiny cap appears to be composed mostly of frozen carbon dioxide, popularly known as dry ice. A new study may explain why. Fresh clues have been found in a computer model of broad circulation patterns that change with the seasons and appear to control how and where water is transported on the Red Planet and why snowstorms are largely limited to the north. Further, the research suggests that the atmospheric circulation may be controlled to some extent by the fact that Mars’ southern hemisphere is generally higher than the surface elevations in the north hemisphere.