When summer hits the south pole on Mars, only half the ice on the pole melts. This leaves behind a strangely off-center chunk of water-and-carbon-dioxide ice, as you can see in this composite satellite image of the pole in summer. After analyzing climate data for several years, researchers have figured out what causes this weird pattern. It all has to do with winds coming off a gigantic crater, Hellas Basin, far to the north. Winds blowing into and out of the 7-km deep crater create "Rossby waves," which in turn affect other weather systems. According to PhysOrg:
These waves reroute the high altitude winds on Mars and force the weather system towards the south pole. In the western hemisphere of Mars, this creates a strong low-pressure system near the south pole, and a high-pressure system in the eastern hemisphere, again near the south pole.
The temperatures in these two systems are different. The low-pressure system is the right temperature to cause carbon dioxide snow, which turns into ground frost. In the high-pressure system there's no snow. So you get ground frost without a snow covering. That means the ice cap is actually half frost, and half carbon dioxide snowy frost. The areas covered in snow don't melt off in summer because they reflect the sunlight. But frost grains can't reflect sunlight as well, and their shape actually exposes them to more sunlight. Which means they're easily melted in summer. So that irregular polar cap you're seeing is the result of two winter different weather fronts coming together — one full of snow, the other causing frost. And in summer, only one side melts away — the non-snowy side. Which means there's always good carbon dioxide skiing on the western side of the Martian south pole, even in summer. Composite image via ESA/ Image Courtesy of F. Altieri (IFSI-INAF) and the OMEGA team. Mars Polar Cap Mystery Solved [via PhysOrg]