Swiss Cheese on a Red Planet

The Martian south polar cap is a layer of carbon dioxide ice, full of pits that make it look like Swiss cheese. The pits form when the Sun heats the ice and makes it sublimate (transform from a solid to a gas). Because it's at the pole, the Sun never gets very high in the sky, so steep slopes get more heat and sublimate faster, causing pits to form and grow. This is balanced by new carbon dioxide frost that forms on flatter areas.