Swiss Cheese Terrain
Since Mars is colder than Earth, there is not just water ice at the poles, but also a concentration of carbon dioxide ice. Some of the carbon dioxide ice at the south pole is there all year long and called the residual cap. This image was taken near the south pole of Mars and shows a characteristic "Swiss cheese" pattern.
This pattern is created when there is relatively high, smooth material that is broken up into these circular-shaped depressions forming the "Swiss cheese" terrain. The depressions are thought to be caused by sublimation, which is when a material goes directly from a solid to a gas phase. Repeated images are taken of areas like this so the changes in depression size and where they form can be monitored through the seasons.