The Mars Global Surveyor snapped this image of the red planet's north pole in May 2000. The ice cap, seen during the planet's early northern summer, is about 680 miles (1,100 kilometers) across (see more images from the Mars Global Surveyor).
Mars's polar caps are made of water ice and frozen carbon dioxide. Like Earth's icy poles, the Martian ice caps fluctuate in size as the seasons change.
In summer the carbon dioxide ice vaporizes, causing the caps to shrink—revealing swaths of the polar surface for the Phoenix probe to dig into.