This photo from the European Space Agency's Mars Express is of the planet's south pole, and the layers of white are frozen water and carbon dioxide. As much as the pole looks smooth here, it's surface is usually compared to swiss cheese.
Photo credit: ESA/DLR/FU Berlin / Bill Dunford
The ESA describes the Mars' ice and winter this way:
The southern cap reaches some 3 km thick in places, and is around 350 km in diameter. This icy region is permanent; in the martian winter another, thinner ice cap forms over the top of it, stretching further out across the planet and disappearing again when the weather warms up.