Since telescopes were first aimed at Mars, astronomers have noticed the planet's distinctive light and dark patches. Some of the patches change position as winds blow dust and sand across the surface. Brighter areas may indicate places that in the recent past have been coated with fine-grained material.
Some dark areas are permanent features, such as the huge ring of dunes around the north polar cap.
Other dark areas have been swept partly clear to reveal older rock beneath. To create this global portrait, technicians combined some one thousand images taken by Viking Orbiters in the 1970s. The brightest area in the image is the north polar cap. Seen here in summer at its minimum size, the permanent cap covers an area about 1.5 times the size of Texas. It is the only known place where frozen water is stable on the Martian surface.