Because it is spring in Mars' northern hemisphere, much of the carbon dioxide frost around the permanent water-ice cap has sublimated, and the cap has receded to its core of solid water-ice several hundred miles across. The abundance of wispy white clouds indicates that the atmosphere is cooler than seen by visiting space probes in the 1970s. Morning clouds appear along the planet's western (left) limb. These form overnight when Martian temperatures plunge and water in the atmosphere freezes out to form ice-crystal clouds.

Towering 16 miles (25 km) above the surrounding plains, volcano Ascraeus Mons pokes above the cloud deck near the western or limb. This extinct volcano, measuring 250 miles (402 km) across, was discovered in the early 1970s by Mariner 9 spacecraft. Other key geologic features include (lower left) the Valles Marineris, an immense rift valley the length of the continental United States. Near the center of the disk lies the Chryse basin made up of cratered and chaotic terrain. The oval-looking Argyre impact basin (bottom), appears white due to clouds or frost.

Seasonal winds carry dust to form striking linear features reminiscent of the legendary martian "canals." Many of these "wind streaks" emanate from the bowl of these craters where dark coarse sand is swept out by winds. Hubble resolves several dozen impact craters down to 30-mile diameter. The dark areas, once misinterpreted as regions of vegetation by several early Mars watchers, are really areas of coarse sand that is less reflective than the finer, orange dust. Seasonal changes in the surface appearance occur as winds move the dust and sand around.

Credit: Philip James (University of Toledo), Steven Lee (University of Colorado), NASA