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 25 kilometers (16 miles) above the surrounding plains, volcano Ascraeus Mons pokes above the cloud deck near the western or limb. Valles Marineris is in the lower left. (Credit: Philip James, University of Toledo; Steven Lee, University of Colorado; and NASA)