Heavy frost deposits coat the ground in and around the 139-kilometer diameter (86-mile diameter) Hooke crater in this Mars Express image. The frost serves as a reminder that Mars still possesses water that moves dynamically between the atmosphere and the surface, mostly in the form of ice and vapor. At present, Mars has thick water-ice polar caps and icy clouds. In many locations, temperatures get low enough overnight for the relative humidity to reach 100 percent, and frost forms on the ground as water vapor condenses. As water has moved around with Mars’ shifting climate over its long history, there may have been many periods of wetting and drying all over the planet.