Special Issue: Water on Mars

The evolution of the Martian surface and atmosphere are increasingly being seen as connected to the behavior of water on all time scales. Over the course of a single day, even the incredibly small amounts of water in the atmosphere can form clouds that affect the energy balance at the surface, the chemistry of the atmosphere, and the geochemistry of the surface layer. On seasonal time scales, atmospheric water is part of the current climate system: It acts as a tracer of dynamical transport by the winds, and the water vapor can diffuse into (and out of) the subsurface, so that it becomes water ice at high latitudes and adsorbed water at lower latitudes. These seasonal processes, operating over many years, are thought to be responsible for large deposits of ground ice at high latitudes and for the evolution of the polar ice caps. On the million- to billion-year time...