The nature of strange ripples of sand on the Martian surface is clearer now thanks to pole-to-pole images returned by a NASA spacecraft. But even with this new information, scientists still are unsure just how the features, which are unlike anything on Earth, came to be. Full story
Ice exposed in two crater bottoms fades from view over the course of the martian summer in this series of HiRISE images taken in late-2008/early-2009. Sublimation of the ice leaves behind a dust layer that gradually thickens to the point where it obscures the ice. Each panel is 75m across.