Images: When Mars was a wetland

Another feature helping scientists understand the history of water on Mars is the presence of enormous gullies. Most of these gullies are found on the walls of craters in the southern hemisphere, where they remain in shadows for much of the Martian winter, accumulating frost.

This is one such crater, located in the Terra Sirenum, pictured here by the Mars orbiter's Context Camera.

Photo by NASA/JPL/MSSS