Gullies at the Edge of Hale Crater, Mars

This image from NASA's Mars Reconnaissance Orbiter shows gullies near the edge of Hale crater on southern Mars.Martian gullies carved into hill slopes and the walls of impact craters were discovered several years ago. On Earth, gullies usually form through the action of liquid water -- long thought to be absent on the Martian surface. Whether liquid water carves gullies under today's cold and dry