The Chasma Boreale, seen above, is about 350 miles long, cutting deep into the north polar icecap of Mars. With walls rising nearly one mile above the floor and ending at the edge of the retreating ice cap, Chasma Boreale is an important Martian feature for geologists. Just like the rings of a tree trunk or ice core data excavated from the poles of Earth, dust samples taken from the canyon could reveal information about the planet's past climate.

The darker spots seen in the composite photograph, which combines a series of images taken between December 2002 and February 2005 by the Thermal Emission System instrument mounted on NASA's Mars Odyssey, indicate deeper patches of sand. The darker the surface -- and deeper the sand patch -- the more the ice beneath has melted.

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Image: NASA.