A deep canyon in the polar cap reveals emerging ancient deposits.
LAYER CAKE. Climatic cycles of ice and dust built the Martian polar caps, season by season, year by year — and then whittled down their size when the climate changed. Here we are looking at the head of Chasma Boreale, a canyon that reaches 570 kilometers (350 miles) into the north polar cap. Canyon walls rise about 1,400 meters (4,600 feet) above the floor. Where the edge of the ice cap has retreated, sheets of sand are emerging that accumulated during earlier ice-free climatic cycles. Winds blowing off the ice have pushed loose sand into dunes, then driven them down-canyon in a westward direction, toward our viewpoint. (A 9 MB version of the image is available.) Credit: NASA/JPL/Arizona State University, R. Luk (vertical exaggeration 2.5x).
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