Chasma Boreale, a long, flat-floored valley, cuts deep into Mars' north polar ice cap. Its walls rise about 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 and driven them down-canyon in a westward direction.This scene combines images taken during the period from December 2002 to February 2005 by the Thermal Emission Imaging