Marsâ€™ northern-most sand dunes are seen as they begin to emerge from their winter cover of seasonal carbon dioxide (dry) ice in this image acquired by the HiRISE camera aboard NASA's Mars Reconnaissance Orbiter Jan. 16, 2014. The steeply sides of the dunes are also ice-free along the crest, allowing sand to slide down the dune. Dark splotches are places where ice cracked earlier in spring, releasing sand.
NASA/JPL-Caltech/Univ. of Arizona/Handout via Reuters