Mars, a dune field at latitude 83.5°, less than 400 km from the north pole. The eternal Martian wind blows the heavy sand into dunes. In the Martian winter, carbon dioxide freezes out of the air. In the summer, that CO2 sublimates; that is, turns directly from a solid to a gas. When that happens the sand gets disturbed, and falls down the slopes in little channels, which spreads out when it hits the bottom. But this disturbs the red dust, too, which flows with the sand.
Around the South Pole of Mars, toward the end of every Martian summer, the warm weather causes a section of the vast carbon-dioxide ice cap to evaporate. Pits begin to appear and expand where the carbon dioxide dry ice sublimates directly into gas. These ice sheet pits may appear to be lined with gold, but the precise composition of the dust that highlights the pit walls actually remains unknown. The circular depressions toward the image center measure about 60 m across
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