The impact crater Rabe is 108 kilometres (67 miles) across. Two features distinguish it: a flat floor with a pit sunk into it, and a large field of dunes. A pair of visible-wavelength images together with numerous infrared ones created this false-colour Thermal Emission Imaging System (THEMIS) taken from NASA's Mars Odyssey spacecraft. The colours portray the overnight surface temperatures: blues indicate cold places, redder tints warm ones. This helps scientists distinguish areas of dust and sand from harder and rockier ground. The technique works because areas mantled in dust cool quickly after sundown, while rocks hold onto daytime heat longer. In the predawn hours, outcrops of bedrock are still glowing with warmth, while dusty ground has long since turned cold and dark.
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