Visitors to the Great Sand Dunes in Colorado must cross Medano Creek (center) before scaling the sculpted mounds of sand. Researchers in the Center for Earth and Planetary Studies conduct research at the Dunes to try and learn more about the formation of Mars. 

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<p>Photographer:&nbsp;Samuel F. A. Cartwright</p>

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Smithsonian's National Air and Space Museum

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Smithsonian Institution

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