Planetary geologist Dr. Jim Zimbelman uses a ruler to delicately cut a cross section through a wetted sand megaripple, a feature formed by the accumulation of larger grains on wind-swept surfaces between dunes. Zimbelman conducted research  the Great Sand Dunes National Park and Preservation in Colorado in the hopes to learn more about the surface 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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