The Answer is in the Crests and Troughs

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Caption:

This microscopic image taken by the Mars Exploration Rover Spirit shows the troughs between the waves of windblown soil that characterize the surface of Gusev Crater, Mars. The fine-grained soil in the troughs, combined with the coarse grains observed in a similar microscopic image taken of the waves' crests, indicate that the waves are geologic features known as ripples, and not dunes. Dunes contain a more uniform distribution of material. This information helps scientists better understand the winds that shape the landscape of Mars. The image was taken on the 41st martian day, or sol, of the rover's mission by its microscopic imager. The observed area is 3 centimeters (1.2 inches) across.

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