Mars 360
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well-preserved, fine-scale layering 'Festoon' Pattern
Opportunity imaged distinctive, smile-shaped features called "festoons," seen on the surface of this rock. The detailed geometric patterns -- nested sets of concave-upward layers -- in sedimentary rocks imply the presence of small, sinuous sand ripples that form only in water on Earth.

Essentially, the festoons are the preserved remnants of tiny (centimeter-sized) underwater sand dunes formed long ago by waves in shallow water on the surface of Mars.

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  Jet Propulsion Laboratory
   California Institute of Technology