Today's Image of Mars shows concave up features called 'festoons' on some rocks imaged by Mars Exploration Rover Opportunity. These geometric features, when found in sedimentary rocks like these, imply the presence of small, winding sand ripples that only form in the presence of water on Earth.
In short, these festoons are preserved remains of tiny sand dunes that formed long ago by ripples in shallow water on the surface of Mars.
Clicking on this photo will take you to the NASA gallery where this image can be found and downloaded.
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