Fossil dunes

A Piece of Mars: This 1.92×1.08 km (1.19x 0.67 mi) scene shows eroded ridges that are, in fact, lithified dunes. They are so old that you might not recognize them as dunes without more context. This doesn’t happen much on Earth, where inactive dunes are quickly eroded, buried, and/or destroyed by other geologic processes, so enjoy this uniquely martian wonder! (HiRISE ESP_046597_1670, NASA/JPL/Univ. of Arizona)