Is it an old fossil barchan dune?
A Piece of Mars: There are many barchans on Mars, those lovely isolated crescent-shaped dunes. In a few places there are what looks like ancient preserved barchans, now lithified. The mound in the center of this 0.96×0.54 km (0.6×0.33 mi) scene shows what may be an example of a fossil barchan. If so, then this is quite unusual. On Earth, dunes are very rarely preserved in their full form, usually having been at least partially eroded away before being preserved. I love how much geology is visible from orbit on Mars! (HiRISE ESP_049955_1665, NASA/JPL/Univ. of Arizona)