Mars is a funny place. It is both familiar and very alien. Take these sand dunes for example.
There are countless examples of dunes on Mars -- features created by the transport of the fine Martian regolith blowing in the wind. The physics behind this movement of material is surprisingly complex, but not too dissimilar to the sand dunes you might find on a terrestrial beach or desert.
However, on analyzing a swarm of dunes inside Mars' Copernicus Crater, the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on board NASA's Mars Reconnaissance Orbiter (MRO) picked out the presence of olivine, a mineral that forms in the presence of water. The High Resolution Imaging Science Experiment (HiRISE) was then used for follow-up observations. The tiny olivine particles have been blown in the wind, collecting in these "raindrop" shaped dunes.
Commonly, Mars dunes take the shape of barchan dunes we find on Earth. Barchans are characterized by a smooth windward-facing edge, with a receding steep slope, often creating a horn-like structure on the downward slope. As can be seen in the image left, some of the dunes do look like classic barchans, whereas many retain the droplet shape.
"Olivine-rich dunes are very rare on Earth, as olivine rapidly weathers to clays in a wet environment," writes Alfred McEwen, University of Arizona planetary scientist and principal investigator of HiRISE. "There is also olivine-rich bedrock in the central peaks of Copernicus Crater on the Moon."
Observations like these not only help us piece together the puzzle of Mars' water history, it also gives us a clue about the active atmospheric and geologic processes that continually shape the red planet's surface.