The High Resolution Imaging Science Experiment camera which is currently whizzing round the Red Planet aboard the Mars Reconnaissance Orbiter has returned this rather awesome picture of sand dunes on the surface.

‘Bad Astronomer’ Phil Plait had a chat with HiRISE’s Alfred McEwen and has a nice explanation of what is actually going on here:

…what [makes] this picture so spectacular are the graceful blue-gray swirls arcing across the dunes. These are caused by dust devils, which are a bit like mini-tornadoes.

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The important thing to note here is that the sand in the craters of Mars is actually dark grey in colour, since it’s made of basalt. The reason it looks red in pictures is because covering the sand is a thin layer of much finer dust, and the dust is what’s red. When a dust devil moves over the Martian surface, it can pick up the very light dust particles, but not the heavier sand grains. So those blue-grey swirls are tracks where the dust devil has vacuumed up the dust, revealing the darker sand underneath.

Image: NASA/JPL/University of Arizona