Photo of Martian dune field

More pictures and commentary Another dose of Martian awesome

That is not a closeup of my chin before I shave. It’s Mars, a dune field in the far north; at latitude 83.5° to be precise, less than 400 km (240 miles) from the north pole. The eternal Martian wind blows the heavy sand into dunes, and you can see the hummocks and ripples from this across the image. The sand on Mars is from basalt, which is a darkish gray color. The red comes from much smaller dust particles which settle everywhere.

But what are those weird tendril thingies?

In the Martian winter, carbon dioxide freezes out of the air (and you thought it was cold where you are). In the summer, that CO2 sublimates; that is, turns directly from a solid to a gas. When that happens the sand gets disturbed, and falls down the slopes in little channels, which spreads out when it hits the bottom. But this disturbs the red dust, too, which flows with the sand. When it’s all done, you get those feathery tendrils. Note that at the tendril tips, you see blotches of red; that’s probably from the lighter dust billowing a bit before settling down.