A stunning image of the sand dunes near the North Pole of Mars, from the Mars Reconnaissance Orbiter’s HiRISE camera:

Phil Plait explains what you’re seeing:

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.

Now, you might think I’m making this all up. How do we know this stuff is flowing downhill like that? Ah, because in this picture we’ve caught it in the act! In this image, a closeup of a region just to the left of center of the big image, you can actually see the cloud of dust from an avalanche as it occurs.

Oh, baby. The cloud is only a few dozen meters across, and can’t be more than a few seconds old.