Image: NASA/JPL/University of Arizona
This is my new favorite picture of Mars.
From Phil Plait:
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.This High Resolution Imaging Science Experiment site has more photos so can zoom in and look for Dejah Thoris.
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.