Pan, one of Saturn's moons, viewed on March 7, 2017, by NASA's Cassini spacecraft:
Pan is about 21 miles wide, orbiting in a gap in Saturn's rings. This view, the closest and most detailed yet, shows its pronounced equatorial ridge, made up of particles from the rings that have accumulated on Pan's equator over centuries.
So - like huge sand dunes.
Update: Jerry Coyne has more.