Windward side shallow, leeside steep. That's the normal way sand dunes are apparently. Is it different on Mars? To get wind direction right from this picture you also need to be sure whether the picture is north up - modern convention, or south up - old convention.

Peter Dodd, Uxbridge , England

The photo actually was taken from about two meters away. It is amply clear that they are hoofprints. If you deny this, you have been bought off by the aliens and their traitor pals there goes the phone again. And again. And again.

George R, Seattle, Wash.

Actually sand dunes can have many forms. The two that are most applicable: Barchans are crescent-shaped with horns pointing downwind while Parabolic are crescent-shaped with horns pointing upwind. These look more like barchans (parabolic are usually anchored with vegetation). The photo should indicate orientation especially since the text references it. Regardless-absolutely stunning.

Melanie Rob, Keystone Heights, FL, USA