The Juventae Chasma on Mars contains dunes of all sizes, from the smallest ripples through the largest draa towering hundreds of meters tall.

A giant draa, superimposed with dunes, transverse aeolian ridges, and tiny ripples. Click for huge version.

Like water waves, sand dune landforms have a wavelength measured from crest-to-crest. The smallest wavelengths are ripples with less than 20 meters between sandy hills, moving up through transverse aeolian ridges at wavelengths of 20 to 70 meters, dunes with wavelengths of 100 meters to a full kilometer, and finally to draa, landforms with more than a kilometer long wavelength.

Ripples are common, frequently superimposed on any other landform with a ready sediment supply, but draa are downright unusual on Mars, only forming in epic sandy seas more common to Dune's Arrakis than our red neighbour. Both transverse aeolian ridges and dunes are relatively common on Mars, with the ridges frequently lighter in colour than their surroundings while the dunes are typically a bit darker.

In this HiRISE image of Juventae Chasma, a giant draa rising several hundred meters tall dominates the view, with all the smaller landforms superimposed on it in rolling, sandy masses. While dune formation is a rapid, dynamic process, it took thousands of (Martian) years for the wind to slowly built this massive wave of sand.

Image credits: NASA/JPL/University of Arizona