Planetary Geomorphology Image of the Month, July 30, 2016: Ryan C. Ewing (Texas A&M University). Ripples cover the surfaces of sand dunes on Earth and Mars. On Earth, ripples formed in typical aeolian sand (e.g., 0.1 and 0.3 mm) range in wavelength between 10 and 15 cm and display a highly straight, two-dimensional crestline geometry. Ripples are thought to develop through a process dominated by the ballistic impacts of saltating sand grains in which wavelength selection occurs through the interplay of grain size, wind speed, the saltation trajectories of the sand grains, and ripple topography.
High Resolution Imaging Science Experiment (HiRISE) images of the surface of Mars show ripples forming at much longer wavelengths ranging between 1 and 5 m and with a variable planform geometry ranging from two-dimensional to sinuous… [More at link]