Coevolving aerodynamic and impact ripples on Earth: unifying bedforms on water, Earth and Mars

Wind-blown sand surfaces on Earth, Mars, and other planetary bodies are covered by multiscale bedforms. The long-standing consensus has been that meter- to kilometer-scale dunes and decimeter-scale ripples on Earth emerge via two distinct physical mechanisms. Dunes evolve from a flat sand bed due to a hydrodynamic instability, as topography and turbulent flow are out of phase. So-called impact ripples are commonly associated with a granular transport instability, related to the spontaneous synchronization of the hopping grains with the emerging surface corrugation. Recent wind tunnel experiments show that on relatively fine monodisperse sand (d = 90microns), centimeter-scale ripples can coevolve with decimeter-scale ripples, suggesting two distinct mesoscale instabilities. This new centimeter-scale ripples are reproduced by direct simulations of granular transport and are thus consistent with impact ripples. We then conclude, in contrast with the existing consensus, that decimeter-scale ripples have a hydrodynamic origin, similarly to large Martian ripples and water ripples. Indeed, their wavelength rescaled by the viscous length is in the same range as ripples in water and Mars. The formation of decimeter-scale ripples as a hydrodynamic instability is captured by existing morphodynamic models assuming the existence of two transport relaxation (or saturation) lengths: a large one, of about 0.5m, that has been proposed to scale with the drag length of sand grains, and a small one, of about 1cm, that is consistent with the average hop length of grain trajectories. We confirmed the values of the small saturation length by measuring the phase lag of the transport rate relative to the calculated bed shear stress.