Two-fold stratification of aeolian transport and its implication for planetary bedforms

Although aeolian sand transport is a major process shaping landscapes on Earth and on diverse celestial bodies, understanding its underlying many-body dynamics is still a central problem in aeolian research. Here, by means of grain-scale simulations and analytical modeling, we elucidate how aeolian conditions favoring bimodal sand transport, with grain bouncing (saltation) driving creeping motion (reptation), results in an recently discovered anomalous scaling of the bulk transport rate. The necessary two-fold stratification of the underlying minimal two-species model in turn raises a number of questions regarding the evolution of multiscale bedforms on Earth, Mars, and other planetary bodies. In fact, according to conventional wisdom, decameter-scale dunes and decimeter-scale ripples emerge via distinct mechanisms on Earth: a hydrodynamic instability and a granular instability. However, recent ambient-air and low-pressure wind-tunnel experiments report the reproducible creation of coevolving centimeter and decimeter-scale ripples on fine-grained monodisperse sand beds, revealing two adjacent mesoscale growth instabilities. Their morphological traits and our grain-scale simulations authenticate the smaller structures as impact ripples but point at a hydrodynamic origin for the larger ones.  This suggests that the aeolian transport layer would have to partially respond to the topography on a scale much smaller than previously thought, but consistent with its inherent two-fold stratification. A corresponding hydrodynamic modelling supports the existence of aerodynamic ripples on Earth and connects them to the so-called large Martian ripples. We thereby open a unified perspective for mesoscale granular bedforms found across the Solar System.