Coevolving aerodynamic and impact ripples on Earth

Wind-blown sand creates multiscale bedforms on Earth, Mars and other planetary bodies. According to conventional wisdom, decametre-scale dunes and decimetre-scale ripples emerge via distinct mechanisms on Earth: a hydrodynamic instability related to a phase shift between the turbulent flow and the topography and a granular instability related to a synchronization of hopping grains with the topography. Here we report the reproducible creation of coevolving centimetre- and decimetre-scale ripples on fine-grained monodisperse sand beds in ambient air and low-pressure wind tunnels, revealing two adjacent mesoscale growth instabilities. Their morphological traits and our quantitative grain-scale numerical 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 comparable to the average hop length, hence faster than previously thought, but consistent with the phase lag of the inferred aeolian sand flux relative to the wind. A corresponding hydrodynamic modelling supports the existence of aerodynamic ripples on Earth, connecting them to megaripples and to the debated Martian ripples. We thereby open a unified perspective for mesoscale granular bedforms found across the Solar System. Wind tunnel experiments and numerical modelling reveal the existence of two distinct ripples on Earth: centimetre-scale impact ripples and decimetre-scale hydrodynamic ripples, akin to those in water and on Mars.