Coevolving aerodynamic and impact ripples on Earth
Hezi Yizhaq, Katharina Tholen, Lior Saban, Nitzan Swet, Conner Lester, S. Silvestro, K. R. Rasmussen, J. P. Merrison, J. Iversen, Gabriele Franzese, Klaus Kroy, Thomas Pähtz, Orencio Durán, Itzhak Katra
Abstract
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.