Coevolving aerodynamic and impact ripples on Earth

Abstract

Windblown sand creates multiscale bedforms on Earth, Mars, and other planetary bodies. According to conventional wisdom, decameter-scale dunes and decimeter-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 centimeter and decimeter-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. Hydrodynamic modelling supports the existence of hydrodynamic aerodynamic ripples on Earth, connecting them mechanistically to megaripples and to the debated Martian ripples. We thereby propose a unified framework for mesoscale granular bedforms found across the Solar System.

References in corpus (10)

• Aeolian sans ripples: experimental study of saturated states
• A unified model of ripples and dunes in water and planetary environments
• Direct numerical simulations of aeolian sand ripples
• A two-species continuum model for aeolian sand transport
• Dissolution instability and roughening transition
• Formation of aeolian ripples and sand sorting
• A lower-than-expected saltation threshold at Martian pressure and below
• Direct validation of dune instability theory
• Megaripple mechanics: bimodal transport ingrained in bimodal sands
• Anomalous Scaling of Aeolian Sand Transport Reveals Coupling to Bed Rheology

Cited by in corpus (3)

• Aerodynamic roughness of rippled beds under active saltation at Earth-to-Mars atmospheric pressures
• Emergence of wind ripples controlled by mechanics of grain-bed impacts
• Quantifying the form-flow-saltation dynamics of aeolian sand ripples