Criticality and Collective Granular Dynamics Control the Emergence of Wind Ripples

Abstract Periodic sediment patterns have been observed on Earth, in rivers and in sand and snow deserts, but also on other planetary environments. On Mars, the observation by rovers of wind or `impact' ripples of the same size as their terrestrial counterpart, while the atmosphere is 80 times lighter, has reopened the controversy about their formation mechanism. Here we show in a numerical simulation that the emergent wavelength of impact ripples is controlled by the mechanics of grain-bed impacts and not by the flying sediment transport above the bed: the distribution of grain trajectories in transport is found to be essentially scale-free, invoking the proximity of a critical point and precluding a transport-related length scale that selects ripple wavelengths. In contrast, when a flying grain strikes the bed, the process leading to grain ejections introduces a previously overlooked collective granular length scale that determines the scale of the ripples. We propose a theoretical model that predicts a relatively constant ripple size for most planetary conditions. Additionally, our model predicts that for high density atmospheres, like on Venus, or for sufficiently large sand grains on Earth, impact ripples propagate upwind. This novel prediction motivates wind tunnel and field experiments to test for the existence of such ‘antiripples’. Our new quantitative model provides fundamental insights about the processes involved in wind-blown (aeolian) sediment transport and may be used to deduce geological and environmental conditions on other planets from the sizes and propagation speeds of impact ripples.