Sediment Motion Initiation: A Modern and Interplanetary Experimental Approach

Aeolian sediment transport has been observed on Earth and Mars, and surface features interpreted to be aeolian bedforms have been imaged on Titan, Pluto, Venus and Comet 67P/ChuryumovGerasimenko. Aeolian transport affects planetary bodies with a wide range of atmospheric properties, even those (such as Comet 67P) which do not possess persistent atmospheres. Several wind tunnel and field experiments have formed the basis of our understanding of particle motion initiation, but these initial studies were limited to terrestrial boundary conditions and materials. The observed planetary aeolian bedforms are formed under a wide range of fluid densities and are composed of a variety of materials with diverse properties. Recent work has demonstrated that extrapolating the results of experiments performed using terrestrial boundary conditions to other planets, such as Mars or Titan, results in inaccurate predictions of particle motion. Particle motion is initiated when the surface shear velocity exceeds a fluid threshold (u*tf), which is the critical shear velocity required to mobilize grains via fluid drag. Once initiated, particle motion terminates when the shear velocity falls below the impact threshold (u*ti; shear velocity required to maintain saltation). The impact threshold (u*ti) is less than u*tf, which adds complexity to attempts to predict particle motion. Observations from the Curiosity rover indicate that grains are being transported below the threshold velocities predicted using current equations for sediment motion, meaning further investigation into the physics of sediment transport in non-terrestrial systems is required. Previous analog work relied on a combination of altering fluid density and using analog sediments. Given the persisting discrepancy between these results and observations, we are taking a novel approach. This study uses the UCLA wind tunnel (operating at ambient atmospheric density and pressure), and a variety of spherical low-density materials as analogs for planetary systems to generalize the expressions for threshold velocities to accurately represent motion initiation in a range of conditions. For example, using scaling parameters from previous studies, table tennis balls are being used as analogs for sediment transport of organics on Titan and nitrogen ice on Pluto.