THE ROLE OF GRAIN SHAPE IN REGOLITH PROCESSES: EXPERIMENTAL RESULTS

Common to all solid planetary bodies is surface regolith, unconsolidated material typically comprised of dust and broken rock fragments. Planetary bodies with atmospheres (Earth, Venus, Mars, Titan) have regolith surfaces that are subject to aeolian and fluvial erosional processes that serve to round individual grains, creating an unconsolidated material with well-rounded grains of high sphericity that exerts a fundamental control on the macroscale behavior of the unconsolidated material. Additionally, common sources of erosion also serve to sort grains by size, often creating sediments that are well sorted and uniform in grain size. Less well understood is the behavior of regolith materials on airless bodies, where minimal erosional processes keep individual grains at a high degree of angularity and low sphericity.

Grain shape distribution influences a material’s overall porosity, compressibility, bulk density, and angle of repose and may in turn influence the materials’ bulk behavior and resultant geomorphic expression. Regolith mass wasting on small airless bodies is expressed in a variety of ways, many similar to fluvial processes, including liquefaction ponding in topographic lows, curvilinear debris flows, and deeply incised gullies. Experiments to understand the behavior of these non-terrestrial type sediments were needed to constrain the geomechanical characteristics observed.

We are currently exploring the role of grain shape on parameters such as porosity, angle of repose, and shear strength with application toward better understanding the surface and near surface regolith properties. Grain size range averages 44 – 90 µm, similar to the moon, but grain shape ranges from spherical to angular and non-spherical. Experimental results suggest that grain sphericity more so than angularity may play a role in explaining Apollo in situ measurements of relative densities of lunar soil >90% at a depth of 30 cm. However, the combined role of sphericity and angularity may influence porosity and angle of repose, which may in turn influence larger scale processes of volatile entrainment and mass wasting. Experimental results are informing current numerical modeling efforts to better understand the role of grain shape in regolith processes.