Granular mechanics behaviors observed in multiple variable-gravity experiments relevant to planetary surfaces

Spacecraft and sample return missions are providing up-close imagery of dusty planetary surfaces, and continued exploration of the lunar surface will soon provide a wealth of observations. Our understanding of these airless planetary surfaces come predominantly from remote sensing observations of the uppermost layers, which record a history of change due to internal and external forces. Images from Bennu and Ryugu have shown surfaces with particle size distributions vastly different to those previously observed on other small bodies such as Itokawa, Eros, and especially the Moon. It is evident that there are particle size-dependent sorting mechanisms that may vary between planetary bodies, potentially including impacts, mass motion or flow, electrostatics, and thermal processes. Microgravity investigations of granular systems are one key to understanding the behavior of regolith, or dusty granular material, that is ubiquitous on planetary surfaces. We will present results from several experiments exploring the behavior of granular materials in microgravity environments, including on parabolic flights, suborbital flights, and the international space station. For all experiments the primary data collection comes from video observations. In some experiments, we observe variations in angle of repose and material compaction with material properties and gravity level. Others provide insight into the bulk behavior of a complex size-distribution of particles that experience vibrations and accelerations that influence the granular sorting and structures observed in the materials.