Leave No Stone Unturned — Modeling Asteroid Regolith Grain Size Evolution

The surface properties of asteroids have been the focus of exploration and scientific study since the beginning of planetary science. The particle size distribution is of particularly interest because it reflects the nature of surface shaping processes and has important implications in remote sensing observations and exploration activities. Here we present a numerical model simulating regolith size distribution evolution considering three fundamental processes: thermal fragmentation, impact-induced fragmentation and ejecta escape, and electrostatic dust removal. Fragmentation processes, including thermal and impact fragmentation, act as a conveyor belt gradually transforming boulders to fine-grained material. Impact ejecta escape and electrostatic dust transport serve as removal processes for fine-grained regolith. Most importantly, the combination of these three processes result in a coupled evolution among different grain size populations — with an effective removal of fine-grained material, larger grain populations are less protected and increasingly eroded. Our preliminary results show that, at 1 AU heliocentric distance, km-sized or smaller bodies where electrostatic dust removal can be active, the fine-grained regolith could be significantly depleted and lead to a lag-deposit-like surface scenery in a few million years timescale. The modeled grain size distribution is in reasonable agreement with measurements from recent asteroid sample return missions. These results are also relevant to other processes important to the evolution of asteroids, including space weathering and orbital evolution, which will be discussed in this presentation.