Regolith flow on top-shaped asteroids

Abstract

We develop a continuum framework of regolith flow on asteroids. We focus on top-shaped asteroids that may be taken as consisting of regolith lying on a solid core. Depth-averaging is employed to model the regolith flow, and effects due to the asteroid’s rotation and its complex gravity field are retained. Angular momentum conservation is invoked to couple regolith flow to the asteroid’s changing shape and spin. This framework is first used to explore the equilibrium of regolith as a function of its friction, and the asteroid’s shape and spin rate. Next, we study regolith flow on top-shaped spinning asteroids and find conditions for the regolith’s shedding or deposition. We also discuss how the regolith’s flow and the asteroid’s spin influence each other. Finally, as an application, we propose and investigate the following evolution history of Bennu: a fast spinning Bennu was slowed down by multiple, impact-induced global landslides to its present spin state. Regolith was shed if the spin was higher than a critical rate. Once the spin rate fell below this critical value, regolith flow from higher latitudes began depositing regolith at its equator, giving Bennu its distinctive shape.