Simulating impact-induced shaking as a triggering mechanism for mass movements on Bennu

Abstract

Observations of near-Earth asteroid Bennu have revealed a dynamic surface composed of unconsolidated material. The OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) mission found numerous locations exhibiting evidence of mass movements of surface material. Mass movements can be a major factor in the surface evolution of a small near-Earth asteroid, and the Small Carry-on Impactor (SCI) experiment on asteroid Ryugu has shown that seismic shaking can trigger it. We selected one mass movement site on Bennu to conduct a detailed survey of the surface boulder arrangement and geomorphology. Using these data, we created dynamical simulations of mass movement events at this site initiated by seismic shaking, and we found a mass flux comparable to estimates from the site survey. The frequency ranges of the shakings are similar to what would be produced by a 0.5-m-diameter impactor, for which the expected crater size is of a scale widely seen on Bennu (32 m). In addition, the simulation exhibited the Brazil nut effect, where finer particles percolate towards greater depth, in this case up to 1.5 m. Our results demonstrate that impact-induced seismic shaking is a viable mechanism for the initiation of mass movements, and a plausible explanation for the scarcity of fine regolith, on Bennu's surface.