Investigating the Surface Evolution of Bennu and Rubble Pile Asteroids through Mass Movements and Seismic Shaking

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, which are major factors in the surface evolution of a small near-Earth asteroid, and can be triggered by seismic events, which in turn can be induced via impact events. In the Chapters 2 and 3 of this thesis, I selected two mass movement sites on Bennu to conduct a detailed survey of the surface boulder arrangement and geomorphology. One site is located in the northern hemisphere (Chapter 2), while the other is in the southern hemisphere within the Bralgah crater, where high resolution imagery exists to allow mapping at a smaller size regime (Chapter 3). My boulder surveys at these two sites reveal a preferential boulder orientation indicative of strong Coriolis force influences. This is supported by further global mapping of mass movement sites (Chapter 4), where the boulder orientation preferences persist in a pattern consistent with a model of mass movements influenced by the Coriolis force. This has significant implications for surface evolution on similar fast rotating small bodies. The surveys also provided context for creating dynamical simulations, using the discrete-element N-body code PKDGRAV, of mass movement events at these sites initiated by seismic shaking. For each of the initial two sites, dynamical simulations demonstrated that the seismic shakings can produce the morphologies observed in the spacecraft data. These include similar mass flux, and at the Bralgah crater site, a flow “wake” feature downhill of a large central boulder. Through the simulations, I also found that material transport is not directly related to the shaking intensity, but instead can be maximized by dominant shaking frequencies. In addition, I relate the shaking parameters I utilized in the simulations to impact events expected on Bennu within its near-Earth lifetime, showing that mass movement could be impact induced. I also show that the distribution of observed mass movement sites is not random against impact craters on Bennu of certain sizes, indicating a connection between the two processes.