Characterization and implications of a mass movement site in Bennu’s Bralgah Crater

Abstract

Rubble-pile asteroids such as Bennu and Ryugu have boulder-covered surfaces and latitude-dependent slope distributions, signifying that mass movement of boulders could contribute significantly to the surface evolution of these small bodies. The OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) mission documented numerous locations on Bennu exhibiting evidence of such mass movements, including one contained within Bralgah Crater. This area was considered (though not used) by the mission as a candidate site for sample collection; thus, high-resolution images collected during reconnaissance of this area enabled us to resolve a smaller minimum boulder size than would be possible elsewhere. Through boulder mapping and topographic analysis, we found evidence of a pileup behind a large central boulder, a flow “wake” downhill with a relative lack of medium-sized boulders, as well as a strong orientational preference of the boulders in this area, with the long axes of the boulders pointing westward of the expected direction of motion. We performed dynamical simulations of seismic shaking using the discrete-element N-body code PKDGRAV to better constrain the conditions that may have formed the landscape. In these simulations, we were able to replicate the pileup of material uphill from the central boulder and the wake downhill from it. Comparisons with previous simulations of seismic shaking–induced mass movements show that material transport is not directly related to the shaking intensity, but instead can be maximized by dominant shaking frequencies. Simulations also showed a preferential orientation emerging, with a strong possibility for preferential orientation parallel to direction of motion. Combined with the orientational preference observed in this and other studies of Bennu's landscape, this indicates that the Coriolis effect could be a major factor in the surface evolution of Bennu and other small bodies. Finally, we estimated the impactor sizes that would instigate our simulated shaking parameters, showing that Bennu could have encountered such impactors within its near-Earth lifetime.