Presentation #400.04 in the session “Plenary Panel: New Results from Bennu and Ryugu”.
NASA’s OSIRIS-REx sample return mission has been investigating the B-type near-Earth asteroid (101955) Bennu for just under two years. Preliminary analyses showed that Bennu is a rubble-pile asteroid that contains globally distributed hydrated materials and abundant boulders on the surface, as well as distinct geologic features including impact craters and longitudinal ridges. These observations informed the selection of primary and backup sites for sample collection, expected to occur in October 2020. It is a mission priority to collect, and analyze upon Earth return, a sample of surface regolith to address fundamental scientific questions about the origin and evolution of Bennu. Analyses of the abundant boulders on Bennu found that they are diverse in several characteristics including morphology, size, and degree of burial. Variations in the boulder abundance and degree of burial in the subsurface allowed us to identify patterns of mass movement across the surface, indicating that material has been transported across the surface of Bennu, from latitudes as high as the polar regions to that of the equator. The apparent direction of mass movement is consistent with the local downslope direction, and material dominantly moves from the mid-latitudes toward the equator, which is the global geopotential minimum. We calculate that up to ~10 m of material may have been removed from the mid-latitudes due to mass movement, leading to the deposition of a layer ~5 m thick in the equatorial region. This transport of material could have led to the preferential erosion of small (<50 m diameter) craters in the mid-latitudes, while preserving those in the equatorial region. Based on modeled changes to the slope distribution over time, we argue that mid-latitude mass movement towards the equator occurred within the past several hundred thousand years in regions that became steeper by several degrees.