Globally distributed mass movement towards the equator on asteroid (101955) Bennu

Abstract:

Near-Earth asteroids (NEAs) have dynamic surfaces characterized by mobile, unconsolidated regolith that responds to local geophysical gradients, even in microgravity environments. The transport of rock fragments on NEAs can lead to distinct morphologies and asymmetric boulder distributions indicating the direction of motion. In this work, we analyze data collected by the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security­­–Regolith Explorer) mission to investigate and quantify mass movement on asteroid (101955) Bennu. We document morphologies indicative of mass movement on Bennu and assess the relationship to slope and other geologic features on the surface. We find evidence of globally distributed mass movement on Bennu (from the equator up to ~70° latitude) on varying spatial scales (ranging from individual meter-scale boulders up to a single debris flow ~100 m long and several meters thick). 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. Mass movement appears to have affected the degree of burial of large (≥30 m diameter) boulders, excavating those in the mid-latitudes and burying those in the equatorial region (±20° latitude). Up to ~10 m of material may have been transported away from the mid-latitudes, which would have deposited a layer ~5 m thick in the equatorial region assuming a stagnated flow model. The equatorial migration of meters of material could have erased small (<50 m diameter) craters and may have contributed material to Bennu’s equatorial ridge. Models of changes in slope over time suggest that mid-latitude mass movement occurred within the past several hundred thousand years in regions that became steeper by several degrees.