Low surface strength of the asteroid Bennu inferred from impact ejecta deposit

Observations of deposits associated with a crater on the rubble-pile asteroid Bennu indicate a surface with low strength that is readily reworked by impact processes. The surface strength of small rubble-pile asteroids, which are aggregates of unconsolidated material under microgravity, is poorly constrained but critical to understanding surface evolution and geologic history of the asteroid. Here we use images of an impact ejecta deposit and downslope avalanche adjacent to a 70-m-diameter impact crater on the rubble-pile asteroid (101955) Bennu to constrain the asteroid's surface properties. We infer that the ejecta deposited near the crater must have been mobilized with velocities less than Bennu's escape velocity (20 cm s(-1)); such low velocities can be explained only if the effective strength of the local surface is exceedingly low, nominally <= 2 Pa. This value is four orders of magnitude below strength values commonly used for asteroid surfaces, but it is consistent with recent estimates of internal strength of rubble-pile asteroids and with the surface strength of another rubble-pile asteroid, Ryugu. We find a downslope avalanche indicating a surface composed of material readily mobilized by impacts and that has probably been renewed multiple times since Bennu's initial assembly. Compared with stronger surfaces, very weak surfaces imply (1) more retention of material because of the low ejecta velocities and (2) lower crater-based age estimates-although the heterogeneous structure of rubble piles complicates interpretation.

Automated summary

Observations of deposits associated with a crater on the rubble-pile asteroid Bennu suggest a surface with low strength that is easily reworked by impacts. The discovery of a downslope avalanche also indicates a surface composed of materials that can be easily mobilized by impacts and may have undergone multiple renewals since Bennu's initial assembly.