Impact-crater ejecta on Bennu indicate a surface with very low strength, submitted to Nature Geosciences

A planetary surface’s resistance to change is generally described as its “strength” (units of stress). The surface strength of small, rubble-pile asteroids, which consist of fragments of larger bodies that were collisionally disrupted, is poorly con-strained due to their wide departure from terrestrial analogs. Here, we report the observation of an ejecta deposit surrounding an impact crater that limits the maxi-mum surface strength of the near-Earth rubble-pile asteroid (101955) Bennu. The presence of this deposit implies that ejecta were mobilized with velocities less than the escape velocity of Bennu, 20 cm/s. Because ejecta velocities increase with sur-face strength, the ejecta deposit can only be explained if the effective strength of the surface material near the crater is exceedingly low, ≤100 Pa. This is three orders of magnitude below values commonly used for asteroid surfaces, but is supported by previous observations of an artificial impact crater on a similar asteroid, Ryugu. Our findings indicate a mobile surface that has likely been renewed multiple times since Bennu’s initial assembly and have far-reaching implications for interpreting observations of Bennu and other rubble piles.