Impact-crater ejecta on Bennu indicate a surface with very low strength

Abstract

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 constrained due to their wide departure from terrestrial analogs. Here, we report the observation of an ejecta deposit surrounding an impact crater that limits the maximum 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 surface 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.

The Formation of Terraces on Asteroid (101955) Bennu

“…According to the FS analysis, the regions of current failure are primarily above 30° of latitude in both the northern and southern hemispheres. These areas are where evidence for surface flows (Barnouin et al, 2019;Jawin et al, 2020;Perry et al, 2022;Walsh et al, 2019) and the terraces described here are observed.…”

“…As a endmember case (Figure 8), we use c = 1 Pa with the plausible ϕ = 40° and T = 3 m. This thickness is derived from the depths of mounds at the centers of three craters on Bennu (R. T. Daly et al., 2020), which offer one direct measure of the approximate depth of the unconsolidated surface layer (Quaide & Oberbeck, 1968). But geomorphic assessments of mass movement (Jawin et al., 2020; Perry et al., 2022) elsewhere on Bennu indicate that surface debris with thicknesses of up to 10 m are readily mobilized. We therefore consider lower values of c ≥ 0.1 for a broader range of plausible surface debris thicknesses (Figure 9), with ϕ = 34° and 40°.…”