The morphometry of small impact craters on Bennu: Relationships to geologic units, boulders, and impact armoring

The Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) mission revealed that the asteroid Bennu has ~1500 impact craters (Bierhaus et al., 2022). Here we use data from the OSIRIS-REx laser altimeter (OLA) to measure the depths, d, and diameters, D, of a sample of small (D ≤ 10 m) impact craters that span a broad range of latitudes and longitudes. d/D in this sample ranges from 0.04 to 0.27, with a mean of 0.13 ± 0.04. Compared to larger (D ≥ 10 m) craters on Bennu, those studied here have a greater average d/D. The mean d/D of craters on Bennu's younger, Rugged Unit is statistically larger than the mean d/D of those on the older, Smooth Unit. One of the studied craters contains an interior mound that may indicate the presence of stronger material at depth. A large fraction of craters studied here are located near large boulders whose lengths are >50% the diameter of the crater. Some of these large boulders cross the crater rim crest, protruding into and perched above the crater interior. In the most extreme cases, the boulder length is ~2.5 times the crater diameter. The d/D of craters near large boulders is bimodal, which could be a consequence of how boulders affect crater formation. Laboratory experiments and results from Hayabusa2's Small Carry-On Impactor experiment indicate that preexisting boulders and heterogeneities affect crater formation and d/D. The abundance of small craters on Bennu makes the asteroid a rich resource for understanding impact processes.