A comparison of boulder morphology in the two geologic units of asteroid (101955) Bennu

Asteroid (101955) Bennu, a carbonaceous rubble-pile asteroid of about 500 m diameter, was visited by the OSIRIS-REx spacecraft in 2018. Images of Bennu's surface show a rugged and a smooth surface unit. Although these regions are geologically distinct, it is unclear if the processes forming them also affect the shape of boulders that are abundantly present on Bennu. We study the boulder morphology of the two geologic units of asteroid Bennu using common shape factors including solidity, roundness, elongation, and circularity to see if they are geologically distinct from each other. We selected a representative number of images taken by the OCAMS camera bundle on board the OSIRIS-REx spacecraft and manually traced boulders in these images to derive the above mentioned morphological shape factors. These were then compared statistically between the two units. A total of 3985 boulders between 0.3 and 9 m were plotted in six different observation areas (five nonoverlapping images were chosen for each observation area, for a total of 30 images). We find that the Smooth Unit contains more and smaller boulders than the Rugged Unit, but no significant differences in boulder shape factors are evident between the two surface units. This indicates that the properties and processes that influence boulder shapes are homogeneous across the surface of Bennu. The average boulder elongation found in this study matches that of fragments created through hyper-velocity impact experiments, in line with Bennu's creation after a catastrophic impact of its parent body. Finally, we found that smaller boulders tend to be smoother and more compact than larger boulders in both units. This suggests that weathering effects that smooth boulders are more efficient on small scales than those that roughen them. We find that the shapes of boulders found on Bennu are independent of the geologic unit they are found in.