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

Kurzfassung

NASA’s Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Ex- plorer (OSIRIS-REx) spacecraft arrived at the near-Earth rubble pile asteroid (101955) Bennu in December 2018, where it took high resolution images of the surface. The im- ages revealed that the surface is covered with boulders of various sizes. The morphology of these boulders can provide valuable information about the body’s history and the mechanical properties of the regolith. In this work, I use OSIRIS-REx images to map the outline of boul- ders on Bennu in two different geologic units: a Rugged Unit and a Smooth Unit. The two units are differentiated by surface texture, shape features and geologic features. This work was implemented using the open-source software QGIS. I compare the two units’ boulder morphology firstly in terms of boulder roughness by looking at the shape factors solidity (to what extent a boulder’s area equals that of its convex hull area) and circularity (to what extent a boulder’s perimeter is similar to the circumference of a circle with the same area). Then I study boulder compactness, by looking at the shape factors elongation (the ratio between its minor and major axis) and roundness (to what extent a boulder’s area resembles that of a circle enclosing the boulder). Despite the geologic differences, I find that there is no significant difference in the boulder roughness and compactness between the two units. Both regions’ boulders possess a large variation of values that overlap significantly. My results match well with laboratory impact experiments, implying that the regolith was created by a catastrophic impact, which is in agreement with Bennu’s status as a rubble-pile asteroid. I also find that the Smooth Unit tends to have smaller boulders (0.579 ± 0.35 m) with more boulders mapped (total 2426) than the Rugged Unit (0.711 ± 0.48 m, total 1774 boulders mapped). Finally, I show that smaller boulders tend to be rounder and less rough than larger boulders in both units. My results imply that boulder morphology is relatively uniform over the surface of Bennu, also indicating that the mechanical material properties associated with the boulder shape (such as for example tensile strength of the assembly, bulk porosity and formation history) are similar in the two units. Although the units are geologically distinct, the boulder morphology is homogeneous.