INSIGHTS INTO REGOLITH DEVELOPMENT ON RUBBLE-PILE ASTEROID (101955) BENNU FROM REMOTE SENSING AND SAMPLE ANALYSIS (Invited Presentation)

Spacecraft exploration of small near-Earth asteroids (NEAs) has revealed their rubble-pile nature. The arrival of the OSRIS-REx spacecraft to the C-complex asteroid Bennu (490 m diameter) came with the surprise discovery of a regolith dominated by large boulders. This boulder-rich surface provided an opportunity to study in detail the constituent building blocks of a rubble-pile asteroid. Remote sensing observations of the boulder population on Bennu led to insights into their thermal and physical properties and hypotheses for the surface evolution of rubble-pile asteroids. These hypotheses are being tested through analysis of the Bennu sample that was returned to Earth by the OSIRIS-REx mission on September 24, 2023.

Here, we focus on the physical evolution of the boulder population on Bennu. The combined census of boulders and craters on Bennu showed that boulders effectively armor the surface against impacts, frustrating the formation of small (<~ 2 m) craters. The effect of the armoring process is also evident in high-resolution images of small craters formed on the surfaces of Bennu boulders. We present evidence for in situ collisional processing of boulders. We leverage the high-resolution images and altimetry data from OSIRIS-REx to better understand the boulder disruption process. Our findings suggest an evolutionary path for boulder fragmentation on asteroids, where (i) an impact generates widespread fractures and flaws within the target, and (ii) subsequently, a lower-strain-rate process (e.g., thermal fatigue) results in the complete fracturing of the boulder, while allowing the fragments to be retained on a micro-gravity surface. We will discuss this evolutionary pathway in the context of evidence from density measurements and X-ray computed tomography data of the Bennu sample. The combined remote sensing and sample analysis results inform our understanding of regolith development on small solar system bodies.