Bennu Sample Analysis Reveals Regolith Production Mechanism

This paper presents a physical analysis of samples returned from the near-Earth asteroid (NEA) Bennu by NASA's OSIRIS-REx mission. It addresses the question of how regolith accumulates on small NEAs, given their extremely low gravity. The study combines detailed physical analysis of Bennu samples, laboratory experiments of impacts into simulant rocks, and 3D numerical simulations of disruptive impacts into boulders. The key finding is that the majority (85% by mass) of impact fragments eject toward and penetrate the asteroid's weak, porous surface, leading to their retention. This contrasts with the previous assumption that impact-generated rock fragments escape into space. The Bennu samples exhibit impact craters up to a few millimeters wide, indicating that impact fragments and impact-processed rocks are retained despite the microgravity environment. Crater depth-to-diameter ratios (d/D) suggest that the Bennu samples (median crater d/D = 0.36 $\pm$ 0.1) are structurally representative of the asteroid's large boulders (median crater d/D = 0.33 $\pm$ 0.08, measured previously). The analyses indicate that most of Bennu's surface rocks (those with diameters $\lesssim$ 20 m) could be products of in situ collisional disruption. This impact-driven mechanism of regolith production likely occurs on other small NEAs with highly porous surfaces. This research has implications for understanding the surface evolution of asteroids and for planning future missions to these bodies.

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