Evidence for Projectile Tunneling at Small Craters on Asteroid (101955) Bennu

Asteroid (101955) Bennu has a diverse cratering record with hundreds of mapped craters ranging from metres to >100 m in diameter. The rock comprising Bennu is a hydrated carbonaceous chondrite lithology based upon its albedo, spectral features and bulk density. The indicated lithology ranks as the second "weakest" of the four broad lithology classes based upon meteoroid collision survivourship (representing a material property such as strength) as indicated by four discrete meteoroid cosmic-ray-exposure age ranges. Bennu's relatively weak lithology may allow for frequent development of projectile tunnels underneath small craters or as standalone features.

Projectile tunnels have been found at small ( ≤ 20 m) terrestrial impact craters. In these impacts, the hypersonic projectiles were relatively strong compared to the impacted material—being able to survive the impact with no disruption, while able to induce a crater-excavating flow field in the target. The surviving projectiles push into the target underneath the crater, making a tunnel of approximately their own diameter before stopping as much as >10 m below the crater floor. Formation of these tunnels crushes the intersected volume of the target to significantly finer sizes than the initially induced shock front/strong elastic wave, leading to a large production of fine particles in and "blown back" from the tunnel. Projectile tunneling is also observed on Earth when low-relative-velocity impacts occur without formation of any explosion crater.

On asteroid Bennu, projectile tunneling is evidenced by 1) dark patches of coating fines located centrally in small craters and 2) as anomalously shaped small craters. For example, a relatively young ~3 m-diameter crater shows a shallow profile, a broader and rounded rim, and a surface draped with a much greater fraction of fines including very fine-grained material. Bennu's rubble-pile structure likely enhances projectile tunneling compared to the terrestrial surface with an ~40% macroporosity indicated from the low bulk density (1,190 ± 13 kg m^-3).

Acknowledgements: A.R.H., F.C., E.M.I., M.G.D. and J.S. acknowledge funding support from the C.S.A. OSIRIS-REx is supported under Contract NNM10AA11C issued through NASA's New Frontiers Program; the OSIRIS-REx team supports all aspects of this mission.