Exogenic basalt on asteroid (101955) Bennu

When rubble-pile asteroid 2008 TC₃ impacted Earth on 7 October 2008, the recovered rock fragments indicated that such asteroids can contain exogenic material^1,2. However, spacecraft missions to date have only observed exogenous contamination on large, monolithic asteroids that are impervious to collisional disruption^3,4. Here, we report the presence of metre-scale exogenic boulders on the surface of near-Earth asteroid (101955) Bennu—the 0.5-km-diameter, rubble-pile target of the OSIRIS-REx mission⁵ that has been spectroscopically linked to the CM carbonaceous chondrite meteorites⁶. Hyperspectral data indicate that the exogenic boulders have the same distinctive pyroxene composition as the howardite-eucrite-diogenite (HED) meteorites that come from (4) Vesta, a 525-km-diameter asteroid that has undergone differentiation and extensive igneous processing^7-9. Delivery scenarios include the infall of Vesta fragments directly onto Bennu or indirectly onto Bennu's parent body, where the latter's disruption created Bennu from a mixture of endogenous and exogenic debris. Our findings demonstrate that rubble-pile asteroids can preserve evidence of inter-asteroid mixing that took place at macroscopic scales well after planetesimal formation ended. Accordingly, the presence of HED-like material on the surface of Bennu provides previously unrecognized constraints on the collisional and dynamical evolution of the inner main belt.