Exogenic basalt on asteroid (101955) Bennu

D. N. Della Giustina, H. H. Kaplan, A. A. Simon, W. F. Bottke, C. Avdellidou, M. Delbo, R.-L. Ballouz, D. R. Golish, K. J. Walsh, M. Popescu, H. Campins, M. A. Barucci, G. Poggiali, R. T. Daly, L. Le Corre, V. E. Hamilton, N. Porter, E. R. Jawin, T. J. McCoy, H. C. Connolly Jr, J. L. Rizos Garcia, E. Tatsumi, J. de Leon, J. Licandro, S. Fornasier, M. G. Daly, M. M. Al Asad, L. Philpott, J. Seabrook, O. S. Barnouin, B. E. Clark, M. C. Nolan, E. S. Howell, R. P. Binzel, B. Rizk, D. C. Reuter & D. S. Lauretta

Nature Astronomy (2020), Published: 21 September 2020


“When rubble-pile asteroid 2008 TC3 impacted Earth on 7 October 2008, the recovered rock fragments indicated that such asteroids can contain exogenic material. However, spacecraft missions to date have only observed exogenous contamination on large, monolithic asteroids that are impervious to collisional disruption. 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 mission that has been spectroscopically linked to the CM carbonaceous chondrite meteorites. 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. 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.”