Exogenic basalt on asteroid (101955) Bennu

Description

When rubble-pile asteroid 2008 TC3 impacted Earth on October 7, 2008, the recovered rockfragments 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 thepresence of meter-scale exogenic boulders on the surface of near-Earth asteroid (101955)Bennu—the 0.5-km, rubble-pile target of the OSIRIS-REx mission [5] which has beenspectroscopically linked to the CM carbonaceous chondrite meteorites [6]. Hyperspectraldata indicate that the exogenic boulders have the same distinctive pyroxene composition asthe howardite–eucrite–diogenite (HED) meteorites that come from (4) Vesta, a 525-km-diameter asteroid that has undergone differentiation and extensive igneous processing [7,8, 9]. Delivery scenarios include the infall of Vesta fragments directly onto Bennu orindirectly onto Bennu's parent body, where the latter's disruption created Bennu from amixture of endogenous and exogenic debris. Our findings demonstrate that rubble-pileasteroids can preserve evidence of inter-asteroid mixing that took place at macroscopicscales well after planetesimal formation ended. Accordingly, the presence of HED-likematerial on the surface of Bennu provides previously unrecognized constraints on thecollisional and dynamical evolution of the inner main belt.

Abstract

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