Characterization of Exogenic Boulders on the Near-Earth Asteroid (101955) Bennu from OSIRIS-REx Color Images

Abstract

Abstract A small number of anomalously bright boulders on the near-Earth rubble-pile asteroid (101955) Bennu were recently identified as eucritic material originating from asteroid (4) Vesta. Building on this discovery, we explored the global presence of exogenic boulders on Bennu. Our analysis focused on boulders larger than 1 m that show the characteristic 1 μm pyroxene absorption band in the four-color MapCam data from the OSIRIS-REx mission. We confirm the presence of exogenic boulders similar to eucrites and find that a mixture of eucrite with carbonaceous material is also a possible composition for some boulders. Some of the exogenic boulders have spectral properties similar to those of ordinary chondrite (OC) meteorites, although the laboratory spectra of these meteorites have a higher albedo than those measured on Bennu, which could be explained by either a grain size effect, the presence of impact melt, or optical mixing with carbonaceous material owing to dust coating. Our Monte Carlo simulations predict that the median amount of OC mass added to the parent body of Bennu is 0.055% and 0.037% of the volume of a 100 and 200 km diameter parent body, respectively. If Bennu was a uniformly mixed by-product of parent body and S-type projectiles, the equivalent mass of OC material would be a sphere with a diameter of 36–40 m (or a volume of 24,200–33,600 m3). The total amount of OC material in the interior of Bennu estimated from the MapCam data is slightly higher (91,000–150,000 m3).