  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-micron 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
mixtures of eucrites 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
byproduct of parent body and S-type projectiles, the equivalent mass of OC
material would be a sphere with a diameter of 36 to 40 m (or a volume of 24,200
to 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).
