In images of near-Earth asteroid (101955) Bennuu acquired by the OSIRIS-REx Camera Suite (OCAMS), we discovered six unusually bright boulders >1.5 m. These boulders are approximately three to six times as bright as the bulk of Bennu’s surface, which has a normal albedo of 4.4%. In MapCam color images, they exhibit a 0.70/0.85 μm band ratio distinct from that of the global average spectrum of Bennu, signifying an absorption at 1 μm that can be diagnostic of either pyroxene or olivine.
To further characterize these bright boulders, we examined spectra collected by the OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS), which indicated that they contain pyroxene. To assess the pyroxene’s composition, we examined the absorption band centers near 1 and 2 μm and applied the Modified Gaussian Model (MGM), which can separate absorptions of high-calcium pyroxene (HCP) from those of low-calcium pyroxene (LCP) to estimate the percentage of HCP. In terms of both estimated HCP% and band centers, the pyroxene-bearing boulders on Bennu spectrally correspond to HED meteorites, which originate from the Vesta family. Bennu most likely inherited the exogenic HED-like pyroxene-bearing material from its parent body, rather than receiving directly in the form of impacts. This is because the parent body could have sustained a larger number of probable impacts given its greater size and lifetime in the main belt, relative to Bennu.
Our discovery of HED-like material on Bennu is complimented by that of ordinary chondrite-like boulders on (162173) Ryugu by the Hayabusa2 mission (Tatsumi et al., this meeting). The finding of differing exogenic lithologies on Bennu and Ryugu provides previously unrecognized constraints on their respective origins and may indicate that they experienced distinct collisional histories.