THE COMPOSITION OF ASTEROID BENNU FROM THE OSIRIS-REX MISSION

NASA's Origins, Spectral Interpretation, Resource Identification, and Security — Regolith Explorer (OSIRIS-REx) mission characterized the surface composition of the carbonaceous asteroid Bennu at visible to infrared wavelengths (~0.4 - 100 µm). Spectral features of hydrated minerals (phyllosilicates) are dominant at both visible to near infrared (VNIR) and thermal infrared (TIR) wavelengths, with ~90 vol.% of the silicates being comprised of phyllosilicates (≤~10 vol.% olivine plus pyroxene). Features of iron oxides are observed in both the VNIR (magnetite, goethite) and the TIR (magnetite). In the 3.2-3.6 µm region, we observe spatially variable evidence for carbonate minerals (some associated with meter-long, cm-wide veins) and organic compounds. A half-dozen isolated, meter-sized boulders exhibit pyroxene signatures consistent with those in the howardite-eucrite-diogenite (HED) meteorites from (4) Vesta. There is evidence for non-uniform deposits of dust (~5-10 µm thick) superposed on a largely boulder-dominated surface. The majority of VNIR features show only small band depth variations across the surface and the TIR features appear to vary dominantly with particle size; no distinctly different lithologies are detected in any of the spectral data. Space weathering may account for Bennu’s visible blue slope.

The observed mineralogy of Bennu is most consistent with an aqueously altered, CI- or CM-like, carbonaceous chondrite (CC) composition. Isolated boulders containing pyroxene are interpreted as exogenic, basaltic material from Vesta, the preserved evidence of inter-asteroid mixing that occurred after the conclusion of planetesimal formation. The manifestation of carbonate veins at scales much larger than has been observed in CC meteorites suggests that Bennu’s parent body experienced fluid flow and hydrothermal deposition on kilometer scales for thousands to millions of years. In October 2020, OSIRIS-REx collected a sample of the surface of Bennu for return to Earth in 2023. We predict that the returned sample will contain the minerals and compounds described here (phyllosilicates, iron oxides, carbonates, and organics). In addition, minerals that are difficult to detect with remote sensing data, such as sulfides, also may be present, as well as exogenous materials.