Evidence for water-rock interaction in VNIR and TIR spectra of B-type asteroid (101955) Bennu

The asteroid (101955) Bennu was selected as the sample return target of NASA's OSIRIS-REx mission based on its classification as a B-type, analogous to hydrated, organic-bearing, carbonaceous chondrite meteorites. Early spectral results from the OVIRS and OTES instruments onboard the spacecraft revealed the surface to be dominated by phyllosilicate minerals like those contained in CI and CM carbonaceous chondrites. Specifically, a water/OH^-absorption in all OVIRS spectra and the silicate bending mode feature in all OTES data are diagnostic of the presence and volumetric dominance of these minerals and indicate that Bennu's parent body experienced some form of water-rock interaction early in Solar System history. Comparisons with laboratory spectra of aqueously altered and heated CM meteorites confirm that the global average spectrum of Bennu is most similar to low petrologic subtype (≤2.5) CM meteorites, with the potential for heating subsequent to aqueous alteration. Data collected by both instruments in the Detailed Survey mission phase have shown that there are spatial variations in the spectral signatures that correlate with both albedo and temperature/thermal inertia. OVIRS spectral slope variations are not yet uniquely interpretable and may be associated with, for example, particle size variations or space weathering. OVIRS also displays complex, variable absorption bands in the 3.2-3.6 µm range that may be attributable to C-bearing compounds (organics) and/or minerals (carbonates) altered by or formed in water-bearing environments. The band area of these spectral features does not vary spatially in a manner that correlates with other spectral features, albedo, or temperature. The majority of OTES spectral variations can be described by two spectral end members, one with a silicate stretching band minimum near 1000 cm^-1and a relatively deep silicate bending band minimum around 440 cm^-1, and the other with a silicate stretching band minimum near 800 cm^-1and a relatively shallow 440 cm^-1band. The detailed mineralogy of these two spectral end members remains a subject of analysis, but their distribution corresponds spatially with differences in temperature/thermal inertia and albedo and likely represents differences in composition and/or particle size, as well as potentially space weathering.