The role of composition in the spectral evolution of carbonaceous asteroids as informed by Bennu sample return

Abstract Near‐Earth rubble‐pile asteroids Bennu and Ryugu are part of the carbonaceous taxonomic complex (C‐complex), and samples returned from both bodies resemble the most aqueously altered carbonaceous chondrites. However, telescopic and spacecraft visible–near infrared (VIS–NIR) reflectance spectra of Ryugu exhibit a red (positive) spectral slope, whereas Bennu has a blue (negative) spectral slope characteristic of the rare B‐type subclass of asteroids. The asteroid spectra also suggest different levels of hydration, with Ryugu dominated by OH and Bennu containing spectral evidence of more H 2 O. To understand what causes these differences, we acquired VIS–NIR reflectance data (~0.3–5 μm) from a variety of Bennu samples over spatial scales of 100 μm to several millimeters. No single sample reproduces the average spectral properties of Bennu, but by evaluating samples of different petrology and physical states—groups of particles, isolated particles, and larger stones—we demonstrate that primary composition, and highly hydrated Mg‐rich phosphate in particular, plays a strong role in controlling the spectral slope and average hydration absorption strength of Bennu materials. Bennu and Ryugu may be dominated by different lithologies originating from different regions of a common planetesimal, thus explaining their different spectral evolution. The spectral characteristics of B‐type asteroids, particularly those with blue slopes at near‐infrared wavelengths and broad hydration features at ~3 μm, may indicate the presence of Mg phosphate and thus a history of complex fluid–rock interactions relevant to prebiotic chemistry.