Diverse lithologies in Bennu stones revealed using X‐ray computed tomography under pristine conditions

Abstract The OSIRIS‐REx mission collected a 121.6 g regolith sample from the surface of asteroid Bennu. Preliminary examination (PE) was conducted jointly by JSC Curation and the OSIRIS‐REx Sample Analysis Team. To enable advanced, non‐destructive characterization during PE, 59 centimeter‐scale stones were individually scanned using X‐ray computed tomography (XCT) inside custom‐built containers to preserve their pristine condition. The XCT data provide three‐dimensional petrographic context that supports PE, sample processing and allocation, and subsequent scientific investigations. We analyzed 14.46% ± 0.03% (17.585 ± 0.038 g) of the total returned sample mass and document extensive lithologic diversity within the collection. The samples are divided into two primary groups, monolithic and clastic, which are further subdivided into nine lithological categories. Using XCT‐derived volumes combined with measured masses, we find that the monolithic group is denser than the clastic group, and some monolithic lithologies exhibit statistically higher average densities, likely reflecting differences in composition and/or internal structure. Shape, quantified using orthogonal caliper measurements and Sneed & Folk and Zingg classifications, does not systematically correspond to lithology. The monolithic‐dark‐veined lithology is an exception, exhibiting consistently flat, bladed shapes. Correspondence between stone morphology, defined as hummocky, angular, or mottled, and lithology varies. Using X‐ray attenuation contrast in conjunction with prior petrographic and compositional studies, we identify veins, large carbonates, candidate chondrules, coarse‐grained pyrrhotite assemblages, ~25 μm long highly‐X‐ray‐attenuating phases that possibly contain platinum group elements and surface micro‐impact craters. These results provide a three‐dimensional petrographic framework for continued characterization of the returned sample and for deciphering Bennu's origin and geological history.