The neodymium and samarium isotope compositions and cosmic ray exposure history of B-type asteroid (101955) Bennu
Zachary A. Torrano, Michelle K. Jordan, Lori N. Willhite, Richard W. Carlson
Geochimica et Cosmochimica Acta, In Press, Journal Pre-proof, Available online 12 March 2026
“We report Nd and Sm isotopic compositions of a sample returned from asteroid (101955) Bennu by the NASA OSIRIS-REx mission to evaluate potential genetic relationships between Bennu, known meteorite groups, and returned samples from asteroid Ryugu, and to track the cosmic ray exposure history of Bennu. We measured Nd and Sm isotopic compositions via thermal ionization mass spectrometry using 1013 Ω amplifiers. We also explore the constraints that determine the sample sizes where such high-gain amplifiers can be expected to provide improved analytical precision to that of 1011 Ω amplifiers. Bennu exhibits negative µ142Nd and µ144Sm isotopic compositions, both consistent with the nucleosynthetic anomalies previously documented for carbonaceous chondrites and Ryugu. Bennu also exhibits a resolvable deficit in 149Sm that is the result of exposure to galactic cosmic rays as demonstrated by the correlation between 150Sm/152Sm and 149Sm/152Sm ratios that fall along the expected neutron capture correlation line. The 149Sm deficit is consistent with an exposure age of 4.4 ± 1.2 Myr assuming sampling of near surface material, which overlaps with the CRE ages inferred for Bennu from other parameters.”
