Neodymium-142 deficits and samarium neutron stratigraphy of C-type asteroid (162173) Ryugu

We report Nd and Sm isotopic compositions of four samples of Ryugu returned by the Hayabusa2 mission, including "A" (first touchdown) and "C" (second touchdown) samples, and several carbonaceous chondrites to evaluate potential genetic relationships between Ryugu and known chondrite groups and track the cosmic ray exposure history of Ryugu. We resolved Nd and Sm isotopic anomalies in small (<20 ng Nd and Sm) sample sizes via thermal ionization mass spectrometer using 10(13) Omega amplifiers. Ryugu samples exhibit resolvable negative mu Nd-142 values consistent with carbonaceous chondrite values, suggesting that Ryugu is related to the parent bodies of carbonaceous chondrites. Ryugu's negative mu Sm-149 values are the result of exposure to galactic cosmic rays, as demonstrated by the correlation between Sm-150/Sm-152 and Sm-149/Sm-152 ratios that fall along the expected neutron capture correlation line. The neutron fluence calculated in the "A" samples (2.75 +/- 1.94 x 10(15) n cm(-2)) is slightly higher compared to the "C" samples (0.95 +/- 2.04 x 10(15) n cm(-2)), though overlapping within measurement uncertainty. The Sm results for Ryugu, at this level of precision, thus are consistent with a well-mixed surface layer at least to the depths from which the "A" and "C" samples derive.