The Heterogenous Hydrogen Isotopic Composition of Bennu Samples

Carbonaceous asteroids are potential contributors to water in the inner solar system and are likely to have contributed to the Earth's water budget. A fundamental tool that provides important constrains on the distribution of water reservoirs at the time of planetary formation is the hydrogen isotopic composition of primitive rocks derived from carbonaceous asteroids. However, only few primitive undifferentiated meteorites (chondrites) are avail-able for certain types of carbonaceous asteroids, and the potential of terrestrial contamination limits our ability to comprehend the distribution of water reservoirs in the protoplanetary disk from such materials.

On September 24, 2023, NASA’s OSIRIS-REx mission delivered to Earth 121.6 g of fresh and pristine carbonaceous regolith from asteroid (101955) Bennu. The petrology of Bennu samples revealed that it the closest meteorite analogs are the chemically primitive chondrites, Ivuna-type carbonaceous chondrites (CIs), and the samples returned by the JAXA’s Hayabusa2 mission from the carbonaceous asteroid (162173) Ryugu. Bennu’s rock fragments underwent extensive aqueous alteration on the parent asteroid and are predominantly composed of hydrated sheet silicates (serpentine, smectite; ~80 vol.%) and other secondary minerals [1-2]. In order to obtain hints on the initial volatile-bearing constituents of Bennu’s parent asteroid (i.e., ices and organics), we have measured the D/H and C/H ratios in chondritic matrices by secondary ion mass spectrometry (SIMS) in individual fine- to intermediate-sized particles of Bennu.