Assessing effects of particle size on water content estimates of carbonaceous chondrites and their parent bodies

The Hayabusa2 and OSIRIS-REx asteroid sample-return missions have recently encountered the hydrous C-class asteroids 162173 Ryugu and 101955 Bennu, respectively. Mission data reveal that these objects are rubble pile asteroids with surfaces that are dominated by boulders. Previous studies of carbonaceous chondrite (CC) powders showed that certain reflectance spectral parameters are highly correlated with hydrogen. As such, additional laboratory studies are warranted to understand how to convert OH/H 2 O absorptions seen in spectral reflectance to estimates of absolute water content of objects that lack fine-grained regolith. This study compares OH/H 2 O reflectance spectral absorption features observed for several intact CM2 chips and size-sieved powder fractions; these data are then compared with independent estimates of bulk water content. Together, the results are used to construct empirical relationships between reflectance band strength, water content, and particle size. We find that for CM2s a variety of band parameter values related to OH/H 2 O increase with decreasing particle size. If particle size is not properly accounted for, this may lead to water content underestimates of as much as 6 wt.%. This highlights the need for improved methods to extract quantitative information from reflectance spectra of regolith-poor (rubble-pile) asteroids as well as areas of the Moon.