Predicting Space Weathering Effects on Primitive Asteroids (101955) Bennu and (162173) Ryugu

Two sample-return missions, OSIRIS-REx and Hayabusa 2, have just arrived at primitive near-Earth asteroids (NEAs) (101955) Bennu and (162173) Ryugu, and it is of interest to predict the effects of space weathering on their surfaces. Most spacecraft-accessible NEAs originate in the inner asteroid belt, and these two sample-return targets most likely originated in the Polana asteroid family (e.g., Campins et al. 2018 and references therein). Spectroscopic studies of primitive inner-belt families offer a preview of the properties expected in the NEAs they produce. So far, primitive asteroids in the inner-belt fall into at least two spectral groups. The first group includes the Polana, Eulalia and Clarissa families, which show considerable spectral homogeneity and no 0.7-μm hydration feature. In contrast, the Erigone and Sulamitis families are spectrally diverse and most of their members show clear 0.7-μm hydration features (e.g., Pinilla-Alonso et al. 2017; Morate et al. 2018). The Clarissa family is considerable younger than the Polana family, at less than 100 million years versus approximately 2000 million years, respectively (Nesvorný et al. 2015), and there are subtle yet significant spectral differences between these two families. These differences are consistent with the space weathering trend suggested by Lantz et al. (2015, 2017 and 2018). This agreement between observations of inner-belt families and laboratory simulations of space weathering has testable implications for Bennu and Ryugu: older terrains would be expected to be bluer than younger surfaces (Campins et al. 2018; de León et al. 2018).