  Many small boulders with reflectance values higher than 1.5 times the average
reflectance have been found on the near-Earth asteroid 162173 Ryugu. Based on
their visible wavelength spectral differences, Tatsumi et al. (2021) defined
two bright boulder classes: C-type and S-type. These two classifications of
bright boulders have different size distributions and spectral trends. In this
study, we measured the spectra of 79 bright boulders and investigated their
detailed spectral properties. Analyses obtained a number of important results.
First, S-type bright boulders on Ryugu have spectra that are similar to those
found for two different ordinary chondrites with different initial spectra that
have been experimentally space weathered the same way. This suggests that there
may be two populations of S-type bright boulders on Ryugu, perhaps originating
from two different impactors that hit its parent body. Second, the model
space-weathering ages of meter-size S-type bright boulders, based on spectral
change rates derived in previous experimentally irradiated ordinary chondrites,
are 0.1-1 Myr, which is consistent with the crater retention age (<Myr) of the
~1-m deep surface layer on Ryugu. This agreement strongly suggests that the
Ryugu surface is extremely young, implying that the samples acquired from the
Ryugu surface should be fresh. Third, the lack of a serpentine absorption in
the S-type clast embedded in one of the large brecciated boulders indicates
that fragmentation and cementation that created the breccias occurred after the
termination of aqueous alteration. Fourth, C-type bright boulders exhibit a
continuous spectral trend similar to the heating track of low-albedo
carbonaceous chondrites, such as CM and CI. Other processes, such as space
weathering and grain size effects, cannot primarily account for their spectral
variation.

Earth and Planetary Astrophysics