High-resolution observations of bright boulders on asteroid Ryugu: 1. Size frequency distribution and morphology

The near-Earth asteroid (162173) Ryugu displays a Cb-type average spectrum and a very low average normal albedo of 0.04. Although the majority of boulders on Ryugu have reflectance spectra and albedo similar to the Ryugu average, a small fraction of boulders exhibit anomalously high albedo and distinctively different spectra. A previous study (Tatsumi et al., 2021Nature Astronomy, 5, doi:https://doi.org/10.1038/s41550-020-1179-z) based on the 2.7-km observations and a series of low-altitude (down to 68 m) descent observations conducted prior to the first touchdown have shown that the spectra of these anomalous boulders can be classified into two distinct groups corresponding to S and C type asteroids. The former originate most likely from an impactor that collided with Ryugu's parent body, whereas the latter may be from portions of Ryugu's parent body that experienced a different temperature history than experienced by the majority of boulder materials. In this study, we analyzed images captured after the first touchdown to determine the quantitative properties of these bright boulders on Ryugu. We measured the sizes of more than a thousand bright boulders and characterized the morphologic properties of the largest ones. Analyses revealed many properties of bright boulders important for the evolution of Ryugu and its parent body. First, the size-frequency distributions of S-type and C-type bright boulders follow a power law with exponents of 1.6 ± 1.3 and 3.0 ± 0.7, respectively. Based on these size-frequency distributions, we obtained the ratios of the total volume and surface area of S-type bright boulders to those of average dark boulders on the Ryugu's surface, that is, 7.1_-5.0^+6.3 × 10^-6 and 1.5_-1.2^+3.2 × 10^-6, respectively, over the diameter range of 0.3 to 3 m. Similarly, the ratio of the total volume and surface area of C-type bright boulders to those of average dark boulders are 4.4_-2.2^+14.0 × 10^-5 and 1.3_-1.1^+9.8 × 10^-3, respectively, at a diameter range of 2 cm to 2 m. Second, the number density of bright boulders inside the artificial crater newly made by the Small Carry-on Impactor (SCI) experiment agrees with the outside number density within a factor of two. Third, many of the bright boulders are embedded in a larger substrate boulder, suggesting that they have experienced mixing and conglomeration with darker fragments on Ryugu's parent body, rather than gently landing on Ryugu during or after its formation by reaccumulation. This observation is consistent with the hypothesis that S-type bright boulders were likely mixed during and/or before a catastrophic disruption. C-type bright boulders embedded in substrate boulders suggests a brecciation process after thermal metamorphism. Furthermore, the embedding of S-type clasts in substrate boulders suggests that brecciation did indeed occur even after a large-scale impact on the parent body. If the brecciation on the Ryugu's parent body occurred over such a long period or over many stages of its evolution, breccias may end up being the dominant constituent materials on Ryugu's parent body. Moreover, the preponderance of breccias may contribute to the globally low thermal inertia of Ryugu.