Different impact histories on Ryugu and Bennu inferred from exogeneous boulders

Near-Earth asteroid (162173) Ryugu is a rubble-pile asteroid, which is likely to originate from a catastrophic disruption of a parent body and re-accumulation by gravitational force [1]. We found enormously bright boulders on the surface of Ryugu based on the high-resolution multi-band observations by the telescopic optical navigation camera (ONC-T) of Hayabusa2 [2, 3]. They could retain the record of the catastrophic disruption and internal structure of the parent body. Data and method: During the MASCOT hovering operation on 3–4 October 2018, we obtained the high-resolution multi-band images (~0.3 m/pixel) around equatorial zone. We have identified 21 bright boulders which have reflectance of 1.5 times more than median value of surface [3]. We measured their reflectance factor at (i, e, alpha) = (30°, 0°, 30°) and normalized spectra with seven color filters with effective wavelength ranging 0.40 to 0.95 µm. Results: The bright boulder are classified into two groups: S-type with absorption towards 1 µm and C/X-type without the absorption. S-type bright boulders were also observed by near-infrared spectrometer (NIRS3) [4]. S-type bright boulders do not present a strong 2-µm band absorption, which is consistent with anhydrous silicates. However, the range of normal albedo, 9-22%, suggests more similar to olivine-rich ordinary chondrites. Discussions: The S-type bright boulders could originate from S-type asteroids, because S-type asteroids are the most abundant in the inner main asteroid main belt, which is Ryugu’s likely source [5]. Collisional mixture might form bright boulders on Ryugu. Since statistically it is difficult to accumulate bright boulders on Ryugu’s surface after the asteroid was formed, they had to be incorporated in Ryugu’s parent body during its catastrophic disruption event. Moreover, the discovery that bright boulders on Bennu and Ryugu with anhydrous spectra exhibit different strength of 2-µm band absorption indicates that they are from different original parent bodies.

1. Watanabe et al. (2019) Science 364, 268.

2. Sugita et al. (2019) Science 364, eaaw0422.

3. Tatsumi et al. (2020) in press

4. Kitazato et al. (2019) Science 364, 272.

5. Campins et al. (2013) ApJ 146, 26.