High-resolution observations of bright boulders on asteroid Ryugu: 2. Spectral properties

Rie Honda, Shingo Kameda, Yosuhiro Yokota, Koki Yumoto, Minami Aoki, Daniella N. DellaGiustina, Tatsuhiro Michikami, Takahiro Hiroi, Deborah L. Domingue, Patrick Michel, Stefan Schröder, Tomoki Nakamura, Manabu Yamada, Naoya Sakatani, Toru Kouyama, Chikatoshi Honda, Masahiko Hayakawa, Moe Matsuoka, Hidehiko Suzuki, Kazuo Yoshioka, Kazunori Ogawa, Hirotaka Sawada, Masahiko Arakawa, Takanao Saiki, Hiroshi Imamura, Yasuhiko Takagi, Hajime Yano, Kei Shirai, Chisato Okamoto, Yuichi Tsuda, Satoru Nakazawa, Yuichi Iijima, Seiji Sugita

Icarus
In Press, Journal Pre-proof, Available online 19 June 2021

LINK

“Highlights

• We conducted spectroscopic analysis of newly found >70 bright boulders.
• S-type bright boulders on Ryugu follow two different space-weathering tracks.
• Ryugu’s parent body may have been hit by more than one large projectile.
• Largest S-type clast embedded in large breccia indicated no serpentine absorption.
• Spectral trend of C-type bright boulders resembles CM/CI heating tracks.”

“Many small boulders with reflectance values higher than 1.5 times the average reflectance have been found on the near-Earth asteroid 162,173 Ryugu. Based on their visible wavelength spectral differences, Tatsumi et al. (2021, Nature Astronomy, 5, doi:doi:10.1038/s41550-020-1179-z) 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 Ryugu’s 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 105–106 years, which is consistent with the crater retention age (<106 years) of the ~1-m deep surface layer on Ryugu. This agreement strongly suggests that Ryugu’s surface is extremely young, implying that the samples acquired from Ryugu’s 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. Furthermore, the distribution of the spectra of general dark boulders, which constitute >99.9% of Ryugu’s volume, is located along the trend line in slope/UV-index diagram that is occupied by C-type bright boulders. These results indicate that thermal metamorphism might be the dominant cause for the spectral variety among the C-type bright boulders on Ryugu and that general boulders on Ryugu may have experienced thermal metamorphism under a much narrower range of conditions than the C-type bright boulders. This supports the hypothesis that Ryugu’s parent body experienced uniform heating due to radiogenic energy rather than impact heating.”