Toward new frontiers : Encounter and synergy of state-of-the-art astronomical detectors and exotic quantum beams

Program B01-3	Muon analysis procedures for carbonaceous material in samples recovered by Hayabusa2 asteroid sample return mission
Principal Investigator	NAKAMURA, Tomoki (Tohoku University)

Hayabusa2 mission was developed based on heritage of Hayabusa mission that realized the first successful recovery of asteroidal sample to the Earth. Hayabusa2 spacecraft was launched in Nov 2014, arrived at C-type asteroid Ryugu around July 2018, and succeeded to collect surface samples in 2019. It was coming back to the Earth in 2020 and initial analysis of the returned samples will start from June 2021. Ryugu is classified to C-type asteroids that are expected to contain high abundance of organic material. Remote sensing observation from the spacecraft showed that the asteroid shows spinning top shape with diameter of 900m and very low density (1.2 g/cc) (Watanabe et al. (2019)), suggesting that Ryugu is not a first-generation asteroid but a rubble-pile asteroid formed by re-accretion of impacted and disrupted first-generation asteroid. The reflectance spectra in the visible and near-infrared wavelength indicate that the surface is very dark and homogeneous and hydrated minerals are distributed on the entire surface (Sugita et al. (2019): Kitazato et al. (2019)).

Fig. 1. Asteroid Ryugu (left: ∼900 m in diameter) and surface rock (right: the figure width is ∼1 m)
Nakamura, the principal investigator of this study, is a member of the initial analysis group, which is the first group to analyze the samples recovered from Ryugu, and is the leader of the team that analyzes coarse (1–5 mm in diameter) particles among the recovered samples. A final rehearsal analysis was performed in April 2021 at J-PARC MLF on a carbonaceous meteorite that is expected to be similar to the Ryugu sample, and it was confirmed that carbon can be quantified with high precision with an exposure to negative muon for about five days.

Nakamura has been engaged in material science research on carbonaceous meteorites flying from C-type asteroids for 30 years and is familiar with these meteorites. In addition, he has participated as a core member in past sample return missions, systematically analyzing comet-recovered samples and asteroid samples recovered by Hayabusa (first) mission to elucidate the materials of primitive solar system bodies and to study the evolution of the early solar system. We would like to elucidate the formation and evolution process of the C-type asteroid Ryugu by comprehensively analyzing the samples recovered by Hayabusa2. In this context, the carbon concentration to be determined in this study is a fundamental elemental abundance that characterizes the C-type asteroid Rryugu, and is an important chemical data that serves as an indicator of the organic content of the asteroid. Since this analysis is non-destructive, it can be performed using about 100 mg of a valuable Ryugu sample. The measured samples will be subjected to various mineralogical and chemical analyses to clarify the formation and evolution history of asteroid Ryugu in the early solar sysytem.

Members

Reference Materials

• T. Morota, T. Nakamura (30^th/95 authors), et al., “Sample collection from asteroid (162173) Ryugu by Hayabusa2: Implications for surface evolution,” Science 368, 654–659 (2020).
• S. Watanabe, T. Nakamura (12^th/88 authors), et al., “Hayabusa2 observations of the spinning-top-shaped carbonaceous asteroid 162173 Ryugu,” Science 364, 268–272 (2019).
• K. Kitazato, T. Nakamura (8^th/58 authors), et al., “Surface composition of asteroid 162173 Ryugu as observed by the Hayabusa2 NIRS3 instrument,” Science 364, 272–275 (2019).
• S. Sugita, T. Nakamura (27^th/120 authors), et al., “The geomorphology, color, and thermal properties of Ryugu: Implications for parent-body processes,” Science 364, eaaw0422 (2019).
• T. Nakamura et al., “Itokawa dust particles: A direct link between S-type asteroids and ordinary chondrites,” Science 333, 1113–1116 (2011).
• T. Noguchi, T. Nakamura, et al., “Incipient space weathering observed on the surface of Itokawa dust,” Science 333, 1121–1125 (2011).
• K. Nagao, R. Okazaki, T. Nakamura, et al, “Irradiation history of Itokawa regolith material deduced from noble gases in the Hayabusa samples,” Science 333, 1128–1131 (2011).
• T. Nakamura, T. Noguchi, A. Tsuchiyama, T. Ushikubo, N. T. Kita, J. W. Valley, M. E. Zolensky, Y. Kakazu, K. Sakamoto, E. Mashio, K. Uesugi, and T. Nakano, “Chondrulelike objects in short-period comet 81P/Wild 2,” Science 321, 1664–1667 (2008).