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

Program B01-1	Development of 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 first successful recovery of asteroidal sample to the Earth. Hayabusa2 spacecraft was launched in Nov 2014, carried our Earth swing by in Dec 2015, and arrived at C-type asteroid Ryugu around July 2018. Ryugu is classified to C-type asteroids that are expected to contain hydrated minerals such as serpentine and organic material. Ground base observations of Ryugu suggested 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 top shape indicates that the asteroid was rotated much faster than the current rotation period (Watanabe et al. 2019). The reflectance spectra in the visible and near-infrared wavelength indicate that the surface mineralogical distribution seems to be heterogeneous, but most regions show reflectance spectra similar to partially dehydrated carbonaceous chondrites, implying that Ryugu surface material experienced hydration and subsequent dehydration by heating (Sugita et al. 2019: Kitazato et al. 2019).

Asteroid Ryugu (left: ∼900 m in diameter) and surface rock (right: the figure width is ∼1 m) After twice rehearsals, the first　touchdown operation was carried out on Feb 22, 2019. The operation was perfectly successful and many fragments, dispersed by a tungsten projectile firing, would have come into a sample container in the spacecraft. The collected samples will come back to the Earth 2020 winter. Six groups of different disciplines were selected and organized for initial analysis of the returned samples. I lead the teams for mineralogy and petrology of the coarse particles and now develop analysis flow to maximize science return form the returned samples. The first results of Ryugu sample analysis will be shown in early 2021.

In the present study we like to develop the muon analysis procedures for carbonaceous material in samples recovered by Hayabusa2 asteroid sample return mission. Successful application of negative muon will reveal that three dimensional distribution of C and N within the samples and bulk N/C ratio of the samples, which in the early solar nebula is proportioanl to the distance from the Sun. These data, together with mineralogical and isotope analysis results, will clarify the nature of the carbonaceous material in C-type asteroid Ryugu and suggest the heliocentric distance from the Sun when Ryugu was formed. Carboneous material of C-type asteroid is one of the candidate for the origin of life on the Earth and therefore muon analysis will gratly contribute to better understanding of contribution of C-type asteroid material to the life on the Earth.

Members

Reference Materials

• S. Watanabe, … , T. Nakamura (12th/88 authors) et al., “Hayabusa2 observations of the spinning-top-shaped carbonaceous asteroid 162173 Ryugu,” Science 364, 268–272 (2019).
• K. Kitazato, … , T. Nakamura (8th/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 (27th/120 authors) et al., “The geomorphology, color, and thermal properties of Ryugu: Implications for parent-body processes,” Science 364, eaaw0422 (2019).
• T. Nakamura, T. Noguchi, M. Tanaka, M. E. Zolensky M. Kimura, A. Tsuchiyama, A. Nakato, T. Ogami, H. Ishida, M. Uesugi, T. Yada, K. Shirai, A. Fujimura, R. Okazaki, S. A. Sandford, Y. Ishibashi, M. Abe, T. Okada, M. Ueno, T. Mukai, M. Yoshikawa, J. Kawaguchi, “Itokawa dust particles: A direct link between S-type asteroids and ordinary chondrites,” Science 333, 1113–1116 (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, T. Nakano, “Chondrulelike objects in short-period comet 81P/Wild 2,” Science 321, 1664–1667 (2008).