[P-PS03] Small Solar System Bodies: New perspectives on the origin and evolution of the Solar System

[PPS03-01] Thermal evolution modelling of Ryugu’s parent asteroid

★Invited Papers

*Shigeru Wakita^1,2, Hidenori Genda³, Tomoki Nakamura⁴, Hisayoshi Yurimoto⁵, Takaaki Noguchi⁶, Ryuji Okazaki⁷, Hikaru Yabuta⁸, Hiroshi Naraoka⁷, Kanako Sakamoto⁹, Shogo Tachibana¹⁰, Sei-ichiro WATANABE¹¹, Yuichi Tsuda⁹ (1.Massachusetts Institute of Technology, 2.Purdue University, 3.Tokyo Institute of Technology, 4.Tohoku University, 5.Hokkaido University, 6.Kyoto University, 7.Kyushu University, 8.Hiroshima University, 9.Japan Aerospace Exploration Agency , 10.University of Tokyo, 11.Nagoya University)

Keywords:Asteroids, Ryugu, Model

To investigate the ancient history of asteroids, their mineralogic and isotopic study is essential. Although we generally use meteorites for this purpose and estimate their parent bodies, it is hard to determine their original asteroids. Sample return missions can bring that information at the same time. The returned samples from asteroid Itokawa given by Hayabusa spacecraft enabled us to estimate the history of parent bodies of Itokawa (Nakamura et al. 2014, Wakita et al. 2014). Hayabusa 2 spacecraft brought from asteroid Ryugu. Based on the initial analysis of Ryugu’s returned samples, they are similar to CI carbonaceous chondrites (e.g., Nakamura et al. 2022, Yada et al. 2021, Yokoyama et al. 2022). Since the size and amount of Ryugu’s returned samples are larger than that of Itokawa, their thermal properties are measurable. These are valuable data to estimate Ryugu’s parent body, in addition to mineralogic and isotopic evidence. Here we model the thermal evolution of Ryugu’s parent asteroid and constraint its formation age that satisfies the analysis data of Ryugu’s returned sample.
Assuming an ice-rock mixture spherical body as a possible parent body of asteroid Ryugu, we numerically solve the heat conduction. To estimate the original size of Ryugu’s parent body, spectroscopic data is helpful. It suggested Ryugu belongs to the Eularia or Polana family (Sugita et al. 2019). We consider their original body as the Ryugu’s parent body. As its size is estimated to be 50 km in radius, we take this size as its rocky component. Chemical modelling of Ryugu sample indicates that the initial water to rock mass ratio (W/R) is 0.2–0.9. Considering this range, the total radius of the parent body ranges from 60 km to 70 km. We use the measured physical and thermal properties of Ryugu sample for the rock in the body. While the main heat source is the decay heat of aluminum-26, we also incorporate the latent heat of the water-ice and the reaction heat of aqueous alteration in our model. We adopt the initial temperature of the body as 70 K, because the Ryugu’s parent body may form at the region where carbon dioxide can condense.
The analysis of Ryugu samples showed that the peak metamorphic temperature should be less than 100 degree Celsius (Nakamura et al. 2022). Additionally, the carbonate formed at 5.2 Myr after CAIs at the temperature of 37 degree Celsius (Yokoyama et al. 2022). To satisfy these data, we found that Ryugu’s parent asteroid formed at 1.8–2.6 Myr after CAI formation, corresponding to W/R ranges of 0.9–0.2. The majority of Ryugu’s sample may originate inside the parent body, 51 km radius from the center in the case of W/R = 0.6, where phyllosilicates can form. Because the cold surface cannot reach the ice melting temperature, the formation of phyllosilicates is limited and thus only valid for least-altered samples. Overall, the major component of Ryugu may originate from about 50 km from the center of the parent body that formed around 2 Myr after the birth of the solar system.

References:
Nakamura et al. (2014) MAPS, 49, 215.
Nakamura et al. (2022) Science, eabn8671.
Sugita et al. (2019) Science, 364, 6437.
Wakita et al. (2014) MAPS, 49, 228.
Yada et al. (2021) Nat. Astron. 6, 214.
Yokoyama et al. (2022) Science, eabn7850.