[P-PS03] 太陽系小天体：太陽系の形成と進化における最新成果と今後の展望

[PPS03-P03] Distributions of thermal inertias of boulders observed by the low-altitude operation of the Hayabusa2

*大杉 歩^1,2、坂谷 尚哉²、金丸 仁明¹、石崎 拓也²、嶌生 有理²、千秋 博紀³、荒井 武彦⁴、出村 裕英⁵、神山 徹⁶、関口 智彦⁷、田中 聡²、岡田 達明^2,1 (1.東京大学、2.宇宙航空研究開発機構 宇宙科学研究所、3.千葉工業大学、4.前橋工科大学、5.会津大学 、6.産業技術総合研究所、7.北海道教育大学)

キーワード：はやぶさ２、温度、岩塊、リュウグウ、TIR、熱慣性

Thermophysical properties of the C-type asteroid 162173 Ryugu were investigated using the thermal infrared imager (TIR) onboard Hayabusa2 [1-3]. Ryugu is a low bulk density, entirely boulder-covered rubble-pile with thermal inertia of 200-400 J m^-2 s ^-0.5 K^-1 (hereafter tiu), which is lower than the 600-1000 tiu of typical carbonaceous chondrites [4, 5]. However, there are still significant uncertainties in determining the thermal inertia of the unconformable boulders. Here, we describe the release paths of the MINERVA rover (MNRV) on September 21, 2018, and the MASCOT lander (MSCT) on October 3, 2018, the first touchdown (TD1-L08E1) operation on February 21, 2019, and the touchdown rehearsal on October 15, 2018 (TD1-R1A), and decent operation for S01 (DO-S01), March 8, 2019, where we analyzed TIR images taken at altitudes below 500 m. We mainly focused on a statistically significant number of boulders taken over a 100 pixels area corresponding to several meters to several tens of meters in diameter. These boulders had different temperatures from each other and had a temperature distribution on the surface due to the shape and orientation of the asteroid. Despite this, the temperatures in each area were Gaussian-distributed, indicating that most of the boulders had similar thermal properties [6-7]. This suggests that the average temperature of each boulder reflects typical thermal properties. The resulting histograms for the thermal inertia values were distributed around 50-500 tiu in all regions. Their peak values showed a similar trend to the global thermal inertia values of Shimaki et al. (2021). Several boulders with very high and very low thermal inertia compared to others reported by Okada et al. (2020) and Sakatani et al. (2021) [3,5]. These histograms did not overlap in TD1-R1A and TD1-L08E1, even though the regions overlapped. This fact indicated that the thermal inertia varied with the changes in solar distance and local time when the images were taken.
Reference: [1] Okada T. et al., SSR, 208, 255 (2017), [2] Watanabe S. et al., Science 364, 268 (2019), [3] Okada T. et al., Nature 579, 518 (2020). [4] Shimaki Y. et al., Icarus 348, 113835 (2020). [5] Sakatani N. et al., Nat. Astron. 5, 766 (2021).[6] Ohsugi et al., JpGU Meeting 2022, PPS03-03, [7] Ohsugi et al., JpGU Meeting 2023, PPS03-05