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

[PPS03-05] Relationships between temperature distributions and thermal inertias of boulders observed by the low-altitude operation of the Hayabusa2

*Ayumu Ohsugi^1,2, Naoya Sakatani², Yuri Shimaki², Masaniri Kanamaru¹, Takuya Ishizaki², Hiroki Senshu³, Takehiko Arai⁴, Hirohide Demura⁵, Toru Kouyama⁶, Tomohiko Sekiguchi⁷, Satoshi Tanaka^1,2, Tatsuaki Okada^2,1 (1.The University of Tokyo, 2.Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3.Chiba Institute of Technology, 4.Maebashi Institute of Technology, 5.University of Aizu, 6.National Institute of Advanced Industrial Science and Technology, 7.Hokkaido University of Education)

Keywords:Hayabusa2, Temperature, boulder, Ryugu, TIR, Thermal inertia

The thermophysical properties of the C-type asteroid 162173 Ryugu were investigated using the Thermal Infrared Imager (TIR) onboard Hayabusa2 [1-3]. TIR can acquire thermal emission images from Ryugu to obtain the thermophysical properties of Ryugu's surface, TIR has a field of view of 16.7º× 12.7ºand an effective detector area of 328 × 248 pixels with a spatial resolution of about 0.051º per pixel [1,3]. Ryugu is a rubble pile body with a low bulk density and entirely covered with boulders, having a thermal inertia of 200-400 J m^-2 s ^-0.5 K^-1 (hereafter, tiu), which is low compared to 600-1000 tiu of typical carbonaceous chondrites [4,5]. However, there are still large uncertainties to determine the thermal inertia of irregularly shaped boulders. Here we analyzed TIR images taken at altitudes below 500 m during the release paths for the MINERVA rover (MNRV) on 21 September 2018 and the MASCOT lander (MSCT) on 3 October 2018, the 1st touchdown (TD1-L08E1) operation on 21 February 2019, the touchdown rehearsals (TD1-R1A) on 15 October 2018, and the decent operation for S01 (DO-S01) on 8 March 2019, and examined the temperature variation of a statistically significant number of boulders taken over an area of 100 pixels or larger, corresponding to several to tens of meters in diameter. These boulders have different temperatures from each other, and there were temperature distributions on the surface due to the geometry and the orientation of the asteroid. Nevertheless, we found that the temperatures in each area are Gaussian distributed and that most of the boulders in each area have similar thermal properties [6]. This suggests that the average temperature of each boulder reflects typical thermal properties. We will present the updates from our previous work [6], including a comparative study of temperature distributions and thermal inertia. 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