坂谷 尚哉

令和2年度宇宙科学に関する室内実験シンポジウム（2021年3月5日. オンライン開催) 2021 Symposium on Laboratory Experiment for Space Science (March 5, 2021. Online Meeting) PDF再処理の為、2023年3月7日に差替 資料番号: SA6000161036 レポート番号: 36

<p>Thermal imaging, or thermography, has revealed the surface physical state of the C-type near-Earth asteroid 162173 Ryugu (Okada et al., 2020). The asteroid is the target body of JAXA Hayabsua2 asteroid sample return mission, and it has been characterized through remote sensing and surface experiments, and will be deeply and accurately investigated by analysis of returned sample. Thermal observations are among such multi-scale observations, providing a new insight into understanding planetary evolution process.</p> <p>Thermal infrared imager TIR (Okada et al., 2017; 2020) was used to take one-rotation global thermal images of Ryugu at every 6° step, from the home position (20 km altitude) or from the Mid-Altitude (5 km altitude). There were two big surprises contrary to the predictions before arrival at Ryugu: i) flat diurnal temperature profiles compared to the case of non-rough surface, and ii) non-cold spots identified for most of boulders. The flat diurnal temperature profiles and its maximum temperature in a day indicate that Ryugu must have very rough surfaces made of highly porous materials, derived from the thermal inertia of 300 ± 100 J K<sup>-1</sup>s<sup>-0.5</sup>m<sup>-2</sup> (hereafter, tiu). Non-cold boulders indicate that boulders are less consolidated or compacted than typical carbonaceous chondrite meteorites, and shows the same thermophysical properties as the surroundings. TIR was also used to take close-up thermal images during the descent operations, and to have proven that the surface of asteroid is covered with fragments of porous rocks, larger than several centimeters in diameter. The typical size of fragments larger than thermal skin depth (~35 mm) results in similar thermal properties between the boulders and their surroundings. We also consider the surface roughness effect (Shimaki et al., 2020) to obtain the maps of thermal inertia ( 225 ± 45 tiu) and the roughness (0.41 ± 0.05) at the same time, corresponding to very rough surfaces made of highly-porous materials. This thermal inertia is basically consistent with the value (282 +93/-35 tiu) by in situ measurement using a thermal radiometer MARA on MASCOT lander (Grott et al., 2019). Furthermore, in the close-up thermal images, there were found boulders colder by 20 °C or more, indicating the thermal inertia of typical carbonaceous chondrite meteorites.</p> <p>Considering these results, we proposed a formation scenario of Ryugu: fluffy cosmic dusts gathered to form porous planetesimals, and then much larger sized but still porous bodies. A low degree of consolidation and alteration has occurred at most of the body, while a higher degree of consolidation or alteration proceeded at the deep interior. Huge meteoritic impacts destroyed and fragmented the bodies, and part of those fragments were re-accreted to form the next generation, rubble-pile bodies (asteroids). Boulders found on Ryugu might have originated from the deep interior of parent bodies, so that most of them are very porous and less consolidated but some of them are relatively dense materials similar to carbonaceous chondrites, which might have originated from the interior. Due to YORP effect, the rotation rate decreased to current one, and the current shape of a spinning top-shape were formed. Analysis of returned sample will make progress in our knowledge of the planetary formation process.</p>

小型・分離型のカメラシステムDCAM3（分離カメラ）は，Hayabusa2が小惑星Ryugu上で衝突実験を行う際に探査機から分離され，近傍から衝突の観測を行なった．DCAM3は，イジェクタ飛散の詳細な様子を高い空間分解能で撮影するための科学観測用カメラシステム「デジタル系（DCAM3-D）」を持つ．このデジタル系は，Hayabusa2がより高いレベルの科学目的を達成するために遅れて搭載が決定されたもので，提案から開発・完成に至るまでの全てが険しい道のりだった．様々な課題を克服して成功を収めた成果は挑戦的な科学観測機器の開発の重要性を訴えている．

TIR(中間赤外カメラ)は,8から12ミクロンの波長帯で熱輻射の2次元イメージングを行う.ターゲット天体の1自転分の撮像から表層物質の熱履歴をもとめ,そこから熱物性を推定する.表層物質の熱物性は,ミッション遂行に必要な情報であるばかりでなく,その後の天体の運命を決める重要な情報である.