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Highly porous nature of a primitive asteroid revealed by thermal imaging
Nature ( IF 64.8 ) Pub Date : 2020-03-16 , DOI: 10.1038/s41586-020-2102-6
Tatsuaki Okada 1, 2 , Tetsuya Fukuhara 3 , Satoshi Tanaka 1, 4, 5 , Makoto Taguchi 3 , Takehiko Arai 6 , Hiroki Senshu 7 , Naoya Sakatani 1 , Yuri Shimaki 1 , Hirohide Demura 8 , Yoshiko Ogawa 8 , Kentaro Suko 8 , Tomohiko Sekiguchi 9 , Toru Kouyama 10 , Jun Takita 11 , Tsuneo Matsunaga 12 , Takeshi Imamura 5 , Takehiko Wada 1 , Sunao Hasegawa 1 , Jörn Helbert 13 , Thomas G Müller 14 , Axel Hagermann 15 , Jens Biele 16 , Matthias Grott 13 , Maximilian Hamm 13, 17 , Marco Delbo 18 , Naru Hirata 8 , Naoyuki Hirata 19 , Yukio Yamamoto 1, 4 , Seiji Sugita 2, 7 , Noriyuki Namiki 4, 20 , Kohei Kitazato 8 , Masahiko Arakawa 19 , Shogo Tachibana 1, 2 , Hitoshi Ikeda 21 , Masateru Ishiguro 22 , Koji Wada 7 , Chikatoshi Honda 8 , Rie Honda 23 , Yoshiaki Ishihara 12 , Koji Matsumoto 4, 20 , Moe Matsuoka 1 , Tatsuhiro Michikami 24 , Akira Miura 1 , Tomokatsu Morota 2 , Hirotomo Noda 20 , Rina Noguchi 1 , Kazunori Ogawa 19, 25 , Kei Shirai 19 , Eri Tatsumi 2, 26 , Hikaru Yabuta 27 , Yasuhiro Yokota 1 , Manabu Yamada 7 , Masanao Abe 1, 4 , Masahiko Hayakawa 1 , Takahiro Iwata 1, 4 , Masanobu Ozaki 1, 4 , Hajime Yano 1, 4 , Satoshi Hosoda 1 , Osamu Mori 1 , Hirotaka Sawada 1 , Takanobu Shimada 1 , Hiroshi Takeuchi 1, 4 , Ryudo Tsukizaki 1 , Atsushi Fujii 1 , Chikako Hirose 21 , Shota Kikuchi 1 , Yuya Mimasu 1 , Naoko Ogawa 1, 25 , Go Ono 21 , Tadateru Takahashi 1, 28 , Yuto Takei 1 , Tomohiro Yamaguchi 1, 29 , Kent Yoshikawa 21 , Fuyuto Terui 1 , Takanao Saiki 1 , Satoru Nakazawa 1 , Makoto Yoshikawa 1, 4 , Seiichiro Watanabe 1, 30 , Yuichi Tsuda 1, 4
Affiliation  

Carbonaceous (C-type) asteroids1 are relics of the early Solar System that have preserved primitive materials since their formation approximately 4.6 billion years ago. They are probably analogues of carbonaceous chondrites2,3 and are essential for understanding planetary formation processes. However, their physical properties remain poorly known because carbonaceous chondrite meteoroids tend not to survive entry to Earth’s atmosphere. Here we report on global one-rotation thermographic images of the C-type asteroid 162173 Ryugu, taken by the thermal infrared imager (TIR)4 onboard the spacecraft Hayabusa25, indicating that the asteroid’s boulders and their surroundings have similar temperatures, with a derived thermal inertia of about 300 J m−2 s−0.5 K−1 (300 tiu). Contrary to predictions that the surface consists of regolith and dense boulders, this low thermal inertia suggests that the boulders are more porous than typical carbonaceous chondrites6 and that their surroundings are covered with porous fragments more than 10 centimetres in diameter. Close-up thermal images confirm the presence of such porous fragments and the flat diurnal temperature profiles suggest a strong surface roughness effect7,8. We also observed in the close-up thermal images boulders that are colder during the day, with thermal inertia exceeding 600 tiu, corresponding to dense boulders similar to typical carbonaceous chondrites6. These results constrain the formation history of Ryugu: the asteroid must be a rubble pile formed from impact fragments of a parent body with microporosity9 of approximately 30 to 50 per cent that experienced a low degree of consolidation. The dense boulders might have originated from the consolidated innermost region or they may have an exogenic origin. This high-porosity asteroid may link cosmic fluffy dust to dense celestial bodies10.



中文翻译:

热成像揭示的原始小行星的高度多孔性

碳质(C 型)小行星1是早期太阳系的遗迹,自大约 46 亿年前形成以来就保存了原始物质。它们可能是碳质球粒陨石2,3的类似物,对于理解行星形成过程至关重要。然而,它们的物理特性仍然鲜为人知,因为碳质球粒陨石往往无法在进入地球大气层后幸存下来。在这里,我们报告了 C 型小行星 162173 Ryugu 的全球单旋转热成像图像,由Hayabusa2 5航天器上的热红外成像仪 (TIR) 4拍摄,表明小行星的巨石及其周围具有相似的温度,衍生的热惯性约为 300 J m -2  s -0.5  K -1 (300 tiu)。与表面由风化层和致密巨石组成的预测相反,这种低热惯性表明巨石比典型的碳质球粒陨石6更多孔,并且它们的周围被直径超过 10 厘米的多孔碎片覆盖。特写热图像证实了这种多孔碎片的存在,而平坦的昼夜温度分布表明强烈的表面粗糙度效应7,8. 我们还在特写热图像中观察到白天较冷的巨石,其热惯性超过 600 tiu,对应于类似于典型碳质球粒陨石6的致密巨石。这些结果限制了 Ryugu 的形成历史:小行星必须是由母体的撞击碎片形成的碎石堆,其微孔率9约为 30% 至 50%,经历了低程度的固结。致密的巨石可能起源于固结的最内层区域,或者它们可能具有外源性。这种高孔隙率的小行星可以将宇宙蓬松的尘埃与致密的天体10联系起来。

更新日期:2020-03-16
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