Europlanet Science Congress 2020
Virtual meeting
21 September – 9 October 2020
Europlanet Science Congress 2020
Virtual meeting
21 September – 9 October 2020
EPSC Abstracts
Vol.14, EPSC2020-12, 2020
Europlanet Science Congress 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Highly porous nature of C-type asteroid 162173 Ryugu revealed by thermal imaging from Hayabusa2

Tatsuaki Okada1,2, Satoshi Tanaka1,2,3, Yuri Shimaki1, Naoya Sakatani4, Takehiko Arai5, Hiroki Senshu6, Hirohide Demura7, Toru Kouyama8, Tomohiko Sekiguchi9, Tetsuya Fukuhara4, and the Hayabusa2 TIR Team*
Tatsuaki Okada et al.
  • 1ISAS/JAXA, Department of Solar System Sciences, Sagamihara, Japan (
  • 2University of Tokyo, Tokyo, Japan
  • 3SOKENDAI, the Graduate University for Advanced Studies, Hayama, Japan
  • 4Rikkyo University, Tokyo, Japan
  • 5Ashikaga University, Ashikaga, Japan
  • 6Chiba Institute of Technology, Narashino, Japan
  • 7University of Aizu, Aizu-Wakamatsu, Japan
  • 8National Institute of Advanced Industrial Science and Technology, Tokyo, Japan
  • 9Hokkaido University of Education, Asahikawa, Japan
  • *A full list of authors appears at the end of the abstract

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.

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-1s-0.5m-2 (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.

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.

Hayabusa2 TIR Team:

Tatsuaki Okada, Satoshi Tanaka, Yuri Shimaki, Sunao Hasegawa, Takehiko Wada (ISAS/JAXA, Sagamihara, Japan), Tetsuya Fukuhara, Naoya Sakatani, Makoto Taguchi (Rikkyo University, Tokyo, Japan), Takehiko Arai (Ashikaga University, Ashikaga, Japan), Hiroki Senshu (Chiba Institute of Technology, Narashino, Japan), Hirohide Demura, Yoshiko Ogawa (University of Aizu, Aizu-Wakamatsu, Japan), Toru Kouyama (National Institute of Advanced Industrial Science and Technology, Tokyo, Japan), Tomohiko Sekiguchi (Hokkaido University of Education, Asahikawa, Japan), Jun Takita (Hokkaido Kitami Hokuto High School, Kitami, Japan), Takeshi Imamura (University of Tokyo, Japan), Tsuneo Matsunaga (National Institute for Environmental Studies, Tsukuba, Japan), Jorn Helbert, Matthias Grott, Maximilian Hamm (DLR, Berlin, Germany), Jens Biele (DLR, Cologne, Germany), Thomas G. Mueller (MPE, Garching, Germany), Axel Hagermann (University of Stirling, UK), Marco Delbo (Observatoire de Cote d'Azur, Nice, France)

How to cite: Okada, T., Tanaka, S., Shimaki, Y., Sakatani, N., Arai, T., Senshu, H., Demura, H., Kouyama, T., Sekiguchi, T., and Fukuhara, T. and the Hayabusa2 TIR Team: Highly porous nature of C-type asteroid 162173 Ryugu revealed by thermal imaging from Hayabusa2, Europlanet Science Congress 2020, online, 21 September–9 Oct 2020, EPSC2020-12,, 2020