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Unveiling Highly-Porous Nature of Primitive Asteroid 162173 Ryugu by Thermal Imager on Hayabusa2

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Unveiling Highly-Porous Nature of Primitive Asteroid 162173 Ryugu by Thermal Imager on Hayabusa2 51st Lunar and Planetary Science Conference (2020) 1352.pdf UNVEILING HIGHLY-POROUS NATURE OF PRIMITIVE ASTEROID 162173 RYUGU BY THERMAL IMAGER ON HAYABUSA2. T. Okada1,2, T. Fukuhara3, S. Tanaka1, M. Taguchi3, T. Arai4, N. Sakatani1, Y. Shimaki1, H. Senshu5, Y. Ogawa6, H. Demura6, K. Suko6, K. Kitazato6, T. Kouyama7, T. Sekiguchi8, J. Takita1,9, S. Hasegawa1, T. Matsunaga10, T. Wada1, T. Imamura2, J. Helbert11, T.G. Mueller12, A. Hagermann13, Jens Biele11, Matthias Grott11, Maximilian Hamm11,14, Marco Delbo15, Y. Yamamoto1, N. Hirata6, N. Hirata15, F. Terui1, T. Saiki1, S. Nakazawa1, M. Yoshikawa1, S. Watanabe16, Y. Tsuda1, and Hayabusa2 Joint Science Team1, 1Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (ISAS/JAXA), 3-1-1 Yoshinodai, Chuo-Ku, Sagamihara 252-5210, Japan, email: [email protected], 2University of Tokyo, Japan, 3Rikkyo University, Japan, 4Ashikaga University, Japan, 5PERC, Chiba Institute of Technology, Japan, 6University of Aizu, Japan, 7National Institute of Advanced Industrial Science and Technology (AIST), Japan, 8Hokkaido University of Education, Asahikawa, Japan, 9Hokkaido Kitami Hokuto High School, Japan, 10National Institute for Environmental Studies (NIES), Japan, 11German Space Research Center (DLR), Germany, 12Max-Planck Institute for Extraterrestrial Physics (MP-E), Germany, 13University of Stirling, UK, 14University of Potsdam, Germany, 15Observatoire de la Côte d'Azur, CNRS, France, 15Kobe University, Japan, 16Nagoya University, Japan. Introduction: The first high-resolution global with regolith that is formed by hypervelocity meteoritic thermal images of an asteroid and the close-up very impacts. On the other hand, the surface of small bodies local thermal images from Hayabusa2 has proven that remains poorly known [10] since a majority of impact the asteroid has very low thermal inertia compared to ejecta might escape due to low gravity, depending on that of typical carbonaceous chondrites, indicating the physical properties of the surface materials, so that highly-porous materials on the surface, as wel as the their surface physical state needs in-depth investigation. highly-porous nature of primitive bodies in general. Global Thermal Images of Ryugu: The first set of Here we show the asteroid thermal images by TIR, the high-resolved global thermal infrared images of an method of data analysis, the derived results of thermal asteroid in history was obtained on 30 June 2018, with inerta, and the speculative discussions. ~18 m/pixel, covering its rotation every 6° [6]. Higher Backgrounds: C-type Near-Earth asteroid 162173 resolution one-rotation thermal image sets have been Ryugu is the target of the JAXA Hayabusa2 asteroid taken from 5 km altitude, with spatial resolution of sample return mission [1], which was launched in 2014, ~4.5 m/pixel, during the Mid-Altitude Observation arrived there in 2018, observed and collected samples Campaign on 1 August 2018. It was surprising that there in 2018-2019, and will return sample to Earth in boulders on Ryugu, including the largest one of ~ 160 2020. C-type asteroids are likely analogues to carbona- m size “Otohime”, showed similar temperatures to the ceous chondrites. Ground-based and space-based re- surroundings while, before arrival, they were expected mote observations [2] of Ryugu characterized the target to be consolidated and observed as “cold spots”. asteroid’s taxonomy as C-type, its rotation period of A brightness temperature image is plotted on the 7.63 hours, its diameter of ~0.85 km, and its global shape model (SHAPE_SFM_200k_v20180804) [3] to thermal inertia from 150 to 300 J m-2 s-0.5 K-1 (tiu, be compared with simulated images for eight uniform hereafter), having been interpreted as a surface covered thermal inertia models from 50 to 1000 tiu. Although with millimeter scale granules. After arrival at Home none of them matches to the observations perfectly, the Position (HP), 20 km from Ryugu, Hayabusa2 globally apparent global thermal inertia of Ryugu is most suita- observed the asteroid [3-6]. Ryugu has a bulk density of ble to ~300 ± 100 tiu [6] (in more detail of the average 1190 ± 20 kg m-3 [3] and its surface is evenly covered and the regional variation, 278 ± 138 tiu, by the latest with boulders [4], without smooth terrains found on calculation [11]). 25143 Itokawa [7]. These facts indicate that Ryugu is a Diurnal temperature profiles have been investigated rubble-pile asteroid with bulk porosity of 50 % or at various sites and geologic features, finding that an higher [3], assuming the asteroid is composed of CI extremely high surface porosity and roughness might (~2420 kg m-3) or CM (~2960 kg m-3) carbonaceous explain these profiles. Thermal inertia of 225 ± 45 tiu chondrites [8]. with roughness of 0.41 ± 0.08 are derived for the glob- Thermal Imaging by TIR: Thermal imaging was al average of Ryugu [11]. conducted using a thermal infrared imager TIR on Close-up Thermal Images of Ryugu: After Sep- Hayabusa2 [9] to investigate the surface physical state tember 2018, Hayabusa2 started descent operations for of Ryugu, such as particle size, porosity, abundance of sampling and for the release of landers. Every time boulders and surface small scale roughness, derived during descents, close-up thermal images were taken from the thermal inertia. The surfaces of the Moon, by TIR down to < 60 m for lander releases and < 20 m Mars, and large satellites are predominantly covered for samplings, at a few cm/pixel at local sites. 51st Lunar and Planetary Science Conference (2020) 1352.pdf During the touchdown rehearsal TD1-R1-A on dust [17] in the early solar system and could have 15 October 2018, the close-up thermal images proved strongly affected planetary formation processes such as that the surface is not covered with soils nor granules cratering and collisional fragmentation by attenuating but dominantly with boulders. This causes similar tem- shock propagation [16]. There still cannot be ruled out perature profiles between large boulders and the sur- that the low thermal inertia and the low density of roundings. Most boulders were from 300 to 310 K, Ryugu are due to materials different from carbona- corresponding to highly-porous materials with 200 to ceous chondrites such as the organic-rich material dis- 300 tiu, as predicted by global thermal images. covered on Comet 67P/Churymov-Gerasimenko [18]. However, We discovered a few boulders below These questions will however be resolved upon sample 280 K, corresponding to 600 to 1000 tiu (typical value return. of carbonaceous chondrites [8]. The surface of Ryugu is dominated by highly-porous boulders, except for Acknowledgments: The authors appreciate all the some dense boulders similar to carbonaceous chondrite members of the Hayabusa2 Project and supporting staff meteorites, which is consistent with the images taken for their technical assistance and scientific discussions. by the camera MasCAM on MASCOT [12], showing This research is partly supported by the JSPS KAKENHI that a large majority of boulders have cauliflower-like No. J17H06459 (Aqua Planetology), and the Core-to- crumbly surfaces, while a minority have flat surfaces, Core program “International Network of Planetary Sci- and the average thermal inertia is consistent with the ences”. MDB acknowledges support from CNES. TGM result derived from in situ measurements of a single has received funding from the European Union’s Horizon 2020 Research and Innovation Programme, under Grant boulder using the radiometer MARA on MASCOT [13], Agreement no 687378, as part of the project “Small Bod- and consistent with the ground observations [2]. ies Near and Far” (SBNAF). AH was supported by STFC Porous Nature of Primitive Bodies: Accounting grant ST/S001271/1. for the asteroid’s bulk density, Ryugu must have a bulk porosity of 50 to 60 % [3], assuming it consists of ma- Data Availability: All the raw thermal images used terials of CI or CM chondrites. The macro porosity in the study [6] taken by TIR on Hayabusa2 (Level 1) and caused by voids between rocks should be below 20 % their brightness temperature converted images (Level 2) for micro porosity of 50 %, or 30 ~ 40 % for micro are available at the Hayabusa2 project data archive site: porosity of 30 %. The former is more likely since the darts.isas.jaxa.jp/pub/hayabusa2/tir_bundle/browse/, with boulders of < 200 tiu are identified by detailed analysis the easy-to-use brightness temperature images in png file [11]. A trend of small asteroids having lower thermal (only in Level 2). inertia compared with typical carbonaceous chondrites [14] might be caused not by regolith as predicted but by References: [1] Tsuda Y. et al. (2013) Acta. Astro- very porous boulders. A similarly low thermal inertia is nautica, 91, 356-362. [2] Mueller T.G. et al. (2017) As- observed for B-type asteroid 101955 Bennu by the tron. Astrophys. 599, A103. [3] Watanabe S. et al. (2019) OSIRIS-REx mission [15]. Science 464, 268-272. [4] Sugita S. et al. (2019) Science The formation history of Ryugu proposed in the 464, 552. [5] Kitazato K. et al. (2019) Science 464, 272- previous study [4] might be constrained [6]. Ryugu 275. [6] Okada T. et al. (2020) Nature, in revision. [8] could be formed from fragments of a parent body that Flynn G.J. et al. (2018) Chemie der Erde 78, 269-298. [9] has experienced a low degree of consolidation of origi- Okada et al. (2017) Space Sci. Rev. 208, 255-286. [10] nally porous materials. This is the direct evidence of Housen. K. R. & Holsapple, K. A. (2011) Icarus 211, less consolidated nature of C-type asteroids formed 856-875. [11] Shimaki Y. et al. submitted to Icarus. [12] from fluffy dust or pebbles in the early solar system. Jaumann R. et al. (2019) Science 465, 817-820. [13] Grott M.
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