Japan Geoscience Union Meeting 2019

Presentation information

[E] Oral

P (Space and Planetary Sciences ) » P-PS Planetary Sciences

[P-PS03] Solar System Small Bodies: A New Frontier Arising Hayabusa 2, OSIRIS-REx and Other Projects

Tue. May 28, 2019 10:45 AM - 12:15 PM A01 (TOKYO BAY MAKUHARI HALL)

convener:Masateru Ishiguro(Department of Physics and Astronomy, Seoul National University), Taishi Nakamoto(Tokyo Institute of Technology), Masanao Abe(Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency), Olivier S Barnouin(Johns Hopkins University Applied Physics Laboratory), Chairperson:Masateru Ishiguro(Seoul National University)

10:45 AM - 11:00 AM

[PPS03-01] The shape and origin of the rubble-pile asteroid Ryugu

★Invited Papers

*Sei-ichiro WATANABE1,5, Masatoshi Hirabayashi2, Naru Hirata3, Naoyuki Hirata4, Rina Noguchi5, Yuri Shimaki5, Hitoshi Ikeda6, Eri Tatsumi7, Makoto Yoshikawa5, Shota Kikuchi5, Hikaru Yabuta8, Tomoki Nakamura9, Shogo Tachibana7,5, Yoshiaki Ishihara10, Tomokatsu Morota1, Kohei Kitazato3, Naoya Sakatani5, Koji Matsumoto11,12, Koji Wada13, Hiroki Senshu13, Chikatoshi Honda3, Tatsuhiro Michikami14, Hiroshi Takeuchi5, Toru Kouyama15, Rie Honda16, Robert Gaskell17, Eric Palmer17, Olivier S. Barnouin18, Patrick Michel19, Paul Abell20, Yukio Yamamoto5, Satoshi Tanaka5, Kei Shirai5, Moe Matsuoka5, Seiji Sugita5,7, Tatsuaki Okada5, Noriyuki Namiki11, Masahiko Arakawa4, Masateru Ishiguro21, Kazunori Ogawa4, Fuyuto Terui5, Takanao Saiki5, Satoru Nakazawa5, Yuichi Tsuda5, Hayabusa2 Science Team (1.Division of Earth and Planetary Sciences, Graduate School of Science, Nagoya University, 2.Auburn University, 3.University of Aizu, 4.Kobe University, 5.ISAS, JAXA, 6.Research and Development Directorate, JAXA, 7.University of Tokyo, 8.Hiroshima University, 9.Tohoku University, 10.National Institute for Environmental Studies, 11.National Astronomical Observatory of Japan, 12.SOKENDAI, 13.Chiba Institute of Technology, 14.Kindai University, 15.National Institute of Advanced Industrial Science and Technology, 16.Kochi University, 17.Planetary Science Institute, 18.John Hopkins University, 19.Universite Cote d'Azur, Observatorie de la Cote d'Azur, 20.NASA, Johnson Space Center, 21.Seoul National University)

Keywords:Planetary exploration, C-type asteroids, Rubble-pile objects, Hayabusa2

In June 2018 Hayabusa2 arrived at near-Earth asteroid (NEA) Ryugu and has been successfully conducting remote-sensing observations from ~20-km or less altitudes and deploying rovers and a lander. Based on images taken by ONC-T, we constructed global shape models of Ryugu [1, 2]. In order to obtain a reliable shape model in a short period, we used two independent methods; the stereophotoclinometry (SPC) technique [3] and the Structure-from-Motion (SfM) technique [4]. Except for boulders and high latitudes, the two models are in good agreement with each other.
The total volume of Ryugu derived from an SPC-based shape model is 0.377±0.005 km3. Gravity measurement revealed that the asteroid’s mass is (4.50±0.06) × 1011 kg. The bulk density is thus obtained as 1.19±0.03 g cm-3. The density is significantly smaller than bulk densities (1.6-2.4 g cm-3) measured for Ch- and Cgh-type having the 0.7-μm absorption [5]. However, it remains within the 0.8–1.5 g cm-3 range measured for BCG-types (B-, C-, Cb-, and Cg-type), which might be related to unheated icy asteroids [5]. The low bulk density of Ryugu is consistent with the spectral type (Cb-type) measured by ONC-T and NIRS3 [2, 6].
NIRS3 observations indicate that hydrated minerals are widely spread on the surface of Ryugu [6]. The presence of water ice, however, is unlikely for NEA Ryugu because the radiative equilibrium temperature (~250 K) is higher than the ice sublimation temperature (~230 K) at its central pressure of ~8 Pa and the thermal diffusion time of Ryugu is estimated to be much shorter than the typical dynamical lifetime of NEAs [1]. Note that the parent body of Ryugu located in the Main Belt might have water ice and low density of Ryugu could be ascribed to loss of volatile components without subsequent compaction. If we adopt the grain densities of CM carbonaceous chondrites (CCs), the derived total porosity is 57-63%. Adopting those of Orgueil CI CC, we predict the total porosity to be 50-52%. The estimated total porosity is even higher than that of rubble-pile asteroid Itokawa (44±4%) [8, 9], indicating that Ryugu is also a rubble pile.
Hayabusa2 reveals that Ryugu is a top-shape asteroid; a prominent elevated ridge around the equator. Although dozens of top-shaped asteroid have been found by ground-based radar, Ryugu is the first top-shape asteroid to be directly observed up close by a spacecraft. Bennu is the second one.
Derived surface slopes to the equipotential surface assuming the present shape and uniform interior demonstrate least variation with latitude if, at some epoch, Ryugu had a spin period of 3.5 hours (about half of the current value) [1]. This suggests that the top shape was formed by centrifugally induced structural failure during a rapid rotation era. The deformation occurred either in the initial re-accumulation stage [10] or in the later quasi-statically spin-up stage (due to the YORP thermal effect) [11]. In the later stage, a deformation process might be induced either on the surface or in the interior, depending on the internal structure. The high porosity nature of Ryugu as well as no significant offset between the centers of figure and gravity suggest that the internal tensile strength of the asteroid is uniform and low.
NIR absorption feature of Ryugu obtained by NIRS3 is similar to those of heated/hocked CI/CM chondrites [6]. However, these meteorites are relatively rare whereas BCG types are abundant, so that another meteoritic counterpart of BCG types are expected. High porosity nature of Ryugu suggests that meteorites originated from BCG-type asteroids would be very fragile and should be destroyed at the atmospheric entry. This is consistent with the scanty of BCG-originated meteorites and a hypothesis that pyroxene-rich interplanetary dust particles (IDPs) are originate from BCG-types based on the mid-IR spectroscopy [5]. Thus, comparative studies of Ryugu and Bennu, not only in material science but also in physical properties and internal structure, are the key to understand the delivery system of water and organic materials from the snow line to Earth.

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