[P-PS08] Formation and evolution of planetary materials in the Solar System

[PPS08-10] Mild shock metamorphism experienced by surface particles of asteroid Ryugu

*Naotaka Tomioka¹, Akira Yamaguchi², Motoo Ito¹, Masayuki Uesugi³, Naoya Imae², Naoki Shirai^4,5, Takuji Ohigashi^6,7, Makoto Kimura², Ming-Chang Liu^8,9, Richard C Greenwood¹⁰, Kentaro Uesugi³, Aiko Nakato¹¹, Kasumi Yogata¹¹, Hayato Yuzawa⁶, Yu Kodama^12,13, Kaori Hirahara¹⁴, Ikuya Sakurai¹⁵, Ikuo Okada¹⁵, Yuzuru Karouji¹¹, Keishi Okazaki¹⁶, Kosuke Kurosawa¹⁷, Takaaki Noguchi¹⁸, Akira Miyake¹⁸, Masaaki Miyahara¹⁶, Yusuke Seto¹⁹, Toru Matsumoto¹⁸, Yohei Igami¹⁸, Hayabusa2 Curation Team, Tomohiro Usui¹¹, Sei-ichiro Watanabe¹⁵, Yuichi Tsuda¹¹ (1.Japan Agency for Marine-Earth Science and Technology, 2.National Institute of Polar Research, 3.Japan Synchrotron Radiation Research Institute, 4.Tokyo Metropolitan University, 5.Kanagawa University, 6.Institute for Molecular Science, 7.High Energy Accelerator Research Organization, 8.UCLA, 9.Lawrence Livermore National Laboratory, 10.The Open University, 11.Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 12.Marine Works Japan, Ltd., 13.Toyo Corporation, 14.Osaka University, 15.Nagoya University, 16.Hiroshima University, 17.Chiba Institute of Technology, 18.Kyoto University, 19.Osaka Metropolitan University)

Keywords:Hayabusa2, Shock metamorphism, TEM, faulting, high-pressure phase

Shock effects related to hydrated asteroids are of particular interest in planetary sciences as this type of asteroid is thought to be one of the major sources of extraterrestrial dust particles. Hydrated materials are likely to be explosively pulverized to become dust particles by the vaporization of volatile components like H₂O during shock heating, and therefore are unlikely to survive as meteorites [1]. This hypothesis was further confirmed by comparing the petrology and mineralogy of recovered samples of anhydrous CV and hydrated CM chondrites using laboratory shock experiments [2,3]. JAXA's Hayabusa2 mission has provided the first opportunity to directly evaluate the shock metamorphism of hydrated asteroidal materials [4]. The present study aims to evaluate the degree of shock metamorphism of Ryugu's surface materials by SEM and TEM, and to assess the hypothesis that massive dust production is driven by dehydration during impact processing on small hydrated asteroids.

Five Ryugu particles examined in this study initially appeared unshocked; however, we found some characteristic features related to shock metamorphism. Particle C0014 exhibits rare, thin straight veins less than 70 µm in length and less than 5 µm in width in SEM observation. In one of these veins, an aggregate of spherical magnetite particles has a thin lens shape, and the aggregate is terminated on both sides by elongated phyllosilicates along the veins. Along the same direction as the vein, another framboidal magnetite aggregate is deformed in simple shear. This feature is likely a fine-grained lithic vein formed by brittle cataclastic deformation: the vein is likely a micro-fault produced by shock metamorphism. This study attempts to evaluate shock-induced faulting as an analogous process to the faulting that causes earthquakes on Earth. By a fault mechanics analysis, the upper bound of the mean stress, which is approximated as the peak pressure, is estimated to be ~2 GPa.

In TEM observation, all the observed particles are dominated by phyllosilicates but lack dehydration textures. This suggests that the shock temperature was below ~500 °C. In the fine-grained phyllosilicate matrix, a unique Fe-sulfide grain was identified in particle A0002. The grain is euhedral and 1.2 µm in size with a pure FeCr₂S₄ composition. SAED patterns from the grain are indexed with a dense monoclinic FeCr₂S₄ mineral known as zolenskyite. Based on a phase equilibrium study of FeCr₂S₄ [5], the low pressure phase daubréelite would have transformed into zolenskyite above ~2 GPa in A0002.

Considering the above mineralogical and petrological features, we conclude the average peak pressure of the Ryugu particles is only ~2 GPa. The complete lack of dehydration textures and mineral features in Ryugu particles observed in the present study demonstrates that Ryugu's surface materials preserved their water as hydroxyl in phyllosilicates throughout the mild impact events that they experienced. This study also suggests that the production of dust particles on Ryugu-like asteroids, due to shock-heating-induced volatilization during such break-up events, is much smaller than previously expected.

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[2] Tomeoka, K., Kiriyama, K., Nakamura, K., Yamahana, Y. & Sekine, T. (2003) Nature 423, 60–62.
[3] Tomioka, N., Tomeoka, K., Nakamura-Messenger, K. & Sekine, T. (2007) Meteorit. Planet. Sci. 42, 19–30.
[4] Yada, T., Abe, M., Okada, T., et al. (2021) Nat. Astron. 6, 214–220.
[5] Tressler, R. E., Hummel, F. A. & Stubican, V. S. (1968) J. Amer. Ceram. Soc. 51, 648–651.