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

[PPS08-07] Mineralogy and space weathering found in the fine-grained fraction of samples returned from the C-type asteroid Ryugu.

*Takaaki Noguchi^1,2, Toru Matsumoto¹, Akira Miyake¹, Yohei Igami¹, Mitsutaka Haruta¹, Hikaru Saito², Satoshi Hata², Yusuke Seto³, Masaaki Miyahara⁴, Naotaka Tomioka⁵, Akira Tsuchiyama^6,7,8, Masahiro Yasutake⁹, Junya Matsuno⁶, Shota Okumura¹, Itaru MITSUKAWA¹, Kentaro Uesugi⁹, Masayuki Uesugi⁹, Akihisa Takeuchi⁹, Satomi Enju¹⁰, Aki Takigawa¹¹, Tatsuhiro Michikami¹², Tomoki Nakamura¹³, Megumi Matsumoto¹³, Yusuke Nakauchi¹⁴, Hisayoshi Yurimoto¹⁵, Kazuhide Nagashima¹⁶, Noriyuki Kawasaki¹⁵, Naoya Sakamoto¹⁵, Ryuji Okazaki², Hiroshi Naraoka², Hikaru Yabuta⁴, Sakamoto Kanako¹⁴, Shogo Tachibana¹¹, Sei-ichiro WATANABE¹⁷, Yuichi Tsuda¹⁴, The Hayabusa2-initial-analysis Sand team (overseas members) (1.Kyoto University, 2.Kyushu University, 3.Kobe University, 4.Hiroshima University, 5.JAMSTEC, 6.Ritsumeikan University, 7.Guangzhou Institute Geochemistry, Chinese Academy of Science, 8.CAS Center for Excellence in Deep Earth Science, 9.JASRI, 10.Ehime University, 11.University of Tokyo, 12.Kindai University, 13.Tohoku University, 14.JAXA, 15.Hokkaido University, 16.University of Hawai‘i at Mānoa, 17.Nagoya University)

Keywords:Ryugu, CI chondrites, Space weathering

Introduction: The JAXA Hayabusa2 spacecraft returned samples from near-Earth C-type asteroid Ryugu. Our Min-Pet Fine sub-team has investigated mineralogy and petrology and space weathering of grains in the fine fraction of the samples. Space weathering is alteration induced mainly by solar wind irradiation and micrometeoroid impact. Here, we report the mineralogy and microstructural and chemical features related to space weathering of the Ryugu grains.
Samples and Methods: Surface morphologies of ~700 grains were observed by field emission scanning electron microscopy (FE-SEM) and focused ion beam (FIB)-SEM. A polished sample of a fragment originating from a large grain A0058 was also investigated. Thin foil samples were prepared by FIB-SEM. We performed transmission electron microscopy (TEM), synchrotron radiation X-ray absorption fine structure (XANES and EXAFS) spectroscopy, nanotomography, and atom probe analysis at 16 universities and laboratories spread across the world.
Results and discussion: Mineralogy of fine fraction of the Ryugu samples. Major minerals of the small Ryugu grains are saponite, serpentine, Fe-Ni sulfides, magnetite, dolomite, and breunnerite. The mineralogy and petrology of the Ryugu FIB sections investigated are similar to CI chondrites, but the samples lack ferrihydrite and sulfates, commonly found in CI chondrites [1]. Considering the effects of terrestrial weathering of CI chondrites, we infer that the mineralogy of investigated Ryugu grains is similar to that of CI chondrites prior to their weathering upon arrival on Earth.
Space weathering of Ryugu samples. Recognizable surface modifications of the phyllosilicate-rich matrix were found on 6% to 7% of the observed grains. A variety of surface modifications are observed: melt splashes, amorphous layers, and melt layers. The amorphous layers form a continuous sheet ~0.1 µm thick composed of amorphous silicate material at the top surface. Their bulk chemical compositions are indistinguishable from those of the underlying phyllosilicate-rich matrix, but they have a higher ratio of Fe²⁺ relative to Fe³⁺ than the interior phyllosilicate-rich matrix. The melt layers have bubbles and numerous submicroscopic (<100 nm) rounded Fe-Ni sulfide beads. These data suggest that both silicate and Fe-Ni sulfides were melted and immiscibly separated into silicate and sulfide melts. Such melt layers have higher Fe and lower Si+Al and Mg contents, as well as a higher ratio of Fe²⁺ relative to Fe³⁺ than the interior phyllosilicate-rich matrix.
The surface morphology of the amorphous layers resembles the surface of an experimental product of Orgueil CI chondrite that was irradiated by 4 keV He⁺. To a first approximation, it appears that solar-wind irradiation likely played an important role in modifying the surface of the phyllosilicate-rich matrix. Structures of the melt layers resemble those of the products from the laser irradiation experiments that simulate micrometeoroid impacts [2], suggesting that they had an important role in forming the melt layers.
Multiple types of surface modifications are observed on some grains. Given textural relationships of these surface modifications and the assumptions that solar wind irradiation is rapid and that micrometeoroid impact processing is slow [3], Ryugu grains preserve a range of stages of space weathering.
The Hayabusa2-initial-analysis Sand team (overseas members): Hope A. Ishii, John P. Bradley, Kenta Ohtaki, Elena Dobrică, Hugues Leroux, Corentin Le Guillou, Damien Jacob, Maya Marinova, Francisco de la Peña, Falko Langenhorst, Dennis Harries, Pierre Beck, Thi H. V. Phan, Rolando Rebois, Neyda M. Abreu, Jennifer Gray, Thomas Zega, Pierre-M. Zanetta, Michelle S. Thompson, Rhonda Stroud, Kate Burgess, Brittany A. Cymes, John C. Bridges, Leon Hicks, Martin R. Lee, Luke Daly, Phil A. Bland, Michael E. Zolensky, David R. Frank, James Martinez, Mingqi Sun
References: [1] Brearley, A. J., Jones, R. H. (1998) Rev. Min. 36, C1. [2] Thompson, M. S. et al. (2019) Icarus 319, 499-511. [3] Vernazza, P. et al. (2009) Nature 458, 993-995.