Japan Geoscience Union Meeting 2023

Presentation information

[J] Online Poster

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

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

Fri. May 26, 2023 10:45 AM - 12:15 PM Online Poster Zoom Room (1) (Online Poster)

convener:Yuki Hibiya(Research Center for Advanced Science and Technology, The University of Tokyo), Noriyuki Kawasaki(Department of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University), Toru Matsumoto(The Hakubi Center for Advanced Research, Kyoto University), Minako Hashiguchi(Nagoya University)


On-site poster schedule(2023/5/25 17:15-18:45)

10:45 AM - 12:15 PM

[PPS08-P11] Microstructural and chemical analyses of impact melt splashes on Ryugu particle surfaces

*Megumi Matsumoto1, Junya Matsuno2, Akira Tsuchiyama2,3, Tomoki Nakamura1, Yuma Enokido1, Masahiro Yasutake4, Kentaro Uesugi4, Akihisa Takeuchi4, Satomi Enju5, Shota Okumura6, Itaru Mitsukawa6, Sun Mingqi3, Akira Miyake6, Hisayoshi Yurimoto7, Takaaki Noguchi6, Ryuzi Okazaki8, Hikaru Yabuta9, Kanako Sakamoto10, Shogo Tachibana11, Sei-ichiro Watanabe12, Yuichi Tsuda10 (1.Tohoku University, 2.Ritsumeikan University, 3.GIG/CAS, 4.JASRI/SPring-8, 5.Ehime University, 6.Kyoto University, 7.Hokkaido University, 8.Kyushu University, 9.Hiroshima University, 10.JAXA/ISAS, 11.University of Tokyo, 12.Nagoya University)

Keywords:asteroid (162173) Ryugu, asteroid surface, x-ray CT

Hayabusa2 spacecraft delivered the C-type asteroid Ryugu samples to the earth in 2020. Initial analyses of the returned samples revealed that the Ryugu samples consist mainly of Mg-rich phyllosilicates, Fe-Ni-sulfides, magnetite, carbonates and other minor minerals, which are most similar to CI chondrites among known meteorite groups [e.g., 1,2]. In this study, we investigated two Ryugu samples, A0067 and A0094, which show evidence of exposure to space and have flat sample surfaces on which a lot of microcraters and impact melt splashes were observed [2,3]. One relatively large crater (A0067-crater#1) and two melt splashes (A0067-melt#1 and A0094-melt#1) were analyzed by scanning electron microscope equipped with energy dispersive X-ray spectroscopy (SEM-EDS), X-ray nano-tomography (XnCT), and scanning transmission electron microscope (STEM) equipped with EDS to investigate the nature of the impactors hit on the asteroid Ryugu.

A0067-melt#1 shows round shape and is ~30 µm in diameter. It is composed of a Mg-Fe-rich glassy silicate main body and an Fe-rich opaque drop (~10 µm) attaching on the glassy silicate. It was extracted from the A0067 sample using focused ion beam technique and analyzed by XnCT and STEM-EDS. The analyses showed that the glassy silicate has a smooth boundary with the Fe-rich opaque drop. The glassy silicate has homogeneous Mg-Fe-rich composition with the ratio Mg/(Mg+Fe) in atom (hereafter Mg#) of ~0.64 and contains small amounts of Fe-Ni metal–sulfide spherules (<100 nm). The Fe-rich opaque drop consists of flower-like crystals of kamacite (~200–300 nm) embedded in the troilite matrix with pentlandite veins (~20 nm in width).

A0094-melt#1 shows hourglass-like morphology (~15 × 5 µm) and is probably made of two Mg-Fe-rich glassy silicate drops connected to each other. XnCT–STEM-EDS analyses revealed that the glassy silicate is compositionally inhomogeneous and shows patchy structure with Fe-rich (Mg# 0.52–0.55) and Fe-poor (Mg#~0.79) glassy silicate regions (2–5 µm in size). The boundaries between the regions are unclear. Spherical voids (a few tens of nanometers to ~2 µm) are abundant both in the Fe-rich and the Fe-poor regions. The Fe-rich region contains spherical and irregular-shaped Fe-Ni sulfides (<500 nm) and olivine grains (1–2 µm). Some carbonaceous aggregates (0.3–1 µm) consisting mainly of spongy carbonaceous material, irregular-shaped Fe-Ni sulfides, and Mg-rich silicates were also observed in A0094-melt#1.

A0067-crater#1 is ~5 µm in diameter. XnCT–STEM-EDS analysis revealed that A0067-crater#1 is ~4 µm in depth and traps small amount of mixture of glassy silicate and Fe-Ni sulfides. The mixture should be an impact melt and shows flow structure consisting of glassy silicate and Fe-Ni sulfide layers (30–250 nm in thickness) stacking with each other. The glassy silicate layer is compositionally inhomogeneous and separated into Mg-Fe-rich (Mg#~0.72) and Si-rich (Mg#~0.39) glasses. The Mg-Fe-rich glass is abundant compared to the Si-rich glass. Both the silicate glasses contain spherical voids (<200 nm) and Fe-Ni sulfide spherules (<100 nm).

The impact melts studied contain abundant Mg-Fe-rich glassy silicates. The major element compositions of the Mg-Fe-rich glassy silicates (including Fe-Ni sulfide and olivine grains) are plotted along with an extension of a line connecting the CI (solar) composition [4] and the Fe-vertex in a (Si+Al)–Mg–Fe ternary diagram. The compositional trend seems to represent a mixing line of the impactors and the Ryugu surface materials. As some of the impact melts studied contain large amounts of Fe-Ni sulfides, some large Fe-Ni sulfide grains should be included in the impactors and/or the Ryugu surface materials. In the meeting, the formation process of each impact melt and the nature of the impactors will be discussed.

[1] Yokoyama et al. (2022) Science. [2] Nakamura et al. (2022) Science. [3] Noguchi et al. (2022) Nat. Astronom. [4] Lodders (2021) Space Sci. Rev.