日本地球惑星科学連合2022年大会

講演情報

[J] 口頭発表

セッション記号 M (領域外・複数領域) » M-IS ジョイント

[M-IS17] 水惑星学

2022年5月25日(水) 13:45 〜 15:15 105 (幕張メッセ国際会議場)

コンビーナ:関根 康人(東京工業大学地球生命研究所)、コンビーナ:福士 圭介(金沢大学環日本海域環境研究センター)、臼井 寛裕(東京工業大学地球生命研究所)、コンビーナ:渋谷 岳造(海洋研究開発機構)、座長:関根 康人(東京工業大学地球生命研究所)、渋谷 岳造(海洋研究開発機構)、福士 圭介(金沢大学環日本海域環境研究センター)、玄田 英典(東京工業大学 地球生命研究所)、臼井 寛裕(東京工業大学地球生命研究所)

13:45 〜 14:00

[MIS17-01] Aqueous alteration of Cb-type asteroid Ryugu

*中村 智樹1松本 恵1天野 香菜1、榎戸 祐馬1、マイク ゾレンスキー2三河内 岳3圦本 尚義4野口 高明5岡崎 隆司6薮田 ひかる7奈良岡 浩6、坂本 加奈子8橘 省吾3渡邊 誠一郎9津田 雄一8 (1.東北大学大学院理学研究科地学専攻、2.NASA/JSC、3.東京大学、4.北海道大学、5.京都大学、6.九州大学、7.広島大学、8.宇宙科学研究所、9.名古屋大学)

キーワード:小惑星、リュウグウ、はやぶさ2

The samples from the asteroid Ryugu, which returned to Earth in the winter of 2020 [1], were stored under pure Nitrogen conditions at the Extraterrestrial Materials Curation Center of ISAS. After basic description [2-3], they were distributed to six initial analysis teams in Japan in June 2021. The initial analysis "Stone" team, which was responsible for the analysis of coarse particles (>1 mm in size), consisted of about 150 scientists from Japan and abroad, and conducted a series of analyses

We analyzed sixteen coarse Ryugu particles 1 ~ 8 mm in size: six from the 1st touch-down site and ten from the 2nd touch-down site. Individual coarse samples were cut by Xe-FIB or wire-saw to expose particular objects or textures to be exposed on the surface based on 3D structure and element distribution obtained by synchrotron X-ray CT analysis and XRF analysis, respectively. FE-SEM/EDS and FE-EPMA/WDS analysis were made on all polished sections. TEM observation of many FIB sections was made to see microstructures and to compare with carbonaceous chondrites. Many other analyses were carried out to characterize mineralogical and compositional properties and summary of the analysis procedures is described in [4].

X-ray CT analysis showed that all sixteen Ryugu particles are composed of fine-grained material, with no chondrules and CAIs larger than 100 μm in size. FE-EPMA observation showed that Ryugu samples are breccias, consisting of many small rock fragments, lithologies of different compositions, and histories. The most common lithology includes Mg-rich saponite and serpentine, dolomite, magnesite, hydroxyapatite, pyrrhotite, and magnetite as main constituents. The mineralogy of this major lithology supports the classification of Ryugu samples as CI chondrites, which experienced extensive aqueous alteration in Ryugu’s parent asteroid.

Based on the obtained mineralogical properties, we performed chemical modeling of aqueous alteration of Ryugu’s parent asteroid [5]. Numerical simulations [6-8] to reproduce thermal history and impact-induced destruction of the Ryugu’s parent asteroid were also carried out using mineralogical and physical properties of the Ryugu samples. In the talk, I would like to explain the conditions of aqueous alteration occurred in the parent asteroid of Ryugu in order to show how the water in the asteroid changed the mineralogy and chemistry of the nebular dust.

References: [1] Tachibana S. et al. (2022) Science, doi: 10.1126/science.abj8624. [2] Yada T. et al. (2021) Nat. Astron. doi.org/10.1038/s41550-021-01550-6. [3] Pilorget C. et al. (2021) Nat. Astron. doi.org/10.1038/s41550-021-01549-z. [4] Nakamura et al. (2022) 53rd LPSC meeting. [5] Zolotov et al. (2022) 53rd LPSC meeting. [6] Genda H. et al. (2022) 53rd LPSC meeting. [7] Kurosawa K. et al. (2022) 53rd LPSC meeting. [8] Wakita S. et al. (2022) 53rd LPSC meeting.