11:15 〜 11:30
[PPS07-09] GEMS-like materials in the aggregate of organic globules in Ryugu grain.
キーワード：リュウグウ、小惑星、有機物、水質変成
Introduction: Insoluble organic macromolecules (IOM) in Ryugu grains are the dominant components of organic matter in Ryugu’s materials [1]. The chemical structures, distributions, and morphologies of the macromolecular organic matter can be tracers of the evolution of organic materials in the interstellar medium, the protosolar disk, and the aqueous environment in the parent body of Ryugu [1, 2]. The presence of organic molecules in phyllosilicate matrix [1, 2] and possibly in the interlayer spaces of phyllosilicate [3] suggests the interaction between minerals, water, and organic matter in the parent body. In this study, we focused on carbon-rich regions in Ryugu samples and examined the chemical structures of organic materials and inorganic minerals.
Methods: The surface features of a Ryugu grain (A0142) was observed using a field-emission (FE)-scanning electron microscope (SEM). Electron-transparent sections were extracted from the regions of interest using a focused ion beam system (FIB). Scanning transmission X-ray microscopy (STXM) imaging was performed to obtain X-ray absorption near edge structure (XANES) spectra at the carbon K-edge (at 280 eV-300 eV). After STXM analysis, the sections were observed using field-emission transmission electron microscope (TEM).
Results: Three carbon-rich regions found in the aqueously well-altered lithologies were extracted by FIB. The widths of the carbon-rich regions were 17×8 µm (area-1), 7×6 µm (area-2), and 35×12 µm (area-4), respectively. TEM analysis revealed that the carbon-rich portions from the area-1 and area-2 show homogeneous textures, while the carbon-rich portion from the area-4 has numerous hollows indicating that this portion consists of the aggregate of organic globules.A comparison of carbon-XANES peak intensities following the method by [4] shows that the organic globules in the area-4 is relatively rich in aromatic carbon (C=C) and poor in aliphatic carbon (C-H) compared to those in the other two areas. Fibrous phyllosilicates are finely intermixed in the organic materials in the area-1 and area-2. On the other hand, the organic materials in the area-4 contain rounded silicate objects with iron-nickel sulfides. Their morphologies and the appearance of sulfide inclusions are similar to GEMS (glass with embedded metal and sulfides) found in interplanetary dust particles (IDPs) [e.g., 5], but they lack iron metals. The majority of these GEMS-like materials show fibrous textures, but some may be amorphous or poorly crystalline.
Discussion: The GEMS-like materials were found in the least altered lithology in Ryugu samples [6], and primitive carbonaceous chondrites [e.g., 7, 8]. They are interpreted to be pre-accretionary phases of their parent bodies. GEMS encapsulated in organic carbons are found in IDPs [5] and ultracarbonaceous Antarctic micrometeorites (UCAMMs) [9], which are thought to be originated from comets. The aggregate of organic materials including GEMS-like objects in our samples resembles IDPs and UCAMMs, further reinforcing the linkage between the building blocks of Ryugu and cometary materials [6]. In addition, the preservation of GEMS-like objects in altered lithologies suggests that complex organic materials act as a barrier to aqueous fluids on the parent body, as proposed in CR2 chondrite [8].
References: [1] Yabuta et al. (2023) Science 379. 6634, eabn9057. [2] Ito et al. (2020) Nature Astronomy 6:1163-1171. [3] Viennet et al. (2023) Geochem. Persp. Let. 25, 8-12. [4] Kebukawa et al. (2020) Geochimica Cosmochimica Acta 271, 61-77. [5] Hope et al. (2018) PNAS, vol. 115, 6608-6613. [6] Nakamura et al. (2022) Science 379.6634, eabn8671. [7] Ohtaki et al (2021) Geochimica Cosmochimica Acta 310, 320-345. [8] Nittler et al. (2019) Nature Astronomy, vo.3, 659-666. [9] Duprat et al. (2010) Science 328, 742-745.
Methods: The surface features of a Ryugu grain (A0142) was observed using a field-emission (FE)-scanning electron microscope (SEM). Electron-transparent sections were extracted from the regions of interest using a focused ion beam system (FIB). Scanning transmission X-ray microscopy (STXM) imaging was performed to obtain X-ray absorption near edge structure (XANES) spectra at the carbon K-edge (at 280 eV-300 eV). After STXM analysis, the sections were observed using field-emission transmission electron microscope (TEM).
Results: Three carbon-rich regions found in the aqueously well-altered lithologies were extracted by FIB. The widths of the carbon-rich regions were 17×8 µm (area-1), 7×6 µm (area-2), and 35×12 µm (area-4), respectively. TEM analysis revealed that the carbon-rich portions from the area-1 and area-2 show homogeneous textures, while the carbon-rich portion from the area-4 has numerous hollows indicating that this portion consists of the aggregate of organic globules.A comparison of carbon-XANES peak intensities following the method by [4] shows that the organic globules in the area-4 is relatively rich in aromatic carbon (C=C) and poor in aliphatic carbon (C-H) compared to those in the other two areas. Fibrous phyllosilicates are finely intermixed in the organic materials in the area-1 and area-2. On the other hand, the organic materials in the area-4 contain rounded silicate objects with iron-nickel sulfides. Their morphologies and the appearance of sulfide inclusions are similar to GEMS (glass with embedded metal and sulfides) found in interplanetary dust particles (IDPs) [e.g., 5], but they lack iron metals. The majority of these GEMS-like materials show fibrous textures, but some may be amorphous or poorly crystalline.
Discussion: The GEMS-like materials were found in the least altered lithology in Ryugu samples [6], and primitive carbonaceous chondrites [e.g., 7, 8]. They are interpreted to be pre-accretionary phases of their parent bodies. GEMS encapsulated in organic carbons are found in IDPs [5] and ultracarbonaceous Antarctic micrometeorites (UCAMMs) [9], which are thought to be originated from comets. The aggregate of organic materials including GEMS-like objects in our samples resembles IDPs and UCAMMs, further reinforcing the linkage between the building blocks of Ryugu and cometary materials [6]. In addition, the preservation of GEMS-like objects in altered lithologies suggests that complex organic materials act as a barrier to aqueous fluids on the parent body, as proposed in CR2 chondrite [8].
References: [1] Yabuta et al. (2023) Science 379. 6634, eabn9057. [2] Ito et al. (2020) Nature Astronomy 6:1163-1171. [3] Viennet et al. (2023) Geochem. Persp. Let. 25, 8-12. [4] Kebukawa et al. (2020) Geochimica Cosmochimica Acta 271, 61-77. [5] Hope et al. (2018) PNAS, vol. 115, 6608-6613. [6] Nakamura et al. (2022) Science 379.6634, eabn8671. [7] Ohtaki et al (2021) Geochimica Cosmochimica Acta 310, 320-345. [8] Nittler et al. (2019) Nature Astronomy, vo.3, 659-666. [9] Duprat et al. (2010) Science 328, 742-745.