Spatial relationship between macromolecular organic matter and organic-bearing phyllosilicates inRyugu grain.
Toru Matsumoto1, Takaaki Noguchi2, Tohru Araki3, Takuji Ohigashi4, Yohei Igami2, and Akira Miyake2
1
The Hakubi center for Advanced Research, Kyoto University.2Division of Earth and Planetary Sciences, Kyoto University,
3
Institute for Molecular Science, 4High Energy Accelerator Research Organization.

Introduction: Regolith samples recovered from the C-type asteroid Ryugu show chemical and petrologicalcharacteristics similar to those of CI carbonaceous chondrites [e.g., 1-3]. They consist of primitive components of the earlysolar system, including water and organic matter, but are highly modified in the parent body by aqueous alteration. Insolubleorganic macromolecules (IOM) in Ryugu grains are the dominant components of organic matter in Ryugu’s materials [3]. The
chemical structures, distributions, and morphologies of the macromolecular organic matter can be tracers of the evolution oforganic materials in the interstellar medium, the protosolar disk, and the aqueous environment in the parent body of Ryugu [3].
Recent analysis of the interlayer spaces of phyllosilicate minerals in Ryugu grains suggested the presence of organic molecules
in clay structures [4, 5]. The infrared absorption band at around 3 µm obtained from Ryugu grains may come from N-Hbearing molecules trapped in the interlayers of phyllosilicates [6]. These results suggest that phyllosilicates have been affectedby organic matter during the aqueous alteration. Detailed investigation of the interlayer spaces of phyllosilicates and their
spatial association with macromolecular organic matter will provide insight into the interaction between minerals, water, andorganic matter in the parent body of Ryugu. In this study, we focused on carbon-rich regions in a Ryugu grain and examined
the spatial relationship between organic materials and phyllosilicates, together with their chemical features.
Methods: A Ryugu grain from the chamber A (grain No. A0142) was distributed by JAXA as a sample for theinternational announcement of opportunity and was used in this study. The surface features of the grain was observed using afield-emission (FE)-scanning electron microscope (SEM) (JSM-7001F). Electron-transparent sections were extracted from theregions of interest using a focused ion beam system (FIB Helios NanoLab G3 CX). Scanning transmission X-ray microscopy(STXM) imaging was performed to obtain X-ray absorption near edge structure (XANES) spectra from the sections. The
bonding structure of carbon was investigated using XANES spectra at the carbon K-edge (at 280 eV-320 eV). After STXM
analysis, the sections were observed using field-emission transmission electron microscope (TEM) (JEOL JEM2100F).
Results: Backscattered electron images of the grain surface show the concentration of organic matter embedded in thephyllosilicate matrix (Fig. 1a). Several carbon-rich spots were found on the grain surface. We investigated the largest carbonrich region with approximately 17×8×9 µm in size (Fig.1ab). Carbon-XANES spectra of the carbon-rich region include major
peaks of aromatic carbon (285 eV) and carboxyl carbon (288.5 eV), and those of phyllosilicate matrix in the vicinity of thecarbon-rich region show peaks of aromatic carbon (285 eV), carboxyl carbon (288.5 eV), aliphatic carbon (287.5 eV), andcarbonates (290.4 eV). TEM analysis showed that organic materials and fibrous phyllosilicates are finely intermixed in thecarbon-rich region. Hereafter, we call the carbon-rich region as the carbon-phyllosilicate aggregate (CPA). Distinct
morphologies of organic materials, such as the organic globules, were not identified in the CPA. The surrounding matrix
consists mainly of fibrous phyllosilicates, magnetite, iron sulfides, and carbonates. High-resolution TEM images showing
layered structures of the fibrous phyllosilicates (Fig. 1c) enabled us to investigate the interlayer distance of the phyllosilicates.
The majority of the fibrous phyllosilicates in the CPA and the matrix have interlayer distances of 0.7~0.75nm, and 1.0-1.2nm.
These values correspond to typical interlayer distances of serpentine and saponite in Ryugu samples, respectively [2]. On theother hand, fibrous phyllosilicates with interlayer distances of 1.23-1.35 nm were occasionally observed in the CPA and in thematrix close to the carbon-rich region (Fig.1c).
Discussion: The enrichment of aromatic and carboxylic carbon in the XANES spectra from the CPA is similar to thespectral features of highly-aromatic or aromatic macromolecular carbon in Ryugu samples [3]. The concentration of aromaticorganic matter may have occurred by redistribution and/or synthesis of organic matter during fluid activity in the parent body.
It is also possible that large chunks of organic matter have been incorporated during the initial accretion of the parent body, inthe form of primitive materials originated from the outer solar system [7]. The appearance of aliphatic carbon in the matrix is
consistent with the previous XANES study of Ryugu samples, in which aliphatic organics were detected in association withphyllosilicates [8]. Previous X-ray diffraction measurements of Ryugu samples detected 001 reflection of smectite at 1.26 nm[4]. The relatively large interlayer distance was interpreted as the expansion of the interlayer space due to the presence oforganic molecules [4]. Accordingly, the interlayer distances of 1.25-1.35 nm found in our sample suggest that organic species

are intercalated into the phyllosilicates. The spatial relationship between the CPA and the possibly organic-bearing
phyllosilicates suggests that macromolecular organic matter could have interacted with surrounding phyllosilicates during
aqueous alteration, as suggested by analyses of carbonaceous chondrites [9, 10].

Figure 1. SEM and TEM analysis of the carbon-rich region of Ryugu grain (A0142). Backscattered electron image (a) and bright-field TEM
image (b) of the carbon-phyllosilicate aggregate (CPA). Arrows indicate the interface between the CPA and surrounding phyllosilicate
matrix. (c) High-resolution TEM image of fibrous phyllosilicates near the CPA. Interlayer distances are shown in the figure.

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