A comparative study on reflectance spectra of the samples collected from the carbonaceous asteroid Ryugu and carbonaceous chondrites

概要

博士論文

A comparative study on reflectance spectra
of the samples collected from
the carbonaceous asteroid Ryugu
and carbonaceous chondrites
（炭素質小惑星リュウグウ回収試料と
炭素質隕石の分光学的特徴における比較研究）

天野

香菜

令和 5 年

博士論文の要約
This thesis discusses reflectance spectral characteristics of rock samples from carbonaceous
asteroids (e.g., samples collected from the carbonaceous asteroid Ryugu, carbonaceous chondrites,
and experimentally-modified carbonaceous chondrites) based on their mineralogical and chemical
properties, and aims to better interpret compositions and origins of carbonaceous asteroids from the
results of ground- and space-based spectroscopic surveys.
Asteroids are remnants of planetary embryos that did not grow into larger bodies (i.e., planets),
and consequently record the materials that formed in the early solar system. Carbonaceous asteroids
are believed to be rich in water and organic matter which are consistent with the origin from a cold
region. Their compositions are crucial for revealing the dynamic evolutionary history of the solar
system.
Reflectance spectroscopy via ground- and space-based telescopes is recognized as being the
primary technique to investigate the compositional distribution of asteroids. However, there is still
considerable ambiguity with regard to the interpretation of observed spectra, especially for Ccomplex asteroids, due to the lack of diagnostic spectral features. Thus, detailed laboratory
measurements of rock samples from carbonaceous asteroids (e.g., carbonaceous chondrites, and
samples returned from asteroids by spacecraft) play a crucial role in linking their mineralogical and
chemical characteristics to spectral properties.
The Japanese spacecraft Hayabusa2 has performed close-up observations of the carbonaceous
asteroid (162173) Ryugu (Sugita et al. 2019; Kitazato et al. 2019) and brought back 5.4 grams of
rock samples to Earth (Morota et al. 2020; Tachibana et al. 2022). The sample return mission has
contributed to revealing the actual nature of a carbonaceous asteroid and re-interpreting the
relationship between carbonaceous asteroids and chondrites. Laboratory analysis of the collected
samples revealed that Ryugu samples consist of hydrated silicates, carbonates, Fe oxides, and Fe
sulfides, suggesting extensive aqueous alteration on the Ryugu parent body (e.g., Nakamura et al.
2022). In relation to meteorites, Ryugu samples are found to be most similar to unheated CI
carbonaceous chondrites in terms of mineralogical and chemical properties (Nakamura et al. 2022;
Yokoyama et al. 2022). However, reflectance spectra of Ryugu samples do not match those of
unheated CI chondrites (Nakamura et al. 2022; Yada et al. 2022). Rather, Ryugu spectra are
consistent with those of CI chondrite samples that were heated experimentally (Sugita et al. 2019;
Kitazato et al. 2019). In this context, this thesis tries to account for the spectral mismatch between
Ryugu samples and unheated CI carbonaceous chondrites despite their mineralogical similarities
based on mineralogical, chemical, and spectral analyses of the samples.
This thesis is divided into two chapters. The first chapter examines spectral and compositional
changes in CI chondrites with heating. I performed a series of heating experiments of Orgueil CI
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carbonaceous chondrites in reducing conditions with varying temperatures (300 to 900 °C) and
duration of heating (50 hours or longer). I conducted reflectance spectroscopy of the heated samples
from ultraviolet (UV) to mid-infrared (MIR) range (0.2-18 µm in wavelength). Spectral
measurements were undertaken without exposing the heated samples to avoid interaction with
atmospheric water and oxygen, which severely affects the spectral properties of the samples. The
heated samples were placed into a customized air-tight cell using a nitrogen-purged glovebox. This
methodology is newly improved in this study. I also conducted characterization of the mineral and
organic composition to be associated with spectral features, including Synchrotron X-ray diffraction
(S-XRD) analysis, Field Emission Scanning Electron Microscope (FE-SEM) observations, Karl
Fischer titration (KFT) methods to determine water contents, Insoluble Organic Matter (IOM)
extraction and transmission infrared spectroscopy of extracted and unextracted materials, Raman
spectroscopy, S canning transmission X ray microscopy (STXM) and X ray Absorption Near Edge
Structure (XANES) analysis, and elemental analysis to determine carbon contents.
As a result, heating at < 300 °C did not cause the decomposition of hydrated silicates in Orgueil
samples, but caused the removal of adsorbed and interlayer water, dehydration of sulfates, and
reduction of iron in phyllosilicates. The decomposition of phyllosilicates and carbonates progressed
by heating at 400-600 °C as well as modification in organic composition. The samples heated at 700
and 900 °C did not exhibit features of hydrated silicates but contained secondary-formed anhydrous
minerals. The unheated Orgueil samples exhibit ~3.5% of reflectance at v-band (0.55 µm in
wavelength) and a sharp drop in reflectance towards UV wavelengths. Visible-Near-IR (Vis-NIR)
reflectance spectra of the Orgueil samples heated at 300 to 600 °C were much darker than those of
the unheated samples, whereas the v-band reflectance increased by heating at 700 and 900 °C. IR
spectra changed according to changes in the mineral composition of the heated samples. The
unheated Orgueil powder samples exhibit a band depth value of >50% for the 2.7-μm OH, 2.9 µm
molecular water, and ~3.4 µm organic and carbonate features. The 2.7-μm band depth decreases with
increasing temperature until 600 °C and the band disappears at 700 °C.
The second chapter begins by examining the spectral properties of various Ryugu samples,
including coarse grains, coarse aggregates, and powder samples. The reflectance spectra of Ryugu
samples were almost uncontaminated by the terrestrial atmosphere thanks to the air-tight procedures
described in the previous chapter. Thus, the spectral data can be directly compared to the remote
sensing data.
Ryugu sample spectra are dominated by Mg-rich phyllosilicates, carbonates, and organic
compounds. Ryugu individual grains do not exhibit large spectral variations, indicating current
Ryugu formed from a portion of the parent body that experienced extensive aqueous alteration.
However, some variations were observed in Vis-NIR reflectance, spectral slope, and the 2.7-μm band
depth probably due to the heterogeneous angular distribution of diffuse reflection from a surface.
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In comparison with meteorites, Ryugu samples show the best spectral match with the Orgueil
samples experimentally heated at 300 °C, rather than with unheated Orgueil. This spectral match
reflects the mineralogical compositions being similar to each other, such as Mg- and Fe2+-rich
phyllosilicates, and the almost complete absence of interlayer water in phyllosilicates. No significant
spectral features of sulfates were observed in Ryugu samples and 300 °C-heated samples. Ryugu
samples originally do not contain sulfates, whereas experimental heating at 300 °C caused
modification, probably dehydration, of Ca- and Mg-sulfates that are observed in unheated Orgueil
samples as terrestrial weathering products. In addition, unheated Orgueil contains abundant adsorbed
and interlayer water and Mg- and Fe3+-rich phyllosilicate composition, which result in the spectral
differences between unheated CI chondrites and Ryugu samples. However, this does not imply
heating at 300 °C of the asteroid Ryugu, inferred from aqueous alteration environments of the Ryugu
parent body which results in originally Fe2+-rich phyllosilicates and no sulfates. Although abundant
interlayer water would occur when saponite formed on Ryugu’s parent body with a relatively high
water-rock ratio, interlayer water in phyllosilicates of the Ryugu surface materials (~tens of
centimeters in depth) can be removed by repeated diurnal heating (~80 °C at TD1 region, ~60 °C at
TD2 region; 674 (Shimaki et al. 2020; Morota et al. 2020) at the current orbit (~1.2 au from the Sun;
(Watanabe et al. 2019)). The mild heating at <100 °C is not enough to decompose phyllosilicates in
Ryugu, but can be effective to remove most interlayer water of phyllosilicates in CI chondrites (Takir
et al. 2013). Collectively, reflectance spectra of CI chondrites have been severely modified in
terrestrial conditions since their falls onto Earth, which accounts for spectral differences between CI
chondrites and Ryugu samples.
A previous study reported that a few Ryugu grains display features indicative of space
weathering, including amorphous layers due to solar wind irradiation and melt layers due to
micrometeoroid bombardment (Noguchi et al. 2023). Among the samples probed, the grains A0067
and A0094 have a thin amorphous layer covering the uppermost part of phyllosilicates, an expected
product of the earliest stage of surface modification due to space weathering (Noguchi et al. 2023).
Such weathering processes probably result in decreases in the 2.7-μm band depth (Matsuoka et al.
2015). The RB peak positions (~10 μm in wavelength) of A0067 and A0094 are shifted towards
longer wavelengths than those of typical Ryugu samples. Similar wavelength shifts of the 2.7-μm
band and RB peak positions were produced by helium ion irradiation experiments on Alais CI
chondrite that simulated solar wind irradiation, probably due to selective sputtering of Mg in
phyllosilicates and amorphization. Heavily space-weathered Ryugu grains, for instance, grains with
melt layers or melt splashes (Noguchi et al. 2023), may have distinct Vis-NIR reflectance spectra
compared to typical Ryugu grains.
I tried to compare the reflectance spectra of Ryugu samples and the asteroid Ryugu obtained by
the onboard instruments. ...