15:45 〜 16:00
[MIS20-07] Diversity of reflectance spectra of asteroid Ryugu samples
キーワード：はやぶさ2、リュウグウ、中間赤外分光法
In December 2020, Hayabusa2 spacecraft brought samples back to Earth from Cb asteroid Ryugu. Initial analysis of the returned samples indicates that Ryugu consists mainly of materials that have experienced pervasive aqueous alteration, but contains some less-altered fragments (Nakamura et al. 2022). The altered major lithology of Ryugu samples resembled CI chondrites based on petrological and mineralogical properties (e.g., Yokoyama et al. 2022).
In this study, we performed spectroscopic measurements and mineralogical observations of 6 millimeter-size Ryugu samples (A0064-02, A0067-01, A0067-02, A0094-02, C0002-plate5 and C0025-03) and 8 millimeter-size samples of CI chondrites (Orgueil and Ivuna) to understand the relationship between mid-infrared (MIR) spectral characteristics and mineral compositions, and to discern the degree of aqueous alteration and mineral compositions from MIR spectra. Both Ryugu and CI chondrite samples are breccia consisting of many sub-millimeter-size fragments. We used micro-FT-IR system for spectroscopic measurements in order to measure MIR spectra of individual fragments which show different degrees of aqueous alteration.
MIR reflectance spectra of Ryugu and CI chondrite samples show that, as the degree of aqueous alteration increases, the reflectance of the Si-O Reststrahlen band at ~10 µm increases and the peak position moves toward shorter wavelengths. The position of 2.7 µm absorption is also shifted toward shorter wavelengths from 2.79 to 2.71 µm as the degree of aqueous alteration increases. These changes of the Reststrahlen band and the 2.7 µm peak position are likely due to an increase of Mg# in phyllosilicates with the progression of aqueous alteration. The elevation of the reflectance of the Reststrahlen band may be due to the increase of crystallinity of the phyllosilicate as the aqueous alteration progressed. These features are considered to be good parameters for estimating the degree of aqueous alteration from the MIR reflectance spectra. SEM/EDS analysis indicates that Mg# of phyllosilicates of probed areas is lower in the CI samples than in the Ryugu samples. This may indicate that the pervasive distribution of nano-size ferrihydrite within phyllosilicates in CI samples, which was formed by terrestrial weathering.
It was also found that the type of carbonates in samples can be inferred from the peak position between 13 and 14 µm. A peak at 13.5 µm is visible when magnesite is abundant enough, and a peak at 13.8 µm is visible when dolomite is abundant. However, the areas with 1-7 area-% carbonate, these peaks may or may not be visible, depending on the relationship with other coexisting phases. These peaks are clearly observed in measured areas that contain 8 area-% carbonate or higher, thus we have concluded that these peaks can be used to identify an area with a high carbonate content, though the abundance of carbonate can not be estimated from the height of those peaks. Furthermore, the peaks at 10.17 µm and 10.45 µm were found to indicate the presence of olivine, which is a measure to distinguish between less-altered and altered fragments. These peaks may or may not be visible for the areas with 1-4 area-% olivine, depending on the relationship with other coexisting phases. These peaks are clearly observed in measured areas that contain 5 area-% olivine or higher, thus we have concluded that these peaks can be used to identify an area with a high olivine content, though the abundance of olivine can not be estimated from the height of those peaks.
In this study, we performed spectroscopic measurements and mineralogical observations of 6 millimeter-size Ryugu samples (A0064-02, A0067-01, A0067-02, A0094-02, C0002-plate5 and C0025-03) and 8 millimeter-size samples of CI chondrites (Orgueil and Ivuna) to understand the relationship between mid-infrared (MIR) spectral characteristics and mineral compositions, and to discern the degree of aqueous alteration and mineral compositions from MIR spectra. Both Ryugu and CI chondrite samples are breccia consisting of many sub-millimeter-size fragments. We used micro-FT-IR system for spectroscopic measurements in order to measure MIR spectra of individual fragments which show different degrees of aqueous alteration.
MIR reflectance spectra of Ryugu and CI chondrite samples show that, as the degree of aqueous alteration increases, the reflectance of the Si-O Reststrahlen band at ~10 µm increases and the peak position moves toward shorter wavelengths. The position of 2.7 µm absorption is also shifted toward shorter wavelengths from 2.79 to 2.71 µm as the degree of aqueous alteration increases. These changes of the Reststrahlen band and the 2.7 µm peak position are likely due to an increase of Mg# in phyllosilicates with the progression of aqueous alteration. The elevation of the reflectance of the Reststrahlen band may be due to the increase of crystallinity of the phyllosilicate as the aqueous alteration progressed. These features are considered to be good parameters for estimating the degree of aqueous alteration from the MIR reflectance spectra. SEM/EDS analysis indicates that Mg# of phyllosilicates of probed areas is lower in the CI samples than in the Ryugu samples. This may indicate that the pervasive distribution of nano-size ferrihydrite within phyllosilicates in CI samples, which was formed by terrestrial weathering.
It was also found that the type of carbonates in samples can be inferred from the peak position between 13 and 14 µm. A peak at 13.5 µm is visible when magnesite is abundant enough, and a peak at 13.8 µm is visible when dolomite is abundant. However, the areas with 1-7 area-% carbonate, these peaks may or may not be visible, depending on the relationship with other coexisting phases. These peaks are clearly observed in measured areas that contain 8 area-% carbonate or higher, thus we have concluded that these peaks can be used to identify an area with a high carbonate content, though the abundance of carbonate can not be estimated from the height of those peaks. Furthermore, the peaks at 10.17 µm and 10.45 µm were found to indicate the presence of olivine, which is a measure to distinguish between less-altered and altered fragments. These peaks may or may not be visible for the areas with 1-4 area-% olivine, depending on the relationship with other coexisting phases. These peaks are clearly observed in measured areas that contain 5 area-% olivine or higher, thus we have concluded that these peaks can be used to identify an area with a high olivine content, though the abundance of olivine can not be estimated from the height of those peaks.