講演情報
[PPS03-P18]Solar UV-induced space weathering on C-complex asteroids implications from spectral and microstructural changes in carbonaceous chondrites
*古川 聡一朗1,2、岡田 達明1,2、保田 慶直3,2、金丸 礼2、矢田 達2、石崎 拓也2、廣井 孝弘4、佐々木 晶5、江頭 勇介5、海田 博司6 (1.東京大学、2.JAXA宇宙科学研究所、3.総合研究大学院大学、4.ブラウン大学、5.大阪大学、6.国立極地研究所)
キーワード：
宇宙風化、C型小惑星、太陽紫外線、スペクトル
Introduction: Space weathering alters the mineralogy and spectral properties of airless body surfaces through micrometeorite impacts and solar wind irradiation, causing spectral differences between asteroids and meteorites. Returned samples and laboratory experiments suggest that these changes result from mineral amorphization, iron reduction, and dehydration [1,2]. Solar ultraviolet (UV) radiation provides an additional energy source that can induce dehydration of hydrous minerals and photochemical alteration of organics [3], particularly affecting C-type asteroids through spectral slope variations and modifications of the ~2.7 µm absorption band [4,5]. Most laboratory UV-weathering experiments have been conducted under ambient or nitrogen-purged conditions, which do not fully simulate airless asteroid surfaces. Here, we perform UV-irradiation experiments on carbonaceous chondrites under vacuum to better simulate space conditions and investigate the progression and timescale of UV weathering.
Methods: We conducted UV-irradiation experiments on carbonaceous chondrites under vacuum to evaluate UV-induced space weathering on C-complex asteroids. Allende (CV3) and Tagish Lake chondrites were irradiated at ~10-4 Pa using a xenon lamp (250–385 nm; Asahi Spectra MAX-303). Sample temperature was estimated using carbon adhesive tape with reflectance similar to that of the carbonaceous chondrites and reached ~88 °C during irradiation, comparable to daytime surface temperatures on Ryugu [6]. UV fluences and equivalent exposure times at 1 AU are summarized in Table 1. Reflectance spectra before and after irradiation were obtained using a µ-FTIR system (IRT-5000 and FT/IR-6100, JASCO), and surface morphologies were examined by SEM (JSM-6510LA, JEOL) to correlate spectral changes with microstructural modification.
Results: FTIR measurements revealed distinct spectral responses between the two carbonaceous chondrites. Allende exhibited a non-monotonic spectral response, with an initial decrease in mid-infrared reflectance followed by recovery toward higher reflectance, while the Christiansen feature (CF) remained largely unchanged. In contrast, Tagish Lake showed more pronounced spectral modifications, including increased mid-infrared reflectance, the Reststrahlen Band (RB) broadening with long-wavelength shift, and weakening of the 2.7 µm absorption band (Figure 1). SEM observations revealed that both Allende and Tagish Lake developed abundant sub-micrometer- to micrometer-scale void-like features and scaly-to-granular textures after irradiation (Figure 2). Some of these features may also reflect measurement-related effects, such as differences in observation areas.
Discussion: The near invariance of the Christiansen feature (CF) in the carbonaceous chondrites suggests that UV irradiation does not significantly alter bulk crystal structures and that modifications are likely limited to thin surface layers. In contrast, changes in mid-infrared reflectance likely reflect variations in scattering properties caused by surface roughening, with possible contributions from removal of organic materials [3,7]. Tagish Lake exhibited more pronounced spectral evolution, likely related to dehydration through removal of adsorbed water and/or structural OH associated with Mg–OH and/or Fe–OH bonds, potentially accompanied by changes in phyllosilicate crystallinity [8,9]. These results highlight solar UV radiation as an effective contributor to space weathering on C-complex asteroids.
References: [1] Noguchi, T. et al. (2023) Nat Astron 7, 170-181. [2] Zhang, P. et al. (2022) A&A 659, A78 (2022). [3] Poggiali, G. et al. (2020) Front. Astron. Space Sci. 7, 18 (2020). [4] Sasaki, S. et al. (2024) JESA 2, 145. [5] Furukawa, S. et al. (2025) Sci Rep 15, 14613. [6] Okada, T. et al. (2020) Nature 579, 518–522. [7] Hapke, B. (2012) Cambridge Univ. Press. 513. [8] Brunetto, R. et al. (2023) ApJL 951, L33. [9] Takase, T. et al. (2025) LPI Contrib. No. 3088, Abstract #5101.
Methods: We conducted UV-irradiation experiments on carbonaceous chondrites under vacuum to evaluate UV-induced space weathering on C-complex asteroids. Allende (CV3) and Tagish Lake chondrites were irradiated at ~10-4 Pa using a xenon lamp (250–385 nm; Asahi Spectra MAX-303). Sample temperature was estimated using carbon adhesive tape with reflectance similar to that of the carbonaceous chondrites and reached ~88 °C during irradiation, comparable to daytime surface temperatures on Ryugu [6]. UV fluences and equivalent exposure times at 1 AU are summarized in Table 1. Reflectance spectra before and after irradiation were obtained using a µ-FTIR system (IRT-5000 and FT/IR-6100, JASCO), and surface morphologies were examined by SEM (JSM-6510LA, JEOL) to correlate spectral changes with microstructural modification.
Results: FTIR measurements revealed distinct spectral responses between the two carbonaceous chondrites. Allende exhibited a non-monotonic spectral response, with an initial decrease in mid-infrared reflectance followed by recovery toward higher reflectance, while the Christiansen feature (CF) remained largely unchanged. In contrast, Tagish Lake showed more pronounced spectral modifications, including increased mid-infrared reflectance, the Reststrahlen Band (RB) broadening with long-wavelength shift, and weakening of the 2.7 µm absorption band (Figure 1). SEM observations revealed that both Allende and Tagish Lake developed abundant sub-micrometer- to micrometer-scale void-like features and scaly-to-granular textures after irradiation (Figure 2). Some of these features may also reflect measurement-related effects, such as differences in observation areas.
Discussion: The near invariance of the Christiansen feature (CF) in the carbonaceous chondrites suggests that UV irradiation does not significantly alter bulk crystal structures and that modifications are likely limited to thin surface layers. In contrast, changes in mid-infrared reflectance likely reflect variations in scattering properties caused by surface roughening, with possible contributions from removal of organic materials [3,7]. Tagish Lake exhibited more pronounced spectral evolution, likely related to dehydration through removal of adsorbed water and/or structural OH associated with Mg–OH and/or Fe–OH bonds, potentially accompanied by changes in phyllosilicate crystallinity [8,9]. These results highlight solar UV radiation as an effective contributor to space weathering on C-complex asteroids.
References: [1] Noguchi, T. et al. (2023) Nat Astron 7, 170-181. [2] Zhang, P. et al. (2022) A&A 659, A78 (2022). [3] Poggiali, G. et al. (2020) Front. Astron. Space Sci. 7, 18 (2020). [4] Sasaki, S. et al. (2024) JESA 2, 145. [5] Furukawa, S. et al. (2025) Sci Rep 15, 14613. [6] Okada, T. et al. (2020) Nature 579, 518–522. [7] Hapke, B. (2012) Cambridge Univ. Press. 513. [8] Brunetto, R. et al. (2023) ApJL 951, L33. [9] Takase, T. et al. (2025) LPI Contrib. No. 3088, Abstract #5101.