2:45 PM - 3:00 PM
[PPS07-17] Diversity of mid-infrared reflectance spectra of samples from asteroid Ryugu.
Keywords:Ryugu, Aqueous alteration, Space weathering, Reflectance spectra
The samples from asteroid Ryugu are the first ever returned to Earth from a C-type asteroid. Analyses of these samples indicate that they are primitive material and their chemical and mineralogical compositions are close to those of CI chondrites (Yokoyama et al. 2022; Nakamura et al. 2022). In addition, the samples show evidence of aqueous alteration on the parent body as well as space weathering, making them important for understanding these processes (e.g., Nakamura et al. 2022; Noguchi et al. 2022).
In this study, we measured reflectance spectra of small (100 μm-1 mm in diameter) Ryugu particles, which were distributed as AO2nd samples, to investigate the range of diversity of mid-infrared (MIR) spectra caused by aqueous alteration and space weathering. We analyzed more than 200 grains from sample A0225 (chamber A; collected at the first touchdown) and more than 100 grains from sample C0213 (chamber C; second touchdown).
As a result, focusing on the Reststrahlen Band (due to the Si-O stretching vibrations in phyllosilicates), we identified two major trends. First, more than 90% of the grains had the peak wavelength of Reststrahelen Band shorter than 10 μm, and the histogram of peak positions showed a unimodal distribution, but the distributions differed between chambers: samples from chamber C showed longer peak wavelengths than those from chamber A. The difference between chambers was also detected by MicrOmega analysis, revealing the gap in the distribution of the (Mg, Fe)–OH absorption at 2.7 μm (Le Pivert-Jolivet et al. 2023). Another study suggested that this difference could be attributed to short-term space weathering induced by UV irradiation (Furukawa et al. 2024). The difference of the Reststrahlen Band observed in our study may similarly be explained by short-term space weathering. Alternatively, the samples from chambers A and C may have experienced different degrees of aqueous alteration on Ryugu’s parent body, and the divergence in their reflectance spectra could reflect this process as well.
Second, some percentage of the grains showed a peak wavelength of the Reststrahlen Band beyond 10 μm. A previous study has suggested that space weathering can shift the Reststrahlen Band to longer wavelengths (e.g., Lantz et al. 2017; Rubino et al. 2024), suggesting that these specific grains have been subjected to space weathering by the solar-wind exposure or micrometeorite impacts. Indeed, when some of these grains were observed using a scanning electron microscope, it was found that they had some surface structures formed by space weathering, like those observed by Noguchi et al. 2023. These grains were found more frequently in the samples from chamber A than in those from chamber C, consistent with the fact that samples from chamber A were collected from Ryugu’s surface and experienced space weathering to higher extent.
In this study, we measured reflectance spectra of small (100 μm-1 mm in diameter) Ryugu particles, which were distributed as AO2nd samples, to investigate the range of diversity of mid-infrared (MIR) spectra caused by aqueous alteration and space weathering. We analyzed more than 200 grains from sample A0225 (chamber A; collected at the first touchdown) and more than 100 grains from sample C0213 (chamber C; second touchdown).
As a result, focusing on the Reststrahlen Band (due to the Si-O stretching vibrations in phyllosilicates), we identified two major trends. First, more than 90% of the grains had the peak wavelength of Reststrahelen Band shorter than 10 μm, and the histogram of peak positions showed a unimodal distribution, but the distributions differed between chambers: samples from chamber C showed longer peak wavelengths than those from chamber A. The difference between chambers was also detected by MicrOmega analysis, revealing the gap in the distribution of the (Mg, Fe)–OH absorption at 2.7 μm (Le Pivert-Jolivet et al. 2023). Another study suggested that this difference could be attributed to short-term space weathering induced by UV irradiation (Furukawa et al. 2024). The difference of the Reststrahlen Band observed in our study may similarly be explained by short-term space weathering. Alternatively, the samples from chambers A and C may have experienced different degrees of aqueous alteration on Ryugu’s parent body, and the divergence in their reflectance spectra could reflect this process as well.
Second, some percentage of the grains showed a peak wavelength of the Reststrahlen Band beyond 10 μm. A previous study has suggested that space weathering can shift the Reststrahlen Band to longer wavelengths (e.g., Lantz et al. 2017; Rubino et al. 2024), suggesting that these specific grains have been subjected to space weathering by the solar-wind exposure or micrometeorite impacts. Indeed, when some of these grains were observed using a scanning electron microscope, it was found that they had some surface structures formed by space weathering, like those observed by Noguchi et al. 2023. These grains were found more frequently in the samples from chamber A than in those from chamber C, consistent with the fact that samples from chamber A were collected from Ryugu’s surface and experienced space weathering to higher extent.