Homogeneity and heterogeneity in Near-Infrared FTIR spectra of Ryugu returned samples

Surface and subsurface materials of C-type near-Earth asteroid 162173 Ryugu were collected and successfully returned to the Earth in the Hayabusa2 mission. Fourier-Transform Infrared Spectroscopy (FTIR) has been conducted to characterize these returned samples as one of the initial description in a non-destructive manner under a purified nitrogen condition without terrestrial contamination. We selected the individual grains and aggregate samples that were not severely influenced by the reflection of incident beam at the sapphire dish and analyzed their reflectance spectra using the primary component analysis (PCA). The result indicates that Ryugu returned samples are highly homogeneous with only a little heterogeneity. The average spectrum of the main PCA group is represented by four absorption bands at 2.7, 3.05, 3.4, and 3.95 µm. The spectral feature is consistent with that obtained from bulk FTIR measurements as reported in Yada et al. (2022), indicating potential presence of hydroxyl, organics and carbonates. Rarely observed types of grains with unique spectra are categorized into three groups: significantly high reflectance, carbonates, and hydroxyl compounds.