Infrared transmission spectra of Ryugu samples and their adsorption properties
Yoko Kebukawa, Hikaru Yabuta, Hisayoshi Yurimoto, Tomoki Nakamura, Takaaki Noguchi, Ryuji Okazaki, Hiroshi Naraoka, Kanako Sakamoto, Shogo Tachibana, Toru Yada, Masahiro Nishimura, Aiko Nakato, Akiko Miyazaki, Kasumi Yogata, Masanao Abe, Tatsuaki Okada, Tomohiro Usui, Makoto Yoshikawa, Takanao Saiki, Satoshi Tanaka, Fuyuto Terui, Satoru Nakazawa, Sei-ichiro Watanabe, Yuichi Tsuda, Hayabusa2-initial-analysis IOM Team
Icarus
Available online 15 May 2026, 117168
“Highlights
- Micro-FTIR transmission spectra show that Ryugu samples have a low mean aliphatic CH2/CH3 peak-height ratio (1.11 ± 0.13), significantly lower than Ivuna CI (~1.6) and lower than previously reported Ryugu values.
- Aliphatic Csingle bondH bands increase by up to ~15% within ~1 h during ambient-condition FTIR measurements, consistent with rapid adsorption of environmental volatile organic compounds (VOCs) on freshly crushed surfaces.
- Ryugu particles are markedly more reactive to VOC adsorption than Ivuna and control antigorite, implying that contamination can occur even in clean environments and must be actively mitigated in sample-return analyses.”
“The Hayabusa2 spacecraft successfully collected samples from two distinct locations on Ryugu, a C-type asteroid, and these samples were stored separately in sample containers in chambers A and C, respectively. Infrared absorption spectra were obtained using Fourier transform infrared (FTIR) microspectroscopy to provide characterization of the Ryugu particles and their organic matter as part of the organic macromolecule initial analysis team. The IR transmission (absorption) spectra of samples from both chambers A and C were nearly identical and consistent with CI chondrites. Some local heterogeneity was observed, such as areas where the IR spectra were dominated by carbonates. The average CH2/CH3 peak height ratio of Ryugu samples was 1.11 ± 0.13, which is lower than that of the Ivuna CI chondrite (~1.6). Furthermore, the CH2/CH3 ratio of Ryugu in this study was significantly lower than the previously reported values for Ryugu. To test the environmental effects, changes in the IR spectra were monitored during analyses and storage. Notably, approximately 15% increases in the aliphatic Csingle bondH peak were observed during the FTIR analysis within ~1 h, when the measurement was conducted under ambient condition, partly due to the adsorption of environmental volatile organic compounds (VOCs), likely machine oil used in the FTIR instruments. However, the effects of VOCs on the CH2/CH3 ratio are not clear in this study. Measurements performed in a heating stage at a mild temperature of 60 °C with N2 flow gave the best spectra, minimizing water and VOC contamination. Porous OH-bearing silicates are known to adsorb VOCs, particularly when fresh surfaces are exposed just after crushing the samples. The Ryugu samples appear to be significantly reactive, adsorbing VOCs very rapidly even in a clean environment with minimal contamination sources. The highly porous and fresh nature of Ryugu samples may be responsible for this behavior.”
