Initial description of Ryugu returned samples: characteristics of individual grains by FT-IR
analysis
Kentaro Hatakeda1,4, Toru Yada1, Masanao Abe1,2, Tatsuaki Okada1,3, Aiko Nakato1, Kasumi Yogata1, Akiko Miyazaki1,Kazuya Kumagai1,4, Masahiro Nishimura1, Yuya Hitomi1,4, Hiromichi Soejima1,4, Kana Nagashima1, Miwa Yoshitake1, Ayako
Iwamae1,4, Shizuho Furuya1,3, Tomohiro Usui1, Shogo Tachibana1,3, Kanako Sakamoto1, Kohei Kitazato5, Hisayoshi Yurimoto61
Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara 252-5210, Japan,
2The Graduate University for Advanced Studies (SOKENDAI), Hayama 240-0193, Japan
3University of Tokyo, Bunkyo, Tokyo 113-0033, Japan,
4Marine Works Japan, Ltd., Yokosuka 237-0063, Japan,
5
The University of Aizu, Aizu-Wakamatsu 965-8580, Japan,
6
Hokkaido University, Sapporo 060-0810, Japan.
A total of ~5.4 g of sample was collected at the surface of the C-type asteroid 162173 Ryugu and successfully returned toEarth [1] [2]. The collected samples were transported to the curation facility in ISAS, Sagamihara, Japan, and stored in purified
nitrogen condition, except for a few grains picked up under vacuum condition, to keep the samples as physically and chemicallypristine as possible. Approximately 100 grains have been picked up from each of those in Chamber A, uppermost centimeterscale layer of Ryugu collected during the first touch-down sampling (TD1), and in Chamber C, surface to subsurface layer (~1m) of Ryugu collected close to the artificial crater during the second touch-down sampling (TD2). Then, initial descriptions,such as weighing, optical imaging, FT-IR spectroscopy in 1-5 µm range, MicrOmega as a hyperspectral NIR microscope, andvisual multispectral imaging, have been performed for characterization of these samples. Here we present the preliminary resultof FT-IR spectral analysis for individual grains.
The measurement system consists of the spectrometric unit outside the clean chamber and the sample chamber connectedto the clean chamber (Figure 1). Although purified nitrogen gas flowed inside the spectrometric unit, absorption bands of O-H(e.g., H2O) and that of C-O (CO2) appeared in the reflectance spectra of reference material (Infragold) measured after the sample
analysis, indicating the influence of spectral absorption in the atmosphere. In addition, the spot size of the incident beam, approx.
1 mm in diameter at the focal position, was slightly larger than the sizes of individual grains in many cases, and these data werecontaminated by the reflectance from the sapphire dish at the bottom of the sample holder. The reflection from the sapphire dishincreased the albedo, changed the spectral slope between 1 and 2.5 µm from positive to negative, and decreased the absorption
band depth (Figure 2).
Except for the samples with significant effect of sapphire dish, reflectance spectra of individual grains generally show asimilar trend to those of bulk samples [3] in the wavelength range between 2.5 and 4 µm. The depth of absorption bands varied
between grains, and the band-depth variation is observed not only between the individual grains but also within the single grain.
We also found grains with unique spectral profiles, of which abundance is ~1% or less. These grains basically contain relatively
large inclusions, which are apparently different from the surrounding matrix.
We are now planning to modify the FT-IR system to extend to the mid to far-infrared wavelength range, and expect to getfurther information on the collected Ryugu samples.

Figure 1. Overview of the FT-IR measurement system

Figure 2. The mixing effect of the sapphire dish reflectance. The larger effect was observed with increasing the area where incident
beam illuminated the sapphire dish; A) Reflectance spectra (%), B) Normalized reflectance at 2.6 µm, C) Normalized reflectancewith continuum at 2.6-3.2 µm and NIRS3 spectral data [4], D) Normalized reflectance with continuum at 3.3-3.6 µm.

References[1] Tachibana S. et al. (2021) LPS, XXXXXII, Abstract #1289. [2] Yada T. et al. (2021) LPS, XXXXXII, Abstract #2008.
[3] Yada T. et al. in press. Nature Astronomy. [4] Kitazato et al. (2019) Science 364: 272.