Visible to Near-Infrared Spectrophotometry of C0002 Powder Samples of Asteroid 162173 Ryugu
in Comparison with ONC-T and NIRS3 Instruments Onboard Hayabusa2 Spacecraft
T. Hiroi1, R. E. Milliken1, K. M. Robertson1, C. D. Schultz1, K. Amano2, T. Nakamura2, H. Yurimoto3, T. Noguchi4, R.
Okazaki5, H. Yabuta6, H. Naraoka5, K. Sakamoto7, S. Tachibana8, T. Yada7, M. Nishimura7, A. Nakato7, A. Miyazaki7, K.
Yogata7, M. Abe7, T. Okada7, T. Usui7, M. Yoshikawa7, T. Saiki7, S. Tanaka7, T. Fuyuto9, S. Nakazawa7, S. Watanabe10, Y.
Tsuda7, S. Sasaki11, H. Kaiden12, K. Kitazato13, M. Matsuoka14, and E. Tatsumi15
1Brown University, 2Tohoku University, 3Hokkaido University, 4Kyoto University, 5Kyusyu University, Japan, 6Hiroshima
University, 7JAXA Institute of Space and Astronautical Science, 8University of Tokyo, 9Kanagawa Institute of Technology,10Nagoya University, 11Osaka University, 12National Institute of Polar Research, 13University of Aizu, 14National Institute ofAdvanced Industrial Science and Technology, 15Instituto de Astrofisica de Canarias.
Samples and Spectrophotometric Measurements
Powder samples of the largest sampled stone C0002 of asteroid
162173 Ryugu returned by Hayabusa2 mission were loaned for visible
and near-infrared (VNIR) spectrophotometric studies. The powder
samples were produced by physical-strength measurements and werepartially contaminated with a glue (glycol phthalate). Part of thepowder was washed with acetone, which turned it into a fine powder(<125 µm estimate). Large glue fragments were handpicked from theremaining coarse powder (125-500 µm estimate) and a bulk powder
sample was produced by combining the two powder samples.
Bidirectional VNIR reflectance spectra (0.3-2.6 µm) of the samples
were measured at NASA Reflectance Experiment Laboratory (RELAB)
using its bidirectional reflectance spectrometer at the standard viewing
geometry of 30° incidence and 0° emergence angles, and theirphotometric properties were measured at a limited number of viewing
geometries. The photometric property of a Labsphere Spectralon
standard was measured relative to the standard viewing geometry at 550
nm in wavelength.
Biconical Fourier-transform infrared (FTIR) reflectance spectra (1100 µm) of the samples were measured in a dry-air purged environment.
The VNIR and FTIR spectra were combined at 2.5 µm in wavelength byscaling the FTIR reflectance to the VNIR reflectance.

Fig. 1. Phase-angle dependency of reflectances of C0002
powder samples at 550 nm in wavelength.

Spectrophotometric Analyses
Phase-angle dependency of the powder samples at 550 nm inwavelength is plotted in Fig. 1, and VNIR-FTIR spectra up to 4 µm are
Fig. 2. VNIR-FTIR spectra (0.3-4 µm) and photos ofplotted in Fig. 2 along with sample photos under a microscope. The C0002 powder samples. Cup diameter is 4 mm.
phase curve in Fig. 1 shows much brighter albedo and lower inclination
than those obtained by ONC-T measurements in proximity of asteroid
Ryugu [1]. It may be due to the difference between the flatter surfaceof the powder sample compared with the rough asteroid surface that
includes shadows. Further photometric modeling analysis is warranted.
The OH and H2O absorption bands near 3 µm in spectra shown inFig. 2 were analyzed in the same manner as [2]. Natural log
reflectance spectra of the powder samples were fit with a linear-inwavelength continuum background and Gaussians (in wavenumber)over the wavelength range of ~2.6-3.6 µm (variable depending onspectral shape). One of the results (fine powder) is shown in Fig. 3.
Because the first two Gaussian bands (labeled as 1 and 2 in Fig. 3) arebelieved to be characteristic of carbonaceous chondrite classes [2], their Fig. 3. Gaussian fitting of C0002 fine powder spectrum.

Band 1 centers and Band 1 / Band 2 strength ratios are plottedin Fig. 4. Ryugu C0002 powders all plot very close tounheated CI1 chondrites and Ivuna heated up to 400°C. This
result is consistent with previous studies of Ryugu being
mostly unheated CI1 chondrite [e.g., 3], and low-degree
heating may not be detected by this method.
Comparison with ONC-T and NIRS3 Observations
Shown in Fig. 5 is a plot of linear combinations of theVNIR-FTIR spectra of the C0002 fine and coarse powder
samples to match average spectra of representative (butdifferent) areas of Ryugu acquired by ONC-T and NIRS3.
Most of the ONC-T band reflectance values, overall spectral
shape, and 2.7 µm absorption band strength could be matched.
However, when the 2.7 µm band region is closely
examined, there are significant mismatches between the NIRS3and C0002 spectra. The NIRS3 spectrum shows absorption
bands at 2.65 and 2.81 µm which are absent in C0002 spectrum.
If this is not due to NIRS3 data calibration errors, there may besome unstable phases lost during the sample recovery.
Another mismatch is the 2.7 µm band center position.
Fig. 6 shows the NIRS3 spectrum and the modeled C0002spectrum with its wavelength resolution matched with that ofNIRS3 spectrum (18 nm). While the NIRS3 spectrum (openblack squares) shows the band center at around 2.72 µm
asymmetrically expanded toward the longer wavelength, themodeled C0002 powder spectrum (solid purple circles) shows
the band center clearly at a shorter wavelength near 2.71 µm.
When the C0002 powder spectra are artificially shifted toward
the longer wavelength by 6 nm before resampling for theNIRS3 resolution, the resulting modeled spectrum (open purplecircles) matches with the NIRS3 spectrum much better. A
difference in band position is consistent with spectra of ejecta
from an artificial crater showing shorter 2.7 µm band positions[4].
Summary and Discussion
If the C0002 powder samples represent the average surface
material of asteroid Ryugu, this study indicates that the global
space weathering may have occurred mainly by solar wind thatcaused the 2.7 µm absorption band center to be shifted toward alonger wavelength [e.g., 5].
If such space weathering
occurred, the 2.7 µm band strength and the visible spectra
measured with ONC-T might have been altered as well. This
would influence the physical interpretation of the model fits
shown in Fig. 5.
References[1] Tatsumi E. et al. 2020. A&A 639:A83. [2] Hiroi T. et al.
2021. Polar Science 29:100723. [3] Yokoyama T. et al. 2022.
Science 10.1126:7850. [4] Kitazato K. et al. 2021. Nature
Astronomy 5:246. [5] Lantz C. et al. 2015. A&A 577:A41.

Fig. 4. Band 1 center vs. Band 1 / Band 2 strength ratio of C0002
powder samples plotted with hydrous carbonaceous chondrites(modified from [2]).

Fig. 5. Comparison of C0002 powder spectra with ONC-T andNIRS3 data.

Fig. 6. Matching of the 2.7 µm absorption band feature between
the modeled C0002 powder spectra and NIRS3 spectrum in Fig. 5.