MicrOmega detections of carbonates in Ryugu returned samples within the Hayabusa 2 JAXA
Extraterrestrial Curation Center
Loizeau, D.1; Bibring, J.-P.1; Brunetto, R.1; Pilorget, C.1; Okada, T.2,3; Carter, J.1;Gondet, B.1; Hamm, V.1; Hatakeda, K.2,4;
Langevin, Y.1; Lantz, C.1; Le Pivert-Jolivet, T.1; Nakato, A.2; Riu, L.2; Usui, T.2,3; Yada, T.2; and Yogata, K.2;
1
Institut d'Astrophysique Spatiale, Université Paris-Saclay, CNRS, France
2
Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Japan 3University of Tokyo,
Bunkyo, Japan, 4Marine Works Japan, Ltd., JapanThe Ryugu samples brought back by the Hayabusa2 spacecraft in December 2020 have been delivered to the JAXA
Extraterrestrial Curation Center [1, 2]. Bulk samples and then individual grains have been picked up and stored into sapphire
dishes, weighted, and analyzed with an optical microscope, FTIR spectroscopy, and MicrOmega hyperspectral imaging [3] for
initial description [2]. The MicrOmega instrument used in the JAXA Extraterrestrial Curation Center is a NIR hyperspectral
microscope. It has a total field of view of 5 mm x 5 mm, with resolution of ~22 µm/pixel in the focal plane. It covers thespectral domain from 0.99 µm to ~3.6 µm. Its capabilities enable the identification of organic matter and of different minerals
in the returned samples [4]. Initial analyses with MicrOmega were first made on the bulk samples from chambers A and C of
the Hayabusa 2 returned capsule, and then on individual grains stored in their sapphire dishes. 137 out of the 205 extracted
grains have been analyzed with MicrOmega as of September 30, 2021 [5].
In the spectral domain of MicrOmega, carbonates have a strong characteristic double absorption band in the 3.3-3.5 µm area,accompanied by two other weaker bands around 2.5 and 2.3 µm. The exact spectral position of these bands varies with thecation content of the carbonate [6]. Iron-bearing carbonates also show a strong absorption below 1.5 µm.
First detections of carbonates were made in grains included in the bulk samples from both chambers A and C. Some small
grains seem to be entirely carbonate-rich and are up to ~450 µm, down to <50 µm in size. Carbonate inclusions were also
detected in larger grains, with sizes up to ~380 µm in a >1.5 mm-sized grain, and down to <50 µm (figure 1-A).
From the first 130 analyzed extracted grains, MicrOmega detected carbonate inclusions with high confidence in 19 of thosegrains. The largest detection was made on grain C0041, covering ~0.25 mm², or ~10% of the visible surface of the grain(figure 1-B). This grain is one of the grains with “White regions” as described in Nakato et al. [7].
In terms of spectral characteristics, they all present a double band at 3.31-3.47 µm and a band centered at 2.71 µm. The largest
grains and inclusions also exhibit spectral bands at 2.51 and 2.30 µm, also characteristic of carbonates, and a deep absorption
below 1.5 µm. Some detections also have a band at 2.77 µm (present in many carbonate reference spectra), and some between
3.07 and 3.10 µm. The presence of a strong absorption below 1.5 µm indicates the likely presence of Fe2+ in the carbonate
mineral, although the position of the bands around 2.3, 2.5 and 3.4 µm is shifted to shorter wavelength compared to a purely
Fe2+ carbonate (siderite), and would better fit Mg-bearing carbonates like dolomite or magnesite. The iron-bearing magnesite
breunnerite is a likely candidate for these detections. Smaller grains and inclusions do not show the absorption below 1.5 µm,
while the other absorptions are centered around the same positions than for the larger grains (figure 1-C). Likely candidates
include dolomite and magnesite.
Such carbonate grains and inclusions within the returned samples from Ryugu are a key to understand the evolution of theasteroid. MicrOmega can help mapping and quantifying the presence of these carbonates throughout the collection.

Figure 1. Example of carbonate detections with MicrOmega on bulks from chamber A and C, and on an extracted grain from chamber C.
Left: MicrOmega images with red pixels where carbonate is detected. A: extracted grain C0041. B: bulk samples from chamber A. C: bulk
samples from chamber C. Right: Average spectra of pixels with carbonate detections within the colored boxes in the left images.

References[1] Tachibana S. et al. (2021) LPS, XXXXXII, Abstract #1289. [2] Yada T. et al. (2021) LPS, XXXXXII, Abstract #2008. [3]Bibring J.-P. et al (2017) Space Sci. Rev. 208, 401-412. [4] Pilorget C. and Bibring J.-P. (2014) PSS 99, 7-18. [5] Pilorget C. etal. (2021) this conference. [6] Hunt G.R. and Salisbury J.W. (1971) Visible and near infrared spectra of minerals and rocks. II.
Carbonates. Modern Geology 2, 23–30. [7] Nakato A. et al. (2021) this conference.