# First compositional analysis of Ryugu samples by the MicrOmega hyperspectral microscope

C. Pilorget, T. Okada, V. Hamm, R. Brunetto, T. Yada, D. Loizeau, L. Riu, T. Usui, A. Moussi-Soffys, K. Hataakeda, et al.

► To cite this version:

C. Pilorget, T. Okada, V. Hamm, R. Brunetto, T. Yada, et al. First compositional analysis of Ryugu samples by the MicrOmega hyperspectral microscope. Nature Astronomy, 2021, 6, pp.221-225. 10.1038/s41550-021-01549-z. insu-03744513

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# First compositional analysis of Ryugu samples by the MicrOmega hyperspectral microscope

C. Pilorget<fnref n="1" />✉, T. Okada²,³, V. Hamm<fnref n="1" />, R. Brunetto<fnref n="1" />, T. Yada², D. Loizeau<fnref n="1" />, L. Riu², T. Usui²,³, A. Moussi-Soffys⁴, K. Hataakeda²,<fnref n="5" />, A. Nakato<fnref n="1" />,², K. Yogata², M. Abe²,⁶, A. Aléon-Toppani<fnref n="1" />, J. Carter<fnref n="1" />, M. Chaigneau<fnref n="1" />, B. Crane<fnref n="1" />, B. Gondet<fnref n="1" />, K. Kumagai²,<fnref n="5" />, Y. Langevin<fnref n="1" />, C. Lantz<fnref n="1" />, T. Le Pivert-Jolivet<fnref n="1" />, G. Lequertier<fnref n="1" />, L. Lourit<fnref n="1" />, A. Miyazaki<fnref n="1" />,², M. Nishimura², F. Poulet<fnref n="1" />, M. Arakawa⁷, N. Hirata<fnref n="8" />, K. Kitazato⁹, S. Nakazawa², N. Namiki⁹, T. Saiki², S. Sugita³, S. Tachibana²,³, S. Tanaka²,⁶,¹⁰, M. Yoshikawa²,⁶, Y. Tsuda²,⁶, S. Watanabe<fnref n="11" /> and J.-P. Bibring<fnref n="1" />

The characterization of objects that have best preserved the mineralogical and molecular phases formed in the earliest stages of the Solar System evolution is key to understanding the processes that led to the formation of the planets in their diversity. The Hayabusa2 mission of the Japan Aerospace Exploration Agency has returned for the first time samples collected at the surface of a C-type asteroid, Ryugu<fnref n="1" />,². They are now preserved at the Extraterrestrial Samples Curation Center of the Japan Aerospace Exploration Agency at the Institute of Space and Astronautical Science in Sagamihara, Japan, where they are submitted to a first round of purely non-destructive analyses. The MicrOmega hyperspectral microscope developed at the Institut d'Astrophysique Spatiale (Orsay, France), which operates in the near-infrared range (0.99-3.65 µm), is performing their mineralogical and molecular characterization down to the scale of a few tens of micrometres. Strong features at 2.7 µm (indicating their OH-rich content) and at 3.4 µm (diagnostic of the presence of organics) dominate at a global scale, but key distinctive signatures have been identified at a submillimetre scale. In particular, carbonates (a fraction of them enriched in iron) as well as NH-rich compounds have been detected. The occurrence of volatile-rich species, likely originating from the outer Solar System, would support Ryugu having preserved both pristine material and altered phases, which are now available for refined laboratory analyses with the potential to draw new insights into the formation and evolution paths of planetary bodies in our Solar System.

On 6 December 2020, the Hayabusa2 mission successfully returned to Earth ~5.4 g of samples collected at the surface of the C-type asteroid Ryugu. Its surface was first sampled on 22 February 2019, then on 11 July 2019 close to a 15 m large artificial crater to possibly access subsurface material³. The collected samples have been labelled ‘bulk A’ and ‘bulk C’, respectively, and are now kept at the Institute of Space and Astronautical Science for preliminary analyses. A first step in their characterization is to perform, within the Curation Facility, non-destructive analyses using a visible-light microscope with five colour filters, a Fourier transform infrared

spectrometer operating in the 1–4 µm range, and MicrOmega, a hyperspectral microscope operating in the near-infrared (NIR) range (0.99–3.65 µm). It is worth noting that characterization by a NIR hyperspectral microscope has never been performed in the preliminary analyses of returned extraterrestrial samples. The primary goal is to achieve a global description of the entire collection, through the analyses of the bulk samples and of a few hundreds of grains extracted from them⁴. The latter stage will support the detailed analyses of a selection of grains by the already-nominated international teams for initial analyses, who started their activity in July 2021, and will contribute to building a catalogue for the distribution of samples to the international community, through Announcements of Opportunity, starting in mid-2022.

MicrOmega<fnref n="5" /> acquires images with 256×250 pixels, and has a useful field of view of ~5×5 mm² and a pixel scale of 22.5 µm. MicrOmega illuminates the samples with a monochromatic beam, scanning over the NIR spectral range, where diagnostic signatures of most candidate minerals and molecules of relevance can be found (for example, mafic minerals, altered phases, ices, aliphatic/aromatic CH and NH-rich compounds; see, for example, refs. ⁶⁻⁹ and references therein). For each pixel the reflectance spectrum is retrieved in up to 400 contiguous spectral channels. Both the negligible amount of illuminating power at less than 10⁻<fnref n="8" /> W per pixel and the lack of contact with the samples allow entirely non-destructive and non-invasive characterization.

Given the much larger than expected amount of material collected, each collection (A and C) was distributed over three sample holders, each 23 mm in diameter (corresponding to six sub-bulks A1 to A3 and C1 to C3). The sample holders, which were previously ultraclean-processed, are made of a sapphire substrate on top of a polished gold mirror, which produces an extremely dark background under the specific MicrOmega illumination conditions (Methods). The data were processed according to the nominal calibration scheme¹⁰.

When analysed at the millimetre scale (Fig. 1) by averaging thousands of pixels, the spectra of both bulk A and bulk C exhibit a pattern similar to those acquired remotely down to the metre scale

<footnote n="1">¹Institut d'Astrophysique Spatiale, Université Paris-Saclay, CNRS, Orsay, France. ²Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), Sagamihara, Japan. ³University of Tokyo, Tokyo, Japan. ⁴Centre National d'Etudes Spatiales (CNES), Toulouse, France.</footnote>

<footnote n="5">⁵Marine Works Japan, Ltd, Yokosuka, Japan. ⁶The Graduate University for Advanced Studies (SOKENDAI), Hayama, Japan. ⁷Kobe University, Kobe, Japan.</footnote>

<footnote n="8">⁸The University of Aizu, Aizu-Wakamatsu, Japan. ⁹National Astronomical Observatory of Japan, Mitaka, Japan. ¹⁰University of Tokyo, Kashiwa, Japan.</footnote>

<footnote n="11">¹¹Nagoya University, Nagoya, Japan. ⁵e-mail: cedric.pilorget@universite-paris-saclay.fr</footnote>
<figure><img src="image_3.png" /><figcaption>**Fig. 1** | Bulk A and C reflectance spectra. Typical MicrOmega Spectra within bulk A (a) and bulk C (b), compared to NIRS3 normalized spectra at the surface ('surface standard') (c) and close to the Small Carry-on Impactor (SCI) crater (d). NIRS3 spectra are from ref. ¹⁵ and are normalized at 2% at the wavelength of 2.0 µm for clarity. Details of the observations used to derive the 'surface standard' and 'SCI crater region' spectra are described in Extended Data Tables 1 and 2 of ref. ¹⁵. A dashed line set at 2.715 µm highlights the position of the absorption maximum in the MicrOmega data.</figcaption></figure>

by the Near-Infrared Spectrometer (NIRS3) instrument on board Hayabusa2 (ref. ¹¹). The global reflectance, 2–3 % over the entire NIR spectral range covered, is extremely low, in agreement with the remote-sensing measurements of the Optical Navigation Cameras (ONCs) and NIRS3 at Ryugu¹¹,¹² and fully consistent with the taxonomic spectral classification of Ryugu as a C-type asteroid¹³,¹⁴. The main spectral feature is the diagnostic OH absorption centred at 2.715 ± 0.005 µm, a position compatible with that of NIRS3 spectra (Fig. 1)¹¹,¹⁵. No significant differences can be observed between bulk A and bulk C for this specific feature. Noticeably, this feature displays a large absorption depth typically varying from 12 to 18%, in contrast to the 7 to 10% band depth from published remote NIRS3 observations. Apart from potential instrumental effects, these discrepancies could originate from geometrical and shadow effects, and/or the sampling process, which may have produced microstructures (for example, via fragmentation)² and/or collected subsurface material in the two occurrences. This would be consistent with the NIRS3 observations that the surface 2.7 µm feature appears deeper close to the impact crater¹⁵. Importantly, each of the six sub-bulk samples present similar average spectra, with variations of mean reflectance within 2–3%. Compared with the regional spectral mapping obtained remotely by NIRS3, we consider that the bulk samples are truly representative of Ryugu's surface, with no bias that could have originated from the specific location of each sampling area.

The degree of compositional homogeneity seems to prevail identically from the global macroscopic scale down to the few millimetre scale. At this spatial scale, MicrOmega spectra also exhibit a broad feature in the 3.3–3.5 µm range that is centred around 3.4 µm (thus referred to as the ‘3.4 µm band’) and present throughout the sub-bulk samples, although with varying band depths of typically a few percent. This global feature, distinct from the dual (3.32–3.45 µm) carbonate ones detected at a local scale (see below), is considered indicative of the large-scale presence of a variety of CH-rich compounds. A fainter ~3.06 µm broad feature is also detected (referred to as the ‘3.1 µm’ absorption), although with varying and much fainter intensities.

Only at a submillimetre scale do heterogeneities show up clearly, either at grain level or as inclusions within grains or both, in some cases comparable to what was observed by the MASCOT camera

<figure><img src="image_4.png" /><figcaption>**Fig. 2** | 3.4 µm band variations at the scale of a few tens of microns.
CH-rich spectrum at a localized spot within bulk A (a) compared to a typical spectrum of a carbonate-rich spot (b), emphasizing the diagnostic difference of the ‘3.4 µm’ feature. Both spectra are 3 × 3 pixels average spectra. Dashed lines set at 3.32 µm, 3.41 µm and 3.455 µm highlight the differences in band position and shape.</figcaption></figure>

(MASCam) at the surface of Ryugu¹⁶. In terms of structures, most grains are similarly dark and exhibit angular morphologies, with no peculiar shapes yet identified at scales >50 µm that are similar to those of meteoritic inclusions such as chondrules. Bright spots, either as isolated grains or larger structures, are clearly distinct photometrically under a given illumination geometry. However, most fade to average dark reflectance when illuminated from different azimuth angles (a few to tens of degrees), and then do not exhibit distinctive spectral signatures.

Submillimetre analyses enable us to extract, from the globally spread 2.7 µm and 3.4 µm absorptions within averaged spectra, the distinct shapes and accurate band centre position relevant to specific grains. Most detections of the 3.4 µm feature point to aliphatic compounds (see, for example, refs. ¹⁷,¹⁸). When resolved at grain level (Fig. 2), a prominent 3.41 µm feature, attributed to CH₂ stretching, dominates over the 3.38 µm CH₃-driven one, suggesting a high CH₃/CH₄ ratio in rather large chains (for example, ref. ¹⁸). The faint 3.1 µm band at a global scale can show up with a much larger intensity, as in the grain shown in Fig. 3 (spectrum b) present within the C2 sub-bulk sample. This grain, a few hundreds of micrometres in size, displays a total reflectance much higher (reflectance factor of ~10%) than that of typical bulk grains. For this specific grain, the spectra acquired in a number of spots exhibit a deep (~45%) 2.7 µm OH band similar in position to the one observed within the bulks, and a strong ~3.06 µm band with a coupled absorption at 3.24 µm. The leading explanation of this 3.1 µm feature involves a nitrogen-rich phase⁹,¹⁹, perhaps hydroxylated. Candidates include NH₄ phyllosilicates, NH₄ hydrated salts and/or nitrogen-rich organics. The spectrum in the 1.9–2.55 µm range has weak structures at ~2.32 µm, ~2.13 µm and ~2.5 µm. The former could be due to Mg–OH-bearing minerals. The occurrence of the 3.1 µm feature at a large scale within the collection, although with varying intensities, would indicate that NH-rich compounds are likely present at small scale in essentially the entire collection.
<figure><img src="image_5.png" /><figcaption>**Fig. 3** | Examples of compositional heterogeneities at sub-mm scale. Average spectrum of a large altered 'OH-rich' grain (a) and of a 'NH-rich' grain (b) within bulk A and bulk C, respectively (right), together with RGB images showing these grains within the analysed bulk samples (left). Top left: R, 2.5 µm; G, 3.2 µm; B, 3.6 µm. Bottom left: R, 2.5 µm; G, 3.05 µm; B, 3.4 µm. Spectra are, respectively, 9 × 9 pixels (a) and 6 × 5 pixels (b) averaged spectra. Dashed lines set at 3.06 µm and 3.24 µm highlight NH-related features.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 4** | Example of carbonate detection. Average spectrum (100 pixels) of a large carbonate grain in sub-bulk A1 (a) with three library reference spectra (siderite (b), dolomite (c) and calcite (d)), together with an RGB image (left) showing the large carbonate grain (R, 2.5 µm; G, 2.7 µm; B, 3.4 µm). Reference spectra are from the RELAB library (siderite, bkr1cb071a; dolomite, bkr1cb017a; calcite, bkr1jb549). Their reflectance factor is divided by 6 for easier comparison with the MicrOmega spectrum (a). Dashed lines set at 2.3 µm and 2.51 µm (two features attributed to carbonates) highlight the similarities and differences in band position and shape.</figcaption></figure>

In addition to these ever-present 2.7 µm, 3.4 µm and 3.1 µm features, MicrOmega detected several types of specific spectral signatures, some in only a very few grains. These have a high potential to help constrain the origin and evolution of Ryugu. Two signatures are highlighted here.

One signature is clearly identified as diagnostic of carbonates and was detected on a large number of occurrences, either as individual grains or inclusions (Fig. 4). We evaluate that <1% of the

MicrOmega pixels are dominated by a carbonate signature, the size of the grains ranging from a few tens to a few hundreds of micrometers. However, up to 5% of pixels show spectra compatible with a minor carbonate component. Thus, carbonates could be present in a larger number of grains, at varying concentration and/or size of inclusions. The largest carbonate-rich grain within the collection, some 400 µm in size, was detected in the sub-bulk A1. Its carbonate signature is highly spectrally dominant throughout the entire
grain. Noticeably, in addition to the characteristic CO₃²⁻-driven 2.3 µm, 2.5 µm and large 3.3–3.5 µm carbonate absorptions, either double-peaked (Fig. 2) or as a plateau (Fig. 4), a deep reddish slope at wavelengths <1.6 µm uniquely points to an absorption by Fe²⁺ as a key component²⁰, potentially coupled to other cations such as Mg or Ca. This latter feature appears in most large carbonate grains that are typically 100 µm in size or larger. However, for some individual grains that are mostly smaller in size, this slope is faint to absent, suggesting carbonates depleted in Fe²⁺. For all the carbonate grains identified, no clear shift of the position of the double peak at ~3.32–3.45 µm could be observed, which may indicate that they have overall very similar cationic chemistry. The similarity of the spectra over the diversity of large carbonate grains within the bulks indicates that instead of originating from a mixture of distinct carbonates, they may have originated from the alteration of similar precursors, as a critical ingredient of Ryugu's parent body. Finally, Ca-rich phases similar to what is identified in meteorites would likely reside as smaller phases, typically 50 µm in size or less.

A second type of spectra is shown in Fig. 3 (spectrum 'a'). Detected either as individual grains (several occurrences within bulk A and C samples, from a few tens to a few hundreds of micrometres in size) or small inclusions (several occurrences a few tens of micrometres in size at most), it exhibits a broad 3 µm OH absorption with a sharp shoulder and OH absorption at ~2.7 µm. It has a distinctive spectral shape overall and weak additional absorption bands in the 2.1–2.2 µm region. The precise position of the 2.7 µm OH feature is slightly shifted upwards by ~10 nm compared to the bulk value. The paucity of more diagnostic absorption features hinders its precise identification, although the reported minor bands and the OH stretch band position collectively point towards an Al–OH-rich compound, such as the Al-hydroxide diaspore. The lack of a clear hydration signature at 1.9 µm argues against H₂O-rich compounds.

The initial spectral characterization of the returned samples by MicrOmega currently points towards Ryugu containing a fascinating variety of grains, including OH-, CH- and NH-rich compounds spread at a global scale, and alteration products, among which highly diagnostic carbonates (for example, ref. ²¹), as also detected on Bennu²². The occurrence of volatile-rich species, likely originating from the outer Solar System, would support Ryugu having preserved some of its building blocks together with their partially altered phases. Within an extremely dark matrix possibly dominated by phyllosilicates, CH-rich inclusions are found widely spread although predominantly at micrometre (subpixel) sizes. A similar pattern has been identified for NH-rich species. A variety of alteration levels is observed through distinct cations trapped within carbonates, closer to what is observed with CI meteorites rather than with CM meteorites²³,²⁴. Noticeably, no chondrules nor refractory inclusions have been identified yet. The Hayabusa2 returned samples, thus, appear to be among the most primordial materials available in our laboratories. The samples constitute a uniquely precious collection, which may contribute to revisiting the paradigms of Solar System origin and evolution.

## Methods

Within the Curation Facility, MicrOmega is mounted in an ultraclean-N₂ purged chamber to prevent H₂O/CO₂ contamination. The instrument is cooled by being mounted on a copper plate maintained at 10 °C, to decrease thermal emission within the instrument and to increase the signal-to-noise ratio. For imaging by MicrOmega, the samples are introduced within a 'sample chamber' part of the main Curation Facility suite¹; this chamber is separated from the MicrOmega chamber by an NIR-transparent sapphire (Al₂O₃) window. The sample chamber, which was extensively ultracleaned (as was the entire facility suite), is continually flowed with N₂. Each sample is deposited on a dedicated sapphire sample holder (23 mm in diameter) set on a polished gold-coated mirror. The sample holder is set on an (x, y, z, θ) remotely controlled moving stage, enabling us to put within the MicrOmega field of view the entire surface of the sample and to illuminate the grains with different azimuth angles.

The MicrOmega instrument sequentially illuminates the sample at different wavelengths with an incidence angle of ~35°. For each illuminated

wavelength, an image is acquired in a 'nadir' configuration on a two-dimensional mercury-cadmium-telluride sensor cooled down to 110 K with a dedicated cryocooler. A hyperspectral (x, y, z) cube is built by scanning the different monochromatic channels in the 0.99–3.65 µm wavelength range. Thanks to the use of an acousto-optic tunable filter (AOTF) as dispersive system, with a full width at half maximum of ~20 cm⁻¹, the wavelengths can be selected and repeated in any order with minimum sampling steps of ~2 cm⁻¹. For the baselined operations, the typical number of spectral channels per cube varies between ~200 and ~400. Importantly, as the AOTF is electronically controlled, it offers the capability to measure and subtract 'on the fly' the background signal (originating from the thermal emission of the sample, the thermal emission of the instrument, the stray light and the dark current) by successively switching the AOTF on and off. This procedure, baselined for routine operations, makes the instrument insensitive to long-term temperature variations during the observation, and very significantly enhances the retrieval of the detected scientific signal. As the dispersive system is part of the illumination path, the thermal background (from both the sample and the instrument) measured by the instrument is integrated over the entire spectral range (and thus does not vary with the wavelength of the illuminating beam).

The wavelength calibration of the illumination beam has been characterized at the Institut d'Astrophysique Spatiale using two reference targets: a Wavelength Calibration Standard from Labsphere and a calcite pellet. To eventually redo calibration within the Curation Facility, reference targets made of Spectralon 99% and Infragold from Labsphere were set in the Curation chamber after an in-depth ultracleaning procedure¹. The potential contribution of the sapphire window to the signal has been removed using acquisitions performed without any sample in the field of view. The radiometric calibration is evaluated to be better than 20% in absolute value and ~1% in relative value (from one spectral channel to the next). The spectral accuracy is evaluated to be better than 5 nm (L.R. et al., manuscript in preparation).

## Data availability

All the data related to this manuscript will be put on the Data Archives and Transmission System (DARTS) after a one-year proprietary period.

Received: 1 June 2021; Accepted: 26 October 2021;
Published online: 20 December 2021

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## Acknowledgements

We are very grateful to have been invited by the Hayabusa2 Project to contribute to their outstanding mission. We thank the French space agency CNES for its full support. The Japan Society for the Promotion of Science (JSPS) provided support through KAKENHI grant no. 17H06459 (T.U. and T.O.) and grant no. 19K03958 (M.A.). We also acknowledge the JSPS Core-to-Core Program 'International Network for Planetary Science'.

## Author contributions

V.H., M.C., B.C., G.L., L.L., J.-P.B. and C.P. contributed to the instrument development. T.Y., T.O., L.L., V.H., L.R., A.N., K.Y. G.L., J.-P.B., C.P., D.L. and R.B. contributed to the installation of the instrument in the Curation Facility and to the protocol development. L.R., V.H., C.L., J.-P.B. and C.P. contributed to the calibration. J.-P.B., C.P., V.H., L.R., T.Y., D.L., R.B., K.H., A.N., K.Y., C.L., T.L.P.-J., G.L., L.L., K.Kumagai., A.M. and M.N. participated in the operations. B.G., D.L., J.-P.B., L.R., R.B., C.P., K.H., A.N., K.Y., J.C., Y.L., T.L.P.-J., C.L., F.P. and A.A.-T. contributed to the data analysis and interpretation. J.-P.B., C.P. and R.B. wrote the draft of the manuscript. All authors were involved in the discussions of the results and the finalization of the manuscript.

## Competing interests

The authors declare no competing interests.

## Additional information

Correspondence and requests for materials should be addressed to C. Pilorget.

Peer review information *Nature Astronomy* thanks Beth Clark, Hannah Kaplan and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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