# Constraints on Solar System early evolution by MicrOmega analysis of Ryugu carbonates

D. Loizeau, C. Pilorget, L. Riu, R. Brunetto, J.-P. Bibring, A. Nakato, A.

Aléon-Toppani, K. Hatakeda, K. Yogata, J. Carter, et al.

► To cite this version:

D. Loizeau, C. Pilorget, L. Riu, R. Brunetto, J.-P. Bibring, et al. Constraints on Solar System early evolution by MicrOmega analysis of Ryugu carbonates. Nature Astronomy, 2023, 7 (4), pp.391-397.
10.1038/s41550-022-01870-1 . hal-04271800

HAL Id: hal-04271800

https://hal.science/hal-04271800v1

Submitted on 4 Oct 2025

**HAL** is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Constraints on Solar System early evolution by MicrOmega analysis of Ryugu carbonates

Received: 30 May 2022

Accepted: 25 November 2022

Published online: 12 January 2023

<figure><img src="image_2.png" /></figure>

Check for updates

D. Loizeau Ⓕ<fnref n="1" />✉, C. Pilonget Ⓕ<fnref n="1" />,², L. Riu<fnref n="1" />,³, R. Brunetto<fnref n="1" />, J.-P. Bibring<fnref n="1" />, A. Nakato<fnref n="4" />,
A. Aléon-Toppani<fnref n="1" />, K. Hatakeda<fnref n="4" />,⁵, K. Yogata Ⓕ<fnref n="4" />, J. Carter<fnref n="1" />, T. Le Pivert-Jolivet<fnref n="1" />,
T. Yada Ⓕ<fnref n="4" />, T. Okada Ⓕ<fnref n="4" />,<fnref n="6" />, T. Usui Ⓕ<fnref n="4" />,<fnref n="6" />, Y. Langevin<fnref n="1" />, C. Lantz<fnref n="1" />, D. Baklouti Ⓕ<fnref n="1" />,
A. Miyazaki Ⓕ<fnref n="4" />, M. Nishimura<fnref n="4" />, K. Nagashima<fnref n="4" />, K. Kumagai<fnref n="4" />,⁵, Y. Hitomi<fnref n="4" />,⁵,
M. Abe<fnref n="4" />,⁷, T. Saiki<fnref n="4" />, S. Tanaka<fnref n="4" />,⁷,⁸, S. Nakazawa Ⓕ<fnref n="4" />, Y. Tsuda<fnref n="4" />,⁷ & S. Watanabe Ⓕ⁹

The samples returned from a C-type asteroid (Ryugu) by the Hayabusa2 mission constitute unprecedented access to carbonaceous material never exposed to Earth's atmosphere that may still contain phases formed in the earliest stages of the Solar System. We present an extensive analysis of a large set of grains and bulks of the Ryugu samples, performed directly within the Japan Aerospace Exploration Agency Curation Center in Japan with the near-infrared hyperspectral microscope MicrOmega, to identify and characterize the carbonate component of the samples, which has recorded early evolutionary steps. We reveal a large presence of carbonates within the collection distributed over two main size-dependent populations: generally small (<100 µm) dolomite-rich and larger (up to hundreds of µm) breunnerite-rich areas, some with complex elongated morphologies. Similarities with C-chondrites suggest that such characteristics may emerge as a general property of primitive materials in the outer part of the asteroid belt. These two carbonate populations likely translate distinct processes and stages of formation in the early Solar System that might have taken place while CO₂ ice was still present (possibly before accretion of the Ryugu parent body) and/or from C-rich phases.

The Hayabusa2 mission returned the very first samples from a C-type asteroid, collected at two sites at the surface of Ryugu, the first one on 22 February 2019 (chamber A sample) and the second one on 11 July 2019 close to a 15 m artificial crater to possibly access subsurface material (chamber C sample). They were brought back by the Hayabusa2 space-craft in December 2020 and delivered to the Japan Aerospace Explora-tion Agency Extraterrestrial Curation Center<fnref n="1" />, preserved from contact with the terrestrial atmosphere. Bulk samples and then individual grains were picked up and stored in sapphire dishes, weighed and analysed with an optical microscope, a Fourier-transform Infrared spectrometer (1–5 µm) and MicrOmega, a 250 × 256 pixel (pixel size 22.5 × 22.5 µm²)

near-infrared hyperspectral microscope (0.99–3.65 µm)², for initial description in a fully non-destructive and non-invasive manner. These instruments have unique access, in a fully controlled, ultraclean and N₂-purged environment, potentially to the entire collection of Ryugu samples, which have never been exposed to the terrestrial environment.

MicrOmega offers unique access to mineralogical mapping over a large portion of Ryugu material. Preliminary analyses performed in the Curation Center have shown that Ryugu samples are made of an extremely dark and OH-bearing matrix in which organics are embed-ded<fnref n="1" />,³. The samples are spectrally quite homogeneous at the scale of a few millimetres: they have very low reflectance (2–3%), an absorption

<footnote n="1">¹Institut d'Astrophysique Spatiale, Université Paris-Saclay, CNRS, Orsay, France. ²Institut Universitaire de France, Paris, France. ³ESAC, ESA, Madrid, Spain.</footnote>

<footnote n="4">⁴Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Japan. ⁵Marine Works Japan, Ltd., Yokosuka, Japan.</footnote>

<footnote n="6">⁶University of Tokyo, Bunkyo, Tokyo, Japan. ⁷The Graduate University for Advanced Studies (SOKENDAI), Hayama, Japan. ⁸University of Tokyo, Kashiwa, Japan. ⁹Nagoya University, Nagoya, Japan. ✉e-mail: damien.loizeau@ias.u-psud.fr;</footnote>
band centred around 2.72 µm attributed to OH stretching within the matrix material, and a weak absorption band centred around 3.4 µm attributed to either or both carbonates and aliphatic CH stretching in a variety of CH-rich compounds³. However, heterogeneities show up at submillimetre scale: in particular, carbonates were identified on a number of grains and inclusions, with up to 5–7% of the pixels of bulk samples (A and C) spectrally compatible with the presence of a carbonate component³.

Carbonates have been reported in few Ryugu grains⁴⁻⁷ and are also present in carbonaceous chondrites (for example, refs. 8–11) as a minor mineral species (around 0.5% (ref. 12) in CI chondrites). Calcite and dolomite, and sometimes aragonite and breunnerite, are reported in CM chondrites (for example, ref. 10), while dolomite and minor breunerite and calcite are observed in CI chondrites (for example, ref. 13). Carbonates are often considered to be aqueous alteration products, thought to have formed shortly after the accretion of the parent bodies from an aqueous solution circulating in the precursor material (for example, refs. 10, 13, 14). In such a scenario, the spatial distribution, size, morphology and composition of the large survey of carbonates within the returned samples would record the spatial and temporal evolution of Ryugu's aqueous alteration.

In this paper, we present the MicrOmega analyses of Ryugu carbonate-rich grains and inclusions performed up to the end of 2021 on samples from chambers A and C of the Hayabusa2 returned capsule, amounting to ~180 individual grains (a few mm in diameter) and 14 ‘sub-bulks’ extracted from the bulk samples never exposed to the terrestrial environment (Methods). This extensive characterization of carbonate occurrences in such conditions and over a large, representative part of the Ryugu returned samples legitimates a comparison with CM and CI chondrites, and offers to constrain the processes that shaped the formation and early evolution of Ryugu material.

## Results

At submillimetre scale, numerous localized surfaces exhibit diagnostic spectral features of carbonates: a strong double absorption band in the 3.3–3.5 µm range due to the overtones of the fundamental $v_3$ asymmetric CO$_3$ stretching bands, sometimes coupled to other weaker bands around 2.5 and 2.3 µm due to combination modes ($v_1 + 2v_3$ and $3v_3$, respectively, with $v_1$ being symmetric stretching¹⁵,¹⁶). Some detections also exhibit a sharper band around 2.8 µm ($2v_3 + v_4$, with $v_4$ being in-plane bending), and a wider band centred around 3.1 µm ($v_3 + v_1 + v_4$ and $2v_2 + 2v_4$, with $v_2$ being out-of-plane bending)¹⁶. These spectra also show with MicrOmega a much higher reflectance (greater than ~10%) than the average sample; they also appear brighter in visible microscope images, where they account for the ‘white regions’, as described in ref. 4. These coupled spectral features are generally detected in the spectrum of the largest (typically >100 µm) carbonate-rich grains and inclusions within the Ryugu samples, while for smaller ones, the identification is based solely on the stronger, diagnostic 3.3–3.5 µm bands. The spectral band position and shape can vary from one detection to another, with band position related to the cation content of the carbonate mineral (for example, refs. 16–18.), although the presence of other compounds—for example, organics—could also induce some spectral effects. An additional strong spectral absorption below 1.5 µm (‘1 µm absorption’) is associated with some carbonate-rich detections, and attributed to the presence of iron-bearing carbonates ($Fe^{2+}$ electronic transition). Noticeably, carbonate-rich spectra in the Ryugu samples also show the 2.72 µm band.

More than 200 carbonate-rich areas (regions of interest (ROIs)) larger than ~45 µm ($2 \times 2$ MicrOmega pixels) have been identified with high confidence among the grains and bulks/sub-bulks observed with MicrOmega. In this study, this lower limit of 4 pixels with carbonate signature was set to reach a high degree of confidence in the detection, which does not preclude the presence of carbonates at a smaller scale. Carbonates were detected either as inclusions present on part of the

surface of larger grains (Fig. 1), or as loose grains, spectrally dominated by carbonate phases. The carbonate-rich inclusions (>45 µm in width) in extracted grains represent 0.4–0.5% of the total surface of the studied grains, not including areas where carbonate may be present but not spectrally dominant. A large majority of ROIs (>85%) are <100 µm wide (Fig. 2). Among these small ROIs, a majority (~63%) correspond to carbonate-rich loose grains (Extended Data Fig. 1), likely produced by the separation of carbonate inclusions from larger grains during the entire process of collection, transport and handling. This would possibly induce an increase in the size distribution towards the smaller fraction. However, in the ~50–500 µm size range, the overall cumulative surface distribution of carbonates has a slope between –1.5 and –2.0, both in bulks and in the inclusions within large grains (Extended Data Fig. 2); it would indicate no significant fragmentation of the loose carbonate-rich grains compared to the inclusions still in the larger grains. This cumulative surface distribution slope is much flatter than the slope of the collected grain size distribution (–3.88 ± 0.25 (ref. 1)), which suffered fragmentation, and also flatter than the average slope (–2.65 ± 0.05 (ref. 19)) obtained for boulders (5–140 m in size) on Ryugu. The carbonate slope index is, however, closer to the power index of boulders smaller than 4 m on Ryugu (–1.65 ± 0.05 to –2.01 ± 0.06) and close to the index of about –2 observed for gravels (>0.02 m to several metres in size) at the local touchdown sites¹, which could also indicate that the carbonate-rich grains studied with MicrOmega did not suffer fragmentation from the sample collection.

The larger carbonate ROIs, a few hundred µm large, show various morphologies. For example, grain C0181 (Fig. 1) reveals complex carbonate-rich structures with apparently brighter crystals (outlined in red) around a large, darker structure (outlined in orange), ~100 µm × ~350 µm. Microscope visible images show, respectively, groups of small, bright crystals and one large, darker crystal. For grain C0041, carbonates are detected on complex, elongated structures (pink arrows) around a main area. This kind of complex structure of carbonate-rich inclusions has not been reported in any other Ryugu grains. On the opposite side of the same grain, carbonates are almost absent, indicating that in this case the carbonates are likely only present as thin layers: the thickness of the carbonate-rich inclusions is in certain cases much smaller than their surface area. Smaller carbonate-rich inclusions (<100 µm) generally appear darker, like in grains A0033 and C0023 (Fig. 1).

Within each ROI, carbonate bands show only limited spectral variations from pixel to pixel, indicating a relative spatial homogeneity of the carbonate chemical composition at the ROI scale. Some variations could, however, be observed within some ROIs, in particular with respect to the 1 µm absorption, and to the relative spectral depth of the different bands. For example, the spectrum of the large crystal in grain C0181 (outlined in orange, Fig. 1) shows a band at 1 µm relatively deeper than for the brighter crystals around it. While a given carbonate composition dominates each carbonate ROI, small spectral variations at the scale of a few tens of microns suggest possible small composition variation, possibly tracing their formation process.

On the other hand, from one carbonate ROI to another, when comparing each ROI's average spectra, spectral differences are clearly observed. In particular, the position of the 3.3–3.5 µm band and the relative depth of its two peaks vary between distinct carbonate ROIs, and absorption below 1.5 µm is observed only in some ROIs. In particular, as seen in Fig. 3, the 3.3–3.5 µm band shows minima at 3.30–3.43 µm in some ROIs (case A, spectra from sub-bulk C0206 and grain C0041 as examples), at 3.31–3.45 µm in others (case B, spectra from sub-bulk C0108 and grain C0046 as examples) and in only a few ROIs at 3.35–3.48 µm (case C, spectrum from sub-bulk A0216 as an example). When the bands at 2.3 and 2.5 µm are present, we also see a spectral shift between case A and case B, with bands at 2.30 and 2.50 µm as compared to 2.32 and 2.51 µm, respectively (Fig. 3). Additionally, spectra corresponding to case A also have absorption below 1.5 µm, detected
<figure><img src="image_3.png" /><figcaption>**Fig. 1** | Examples of carbonate ROIs. a–d, MicrOmega infrared images of four grains at 2.5 µm. e–h, RGB composites (R: 3.34 µm, G: 2.5 µm, B: 1.2 µm), highlighting carbonates in blue/green. Pink arrows indicate elongated carbonate inclusions, suggesting formation as veins. i, Spectra extracted from the ROIs indicated in colour and with white arrows on the MicrOmega images.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>**Fig. 2** | Carbonate size distribution. a–c, Histograms of the distribution of the carbonate ROIs in the analysed samples, by carbonate species, and between chambers A and C, in initial bulks (a), extracted sub-bulks (b) and grains (c) (Methods). Diameters are given for equivalent circular ROIs, but shapes can be very irregular.</figcaption></figure>

by a strong positive slope between 1.2 and 1.5 µm (Fig. 3). To identify the composition of the different carbonate ROIs, we systematically compared MicrOmega ROI spectra to carbonate reference spectra (aragonite, breunnerite, calcite, dolomite, magnesite and siderite) and used a chi-square goodness-of-fit test, as well as visual inspection of the spectral features and their position (Methods and Extended Data Fig. 3). When comparing these endmembers' spectra (cases A, B and C) from the MicrOmega observations with carbonate reference

spectra, case A fits very well with (Mg, Fe)-carbonates like breunnerite, case B is similar to (Ca, Mg)-carbonates like dolomite, while case C corresponds to CaCO₃ calcite spectra. Small variations in the position and shape of the bands of the ROI spectra could, however, be observed for some ROIs, suggesting (1) possible slight variations in the cations' composition around these endmembers, resulting in different carbonate species in a single ROI, and/or (2) carbonates intimately mixed with other species presenting signatures in the 3.2-3.6 µm area, primarily
<figure><img src="image_5.png" /><figcaption>**Fig. 3** | Ryugu spectral carbonate endmembers. **a–c.** Spectral carbonate endmembers identified in the Ryugu samples with MicrOmega (b and solid lines in c) using examples from different grains and sub-bulks, and comparison with reference spectra (a and mixed lines in c). Spectra in c are continuum removed. Spectra extracted from sub-bulk C0206 and grain C0041 fit with the breunnerite</figcaption></figure>

reference spectrum; band positions at 3.3–3.5 µm are slightly shifted to higher wavelengths for spectra from sub-bulk C0108 and grain C0046, better fitting with the dolomite reference spectrum; a bigger shift for the spectrum from sub-bulk A0216 fits with the calcite reference spectrum.

organics. Neither pure magnesite-like nor pure siderite-like spectra provide a good fit so far.

These carbonate species have also been detected with complementary techniques in a few Ryugu grains extracted from the collection: (Mg, Fe)-carbonates in C0041 (ref. 4), C0009 and a few other grains⁵,⁶; dolomite in C0041 (ref. 4), A0026 (ref. 7), A0037 and a few other grains⁵,⁶; and Ca-carbonate in C0009 (ref. 5).

Dolomite-like spectra are the most numerous with ~80% of the carbonate ROIs, while breunnerite-like spectra represent ~20% of the ROIs (Fig. 2). However, the breunnerite-like ROIs are generally larger in surface area than the dolomite-like ROIs, resulting in a larger total surface area of breunnerite-like than dolomite-like carbonates (two-thirds similar to breunnerite and one-third similar to dolomite, but with a size lower limit of detection of 45 µm). The detection of calcite, associated only with two small ROIs (~45 µm wide), may suggest a wider variety of carbonates for inclusions and grains smaller than the size limit of this study.

No carbonate ROI was detected with both (Mg, Ca)- and (Mg, Fe)-carbonates in contact with each other. Both kinds of carbonates could, however, be detected on grain C0041 (Fig. 1), with the large (Mg, Fe)-carbonate complex inclusion on one face of the grain and a smaller dolomite inclusion on the opposite face.

## Discussion

Carbonate grains and inclusions detected with MicrOmega in the Ryugu samples appear much brighter than the bulk matrix material (~3% or less), reaching 15–20% for the brightest ROIs, although below 10% for most of them. This is, however, much lower than for pure terrestrial carbonate powders (a few tens to a few hundreds of microns in size, of the same order as carbonate grains in Ryugu samples), which show albedo reaching over 50%; dolomite, for example, reaches 90% (Fig. 3), although one should note that differences in grain size and instrumental set-up induce differences in measured reflectance for these reference

samples. In addition, MicrOmega spectra of carbonates throughout the collection show an absorption band at ~2.72 µm (Figs. 1 and 3)—at the same position as the one present throughout the matrix—that is not present in pure carbonate spectra. Possible explanations include: (1) presence of dark matrix material viewed in transparency through the carbonates, (2) multiscattering of carbonate crystals and the surrounding matrix and/or (3) intimate mixture between carbonate crystals and some matrix compounds (possibly including dark CH-rich phases) at subpixel scale. In case (1), we would expect the relative depth of the 2.72 µm band to decrease when the 3.3–3.5 µm band increases—that is, when the carbonate signature is stronger—but such behaviour is not observed. In case (2), large carbonate ROIs would tend to show a weaker 2.72 µm band, which is not observed either. Hence, MicrOmega observations would favour case (3), which would also explain the lower reflectance of Ryugu carbonates compared to pure species. This intimate ‘optical’ mixing might record, at least partially, the formation process of the relevant phases.

The presence of other compounds intimately mixed with the carbonates at scales smaller than the pixel (~22 µm) would be consistent with observation from ref. 5 of magnetite and phyllosilicate grains within a dolomite grain, as well as that of ref. 20, in which it was observed that phyllosilicates, carbonates and organic matter are intimately mixed at a scale <50 µm in small fragments of grain A0064. Mixture with phyllosilicates in addition to magnetite and/or organics below the MicrOmega pixel level would account for the low reflectance and presence of the 2.72 µm band. These observations are in agreement with either a possible partial dissolution of the carbonates when the phyllosilicates formed¹⁰ or a formation of at least part of the magnetite and phyllosilicates before the carbonates’ formation⁵.

The flat surface size distribution indicates that large carbonates are an important component of Ryugu materials, a property likely related to the carbonate formation mechanism itself, and not to subsequent fragmentation, during Ryugu’s parent body(ies) disruption
or during sample collection. Within the >5 g of returned material from Ryugu, the largest carbonate-rich grains or inclusions reach ~500 µm in size, but we may imagine larger carbonate inclusions in Ryugu. At the surface of Ryugu, the lander MASCOT has also observed bright inclu- sions, some of which may correspond to carbonate-rich inclusions. Their sizes were measured from a distance of <25.5 cm over a surface of ~185 cm² (ref. 21): they are mainly <1 mm and up to 3 mm.

The presence of both carbonate compositions throughout the entire collection, in samples from both chambers A and C (Fig. 2), sug- gests a spatial homogeneity of the material on Ryugu, consistent with accretion model observations showing that Ryugu has only sampled a localized area of its parent body²². The largest number of detections from chamber C compared to chamber A could reflect either a possible bias due to higher grain fragmentation induced by the impact on the second site of collection, revealing a higher number of inclusions, or macroscopic (greater than a few cm) variations in the carbonate spatial distribution on the asteroid. In any case, our observations would imply that the different carbonates detected in the Hayabusa2 samples are sourced from the same area of formation, and suggest a relative spatial homogeneity of the local environments (for example, available cations or temperature) in which carbonates formed.

The MicrOmega analyses of an extensive set of Ryugu mate-
rial show the presence of a variety of carbonate-rich inclusions and
grains, with sizes up to a few hundred microns. Among the more than
200 carbonate detections in this study, their composition is domi-
nated by (Mg, Ca)- and (Mg, Fe)-carbonates. (Mg, Ca)-carbonates
are more ubiquitous and tend to dominate the composition of less than
-100 µm carbonate-rich areas, while only (Mg, Fe)-carbonates can
be observed for the larger ones. These two carbonate compositions
and sizes are similar to what has been observed in CI chondrite mete-
orites (for example, refs. 23, 24), although we show that in the Ryugu
samples (Mg, Fe)-carbonates appear for MicrOmega as a major car-
bonate component. We also reveal the spatial heterogeneity of the
(Mg, Fe)-carbonates within the Ryugu sample collection, which may
explain their absence in many individual grains.

From these observational results, we discuss two scenarios:
(1) both carbonates were formed during a single 'alteration' event, with
fluid and cation availability progressing with time and/or varying with
space; or (2) both carbonates formed following two distinct processes,
possibly separated in time.

By analogy with dolomite in CI chondrite meteorites, (Mg, Ca)-carbonates would have formed within small pores present in the precursor material, early in the Solar System, after the accretion of the parent body¹⁰, or possibly even before, within the protosolar disc. Few dolomite grains from Ryugu particles have been dated to <6.8 Myr after calcium–aluminium-rich inclusion (CAI) formation²⁵, or even <1.4 Myr (ref. 26), indicating a very early formation.

The presence of (Mg, Fe)-carbonates as larger grains/inclusions, in some cases as a thin layer (for example, C0041; Fig. 1), suggests a formation between grains or in fractures, possibly after an impact on Ryugu's parent body, in analogy with the case of fracture-filling dolomite and calcite in CM chondrite meteorites, as discussed in ref. 10.

In addition, we did not detect any example of transition from one
composition to another; carbonates of distinct compositions seem
to be spatially separated. These observations would suggest that the
two Ryugu main families of carbonates formed through two distinct
processes. At what scale they were distinct in time is still questionable.
For comparison, in CI chondrites, the date of breunnerite formation
may not be distinguishable from that of dolomite formation¹⁴.

Possible distinct episodes of carbonate formation may be linked
to the availability of different sources of carbon. Sources of carbon
in studied meteorites remain poorly constrained. For example, for
calcites and dolomites in CM chondrites and Tagish Lake meteorites,
ref. 27 propose CO₂ as the main reservoir, with possible contributions
from CO and CH₄ ices, and from organic matter. CO₂ ice was likely

more widely available in the early stages of protosolar disc evolution,
in cometary nuclei accretion, while organic molecules, even in the
form of organic grains, may have been present all along the evolution
towards Ryugu's accretion. The availability of these two distinct carbon
sources, together with that of reactive cations such as Mg, Fe and Ca,
has thus substantially varied in time, and potentially induced distinct
carbonate formation processes responsible for the occurrence of two
carbonate families detected in Ryugu samples.

Similarities between carbonate properties, small (Mg, Ca)-carbonate and large (Mg, Fe)-carbonate, in Ryugu samples and in CI chondrite meteorites<sup>23,24</sup> tend to confirm that at least part of the formation processes of their carbonate content might be analogous, as is also suggested by the absence of large chondrules or CAIs and the position of the 2.72 µm feature in the sample's spectra<sup>1,3</sup>. This similarity can be put in the larger context of asteroidal studies.

Ryugu is a small, C-type near-Earth asteroid, probably formed in the main belt within the Polana and/or Eulalia asteroid families<sup>28,29</sup>, by the reassembly of fragments from a previous larger body (the primary parent body). It has been shown that primitive asteroids (C- and P-types) account for more than half of the mass of the main-belt and Trojan asteroids<sup>30</sup>, and the C-types in particular are thought to represent a very significant fraction of the asteroid mass in the outer belt, for all asteroid sizes<sup>31</sup>. Strong indications exist that the collected Ryugu grains from the two sampling sites are representative of the composition of the whole asteroid<sup>1,3</sup>. This suggests that bulk properties of the Ryugu collection are probably common characteristics of C-types.

The existence of two families of carbonates may thus emerge as a
general property of primitive materials in the outer part of the asteroid
belt, and more generally of a large fraction of the mass in the early Solar
System. The existence of different conditions for carbonate formation
and/or the potentially different nature of their precursors would thus
represent a general property of the early Solar System.

**Methods**

The MicrOmega instrument is a hyperspectral microscope working in the 0.99–3.6 µm spectral range, with a 20 cm⁻¹ spectral resolution (with a typical spectral sampling of 20 cm⁻¹, and around 10 cm⁻¹ in the 3.3–3.6 µm range). The three-dimensional (x, y, z) hyperspectral cubes are built by sequentially illuminating the sample, imaged onto a two-dimensional detector, with a monochromatic light². The illumination incidence angle is 35°, while the sample is observed from the top (nadir geometry). The monochromatic images, typically a few hundred, are piled up, forming the three-dimensional hyperspectral cube. The pixel size is 22.5 × 22.5 µm² in the focal plane. Spatial oversampling can be achieved by acquiring several cubes with a shift in position of a fraction of a pixel in both directions. This approach provides an improved resolution corresponding to a pixel size of ≤14 µm. It was used for the images in Fig. 1.

The MicrOmega instrument is mounted within the Curation Center at the Institute of Space and Astronautical Science (Sagami-hara, Japan) in an ultraclean, N₂-purged chamber to prevent H₂O and/or CO₂ contamination³. For imaging by MicrOmega, the samples are introduced within a sample chamber part of the main Curation Center suite¹; this chamber is separated from the MicrOmega chamber by a near-infrared-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 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. Such capability offers the ability to observe different sides of the grains, which can be in shadow for certain illumination configurations. The calibration process is fully described in ref. 32. In particular, 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.

Each bulk sample from chambers A and C (corresponding to a total mass of 5.4 g) has been distributed over three sample holders (corresponding to a total of six sub-bulk samples: A1, A2, A3, C1, C2 and C3). From these sub-bulks, individual grains (a few mm in size), as well as small sub-bulks (typically a few tens of mg), have been extracted for more detailed analysis by the instrument suite within the Curation Center (including MicrOmega). The dataset used in this study corresponds to the acquisitions performed between 26 December 2020 and 31 December 2021 on all six bulk samples from chambers A and C (containing all the collected material from Ryugu), as well as 177 individual grains and 14 small bulks extracted from them. Among the 177 grains, five have been turned over for complementary characterizations of the other face (A0017, C0041, A0042, C0094 and C9000).

The detection of the carbonates present within the samples was done by the analysis of the 3.3–3.5 µm diagnostic feature completed by visual inspection of the reflectance spectra. More specifically, a spectral parameter determines the cumulative depth of two bands, one around 3.35 µm and one around 3.48 µm, against a continuum taken between 3.24 and 3.57 µm. This spectral parameter highlights the 3.3–3.5 µm feature for most carbonate compositions. Visual inspection includes checking a correct match with a carbonate spectrum for each pixel of the carbonate-rich area, that is, when a clear doublet around 3.3–3.5 µm is present in the spectrum, with sharp spectral drops on each side of the band, and with either a flat bottom of the band or with two minima on each side of the bottom (and possibly with a third minimum in between). When the same carbonate-rich area was detected on different observations of a given sample, its size was determined by taking the largest one among the different observations. The characterization of the composition of the carbonates was then performed by using a chi-square goodness-of-fit test over the 3.2–3.6 µm spectral range, as well as visual inspection of the different spectral features (including the 1 µm band related to the presence of iron) and their position. The reference spectra of the different endmembers, used for the chi-square goodness-of-fit test, were taken from the NASA Reflectance Experiment Laboratory³³ spectral library (BKRICB017A for CaMg(CO₃)₂ dolomite CB-EAC-017-A, LACB09A for CaCO₃ calcite CB-EAC-009-A, BKRICB020A for CaCO₃ aragonite CB-EAC-020-A, BKRICB070A for MgCO₃ magnesite CB-EAC-070-A and LACB08B for FeCO₃ siderite CB-EAC-008-B), except for the (Mg,Fe)CO₃ breunnerite, whose spectrum was acquired on a PerkinElmer Spectrum 100 infrared spectrometer present at Institut d'Astrophysique Spatiale. This breunnerite was sampled from a large breunnerite crystal extracted from a talc-rich rock from Vizze Valley, Trentino-Alto Adige, Italy. It was manually crushed in an agate mortar to obtain a powder with grain size <200 µm used for the infrared spectrometer. Some grains were analysed by energy dispersive spectroscopy with a secondary electron microscope (ICMMO, Paris-Saclay). The Mg:Fe atomic ratio is about 75:25, matching that of a breunnerite, and the sample contains <2% Ca, showing a very limited Ca influence on the spectrum. In addition, Si atomic % content is <2%, showing very limited contamination by talc. Each endmember reference spectrum was compared to the carbonate-rich spectrum (averaged over the pixels that display a carbonate signature) after a normalization of the 3.3–3.5 µm band using the reflectance factor 3.2 µm (outside the band) and 3.4 µm (inside the band), followed by the application of an offset to reach a similar reflectance factor at 3.2 µm (Extended Data Fig. 3). It is important to note that the chi-square goodness-of-fit test can be biased by some artefacts or other spectral biases, especially at small scales, where the spectral quality tends to decrease. In addition to this test, we thus performed for each ROI a check of (1) the position of the edges of the 3.3–3.5 µm band, as well as the different peaks, and (2) the presence or absence of an absorption below 1.5 µm (diagnostic of the presence of iron). The type of carbonate could be assessed with confidence in

177 cases over a total of 238 carbonate-rich areas (74%). The remaining cases only concern small detections (≤15 pixels, most being between 4 and 6 pixels and tending to display noisy spectra or spectra with a peculiar overall shape) and were not taken into account in Fig. 2 and Extended Data Fig. 1.

## Data availability

Each of these carbonate detections will be available in the catalogue of Ryugu samples available at https://darts.isas.jaxa.jp/curation/hayabusa2/, including the location of the detection and its average spectrum.

## References

1. Yada, T. et al. Preliminary analysis of the Hayabusa2 samples returned from C-type asteroid Ryugu. *Nat. Astron.* **6**, 214–220 (2021).

2. Bibring, J.-P., Hamm, V., Pilorget, C. & Vago, J. MicrOmega team, the MicrOmega investigation onboard ExoMars. *Astrobiology* **17**, 621–626 (2017).

3. Pilorget, C. et al. First compositional analysis of Ryugu samples by the MicrOmega hyperspectral microscope. *Nat. Astron.* **6**, 221–225 (2021).

4. Nakato, A. et al. Morphological classification of Ryugu returned samples based on Curation initial description. 53rd Lunar and Planetary Science Conference, The Woodlands, TX, 7–11 March 2022. LPI Contribution No. 2678, 2022, id.1810.

5. Yamaguchi, A. et al. Petrology of asteroid Ryugu particles allocated to the Phase2 Curation Kochi Team. 53rd Lunar and Planetary Science Conference, The Woodlands, TX, 7–11 March 2022. LPI Contribution No. 2678, 2022, id.1822.

6. Yurimoto, H. et al. Chemical and isotopic characterization of asteroid Ryugu. 53rd Lunar and Planetary Science Conference, The Woodlands, TX, 7–11 March 2022. LPI Contribution No. 2678, 2022, id.1377.

7. Lantz, C. et al. Visible and IR hyperspectral imaging of Ryugu samples compared to meteorites and to remote sensing of Bennu and other primitive asteroids. 53rd Lunar and Planetary Science Conference, The Woodlands, TX, 7–11 March 2022. LPI Contribution No. 2678, 2022, id.1866.

8. Johnson, C. A. & Prinz, M. Carbonate compositions in CM and Cl chondrites and implications for aqueous alteration. *Geochim. Cosmochim. Acta* **57**, 2843–2852 (1993).

9. De Leuw, S., Rubin, A. E. & Wasson, J. T. Carbonates in CM chondrites: complex formational histories and comparison to carbonates in Cl chondrites. *Meteorit. Planet. Sci.* **45**, 513–530 (2010).

10. Lee, M. R., Lindgren, P. & Sofe, M. R. Aragonite, breunnerite, calcite and dolomite in the CM carbonaceous chondrites: high fidelity recorders of progressive parent body aqueous alteration. *Geochim. Cosmochim. Acta* **144**, 126–156 (2014).

11. Alexander, C. O. D., Bowden, R., Fogel, M. L. & Howard, K. T. Carbonate abundances and isotopic compositions in chondrites. *Meteorit. Planet. Sci.* **50**, 810–833 (2015).

12. Alfing, J., Patzek, M. & Bischoff, A. Modal abundances of coarse-grained (> 5 µm) components within Cl-chondrites and their individual clasts–mixing of various lithologies on the Cl parent body(ies). *Geochemistry* **79**, 1255–1292 (2019).

13. Endress, M., Ernst Zinnert, E. & Bischoff, A. Early aqueous activity on primitive meteorite parent bodies. *Nature* **379**, 701–703 (1996).

14. Steele, R. C., Heber, V. S. & McKeegan, K. D. Matrix effects on the relative sensitivity factors for manganese and chromium during ion microprobe analysis of carbonate: implications for early Solar System chronology. *Geochim. Cosmochim. Acta* **201**, 245–259 (2017).
15. Clark, R. N., King, T. V. V., Klejwa, M., Swayze, G. A. & Vergo, N. High spectral resolution reflectance spectroscopy of minerals. *J. Geophys. Res. Solid Earth* **95**, 12653–12680 (1990).

16. Bishop, J. L. et al. Spectral properties of anhydrous carbonates and nitrates. *Earth Space Sci.* **8**, e2021EA001844 (2021).

17. Hunt, G. R. & Salisbury, J. W. Visible and near infrared spectra of minerals and rocks. II. Carbonates. *Mod. Geol.* **2**, 23–30 (1971).

18. Gaffey, S. J. Spectral reflectance of carbonate minerals in the visible and near infrared (0.35–2.55 um): anhydrous carbonate minerals. *J. Geophys. Res.* **92**, 1429–1440 (1987).

19. Michikami, T. et al. Boulder size and shape distributions on asteroid Ryugu. *Icarus* **331**, 179–191 (2019).

20. Dionnet, Z. et al. Small Ryugu fragments analyzed by IR micro-spectroscopy and tomography: a description of the 3D heterogeneity at the micrometric scale. 53rd Lunar and Planetary Science Conference, The Woodlands, TX, 7–11 March 2022. LPI Contribution No. 2678, 2022, id.1534.

21. Schröder, S. et al. Spectrophotometric analysis of the Ryugu rock seen by MASCOT: searching for a carbonaceous chondrite analog. *Planet. Sci. J.* **2**, 58 (2021).

22. Michel, P. et al. Collisional formation of top-shaped asteroids and implications for the origins of Ryugu and Bennu. *Nat. Commun.* **11**, 2655 (2020).

23. Fredriksson, K. & Kerridge, J. F. Carbonates and sulfates in C1 chondrites: formation by aqueous activity on the parent body. *Meteoritics* **23**, 35–44 (1988).

24. Endress. M. & Bischoff, A. Carbonates in C1 chondrites: clues to parent body evolution. *Geochim. Cosmochim. Acta* **60**, 489–507 (1996).

25. Yokoyama, T. et al. Samples returned from the asteroid Ryugu are similar to Ivuna-type carbonaceous meteorites. *Science* https://www.science.org/doi/10.1126/science.abn7850 (2022).

26. McCain K. A. et al. Early fluid activity on the Ryugu parent asteroid inferred from 53Mn/53Cr ages of Ryugu carbonate. 85th Annual Meeting of The Meteoritical Society, 2022. LPI Contribution No. 2695.

27. Fujiya, W. et al. Migration of D-type asteroids from the outer Solar System inferred from carbonate in meteorites. *Nat. Astron.* **3**, 910–915 (2019).

28. de Leon, J. et al. Expected spectral characteristics of (101955) Bennu and (162173) Ryugu, targets of the OSIRIS-REx and Hayabusa2 missions. *Icarus* **313**, 25–37 (2018).

29. Tatsumi, E. et al. Spectrally blue hydrated parent body of asteroid (162173) Ryugu. *Nat. Commun.* **12**, 5837 (2021).

30. DeMeo, F. E. & Carry, B. The taxonomic distribution of asteroids from multi-filter all-sky photometric surveys. *Icarus* **226**, 723–741 (2013).

31. DeMeo, F. E. & Carry, B. Solar System evolution from compositional mapping of the asteroid belt. *Nature* **505**, 629–634 (2014).

32. Riu, L. et al. Calibration and performances of the MicrOmega instrument for the characterization of asteroid Ryugu returned samples, *Rev. Sci. Instrum.* **93**, 054503 (2022).

33. Milliken, R. E., Hiroi, T. & Patterson, W. The NASA Reflectance Experiment Laboratory (RELAB) facility: past, present, and future. 47th Lunar and Planetary Science Conference, The Woodlands, TX, 21–25 March 2016. LPI Contribution No. 1903, id.2058.

## Acknowledgements

We thank the whole Hayabusa2 team for the quality and quantity of samples returned from Ryugu, which are already a landmark achievement in the science of asteroids. We thank the French space agency CNES for its full support. T.Y. received support from the Japan Society for the Promotion of Science KAKENHI, grant number JP18K03830. We would like to offer special thanks to B. Gondet, who, although no longer with us, contributed greatly to this work.

## Author contributions

D.L., C.P., L.R., R.B., J.-P.B., A.N., K.Y., T.Y., T.O., T.U. and M.A. designed, conceived and planned the experiment at the Curation Center. D.L., C.P., L.R., R.B., J.-P.B., A.N., K.H., K.Y., T.L.P.-J., T.Y., T.O., C.L., A.M., M.N., K.N., K.K. and Y.H. performed the MicrOmega analyses. D.L., C.P., L.R., R.B., J.-P.B. and J.C. analysed the data concerning the carbonate detections. D.L., C.P., L.R., R.B., A.N., A.A.-T., J.C., T.L.P.-J., Y.L., C.L., D.B., T.S., S.T., S.N., Y.T. and S.W. contributed materials and analysis tools to the study.

## Competing interests

The authors declare no competing interests.

## Additional information

Extended data is available for this paper at
https://doi.org/10.1038/s41550-022-01870-1.

Correspondence and requests for materials should be addressed to D. Loizeau.

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

Reprints and permissions information is available at
www.nature.com/reprints.

Publisher's note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.

© The Author(s), under exclusive licence to Springer Nature Limited 2023
<figure><img src="image_6.png" /></figure>

**Extended Data Fig. 1** | Carbonate size distribution between grain inclusions and loose grains. Histograms of the distribution of the carbonate ROIs in the analysed samples, by carbonate species, and between carbonates as grain inclusion and as loose grain. Diameters are given for equivalent circular ROIs, but shapes can be very irregular. ‘Inclusions’ are for carbonate ROIs detected within a part of a larger grain, ‘loose grains’ are for grains that appear entirely carbonate-bearing. Extracted grains (mostly > 1 mm in width) are deposited in individual sample holders. Small grains (mostly <100 µm in width) are found with them in

the same sample holders, either transported together with the main grain during extraction from the bulks, or detached from the main grain during movements of the sample holder. In the category ‘extracted grains’, the large carbonate inclusions (plain colour) are within the main grains, but small carbonate loose grains (hashed colour) are also detected around. The majority of carbonate inclusions are in the extracted grains, which is expected as only the largest grains were extracted. In the sub-bulks, which mostly contain a finer fraction of the grains, carbonate ROIs correspond mostly to loose carbonate grains.
<figure><img src="image_7.png" /></figure>

**Extended Data Fig. 2** | Cumulative size distributions of the carbonate ROIs in the analysed samples. Cumulative size distributions of the carbonate ROIs in the analysed samples, one from all detected carbonates in the bulks (grey), and the other from carbonate inclusions within the characterized individual grains

(black). The power indices of the distributions are respectively indicated in black and grey, and may be compared with the previously derived power index of the size distribution of Ryugu's collected particles of -3.88¹, and the one of Ryugu's boulders >5 m of -2.65¹⁹.
<figure><img src="image_8.png" /></figure>

**Extended Data Fig. 3** | Comparison of the 3.3–3.5 µm band with different reference endmembers. Comparison of the 3.3–3.5 µm band from two carbonate-rich areas with different reference endmembers after normalization and application of an offset. Left: spectrum from grain C0041 (average over 342 MicrOmega pixels) with a best fit with an (Mg,Fe)CO₃ carbonate breunnerite; right: spectrum from grain A0033 (average over 56 MicrOmega pixels) with a best fit with an (Mg,Ca)CO₃ carbonate dolomite. Results of the goodness-of-fit test, indicating in each case what endmember provides the best fit (right: C<sub>aragonite</sub> = 0.00699543, C<sub>breunnerite</sub> = 0.00178846, C<sub>calcite</sub> = 0.00613925, C<sub>dolomite</sub> = 0.000463486, C<sub>magnesite</sub> = 0.00604942, C<sub>siderite</sub> = 0.00483010; left: C<sub>aragonite</sub> = 0.0555209, C<sub>breunnerite</sub> = 0.00264406, C<sub>calcite</sub> = 0.0489774,

<figure><img src="image_9.png" /></figure>

C<sub>dolomite</sub> = 0.0175524, C<sub>magnesite</sub> = 0.0301949, C<sub>siderite</sub> = 0.0329548). The final identification also includes a check of (1) the position of the edges of the 3.3–3.5 µm band, as well as the different peaks, and (2) the presence or not of an absorption below 1.5 µm (diagnostic of the presence of iron). MicrOmega spectral sampling is 10 cm<sup>-1</sup> in this spectral domain. Such spectral sampling allows us to see a shift of the minima between the carbonate spectra. For example, from breunnerite to dolomite, there is a shift from 3.43 to 3.45 µm of the second minimum of the 3.3–3.5 µm doublet band. This shift corresponds to a 17 cm<sup>-1</sup> difference in wavenumber, thus corresponding to 1–2 spectral channels, which is sufficient to discriminate between these two endmembers.