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.
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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.
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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.
The authors declare no competing interests.
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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.
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.
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: Caragonite = 0.00699543, Cbreunnerite = 0.00178846, Ccalcite = 0.00613925, Cdolomite = 0.000463486, Cmagnesite = 0.00604942, Csiderite = 0.00483010; left: Caragonite = 0.0555209, Cbreunnerite = 0.00264406, Ccalcite = 0.0489774, Cdolomite = 0.0175524, Cmagnesite = 0.0301949, Csiderite = 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–1 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–1 difference in wavenumber, thus corresponding to 1–2 spectral channels, which is sufficient to discriminate between these two endmembers.
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Loizeau, D., Pilorget, C., Riu, L. et al. Constraints on Solar System early evolution by MicrOmega analysis of Ryugu carbonates. Nat Astron (2023). https://doi.org/10.1038/s41550-022-01870-1