Tracing water's reach: Carbonate mineralogy of Ryugu and Bennu as seen by MicrOmega

Introduction: Aqueous alteration was a fundamental early geological process in the Solar System, significantly shaping the mineralogical composition of primitive asteroids. Carbonate minerals serve as important tracers for understanding the physicochemical conditions during these alteration events on early planetesimals [1, 2]. This study presents a comprehensive characterisation of carbonate minerals found in the returned samples from asteroids (162173) Ryugu and (101955) Bennu [3, 4], which provide pristine materials analogous to CI chondrites but without terrestrial alteration. Sample analysis was performed using the MicrOmega instrument, operated collaboratively between ISAS, Japan, and IAS, France, at the JAXA Extraterrestrial Sample Curation Center in Sagamihara, Japan [5].

Method: The Hayabusa2 mission returned a total of 5.42g of Ryugu material, split over the chambers A (surface sample) and C (at least in part subsurface sample after artificial cratering) [3]. In August 2024, NASA provided approx. 0.6g of Bennu samples to the ISAS curation facility, containing larger and smaller grains from both inside and outside the OSIRIS-REx sample container.

MicrOmega is a near-infrared hyperspectral microscope, operating between 0.99-3.60µm with a spatial resolution of 22.5µm per pixel on a 250x256 pixel grid [6]. Sample measurements are performed in N₂-purged chamber, preventing terrestrial contamination. Between December 2020 and June 2024, we have imaged the entire returned Ryugu mass with MicrOmega as bulk samples and about half in more detail in smaller “sub-bulks” and as individual grains. Since September 2024, we are analysing both sub-bulk samples and individual grains of Bennu with MicrOmega under identical measurement conditions as Ryugu. Measurements include multiple observing geometries (varying azimuth and focus depth) for each sample.

Carbonate-rich pixels were identified using absorption depth criteria and χ2 comparison with synthetic carbonate reference spectra, focusing on the characteristic doublet feature around 3.4µm sensitive to carbonate composition [7, 8]. Pixel detections were grouped into contiguous regions-of-interest, assumed to represent individual carbonate inclusions, and assigned a dominant species (e.g., dolomite, breunnerite, calcite) based on the best spectral match, refined by visual inspection.

Results: We identified 709 distinct carbonate inclusions in Ryugu and 90 in Bennu, with typically two or more measurements by MicrOmega. Inclusion sizes range from single pixel (~20µm in equivalent diameter, i.e. diameter of a circle with equal area) to several hundred pixels (several hundred µm in equivalent diameter). On both asteroids, dolomite (CaMg(CO₃)₂) and Mg-rich breunnerite ((Mg,Fe)CO₃) are the dominant carbonate phases, comprising slighty less than two thirds and one third of inclusions respectively, with minor amounts of calcite (CaCO₃) (~5%). Fe-rich breunnerite and tentative Mg-siderite were also found in Ryugu. The mean compositions (inferred from spectral features) of dolomite and breunnerite match closely between the two bodies, as do their relative abundances in terms of total pixel area (~1:1 dolomite:breunnerite ratio overall). These assemblages resemble those in CI chondrites. On both asteroids, carbonates are intimately mixed with phyllosilicates, showing characteristic 2.7µm absorptions. The shape of the 2.7µm signatures in carbonate assemblages differ between both asteroids, suggesting different matrix environments. Breunnerites are the largest inclusions (median ~100-120µm equivalent diameter), followed by dolomites (~70-90µm), while calcites are small (generally one or two pixel, though likely smaller than MicrOmega’s pixel scale of 22.5µm). We observe a spatial separation of dolomite and breunnerite inclusions in samples from both asteroids; they generally do not occur in close proximity. Zoning within inclusions is not detected at the MicrOmega pixel scale. We further note a significant difference between Ryugu's chambers: Chamber C has considerably more breunnerite, while Chamber A is richer in dolomite.

Discussion: The similarity in carbonate assemblages suggests convergent alteration pathways on the parent bodies of Ryugu and Bennu, despite (minor) differences in their isotopic signatures (H, N, O), indicative of different formation environments [4]. Nevertheless, we observe slight differences in the host matrix of the carbonates, based on the shape of the 2.7µm band. We interpret the consistent spatial separation of dolomite and breunnerite at the 10-100µm size scale as remnant of a spatially heterogeneous local water-rock (W/R) ratio that persisted over time. We propose that intermediate W/R ratios favoured dolomite precipitation, while high local W/R ratios suppressed dolomite formation due to Ca²⁺ dilution and promoted breunnerite precipitation. This W/R-driven mechanism explains the large-scale spatial patterns observed with MicrOmega, however, we note that this pattern breaks at smaller size scales (<10µm), where dolomite-breunnerite assemblages have been observed by other studies [9, 10]. The heterogeneity between Ryugu's chambers suggests they sampled different mixtures of parent body lithologies experiencing different dominant W/R conditions, reinforcing the concept of limited large-scale mixing. We observe no systematic difference in the 2.7µm absorption around dolomites and breunnerites, aligning with isotopic data suggesting Mg incorporation into clays prior to carbonate formation [9].

Conclusion: MicrOmega hyperspectral analysis reveals remarkably similar carbonate populations on Ryugu and Bennu, dominated (at the MicrOmega size scale of 10-100µm) by dolomite and breunnerite with minor calcite, suggesting convergent aqueous alteration pathways. The consistent spatial separation of dolomite and breunnerite suggests the controlling influence of local W/R ratio heterogeneity. Different dolomite-breunnerite ratios between Ryugu's chambers indicate heterogeneity at the sampling scale. These results highlight the critical role of local physicochemical conditions, particularly W/R ratio, in governing carbonate speciation during aqueous alteration on primitive asteroids.

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