Reconstructing the aqueous environment of the Ryugu parent body through the speciation of multiple elements

Abstract

The parent body of the C-type asteroid Ryugu experienced aqueous alteration. The past aqueous environments have been reconstructed using mineralogies and geochemical modeling, often assuming Na-Cl-rich fluid compositions. Here, we reconstruct Ryugu’s aqueous environment using chemical equilibrium modeling constrained by Electron Probe Microanalyzer (EPMA) and X-ray absorption near-edge structure spectroscopy. Our results suggest that Ryugu water possessed lower salinity than previously estimated and was highly reducing (Eh: −0.5 to −0.8 V) and strongly alkaline (pH: 10.3 to 11.5) conditions conducive to prebiotic organic reactions. The inferred high hydrogen fugacity, lack of NaCl, and low salinity recorded in Ryugu sample suggest that aqueous alteration occurred beneath a surface ice layer, with subsequent ice sublimation driving water loss. These findings refine our understanding of C-type asteroids evolution and the quantity and nature of water delivered to the early Earth.

Analysis of the elemental species composition of returned samples from the C-type asteroid Ryugu indicates that Ryugu’s aqueous environment was highly reducing and strongly alkaline, conditions conducive to prebiotic organic reactions.

Introduction

C-type asteroids originally contained various volatile substances, such as water in the form of ice and carbon compounds. Subsequently, heat from radioactive aluminum (Al)−26 and other nuclides led to the melting of ice into liquid water, which facilitated the aqueous alteration in primary minerals within the parent body of the asteroid, yielding minerals such as phyllosilicates, carbonates, pyrrhotite (FeS), magnetite, and organic matter generally found in the Ryugu samples. Previous studies made substantial advancements through direct measurements of various minerals and organic substances in samples returned by the Hayabusa-2 and OSIRIS-REx missions from the C-type asteroids Ryugu and Bennu, respectively^1–9. In this study, we focused on Ryugu in this study. During aqueous alteration, various substances, including mineral-derived components, dissolved into the water from surrounding original minerals and other sources. The dissolved species underwent various chemical reactions in water, leading to the formation of phyllosilicates, mainly saponite and lizardite, which are the major minerals found in Ryugu^1–3,10. The organic matter in Ryugu is also believed to have undergone aqueous alteration, resulting in the formation of a diverse range of organic compounds^5,11.

In organic chemical reactions, pH and redox potential (Eh; vs. SHE) significantly influence reaction progress. In terms of pH, for example, the abiotic synthesis of nucleobases, such as uracil, previously identified in Ryugu¹², favored alkaline condition¹³. In general, Eh is crucial for the abiotic synthesis of organic matter, as reducing environments facilitate the formation of organic compounds, whereas oxidizing conditions hinder such processes¹⁴. A reducing environment also minimizes oxidative degradation, allowing organic compounds to accumulate and persist over time. Therefore, estimating the aqueous environment is critical in studies on the origin and evolution of organic compounds. However, no studies have quantitatively examined these factors in Ryugu’s aqueous environment through the identification of relevant chemical reactions via chemical speciation².

Given the scarcity of liquid water in carbonaceous chondrite meteorites or Ryugu samples (except for fluid inclusion reported in Nakamura et al.²), attempts have been made to reconstruct the aqueous environment through mineral composition and chemical equilibrium calculation. Zolensky¹⁵ used EQ3/6, a modeling software for chemical and mineralogical interactions, to calculate the chemical equilibria in water within CI and CM chondrite parent bodies at various total dissolved carbon concentrations. They estimated that aqueous alteration occurred under basic and strongly reductive conditions in both cases. Zolotov¹⁶ estimated that the pH of water interacting with CI-type solids at a water-rock ratio of 1:1 is approximately 10. Nakamura et al.² performed chemical equilibrium calculations to determine the variations in dissolved ion and mineral compositions of water in the Ryugu parent body as a function of the water-rock ratio. As a result, the pH of the water was estimated to be above pH 8.5. Alternatively, another approach assessed the aqueous environment by adding water to Ryugu samples and analyzing the leachate¹⁷, suggesting that the pH of Ryugu water was slightly acidic. The resulting aqueous condition may differ from those of the actual Ryugu parent body, as the water-rock ratio exceeds that of the Ryugu parent body in the experiment¹⁷, facilitating the oxidation of various species even in the anaerobic chamber.

Therefore, to quantitatively reconstruct the aqueous environment during aqueous alteration, it is essential to analyze the chemical species and quantify each element in the Ryugu samples, because these analyses enable us to conduct chemical equilibrium calculations that consider the Eh-pH conditions. Unlike previous studies², we attempted to reconstruct the Eh-pH condition using actual chemical and mineralogical data from Ryugu. In other words, we performed chemical equilibrium calculations by replacing the conditions assumed in previous studies with constraints derived from actual experimental data (Supplementary Fig. 1). In this approach, solid-water distributions of various elements, including precipitation-dissolution and adsorption-desorption reactions, play a key role. Thus, the chemical reactions underlying these equilibria must be identified. For this purpose, speciation analysis of major elements was conducted using X-ray absorption near-edge structure (XANES) spectroscopy for sodium (Na), potassium (K), calcium (Ca), and chlorine (Cl), regarded as the major dissolved species in the water. Several studies have suggested that the water on Ryugu’s parent body and CI-type chondrites is of the NaCl-type, in which the primary cation and anion are Na⁺ and Cl⁻, respectively^2,16,17. In fact, Na, with a concentration similar to that of CI-type chondrite¹⁸, has been detected in Ryugu³, but this study will show that the fraction of Na present as NaCl is less than 10% of the total Na in Ryugu^3,19. Thus, the main species of Na should be identified in the Ryugu sample. In addition, speciation of Cl is highly important because Cl⁻ in water is considered the main counteranion of various major cations in Ryugu water, such as NaCl, KCl, and CaCl₂. However, reliable data on the speciation of Cl in Ryugu, or in CI chondrites, are lacking.

The main Na species in the Ryugu sample can be Na⁺ ion-exchanged into phyllosilicate, notably saponite. In such cases, the concentration of each dissolved ion can be estimated by the ion-exchange reactions between liquid water and the interlayers of phyllosilicate, using the selectivity coefficient²⁰. This feature facilitates the estimation of the ratio of each cation dissolved in Ryugu by comparing the ratio adsorbed in the interlayer²¹. The Eh-pH conditions in Ryugu can be reconstructed using the estimated dissolved cation ratios and the solubility of various minerals present in the Ryugu samples.

Therefore, this study aimed to reconstruct the aqueous environment of Ryugu, particularly Eh and pH, by analyzing the speciation and concentrations of various elements in the Ryugu parent body (Supplementary Fig. 1). Moreover, the ratio of NaCl to total Na and Cl was deemed important in discussing the mechanisms of water loss from Ryugu, including evaporation from liquid water and/or sublimation of ice. For these purposes, the speciation analysis of various elements was conducted using bulk and local-scale XANES analysis in this study.

Results

Field emission (FE)-Electron Probe Microanalyzer (EPMA) observations of polished Ryugu C0120 surfaces (Fig. 1) revealed the absence of less-altered fragments consisting of olivine and pyroxene²; all areas exhibited lithology characterized by aqueous alteration, often indicated as altered region or highly-altered region^2,22. In the C0120 particle, the carbonates were primarily dolomite, with some Fe-bearing dolomite and breunnerite, which was consistent with previous studies^2,22. Calcite was not detected in the prepared thin sections. These results showed that our sample consisted of the altered major lithology of Ryugu classified in Nakamura et al.². Since less-altered fragments have rarely been reported in A particles in previous studies, these particles were treated as an altered region. We measured bulk XANES spectra at Na, K, Ca, Al, and Cl K-edges for the sample with the major lithology to obtain the average ratios of various chemical species present within the particles, which indicated that the final aqueous water environment during alteration was before complete freezing or evaporation of liquid water in Ryugu.

Fig. 1. BSE image of the polished surface of Ryugu C0120 embedded in epoxy resin.

a Overall BSE image of Ryugu C0120. There were no less-altered or least-altered regions in this sample. Major minerals were indicated (Po: pyrrhotite; Do: dolomite; Mag: magnetite; Phys: phyllosilicate). b The enlarged image of the coarse-grained phyllosilicate region is shown. The fibrous and bright portions correspond to coarse-grained phyllosilicates. The numbered areas in the figure indicate the locations where the chemical compositions of the phyllosilicates were analyzed. More than 100 points were measured, and the compositions were calculated after excluding regions enriched in impurities such as pyrrhotite, magnetite, and carbonates (Table 7). A blue rectangle area in Fig. 1b was used to prepare a focused ion beam this section for high-resolution transmission electron microscopy (HR-TEM) observation, and the image is shown in Supplementary Fig. 2.

As indicated above, the Ryugu samples consist predominantly of phyllosilicates, mainly consisting of saponite and serpentine (Supplementary Fig. 2), which suggests that the low-temperature aqueous alteration was the dominant process on the parent body^2,22. Although minor components with distinct alteration histories are present, including less-altered and least-altered fragments^2,22, as well as particles affected by localized acidic hydrothermal alteration²³, these are interpreted as fragments derived from spatially heterogeneous domains or localized events, subsequently mixed during disruption and reaccumulation of the rubble-pile body. Therefore, the Eh–pH conditions reconstructed in this study are not intended to represent a single, universal fluid condition for all lithologies, but rather the aqueous environment responsible for the formation of the major altered lithology at the final state of the aqueous alteration, which comprises the bulk of the Ryugu samples^2,22.

Specifically, the FE-EPMA analysis presented below focused on coarse-grained phyllosilicates^22,24. It is well established that the coarse-grained phyllosilicates studied here (Fig. 1 and Supplementary Fig. 2) were formed in the late stage of formation of various minerals, which was later than the formation of fine-grained phyllosilicates^19,22. Furthermore, it has been suggested that a part of coarse-grained phyllosilicates was formed by the dissolution and reprecipitation of fine-grained phyllosilicates. Given that these coarse-grained phyllosilicates were the final minerals to precipitate, their chemical compositions are expected to reflect the environmental conditions at the final stage of the aqueous alteration. Therefore, we focus on coarse-grained phyllosilicates for the FE-EPMA analysis.

Na and K K-edges XANES

The speciation of Na, K, and Ca was determined by their K-edge XANES, following methods established in previous studies^25–27, and XANES spectra and results of linear combination fitting (LCF) analysis of XANES, including ratios of the host phases of these elements estimated by LCF-XANES, are shown Figs. 2, 3 and Supplementary Tables 1–3. For Na, NaCl and Na-adsorbed saponite and Na-adsorbed lizardite were considered as possible species. Scanning Transmission X-ray Microscopy (STXM) analysis revealed that the distribution of Na correlates with that of Mg (Supplementary Fig. 3), suggesting that Na distribution was controlled by Mg-bearing phyllosilicates. The Na K-edge XANES spectra of the two Ryugu particles (A0477 and C0120) were almost identical to that of the Na-adsorbed saponite with two peaks at 1071.8 and 1075.5 keV and a pre-edge peak at 1068.5 eV (Fig. 2 and Supplementary Table 1). The spectra are very different from that of NaCl with two intense peaks at 1070.8 and 1073.9 eV. LCF-XANES analysis showed that the contribution of other Na species, such as Na₂CO₃ and NaCl (previously identified by Matsumoto et al.¹⁹), was negligible as a major Na species. The R values used to evaluate the goodness of fit (see Methods for details) are all significantly lower than 0.05 (Supplementary Tables 1–5), indicating that the estimated fractions of each species are robust when their contributions exceed ~5 mol% (i.e., Na adsorbed on saponite, or Na-saponite). In contrast, the quantification of minor species, such as Na₂CO₃ and NaCl in sample A0477, is subject to larger uncertainty and should therefore be interpreted with caution. Furthermore, our adsorption experiments (details in Supplementary Information section 1) showed minimal Na adsorption onto lizardite, the dominant polytype of serpentine in Ryugu¹⁰. This is consistent with the findings of Bertsch and Seaman²⁸, who observed no Na⁺ adsorption on serpentine, and is reasonable considering the very low cation-exchange capacity of serpentine-group minerals compared with smectite^29,30. Therefore, cation adsorption onto lizardite must be substantially lower than that for saponite.

Fig. 2. Na K-edge XANES of Ryugu and standard samples.

(Sap: Saponite) a, b Linear combination fitting (LCF) of Na K-edge XANES of Ryugu C0120 and A0477. c Na K-edge XANES of reference materials. d K K-edge XANES of reference materials. e, f LCF of K K-edge XANES of Ryugu C0120 and A0477, indicating that most of K in the sample is adsorbed on saponite with less than 10% of chloride.

Fig. 3. Ca K-edge XANES of Ryugu and reference materials.

a, b LCF of Ca K-edge XANES of Ryugu C0120 and A0477. For C0120, relative ratios of dolomite, apatite, Ca-saponite, and calcite were 0.65, 0.21, and 0.15, respectively; for C0477, those of dolomite and Ca-saponite were 0.78 and 0.22, respectively. c Ca K-edge XANES for reference materials. d Coarse phyllosilicate region from which the FIB thin section was extracted. The yellow box is the region of interest for the Ryugu sample spectrum. e Ca L_3,2-edge XANES of standards and Ryugu C0120.

Similarly, K K-edge XANES spectra of A0477 and C0120 were almost identical to that of the K-adsorbed saponite with one peak at 3613.5 eV and a shoulder peak at 3610 eV (Fig. 2). The spectra differ from that of KCl with two intense peaks at 3608.6 and 2612.2 eV. LCF analysis using KCl and K-adsorbed saponite as reference standards showed that the K K-edge XANES of the Ryugu samples was nearly identical to that of the K-adsorbed saponite (Fig. 2 and Supplementary Table 2). The contributions from other species, such as KCl, K₂CO₃, and K₂SO₄, were minimal. Additionally, almost no K was adsorbed on lizardite.

To further increase the number of samples for LCF-XANES analysis and to confirm the dominance of saponite as main hosts of Na and K, their K-edge XANES spectra for another sample collected from aggregate sample (A0477) were measured and fitted by the combination of (i) NaCl/KCl and (ii) those adsorbed on saponite, montmorillonite, or lizardite prepared in laboratory (see “Methods” for more details). The results (Supplementary Fig. 4) demonstrated that those adsorbed on saponite gave the best fit (indicated by the lowest R-value) among the three adsorbents, showing that saponite is the main host phase for both Na and K in Ryugu.

Ca K and L-edges XANES

The bulk Ca K-edge XANES spectra of Ryugu (Fig. 3 and Supplementary Table 3) were similar to that of dolomite, but they are not identical. Consequently, the spectra were fitted by LCF using various Ca-bearing minerals previously identified in Ryugu samples, including dolomite, calcite, apatite, and Ca-adsorbed saponite. The results indicated that dolomite was the primary component (65–78%; Supplementary Table 3), and these findings remained consistent even using dolomite standards from different origins (Supplementary Fig. 5). In addition, a portion of the Ca was estimated to be adsorbed onto saponite (15–22%; Supplementary Table 3). To confirm the adsorption of Ca on saponite, we prepared an FIB thin section of coarse phyllosilicate region from a polished Ryugu sample and analyzed it by STXM with Ca L_3,2-edge XANES (Fig. 3). This area contained few impurities, allowing us to obtain XANES spectra specifically obtained from the phyllosilicate region. The results demonstrated that Ca was adsorbed in the coarse phyllosilicate area consisting of saponite and lizardite, uniformly distributed within the area without contributions of carbonates and phosphates. Calcium L_3,2-edge XANES spectrum of the area (C0120) was very similar to that of Ca-saponite or Na, Ca-saponite (Fig. 3e). Considering that lizardite has a limited cation-exchange capacity to adsorb cations as described above, and the purity of the coarse phyllosilicate region reported in previous studies^2,22, we concluded that most Ca was adsorbed on saponite within the phyllosilicate region.

The representativeness of the analyzed samples (A0477 and C0120) for the speciation of Na, K, and Ca is supported by (i) mineralogical observations by FE-EPMA (Fig. 1) and (ii) the analytical scale. The latter factor stems from the fact that our bulk XANES measurements were conducted with a beam size of approximately 1 × 1 mm². Given that the typical grain size of secondary minerals in Ryugu is less than 50 µm, each XANES spectrum represents a statistically averaged composition derived from thousands of mineral grains. This ensures that the LCF-XANES spectra reflect the bulk chemical speciation of the Ryugu parent body rather than local heterogeneities for each element.

Al K-edges XANES

As reference materials for Al K-edge XANES, we measured α-Al₂O₃ and minerals expected to be present on Ryugu, such as saponite (Australian saponite and synthetic saponite), montmorillonite (SWy-2 and Kunipia-F), and lizardite as reference standards, summarized in Supplementary Table 6. The layer charge of saponite was determined by the extent of isomorphous substitution of Si⁴⁺ by Al³⁺ in the tetrahedral structure, which was important for calculating the total equivalent concentrations of cations within the interlayer. Additionally, Al³⁺ could occupy not only tetrahedral sites but also octahedral sites in the phyllosilicates. These structural differences were distinguished by Al K-edge XANES, which reflected the symmetry of Al³⁺ (Fig. 4). Here, we found that Al³⁺ in tetrahedral coordination exhibited a strong peak at 1564 eV with a secondary peak at 1572 eV. By contrast, Al³⁺ in octahedral coordination has a strong peak at 1565 eV with a second one at 1569 eV^31,32. The two peaks found in the XANES spectra of Ryugu samples indicated that almost all of Al³⁺ in Ryugu were incorporated into the tetrahedral sites of phyllosilicates (Fig. 4 and Supplementary Table 4). In saponite, Al³⁺ was predominantly present in tetrahedral coordination, particularly when formed under alkaline pH^33,34, Al³⁺ mainly occupied the octahedral site in serpentine³⁵. In the Ryugu samples, we found almost no peaks corresponding to octahedral Al³⁺. Therefore, most of the Al³⁺ in the Ryugu samples is present in the tetrahedral site of saponite.

Fig. 4. Al K-edge XANES of Ryugu and standard samples.

a, b LCF of Al K-edge XANES of Ryugu C0120 and A0477. Ryugu spectra do not have the peak derived from Al in the octahedral site. c Reference spectra for Al K-edge XANES. Even the saponite, which does not have octahedral Al, has a small octahedral peak in 1569 eV. Lizardite and other octahedral Al in various minerals have a strong peak at 1569 eV. Kunipia-F and SWy-2: montmorillonite; NAu-2: nontronite.

Cl K-edge XANES

Chlorine K-edge LCF-XANES analysis was performed using NaCl, KCl, Fe and Mg chlorides and hydroxychlorides and the Cl-bearing lizardite synthesized in this study as reference materials (Fig. 5). The XANES spectra of the chlorides and hydroxychlorides were measured using a fluorescence detector (SDD), whereas the conversion electron yield (CEY) mode was employed for the Ryugu samples and the Cl-lizardite to mitigate high background signals, as described in the “Methods” section. For Cl in NaCl, one peak and a shoulder were observed at 2835 eV and around 2830 eV, respectively, which were not present in the Ryugu sample. These results indicated that NaCl is not the dominant Cl species. Instead, the Ryugu spectrum was similar to that of the Cl-lizardite. In addition, it is unlikely that Cl is fixed on saponite, since (i) the addition of saponite did not improve the fitting (Supplementary Table 5) and (ii) Cl adsorption to saponite is not favorable due to the large negative charges in the saponite. Based on LCF, approximately 90% of the Cl K-edge XANES in the Ryugu sample was Cl-lizardite, in which Cl substitutes for OH, with the remaining fraction most likely attributable to NaCl (Fig. 5 and Supplementary Table 5).

Fig. 5. Cl K-edge XANES of Ryugu and reference materials.

a LCF of Cl K-edge XANES of Ryugu A0477 measured in electron yield mode, indicating that most of Cl exists in lizardite. b Cl K-edge XANES of reference materials. PVC polyvinyl chloride; PVDC polyvinylidene chloride. The structure of Cl XANES of the Ryugu sample is similar to that of Cl-Lizardite, but not to NaCl. XANES of Cl-Lizardite is similar to that of Cl bound to magnesium hydroxide due to the similar chemical environment between the two species.

FE-EPMA analysis

FE-EPMA analysis revealed several areas of phyllosilicate structures that appeared relatively bright in the back-scattered electron image (Fig. 1b). In these regions, the undulating texture of the phyllosilicates was clearly visible, while minerals such as pyrrhotite, which are ubiquitous in other regions, were entirely absent. These areas are considered aggregates of nearly pure phyllosilicates with minimal impurities. More than 100 points within these regions were selected for FE-EPMA analysis. The Na/K ratios in the layered silicate regions of the Ryugu sample were approximately 30% lower than the Na/K ratios in the bulk composition of the entire Ryugu sample determined by X-ray fluorescence spectroscopy and ICP-MS³⁶. This discrepancy is likely attributable to the high susceptibility of Na to beam damage during EPMA analysis³⁷. In the compositional calculations, measurement points with high sulfur or exceptionally high Ca concentrations were excluded to estimate the average chemical composition of phyllosilicates without contributions from pyrrhotite and carbonates (Supplementary Fig. 7). Raw chemical compositions of the coarse-grained phyllosilicates, after excluding analyses affected by pyrrhotite, carbonate, and magnetite, are presented in Supplementary Table 7. Based on the partitioning of octahedral cations (Mg + Fe), the molar ratio of saponite to lizardite in the coarse-grained phyllosilicates examined in this study is estimated to be 0.55:0.45. Details of the calculation were given in Supplementary Table 7. In addition, our adsorption experiment results indicated that the ion-exchange capacity for Na⁺, K⁺, Mg²⁺, and Ca²⁺ in lizardite was negligible compared with that of saponite, as described above. Furthermore, we assumed that (i) the ion-exchange capacity of saponite is equal to its layer charge, determined by the extent of Al³⁺ substitution in the tetrahedral structure, and (ii) all Al³⁺ is incorporated into these tetrahedral sites of saponite. The amount of Mg²⁺ adsorbed in the interlayer of saponite was estimated by calculating the difference between the total layer charge and the combined equivalent concentrations of Na⁺, K⁺, and Ca²⁺. The estimated cation compositions derived from the equivalent charge are presented in Table 1. The cation exchange capacity of the Ryugu sample was determined to be 414 meq/kg, which is consistent with 400 meq/kg assumed in the chemical modeling study by Tokumon et al.³⁸.

Table 1.

Estimated cation equivalent compositions in the interlayer of saponite

Cation	Na+	K+	Mg2+	Ca2+
Ratio in the interlayer	0.64	0.04	0.07	0.25

Chemical equilibrium calculation in Ryugu water

To evaluate the salinity of the fluid which depends on the water-rock ratio derived from porosity, we defined a salt concentration parameter, R_Cl, as the ratio of soluble chlorine (expressed as weight % of NaCl to total Cl in the sample) to the water-rock ratio (v/v):

$$R_{\mathrm{Cl}}=\frac{{\mathrm{Cl}}_{\mathrm{NaCl}}/{\mathrm{Cl}}_{\mathrm{Ryugu}}}{\mathrm{W}/\mathrm{R}\left(\mathrm{v}/\mathrm{v}\right)}$$ 1

We assumed that the minerals identified in the FE-EPMA analysis and previous studies reached thermodynamic equilibrium, and that the interlayer cations of saponite were in equilibrium with the dissolved cations. Details regarding the minerals assumed to be in precipitation-dissolution equilibrium are provided in the “Methods” section and Supplementary Fig. 1. We calculated the dissolved ion concentrations in equilibrium with the cation composition of the saponite interlayers (Table 1). Calculations for precipitation-dissolution equilibria were performed as a function of (i) the activity of SiO₂(aq), (ii) the activity of ferrous iron (Fe²⁺) in Ryugu water, and (iii) the relative abundance of the water-soluble Cl component, defined as R_Cl (Fig. 6).

Fig. 6. Eh-pH diagram of estimated aqueous environment calculated from GWB SpecE8.

Each point in the same line indicates the fixed activity of SiO₂(aq), common logarithms of the activity of SiO₂(aq) are −4.5, −5.5 and −6.5, from left to right, respectively. a Conditions under which 10% of chlorine in Ryugu is soluble (=NaCl), and water-rock ratios (v/v) are 1 (= R_Cl = 0.1). Each line indicates the fixed activity of ferrous ions (Fe²⁺) in Ryugu water. “Siderite” means the condition in which ferrous ions are at equilibrium with siderites. b Conditions under which ferrous iron is at equilibrium with siderite. Each line indicates the salt concentration parameter, R_Cl (= defined as the ratio of soluble chlorine to the water-rock ratio (v/v) in Ryugu).

The estimated redox potential (Eh) was highly reducing (low Eh) overall; in some cases, conditions were sufficiently reducing to fall below the water stability limit, potentially leading to the decomposition of water and the generation of H₂ (Fig. 6). The most plausible equilibrium conditions were estimated based on the following assumptions: (i) the water/rock volume ratio of Ryugu is approximately 1, assuming a porosity of 50%^1,39, and (ii) about 10% of Cl was water-soluble based on the NaCl fraction relative to total Cl derived from Cl K-edge XANES. Under this condition, R_Cl equals 0.1. At this value, if the activity of Fe²⁺ in the water exceeded 1.0 × 10⁻⁸, the Eh remains below the water reduction line across the range of dissolved SiO₂ concentrations (Fig. 6a). The major ions in Ryugu water are of the Na⁺-Cl⁻-CO₃⁻ type (Table 2), with a total cation/anion concentration of approximately 7 meq/kg. This value is roughly 1/100 the concentration of terrestrial seawater (ca. 0.6 eq/kg). Notably, these total concentrations are significantly lower than those previously estimated for CI chondrites with similar water/rock ratios, by a factor of 10–100^2,16.

Table 2.

Estimated concentrations of aqueous species dissolved in the water of the parent body of Ryugu, assuming NaCl/Cl_total = 0.1, activity of SiO₂(aq) = 1.0 × 10^−5.5, and siderite equilibrium condition

Cation	Na+	K+	Mg2+	Ca2+
Activity	6.96 × 10−3	2.20 × 10−5	5.15 × 10−5	1.19 × 10−5
Anion	Cl−	HCO3−	NaCO3−	CO32−	H3SiO4−	OH−
Activity	3.90 × 10−3	4.14 × 10−4	3.71 × 10−5	1.30 × 10−3	1.19 × 10−5	1.26 × 10−4

This difference is mainly due to the fact that the adsorption of Na⁺ and Cl⁻ onto phyllosilicates was not considered in previous studies. The primary focus of this study is the quantitative estimation of pH based on direct analyses of the speciation and concentrations of various elements in the sample, which are essential to describe the chemical equilibria. Specifically, the most important point of this study is to estimate the Eh-pH condition at the final stage of the aqueous environment on Ryugu based on the chemical equilibria calculations using actual data of speciation and concentrations of major ions. Previous thermodynamic modeling studies^2,15,16, which can be described as “forward models”, assumed initial states (e.g., original mineral compositions, water-rock ratio) to estimate the aqueous environment. However, it is possible that the assumptions made in the forward model did not reflect the actual chemical environment. The strong point of this study is that we identified the species to exclude these assumptions used in the forward model. For example, our study clearly demonstrates that (i) Na is not present as NaCl but mainly as Na adsorbed on saponite and (ii) the water-soluble Cl ratio is not 100%, but about 10%.

Consequently, the estimated pH became highly alkaline (>10.5) when less than 10% of the total Cl occurred as NaCl. When 100% of Cl was assumed to be soluble Cl (e.g., NaCl), the estimated pH shifted to a lower range below 9.8. This difference in pH is critical when considering the pH-dependent speciation of ammonia/ammonium ions during aqueous alteration, as discussed in the following section. In the Supplementary Information (section 3), a discussion of other species that can be associated with phyllosilicates was given, which does not disturb the ion-exchange equilibria considered in this study.

Discussion

Chlorine in Ryugu: comparison with previous studies

The abundance of carbonate minerals in Ryugu samples suggests that the water on Ryugu’s parent body was under basic pH condition^2,16. Therefore, we initially assumed that the Cl and hydrochloride salts of Fe and Mg, as well as minerals frequently found in evaporites such as NaCl, were the dominant Cl species in the Ryugu samples. However, SEM observations of thin sections and TEM observations of several FIB thin sections from the Ryugu samples failed to detect any Cl signal, which is reasonable if we assume that Cl is relatively uniformly distributed throughout the sample, given the bulk concentration of Cl in Ryugu (= 776 mg/kg)³⁶. Although an example of the presence of NaCl with a size smaller than 200 nm exists within Na₂CO₃ (natron) in Ryugu¹⁹, the majority of Cl may be uniformly distributed throughout the sample.

Similar issues have been discussed regarding serpentines in terrestrial environments. Serpentinization in oceanic crusts creates a highly reducing environment via the reaction between seawater and olivine and/or pyroxene^40,41. These serpentines often contain Cl, and in such cases, Cl incorporated into lizardite (Cl-lizardite) may also be a primary Cl species in the serpenties⁴². Chlorine K-edge XANES analysis suggests minimal to no Cl derived from evaporites (e.g., NaCl) in the Ryugu sample, strongly indicating that Cl-substituted serpentine is the predominant Cl species. We estimated that Cl is incorporated at a rate of 0.005 per element in the octahedral site, based on the findings of Anselmi et al. (2000) regarding the formation of Cl-lizardite. Considering that (i) Ryugu samples contained about 0.004 Cl relative to 1 Mg in bulk XRF analysis³⁶, (ii) our EPMA analysis showed that the Mg/Fe ratio in the phyllosilicate region was approximately 5:1, and (iii) the molar ratio of lizardite to saponite in Ryugu was roughly 3:2³, the Cl content in the Ryugu sample supported the hypothesis that 90% of Cl is present within lizardite.

Huang (2018) reported that Cl present in lizardite comprises two components: one that is easily adsorbed and separated, influenced by the concentration of NaCl in aqueous solution, and another that is independent of the NaCl concentration⁴³. Unlike smectite, lizardite possesses a characteristic that complicates the opening of the interlayer. When olivine changes to lizardite through the serpentinization reaction, if the hydroxyl groups in the lizardite interlayers are substituted with chlorine, the resulting structure may impede the subsequent release of Cl. Thus, Cl in the Ryugu samples exists as Cl in this type of lizardite structure. This Cl is thought to persist in the structure without being released during aqueous alteration after the formation of lizardite. According to Huang et al., the majority of Cl in serpentine is in a form that is difficult to release, suggesting that some of the Cl in the lizardite of Ryugu may also exist in similar forms⁴⁴. This result supports our assumption that the ratio of soluble Cl to total Cl in the Ryugu sample is about 10%.

Matsumoto et al. (2024)¹⁹ found natron in the veins of the Ryugu samples. Natron is an evaporite typically found in alkaline salt lakes and other highly alkaline environments. Fine NaCl crystals have been discovered in the natron region of Ryugu. However, the discovered natron region has a very low Cl ratio of Cl/Na = 2 × 10⁻³. According to Yokoyama et al.³⁶, the bulk Cl/Na ratio of Ryugu is 0.082 ± 0.014, showing that other Cl species exist in Ryugu. Lizardite and other serpentines are known to incorporate Cl into their structures and to adsorb Cl⁻ ions. The amount of adsorbed Cl⁻ is thought to increase in proportion to its aqueous concentration. As the water-rock ratio decreased because of freezing on Ryugu’s parent body, the concentrations of Na⁺ and Cl⁻ increased. However, during this process, the Cl⁻ ion could have been incorporated into the structure of lizardite, preventing a proportional increase in Cl⁻ concentration relative to Na⁺. This process would also lead to the release of OH⁻ ions, which could then react with HCO₃⁻ to form CO₃⁻, promoting natron precipitation. Under this assumption, elevated Cl⁻ concentrations in solutions would result in increased Cl⁻ adsorption, suggesting that a Cl-rich lizardite region should exist within Ryugu samples. However, apart from the natron area, no such Cl enrichment was identified in the Ryugu samples. In our study, the estimated amount of water-soluble Cl is approximately 1/10 of that in Nakamura et al.²: the salinity of the solution was an order of magnitude lower than that estimated in the previous study². The incorporation of Cl⁻ into lizardite occurred at the final stages of aqueous alteration, minimizing NaCl precipitation.

Importance of quantitative evaluation of alkaline pH: ammonium retention and abiotic synthesis of organic matter

Based on the chemical equilibria calculations, we estimated that the pH of Ryugu water was within a highly alkaline range (10.3 < pH < 11.6) when we assume R_Cl < 0.25. This pH range is significant for the retention of ammonium ions and abiotic synthesis of organic matter in Ryugu. Some researchers suggested that Ryugu’s parent body formed beyond the snow line of water ice and CO₂^1,2. Asteroids originating in that region are composed of water and CO₂ ice, and they may also contain ammonium ion and/or ammonia ice, suggesting that Ryugu’s parent body could have been rich in ammonia. Certain organic compounds that can be synthesized from ammonia have been found in Ryugu samples⁵. However, ammonium-bearing saponite, which would be expected if the parent body was rich in ammonia, has not yet been identified in Ryugu samples. Glavin et al.⁷ reported that Ryugu samples contain much lower amounts of ammonia (<0.1 μmol/g) compared with other carbonaceous chondrites and Bennu (= 140 μmol/g). Our results indicated that the water on Ryugu’s parent body exhibited highly alkaline conditions (pH > 10.3). Therefore, even if the parent body accumulated ammonia, the majority could not have been converted to NH₄⁺, because its pKa (= 10.1 at 0 °C calculated by The Geochemist’s Workbench; Bethke, 2008⁴⁵) was lower than 10.3. Chemical modeling³⁸ in the work of Tokumon et al. also showed that neutral NH₃ dissolved in water is predominant relative to NH₄⁺ adsorbed on saponite in the aqueous solution of Ryugu, assuming pH > 9.5. Thus, the alkaline pH above 10.3 estimated in this study strongly showed that the very low abundance of ammonia was due to the highly alkaline conditions, whereas the high amount of NH₄⁺ in Bennu might be due to its lower pH compared with Ryugu, which was already suggested in Tokumon et al.³⁸. However, if the NH₄⁺ fraction in Mg-phosphates is dominant as NH₄⁺/NH₃ species, the conclusions based on the adsorption onto saponite may need to be reconsidered. This is because the incorporation of NH₄⁺ into phosphate stabilizes NH₄⁺, subject to little influence of pH increase for its desorption, compared with the reversible adsorption-desorption reaction of saponite. Further detailed studies, including speciation of NH₄⁺/NH₃, are required to establish the potential pH difference between Bennu and Ryugu.

The highly alkaline pH conditions of Ryugu are crucial for evaluating the possible abiotic synthesis of organic matter in Ryugu. For example, the abiotic synthesis of nucleobases, such as uracil and others already identified in C-type asteroids Ryugu and Bennu^7,12, is known to be favored under alkaline conditions¹³. Furthermore, the formose-type reaction, which generates a variety of sugars and aldehydes from simple carbon compounds such as formaldehyde (abundant in the parent bodies of C-type asteroids), is also favored under high pH conditions^46,47. Moreover, the highly reducing conditions identified in this study would suppress the oxidative degradation of formaldehyde, potentially prolonging its lifetime in the aqueous environment and enhancing its availability as a precursor for the formose-type reaction. The resulting aldehydes could also serve as precursors for amino acid formation via Strecker-type reactions and related pathways in the presence of NH₃ and HCN⁴⁸. Therefore, the reconstructed aqueous environment likely supported coupled organic synthesis pathways involving carbon–carbon bond formation and subsequent nitrogen incorporation.

How did aqueous alteration proceed on the Ryugu parent body?

In our calculations, the water-soluble component of Cl (= NaCl in Fig. 6b) in the Ryugu samples was less than 10%, assuming a water-rock ratio of 1:1 (or 50%) during aqueous alteration (R_Cl = 0.1). Under these conditions, our estimates suggested that the pH of Ryugu’s water ranged from 10.5 to 11.5, and the Eh ranged from −0.5 to −0.8 V (Fig. 6). An increase in the water-soluble component of Cl to 50% resulted in a reduction in pH by 0.5 unit, which is not likely based on the Cl K-edge XANES result. Conversely, the water-rock ratio may be lower than 1:1, such as 3:7²². The 30% water-rock ratio corresponded to an increase in the R_Cl to ca. 0.23, indicating a decrease in pH by 0.2 unit (Fig. 6b). All these results showed that the pH condition was more alkaline than pH = 10.3 for any Fe²⁺ range assumed in Fig. 6.

The aforementioned Eh range is near the equilibrium boundary for water reduction, suggesting that Ryugu’s water may have undergone a reduction reaction, and hydrogen (H₂) gas may have been abundant. As indicated by the Na and Cl K-edge XANES results, little evaporites such as NaCl were detected in the Ryugu samples, and these elements were primarily present as ion-exchanged species within phyllosilicates. Additionally, chemical equilibrium calculations indicated that the salt concentration of Ryugu’s water was relatively low ([Na⁺] < 10 mmol/L), remarkably below NaCl saturation. On Ryugu’s parent body, liquid water or ice is thought to have been lost through evaporation and/or sublimation^19,49. The results of this study indicate that chlorides did not form in the Ryugu samples, at least within the altered lithology. Moreover, the high H₂ fugacity revealed that H₂ was trapped beneath the frozen surface ice layer⁵⁰, which affected the products of aqueous alteration depending on whether H₂ gas is present at the onset of alteration. The physicochemical conditions of Ryugu’s alteration were consistent with those reported by Shibuya et al.⁵⁰: a temperature of around 0 °C; a water-rock ratio below 1; and the presence of abundant CO₂, NH₃, and H₂S, along with H₂ around 1 bar. Our calculations suggested that, at least in the final stage of alteration, H₂ gas fugacity even exceeded 1 bar, indicating that the sample underwent aqueous alteration beneath the surface ice of the parent body, followed by freezing from the surface downward. Thereafter, the asteroid-like Ryugu finally lost its water by sublimation in space during/after its migration into the inner solar system (Fig. 7).

Fig. 7. Schematic of the sublimation process of water ice in Ryugu suggested from the present study.

Estimated results of Ryugu seawater (− 0.8 <Eh/V <− 0.5, pH: 10.3 ~ 11.5) show that the formation of evaporites (as on the Earth) is not the main process for the major elements in water, but adsorption on phyllosilicates is important for Na, K, Ca, and Cl. Low Eh value indicates the highly reducing and alkaline conditions of Ryugu with a H₂-rich environment by reactions (i)–(iii), which may be created by the surface ice layer in the parent body of Ryugu (Shibuya et al., 2024).

The chemical equilibrium calculation in this study is considered to record the major element concentrations when liquid water on Ryugu refroze after aqueous alteration²¹. The low salt content of Ryugu water (Table 2) compared to other studies^2,16 due to the presence of Na⁺ and Cl⁻ adsorbed on phyllosilicate, also suggests that Ryugu lost water primarily through the sublimation of ice rather than evaporation of liquid water. The speciation of various elements in this study also suggests that Ryugu does not contain evaporites, such as chlorides, which are known to form on the surface of Ceres⁵¹. This indicates that Ryugu did not originate from the hydrated regolith on the surface of its parent body, but rather from its hydrated core deep within it⁵². By contrast, evaporites have been found in the Bennu samples based on the discovery of a sequence of various evaporite minerals, despite the hypothesis that Bennu and Ryugu share a similar origin. The presence of evaporites suggested that Bennu’s formation process might have differed to some extent from that of Ryugu⁸. In particular, our results suggest distinct water-loss processes, with water being lost primarily through ice sublimation on Ryugu and through evaporation of liquid water on Bennu. Consequently, evaporites such as chlorides did not readily form on Ryugu but may have been abundant on Bennu. If we assume that the rate of losing water is much slower for the sublimation of ice rather than for the evaporation of liquid water, the amount of water supplied to the Earth during the heavy bombardment period can be larger for the Ryugu parent body than for Bennu. This implication was consistent with a recent study on the preservation of water ice in Ryugu for over 1 billion years, based on the Lu-Hf isotopic system⁵³.

Methods

Standard samples

The origins and characteristics of the standard samples used in this study are summarized in Supplementary Table 6. As for the preparation of cation-adsorbed standards, Na⁺-, Ca²⁺-, and K⁺-saturated saponite samples were prepared by equilibrating 0.2 g of Australian saponite with 1 M solutions of NaCl, CaCl₂, and KCl, respectively. The pH of each suspension was adjusted to 9 using NaOH and/or HCl solutions. These saponite samples were collected by suction filtration, washed five times with 1 mL of Milli-Q water, and dried. To prepare the chlorine (Cl) adsorption standard, 0.2 g of lizardite was added to 10 mL of 0.1 M NaCl. The mixture was adjusted to pH 10 with NaOH, stirred, and the solid was then collected via suction filtration and air-dried.

Sample preparation and beamlines used in this study

Two samples, Ryugu A0477 and C0120, used in this paper were distributed through the Announcement of Opportunity for Ryugu Samples (AO3 and AO1, respectively), organized by the Astromaterials Science Research Group (ASRG), ISAS, JAXA. The A0477 and C0120 were forms of aggregate and particles with a 3 mm size, respectively, distributed by JAXA and were employed in this study (Supplementary Fig. 6). The A0477 sample consisted of very fine, sand-like particles with a diameter of 2 mm or less. After the particles were separated in a glove box (P_O2 < 10 mbar), they were used for various analyses. Some of the particles were loaded on the carbon tape or pressed onto an indium plate and used to obtain bulk XANES spectra at BL-9A and BL-19B in Photon Factory (PF), Institute of Materials Structure Science, KEK (Tsukuba, Japan). The beam sizes of the two beamlines were about 1 × 1 mm². Considering that the sizes of most minerals in Ryugu were less than 50 μm, the XANES spectra obtained at these beamlines reflected the bulk species of each element. The mixing molar ratios of various species present in the Ryugu sample were determined by linear combination fitting (LCF) of the XANES spectra of reference materials.

Ryugu C0120 was sealed in a glove box purged with nitrogen gas, and X-ray CT scans were performed at beamline BL20XU at SPring-8 (Hyogo, Japan) to obtain a 3D shape of the sample and examine the internal structure. From the 3D data obtained, molds that fitted the sample were made using a 3D printer. The sample was then mounted in the mold in a glove box and cut with a diamond wire saw. Thus, a smooth sample surface for XANES measurements was prepared. Particles produced from crushed materials during the cutting process with the diamond wire saw were collected and used for subsequent measurement. One particle (size: 1 mm) derived from the cutting of Ryugu C0120 using the diamond wire saw was embedded in Epofix resin. Given that Ryugu samples contain many voids, as indicated by previous research, we ensured that the resin penetrated even the microvoids within the samples by adding ethanol to Epofix resin following Nakashima et al.⁵⁴. The sample in the resin was polished dry using alumina or diamond-coated lapping film. This sample was used for EPMA analysis.

Small particles of Ryugu A0477 (less than 500 μm) and a polished piece of Ryugu C0120 were placed on carbon tape and introduced into a focused ion beam (FIB) system (FB-2100, Hitachi, Japan) to obtain ultra-thin sections. Tungsten deposits were used to protect the area of interest and extract thin sections from it. Each section was processed to a thickness of about 100 nm. This section was employed for analysis by scanning transmission X-ray microscopy coupled with XANES spectroscopy (STXM-XANES) using the BL-19A beamline in PF.

Finally, speciation of sulfur in Ryugu was also conducted for the bulk sample and local area in phyllosilicate regions, which were obtained at BL27SU and BL08W at SPring-8 and NanoTerasu (a new synchrotron radiation facility in Sendai, Japan), respectively. The beamlines shown above were summarized in Supplementary Table 8.

FE-EPMA analysis

The polished surface of C0120 was analyzed with an FE gun electron microprobe analyzer (FE-EPMA JEOL JXA-8530F) at the Department of Earth and Planetary Science, The University of Tokyo, after carbon coating. We first observed various minerals within the polished sample using backscattered electron imaging and chemical analysis. The Ryugu sample contained a region of coarse-grained phyllosilicate with few impurities, such as pyrrhotite, and this region encompassed a structure called “coarse phyllosilicate”, which is characterized by relatively bright reflections in the backscattered electron image²². These regions were as large as several tens of micrometers. To analyze the cation composition in the interlayer of phyllosilicates using EPMA, we identified several coarse phyllosilicate regions and conducted composition analysis at 15 kV with a beam current of 12 nA². Sodium diffuses because of beam damage, preventing accurate concentrations. Therefore, to perform quantitative analysis by EPMA, we minimized beam damage by increasing the beam size to 1 µm. Although the beam diameter was 1 µm, the characteristic X-ray generation region was thought to be approximately 10 µm in diameter. Therefore, we selected regions where the phyllosilicate domains extended beyond 10 µm. We then calculated the average composition of the data obtained, excluding areas with a high concentration of impurities, such as carbonates and pyrrhotite. Assuming that the negative charge on the interlayer of saponite depends on the amount of Al³⁺ that replaces silicon (Si⁴⁺) in the tetrahedral structure, the analysis of Al is crucial. We also determined interlayer cation ratios from the obtained composition of Na, K, and Ca.

XANES analysis

Bulk XANES spectra of the Ryugu samples at Na, K, Ca, Al, and Cl K-edges were measured in fluorescence mode using SDD detectors (Amptek), as the concentrations of these elements in the samples were less than 1 wt.% without serious concerns about the thickness effect⁵⁵. The XANES spectra for the majority of standard materials with high concentrations of the target element were measured mainly in the electron yield (EY) mode. The micro-scale speciation of Ca and Al for the FIB section of Ryugu A0477 was also analyzed at the Ca L_3,2-edge and Al K-edge via STXM-XANES at BL-19A in PF. The energy calibration of these absorption edges was conducted as shown in Supplementary Table 8. For the interpretation of XANES spectra, LCF was performed using Athena⁵⁶ or Larch⁵⁷. For each element, reference materials were also measured for energy calibration, along with other compounds that may be present in the sample. The uncertainties in the component fractions correspond to one standard deviation (1σ) estimated from the covariance matrix of the least-squares fit (Supplementary Tables 1–5), and the corresponding ±2σ interval represents approximately 95% confidence. These uncertainties reflect only statistical errors from the fitting procedure and do not include systematic uncertainties related to the choice of reference spectra. Consequently, we included potential species reported in previous studies, particularly for minor components (e.g., Na₂CO₃ and NaCl for Na speciation). The detection limit for individual components in LCF-XANES is typically 5–10%, depending on spectral quality and the similarity among reference spectra^58,59. To quantify the goodness-of-fit, the R-factor and the reduced chi-square (χ²) were employed. The R-factor, which represents the overall misfit relative to the norm of the data, is defined as

$$R={\sum }_{E=E_1}^{E_2}{\left(A_S\left(E\right)-A_M\left(E\right)\right)}^2/{\sum }_{E=E_1}^{E_2}{\left(A_S\left(E\right)\right)}^2$$ 2

where A_S and A_M are the normalized absorption intensities of the sample and the model (linear combination (LC) of standard spectra), respectively, for the energy range from E₁ to E₂. Additionally, the reduced χ² was determined to compare the relative quality of different fitting models, defined as

$$\mathrm{\chi }2=1/\mathrm{\nu }\left\{{\sum }_{E=E_1}^{E_2}{\left(A_S\left(E\right)-A_M\left(E\right)\right)}^2/{ϵ}_E^2\right\}$$ 3

where ν is the number of reference standards employed for the fitting and ${ϵ}_E$ is the estimated measurement noise. Note that the absolute values of χ² are significantly smaller than 1 because the data are normalized to unit step height, and the parameter represents the minimized residual sum of squares rather than a strictly weighted statistical chi-square. Therefore, an R-factor below 0.05 and a minimized χ² were used as criteria for statistically robust fits⁶⁰.

Background signals for Cl were detected from the beamline and SDD detection system. Therefore, the conversion electron yield (CEY) method was used at BL-9A for low-Cl materials to exclude any background signal from the beamline, as this method selectively measured Auger and secondary electrons emitted from the irradiated sample. This technique enabled the measurement of the XANES spectrum of Cl in Ryugu without any background interference from the beamline and detection system.

Chemical equilibria calculation

The basic concept of the chemical equilibria calculation was shown in Supplementary Fig. 1. For this calculation, SpecE8 Program in Geochemist’s Workbench^® (GWB) software⁴⁵ was used to calculate the concentrations of various dissolved ions under equilibrium conditions to estimate the aqueous environment during the aqueous alteration of Ryugu’s parent body, with focus on redox potential (Eh) and pH. These calculations were constrained by the solubility equilibria of various minerals observed in the Ryugu sample, chemical compositions of phyllosilicate determined by FE-EPMA, and relative ratios of chemical species obtained by bulk XANES analysis (Supplementary Fig. 1). Additionally, cation-exchange partition coefficients (selection coefficient) between the montmorillonite interlayer phase and bulk water²⁰ were applied to saponite based on previous studies^20,21. This approach is justified because montmorillonite and saponite belong to the smectite group of phyllosilicates and share a similar structure. In the solubility equilibrium calculations, we assumed that minerals identified in FE-EPMA analysis and previous studies reached equilibrium. The following minerals were specifically considered: serpentine (lizardite), magnetite, proto-dolomite, siderite (as a surrogate for breunnerite, an iron (Fe)-magnesium carbonate), pyrrhotite, and MSH12 (a low-crystallinity layered silicate suggested in previous studies)^2,22. The activity of ferrous ion (Fe²⁺) was fixed at values ranging from 1.0 × 10⁻⁷ to 1.0 × 10⁻⁹ M, or alternatively, assumed to be in equilibrium with siderite to account for Fe chemistry⁶¹. The dissolved silicate concentration was set between two constraints: below the threshold for quartz precipitation and above the level required for serpentinite formation, as quartz has not been detected in Ryugu.

The initial concentration of dissolved cations was adjusted to correspond with the cation composition in the smectite layers, which was determined using the distribution coefficient of cations between the smectite layers and the actual composition of Ryugu saponite³. In these calculations, solubility equilibria were assessed within the framework of redox reactions, particularly the valence changes in ferrous and ferric ions, as well as the redox reactions involving sulfide ions. Specifically, dissolved Fe²⁺ and Fe³⁺ concentrations were treated as separate elements, and their final ratio was used to estimate the Eh of the system. We assumed the concentration of Cl⁻ based on the percentage of water-soluble components of Cl identified via XANES analysis. Ion balance in the aqueous phase was maintained with the assumption that Cl⁻ and carbonate are the primary anionic species; nevertheless, the contribution of carbonate was found to be negligible compared with Cl⁻ as an anion.

Author contributions

Y.T. and T.K. conducted the initial XANES analysis of Na, K, and Ca, which led to the concept of this study. T.K., K.F., and Y.T. outlined the design of this study. T.K., T.O., A.T., and A.Y. conducted the XAFS experiments. Y.I., S.Y., Y.N., R.N., M.K., and M.O. also conducted a part of the XAFS experiments. H.S. and M.U. contributed to the sample preparation. T.K. and H.Y. contributed to the EPMA analysis. M.M. and T.N. conducted HR-TEM analysis. Y.K., T.S., S.T., T.I., T.U., and Y.S. contributed to interpreting the data. All authors discussed the results and commented on the manuscript.

Peer review

Peer review information

Nature Communications thanks John Bridges, Devin Schrader and the other anonymous reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

Funding

This work was supported by the JSPS KAKENHI Grant Numbers 26K21720, 26A302, 25K00227, 25K00022, 24H00268, 24K21564, 24K22346, 23H03986, 22H00166, and 22KJ1077. This work was supported by JSPS Program for Forming Japan’s Peak Research Universities (J-PEAKS) Grant Number JPJS00420230006.

Data availability

XANES spectra data are available at https://doi.org/10.5281/zenodo.21897801⁶². The Hayabusa2 project is releasing raw data on the properties of the asteroid Ryugu from the Hayabusa2 Science Data Archives (DARTS, https://www.darts.isas.jaxa.jp/planet/project/hayabusa2/) for Optical Navigation Camera (ONC), Thermal InfraRed Imager (TIR), Near InfraRed Spectrometer (NIR), Light Detection And Ranging (LIDAR), SPICE kernels, and PDS4. All other raw and processed data are available upon request to the corresponding author, YT.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at https://doi.org/10.1038/s41467-026-78348-5.