The spatial distribution of soluble organic matter and its relationship to minerals in the asteroid (162173) Ryugu.

Abstract

We performed in-situ analysis on a ~1 mm-sized Ryugu grain A0080 returned by the Hayabusa2 spacecraft to investigate the relationship of soluble organic matter (SOM) to minerals. The DESI-HRMS (desorption electrospray ionization-high resolution mass spectrometry) imaging using methanol spray identified more than 200 soluble organic compounds, which were assigned as CHN, CHO, CHO-Na (sodium adducted), and CHNO in molecular composition. Heterogeneous spatial distribution was observed for different compound classes of SOM as well as among the same alkylated homologues on the sample surface.

The A0080 sample showed similar mineralogy to that of CI chondrite and contained two different lithologies, which are rich in magnetite, pyrrhotite, and dolomite (lithology 1) and poor in those minerals (lithology 2). CHN compounds were relatively concentrated in lithology 1 than in lithology 2, on the other hand, CHO, CHO-Na, and CHNO compounds were distributed in both lithologies. Such different spatial distribution of SOM is the result of interaction of the SOM with minerals, during precipitation of the SOM via fluid activity, or could be due to difference in transportation efficiencies of SOMs in aqueous fluid. However, organic-related ions measured by ToF-SIMS did not coincide with the spatial distribution revealed by DESI-HRMS imaging, indicating that the ToF-SIMS data would be mainly derived from methanol-insoluble organic matter in A0080.

Alkylated homologues of CHN compounds with large C number appeared more abundant in lithology 2 than lithology 1. In contrast, fragments of the Murchison meteorite showed different features to of A0080, implying different formation or growth mechanisms for the alkylated CHN compounds by interaction with fluid and minerals on Murchison parent body and asteroid Ryugu. This difference might be mainly attributed to the carbonate grains, which would have played as a catalyst for CH₂ growth of CHN compounds. Future in-situ analysis of CI chondrite will provide more reliable constraints for the history of soluble organic compounds in asteroid Ryugu. 

Introduction

Recent nontargeted analysis using high-resolution mass spectrometry (HRMS) (Schmitt-Kopplin et al., 2010; Naraoka et al., 2017) has found high diversity and complexity of SOMs in CCs. Such extraterrestrial SOMs are expected to record the chemical evolution of the solar system from the presolar molecular cloud phase to present geological activities as well as prebiotic evolution of organics.

The Hayabusa2 spacecraft collected samples from two touchdown sites on C-type asteroid (162173) Ryugu and returned them to Earth on December 6, 2020 (Tachibana et al., 2022). One of the science goals is to understand the role of C-type asteroids on delivery of water and organic matter to the proto-Earth (Tachibana et al., 2014; Watanabe et al., 2017; Tachibana, 2021). Initial spectroscopic investigation of returned samples with MicrOmega, a hyperspectral microscope, in the clean chamber at the JAXA curation facility, showed C-H vibration features of organic matter in the near-infrared wavelength range (Pilorget et al., 2021). MicrOmega found heterogeneity in spectroscopic features at submillimeter scale, and one particle potentially enriched in N-H bond was reported (Pilorget et al., 2021). Chemical, isotopic, and mineralogical or petrological analyses of asteroid Ryugu samples have similar characteristics as those of CI chondrites (Nakamura et al., 2022; Yokoyama et al., 2022; Yurimoto et al., 2022). Organic analysis was performed for SOM and insoluble organic matter (IOM), revealing a high variety of organic compounds as well as chemical and isotopic heterogeneities of organic matter from Ryugu grains (Naraoka et al., 2022; Yabuta et al., 2022). The solvent extracts from Ryugu aggregate sample A0106, collected at the first touchdown site, showed a high molecular diversity consisting of C, H, N, O and/or S compositions by high-resolution mass spectrometry (HRMS) analysis, including the high abundance of poly-sulfur bearing species (Naraoka et al., 2022; Schmitt-Kopplin et al., 2022). Various soluble organic compounds including aliphatic amines, carboxylic acids, polycyclic aromatic hydrocarbons, amino acids, and alkylated N-bearing heterocyclic compounds were also detected (Aponte et al., 2022; Parker et al., 2022).

Macromolecular organic matter with complex structure and various morphology is ubiquitously present in Ryugu samples (Yabuta et al., 2022). Several functional groups, such as C = O, C = C, and Si–O were assigned by micro-Fourier transform infrared spectroscopy (µ-FTIR) and atomic force microscope-based infrared spectroscopy (AFM-IR) on the intact grains A0108 and C0109, which were from the first and second touchdown sites, respectively. These microscopic analyses also showed the close association of macromolecular organics with mineral components in the grains (Yabuta et al., 2022) as observed in carbonaceous chondrites (Pearson et al., 2002; Kebukawa et al. 2010; Le Guillou et al., 2014; Noun et al., 2019).

The isotopic analysis revealed that the distributions of D/H ratio of Ryugu IOM were within the range of CI, CM, and Tagish Lake (C2_ungrouped) chondrites, whereas the ¹⁵N/¹⁴N ratio was similar to that of CI chondrites (Yabuta et al. 2022). Furthermore, ¹³C-rich presolar grains were identified in a Ryugu grain C0109-2 from the second touchdown site and their abundance (~ 30 ppm) was identical to that for CI, CM and CR chondrites (Nittler et al., 2022). These results also suggest a potential Ryugu-CI chondrite connection. Furthermore, the presence of isotope heterogeneity in Ryugu organic matter suggests the characteristics of organics in the preaccretion phase was not completely erased by parent body alteration processes.

In this study we further reveal the association of organic matter with minerals on Ryugu at the molecular level. We performed in-situ analysis of SOMs on an intact Ryugu sample using desorption electrospray ionization (DESI) technique coupled with HRMS (Naraoka and Hashiguchi, 2018; Hashiguchi and Naraoka et al., 2019a, 2019b), time of flight-secondary ion mass spectrometry (ToF-SIMS), and mineralogical observation to investigate the relationship of organic compounds and minerals.

Sample And Experiments

The Ryugu grain (A0080) used in this study, collected at the first touchdown site, has a flat surface and ~ 1 mm in size (Figure A1). Because of its fragile nature, the grain was embedded in a soft alloy of Bi, Sn and In without any blazing or polishing (Hashiguchi and Naraoka, 2019a) (Fig. 1a). Organic molecular imaging was performed on area 2.8 mm × 2.8 mm of the sample surface using DESI-HRMS (High-resolution mass spectrometry) system, which is a 2D-DESI ion source (Omni Spray Source 2D, Prosolia Inc.) equipped with a hybrid quadrupole-Orbitrap mass spectrometer (Q-Exactive Plus, Thermo Scientific) at Kyushu University. The spray solvent was 100% methanol (FUJIFILM Wako Pure Chemical Corporation (LC-MS grade) with a flow rate of 3 µl/min. The spot size was ~ 150 µm (note that the spot size is not directly equal to the spatial resolution of the imaging analysis). The desorbed positive ions (m/z 50 − 500) were collected in full scan mode with a mass resolution of 140,000 (m/Δm at m/z = 200). During the imaging, a lock mass mode was employed to calibrate the accurate mass using dioctyl phthalate ([C₂₄H₃₈O₄ + H]⁺ = 391.2843) derived from tubing. Other detailed analytical conditions are described in Naraoka and Hashiguchi (2018) and Hashiguchi and Naraoka (2019b). Criteria for identification of mass peaks from the sample surface was the intensity ratio > 10 at the sample surface (inside of the dotted line in Fig. 1b) to the outside, which contained background ions from spray solvent and/or surrounding air. The obtained mass spectral data were converted to a text data file for imaging by Firefly software (Prosolia Inc.) and the DESI images were visualized using BioMAP (maldi-msi.org). For the image analysis, the mass resolution of m/z of ± ~ 0.001 was adapted. The chemical formulae of identified peaks were assigned using ¹²C, ¹³C, H, ¹⁴N, ¹⁶O, ²³Na, and ³²S, then filtered by ± 3 ppm mass precision.

After the imaging, the ToF-SIMS measurement was performed on the same surface of A0080 using a TRIFT III spectrometer (ULVAC-PHI, Inc.) at Nagoya University. Positive and negative spectra were obtained using an Au⁺ beam. An accelerating voltage and current of the primary ion were 22 kV and 2.6 nA, respectively. The ion beam pulse width was set at 1.4 ns (bunched mode) and 9.5 ns (non-bunched mode) for spectral and image analysis, respectively.

After the ToF-SIMS analysis, detailed mineralogical observation of the grain surface was performed using a field-emission scanning electron microscope (FE-SEM) (Hitachi, SU 6600) equipped with an EDS (HORIBA, EMAX Energy) at Nagoya University. The topography of the sample surface was also observed using a 3D laser microscope (Nikon, A1RMP) at Nagoya University Equipment Sharing System (NUSS).

We also analyzed an antigorite grain as a procedural blank in the series of analyses, which was heated in air at 450 ºC for 3 hours and then methanol washed and studied using the same methods as described above. Murchison carbonaceous chondrite (CM2) fragments were analyzed by DESI-HRMS using methanol spray of 2 µl/min for comparison. The Ryugu grain and the blank sample were stored in a pure N₂-purged container or sealed glass vial for blank and meteorite sample during transportation between institutes.

Data analysis was performed using Image J and Adobe® Photoshop® for DESI-HRMS imaging and FE-SEM data and Wincadence software (Ulvac-phi. Inc) for ToF-SIMS data.

Results

Organic compound species identified by in-situ DESI-HRMS analysis of A0080

A set of optical, total ion, and specific ion images of A0080 are shown in Figure 1 with a mass spectrum obtained from the entire grain surface. Ion signals were observed both from the sample surface and its outside. Peaks detected from outside the sample came from surrounding metal and/or surrounding air. For instance, an ion of m/z 312.362 (C₂₁H₄₆N⁺: Fig. 1c) was detected only from the surrounding metal alloy and should not be indigenous to A0080.

More than 200 positive ions were identified in m/z 80–400 from the grain surface of A0080 by DESI-HRMS imaging and assigned to CHN, CHO, CHONa, and CHNO compounds within ±3 ppm in mass precision. Sulfur-bearing species, which are abundantly present in Ryugu SOMs (Naraoka et al., 2022; Schmitt-Kopplin et al., 2022), were not identified likely because the DESI imaging was made for positive ions. 

The CHO compounds were dominated among the identified species both in terms of signal intensity (~65 % of the total ion counts) and number (Fig. 2a, Table A1). The CHN, CHO, CHONa, and CHNO compounds include their CH₂ families (alkyl homologues) such as C_nH_2n+2N⁺, C_nH_2n-4N⁺, and C_nH_2n-1N₂⁺ for CHN (n = 4 to 22), C_nH_2n-8O₄⁺, C_nH_2nO₄⁺, and C_nH_2n+2O₅⁺ for CHO (n = 4 to 16) (Table A2). The abundant CH₂ families of CHN compounds were also identified from analysis of solvent extracts from carbonaceous chondrites (Naraoka et al., 2017; Isa et al., 2021) and Ryugu grain A0106 (Naraoka et al., 2022; Orthous-Daunay et al., 2022) and in-situ DESI imaging of carbonaceous chondrites (Naraoka and Hashiguchi, 2018; Hashiguchi and Naraoka 2019b).

In contrast to the presence of Mg-containing metalorganic compounds in several   carbonaceous chondrites including Murchison and Tagish Lake (Ruf et al., 2017; Hashiguchi and Naraoka, 2019a), the Mg-containing organic species were not detected from A0080. This result would indicate that Ryugu has experienced lower degree of thermal metamorphism relative to Murchison (Ruf et al., 2017), of which details will be discussed below. Based on the result of less Mg concentration in H₂O extracts of Ryugu sample than Orgueil (Yoshimura et al., 2022), soluble Mg in asteroids Ryugu have probably been consumed to form dolomite and breunnerite grains (Yokoyama et al., 2022). Among these identified organic species, total of 35% of them (78% of CHN compounds, 11% of CHO compounds, 52% of CHNO compounds, and 7% of CHONa compounds, respectively) were reported as positive ions in the methanol extract of the Ryugu aggregate sample (A0106) using nano-liquid chromatograph (LC) equipped with a nano-ESI and nanoAmide column (Naraoka et al., 2022), while others are newly identified in this work. The difference in the compound distribution may be attributed to 1) heterogenous occurrence of the SOM compounds between Ryugu samples (A0106 vs. A0080), and/or 2) lower analytical sensitivities of DESI-HRMS rather than nano-ESI.  

Spatial distribution of SOM related to mineralogy of A0080 

The detected organic ions are heterogeneously distributed on the sample surface and are concentrated in specific regions on the surface. The grain surface after the DESI-HRMS imaging appeared rougher than the original surface, and had up to about 150 µm of roughness (Fig. A2). This is because of the loss of small fragments from the surface by the spraying of methanol solvent and/or N₂ gas flow used for the DESI system. The distribution of ion signals were affected by the surface roughness and were generally highest around the highest region of the grain surface (Fig. A2). Despite the roughness effect on ion detection, total ion signals of CHN, CHO, CHONa, and CHNO compounds showed different spatial distributions on the sample surface (Fig. 2b). For example, most of the CHN compounds appear to be concentrated lower region of the sample surface shown in Fig.2b relative to CHO compounds that are concentrated in the upper region. CHNO and CHONa compounds were detected in the intermediate region.

FE-SEM-EDS analysis showed that the A0080 consists mainly of phyllosilicates, magnetite, pyrrhotite, and dolomite larger than 10 µm in size, and there are two distinct lithologies in A0080; one is enriched in large (> 10 µm) magnetite, pyrrhotite and carbonate (dolomite) grains (lithology 1) and the other is poor in those minerals (lithology 2) (Figs. 3b-c). There was neither chondrule nor Ca-Al-rich inclusion (CAI) on the A0080 surface. In both lithologies, magnetite showed various morphologies, such as single hexagonal crystals and spherical (including framboids for magnetite) and screw-shaped crystals called plaquettes (Figs.3e, f) (Kimura et al., 2013; Gounelle and Zolensky, 2014). Pyrrhotite also occurs as a single hexagonal crystal, which is up to ~ 30 µm, and irregular or spherical shape (Figs.3e, f). The textures of magnetite and pyrrhotite in A0080 were similar to those in CI chondrites (Gounelle and Zolensky, 2014). Such mineralogical and petrological feature of A0080 is almost consistent with that for the most common lithology of other Ryugu grains (Nakamura et al., 2022; Yokoyama et al., 2022). 

In the high SOM signal region of the grain, the CHO compounds distributed in both lithology1 and 2, while CHN, CHONa, CHNO compounds were enriched in smaller regions, especially in lithology 2 (Figs.3a-d, Fig.4). Furthermore, the spatial distribution of each species was different among the same alkyl homologues (Fig.5) of these compounds. Figure 6 shows the ion intensity ratio of the alkyl homologues with different C numbers from lithology1 (ROI 1) to lithology 2 (ROI 2). There was no clear correlation in abundance of CHN, and CHO, CHNO between different two lithologies and their C number in the same CH₂ families (Figs.6b–d). 

ToF-SIMS analysis

Ion images obtained by ToF-SIMS showed heterogeneous distributions of both inorganic and organic ions (Fig. A3). Spatial distributions of inorganic elements (Si⁺, Mg⁺, and Fe⁺) were very similar to each other, suggesting that the image heterogeneity reflected the roughness of the sample surface rather than by the mineral distribution (e.g., pyrrhotite and magnetite) as shown in previous studies of ToF-SIMS analysis on carbonaceous chondrite (Simkus et al., 2015; Noun et al., 2019). The ToF-SIMS data obtained by spectral mode was used to compare the abundance of organic-related ions including positive ions (¹²C⁺, ¹²CH₂⁺, C₃H₉⁺, CNO⁺, and CNNO⁺) and negative ions (CH^–, CN^–, CO₂^–, CO₃^– ,CO₄^–, and C₃H₉^–), in three regions; Area A (enriched in CHN, CHO, CHONa, and CHNO), area B (boundary of CHO-rich region and CHNO/CHN-poor region), and area C (CHO-rich region and CHNO/CHN-poor region) were identified by DESI-HRMS imaging and FE-SEM observation (Fig. 7). The CNO⁺ and CNNO⁺ ions detected by ToF-SIMS were more abundant in the CHO-rich region and CHNO/CHN-poor region that was identified by DESI-HRMS imaging (area C > area B > area A). The intensity of ¹²C⁺ ions was almost similar in the three areas. Negative ions CN^–, CO₃^– and CO₄^– also showed a similar trend as CNO⁺ and CNNO⁺ (Figs. 7b and 7c). In contrast, CO₂^– and C₃H₉^– were less in area B than in areas A and C. 

Comparison for spatial compound distribution in the Murchison meteorite

  More than 300 compounds were identified from two Murchison fragments (fragment 1 and fragment 2) by DESI-HRMS imaging using methanol spray (Table A1). These compounds were assigned to CHN, CHO, CHONa, and CHNO in molecular compositions. For both fragments, CHNO compounds were most abundant in relative intensity (75­–80 %) although CHN compounds were the most in the number of species. The CHN compounds in Murchison include 13 families of alkyl homologues, which are more abundant than those in A0080. Even though some CHNO species were identified as alkyl homologues, most CHNO species have no CH₂ families. Alkyl homologues of CHO and CHONa were not identified in Murchison (Table A2). Total ion of CHN, CHO, CHONa, and CHNO showed different spatial distributions in the sample surfaces (Fig. 8). Alkyl homologues of CHN compounds showed different ion intensities and almost similar spatial distribution in the same fragments (Fig. 9a and 9b) as distinct from A0080. In contrast, the CHNO compounds were differently distributed in the same alkyl homologues as observed in A0080 (Fig. 9c and 9d).  

FE-SEM-EDS observation indicated that the identified SOM species were distributed in the matrix of both fragments, where few chondrules and cracks were present (Figs.10a–d). Fe-sulfide, magnetite, and Ca sulfate occurred in both fragments, and large (> ~10 µm) carbonate was richer in fragment 1 than fragment 2 (Figs. 10c, 10e-f). The carbonate grains were Ca-carbonate, sometimes partially containing sodium, and appeared randomly distributed in the matrix of fragment 1. The CHO, CHONa, and CHNO compounds seemed to be more concentrated in large (> 20 µm) Fe-sulfide-poor region in fragment 1 (upper right region) than CHN compounds (Figs. 10c-d). However, these compounds were also detected from the surrounding of Fe-sulfide (the center region of fragment 1). The spatial distribution of SOMs seemed to be uncorrelated with specific minerals in each fragment, consistent with our previous report (Hashiguchi and Naraoka, 2018). The relationship of C number of alkyl homologues of CHN compounds and distribution of carbonate in the two Murchison fragments is distinct from the observation of A0080. The intensity ratios of alkyl homologues of CHN and CHNO between fragment 1 (carbonate-rich) and fragment 2 (carbonate-poor) are compared in Fig. 11. The intensity ratios of alkyl CHN homologues in fragment1 to fragment2 increased with the C number of alkyl CHN homologues for C_nH_mN⁺ (Fig.11b). On the other hand, C_nH_mN₂⁺ compounds showed a similar intensity ratio between fragment 1 and fragment 2. No clear correlation was found for CHNO compounds (Fig.11c). 

Discussion

Molecular diversity of identified SOMs in A0080

DESI-HRMS imaging revealed a variety of methanol-soluble organic matter on the surface of A0080. Molecular diversity and distribution pattern of organic compounds were reported from the solvent extracts of Ryugu aggregate sample A0106 (Naraoka et al., 2022; Orthous-Daunay et al., 2022; Schmitt-Kopplin et al., 2022). Alkylated homologues (-CH₂) of N-heterocycles having a core structure with CH₂ bonds such as piperidine (C_nH_2n+2N⁺), pyridine (C_nH_2n-4N⁺), and imidazole (C_nH_2n-1N₂⁺), were identified in the methanol extract of the A0106 (Naraoka et al., 2022). Such N-heterocycles were also reported previously from the extracts of the Murchison meteorite (Naraoka et al., 2017). Identification of the alkylated CHN compounds from A0080 and Murchison by DESI-HRMS in this study is consistent with the detection of those compounds in the solvent extracts.

The distribution of molecular species, however, is different between A0080 and Murchison. A0080 was dominant in CHO compounds compared with Murchison, being rich in CHN and CHNO compounds (Fig. 2a and Table A1). Abundant alkyl homologues of CHN compounds such as the C_nH_2n-4N⁺ composition with a wider range of C number were identified from Murchison than in A0080 (Table A2). High-mass resolution mass spectra of negative ions from the methanol extract of the Ryugu A0106 sample obtained by Fourier transform ion cyclotron mass spectrometry (FTICR-MS) did not show an obvious CHO-composition enrichment relative to Murchison (Schmitt-Kopplin et al., 2022). Furthermore, the distribution pattern of alkylpyridine (C_nH_2n-4N⁺) identified from methanol extract of A0106 showed a wider C range relative to Murchison, in contrast to the present result in this study for A0080. Such difference in relative abundance is probably caused by different hydrothermal activity and/or different history of solar radiation and cosmic ray irradiation between Ryugu and the parent body of the Murchison meteorite (Orthous-Daunay et al., 2022). In contrast, a similar distribution was found in C_nH_2nN⁺ between methanol extracts of A0106 and Murchison (Orthous-Daunay et al., 2022). The difference of molecular diversity of A0080 compared to the Murchison fragments found in this study was not consistent with data of methanol extract of A0106 completely, and that result would be attributed to sample heterogeneity, in other words, heterogeneity of SOM distribution in the carbonaceous chondrite (Naraoka and Hashiguchi, 2018; Hashiguchi and Naraoka, 2019b; this study) and in Ryugu sample.

The effect of space weathering should be considered for the result from Ryugu A0080 grain. Sample in Chamber A of Hayabusa 2 spacecraft investigated in this study was obtained from the surface of the Ryugu asteroid and have been experienced space weathering due to solar wind sputtering or micrometeorite bombardment (Matsumoto et al., 2022). It is suggested that space weathering could impart a decrease of D/H in organic compounds by H implantation of Ryugu IOM (Remusat et al., 2022). Destruction of the chemical bonds of SOM could have occurred by UV irradiation, which may result in the loss of H₂, methane, water, or aliphatic features (Orthous-Daunay et al., 2019). On the other hand, molecular analysis of organic matter of Murchison meteorite was performed from inside of the sample, which was not severely affect by space weathering. Such effect of space weathering is a possible mechanism for different molecular diversity of A0080 compared to the Murchison meteorite. 

Spatial distribution of SOM in A0080 and implication for interaction with minerals or aqueous fluid.

Organic compounds identified from A0080 and Murchison meteorite by DESI-HRMS imaging in this study are methanol-soluble organic matter. Ryugu samples experienced by aqueous alteration on the asteroid Ryugu based on their mineralogy (Nakamura et al., 2022; Yokoyama et al., 2022; Yurimoto et al., 2022), and identified organic compounds should have been dissolved in aqueous fluid during hydrothermal aqueous alteration on the Ryugu and Murchison parent body. The FE-SEM-EDS observation revealed that large (>10 µm) sulfide, magnetite, carbonate grains were heterogeneously distributed in A0080, as seen in lithology 1 and 2. These minerals, particularly magnetite and carbonate, were produced during aqueous alteration and SOM distribution were related to the mineral distribution (more concentrated in lithology 1 or lithology 2, as discussed above). Therefore, different spatial distribution of the SOMs (CHN, CHO, CHONa, and CHNO: Fig.2) imply that the heterogenous spatial distribution of SOM was produced by activity on Ryugu parent body, for example interaction of minerals, and fluids which contains SOMs. Mineral precipitation and compound adsorption with surrounding minerals is one of the key processes for different compound distribution. There are three possible mechanisms are a) presence of multiple fluids with different chemical compositions (abundance of CHN, CHO, CHONa, and CHNO) during aqueous alteration, which can be ascribed to the different chemical compositions of interstellar ice grains accreted to the parent body, and distribution of each fluid to different range of the body, b) different timing of SOM precipitation, and c) different transportation efficiency of organic compounds by fluid flow and adsorption effect onto surrounding minerals. These will be considered individually below.

a) Variety of chemical compositions in the initial ice grains is not implausible, based on chemical compositions of cometary ice (Goesmann et al., 2015, Bockelée-Morvan and Biver, 2017), and ice grains with various chemical compositions should have presented based on the stability of molecules at heliocentric distance and time during evolution solar nebula (e.g., Dodson-Robinson et al., 2009). However, CHO, CHN, CHONa, and CHNO compounds were identified in a small region of a few hundred µm and more than half of the region for these SOMs were overlapped. Therefore, the presence of different aqueous fluids without mixing is unlikely to produce the different distribution of SOM in such a restricted such small region associated during with alteration processes.　

b) Organic ions from ice grains, and cations (e.g., Mg²⁺, Fe^{2+ or 3+}, and Ca²⁺) generated by dissolution of anhydrous or amorphous silicates during aqueous alteration (Brearley, 1995, Howard et al., 2009) on the asteroid Ryugu parent body. These organic ions in fluids would have been precipitated nearby in secondary minerals, such as carbonate and phosphate when the aqueous fluid was consumed by the alteration process (Le Guillou et al., 2014). 

The influence of formation and growth of carbonate by the presence organic matter have been investigated in previous experimental studies, for example focused on carboxylic acids (Wada et al., 1999; Wada et al., 2001; Roberts et al., 2013) or alcohol (Dickinson and McGrath 2003). A study on the abiotic synthesis of dolomite at low temperature showed that the carboxyl-group in organic matter can catalyze precipitation through complexation between the carboxyl groups and Mg²⁺ followed by dehydration to make Mg²⁺ available for dolomite precipitation at ~25 ºC (Roberts et al., 2013). Aqueous alteration temperature of Ryugu was reported at ~40 ºC, determined by oxygen-isotope thermometry of coprecipitated dolomite and magnetite (Yokoyama et al., 2022; Yurimoto et al., 2022), and that is consistent with the precipitation of dolomite catalyzed by carboxylic organic matter on Ryugu asteroid during aqueous alteration. Alcohol such as methanol, ethanol, and 1-propanol, which may be detected as CHO compounds in this study, also lead preferential nucleation and growth of calcite (Dickinson and McGrath, 2003), which could have form to dolomite by incorporation of Mg²⁺ from fluid.

The CHO compounds in A0080 were more abundant in lithology 1 than other SOMs. On the other hand, CHN, CHNO, and CHONa compounds were more concentrated in lithology 2 rather than in lithology 1, and the distribution of these compounds mostly overlapped (Fig. 3d). Based on this observation, a possible scenario of the SOM precipitation could be derived by the consumption of fluid when the fluid flow from lithology 2 to lithology 1. First, CHO compounds in aqueous fluid catalyzed dolomite precipitation and some parts of the CHO compounds were precipitated in lithology 1 together with dolomite and Mg²⁺ ions to form dolomite grains (and Fe ions to form magnetite grains) was consumed from the fluid at that time. Sequentially, the fluid moved to lithology 2 and was gradually dried up, then the remaining CHO compounds in addition to CHN, CHONa, and CHNO compounds were also precipitated by fluid consumption. The CHO compounds showed high ion intensities in lithology 1, thus, abundant CHO compounds seemed to have been precipitated during dolomite precipitation. 

c) The fluid activity could produce the heterogeneous distribution of soluble organic compounds (Naraoka and Hashiguchi, 2018; Hashiguchi and Naraoka, 2019b; Potiszil et al., 2020; Muneishi and Naraoka, 2021). Transportation efficiency of SOMs by fluid flow varies with its affinity to for the aqueous phase (Potiszil et al., 2020) and adsorption onto surrounding minerals such as phyllosilicates. This process is like chromatography between the aqueous phase (mobile phase) and minerals (solid phase) (asteroidal chromatography). Geochromatographic phenomena between clay minerals and/or solubility in H₂O to produce fractionation of N-heterocycles have been observed in terrestrial environments (Yamamoto, 1992) and postulated in parent bodies of carbonaceous chondrites (e.g., Wing and Bada 1991). 

The adsorption effect of organic compounds on minerals was previously observed, especially on clay minerals such as phyllosilicates by ion exchange (Bolger, 1983; Hashizume, 2015; Awad et al., 2019). Wada et al. (2001) reported the stronger affinity of carboxylic acid to CaCO₃, which is resulting in inhibition of the CaCO₃ growth. If adsorption effect was the main process that caused the different spatial distribution of SOMs in A0080, the relationship between more specific minerals and organic compounds expected to be observed. However different transportation rates between CHO, CHN, CHNO (CHONa) compounds by the interaction between fluid and surrounding minerals had also been possible (Muneishi and Naraoka, 2021), resulting in the different spatial distribution of these SOM. Further investigation for transportation mechanism of CHO, CHN, CHNO compounds on minerals such as dolomite grains will provide more firm interpretation.

CHO compounds seemed to have affected distribution of SOM on Ryugu by process of b) or c). The CHO compounds were detected from Ryugu as positive ions in this study, on the other hand, carboxylic acids are mainly ionized as negative ions by electrospray ionization (ESI). Therefore, these CHO compounds may be corresponding alcohol or perhaps ether, and heterogeneous SOM distribution in Ryugu would imply interaction of these organic species in fluid with mineral including mineral precipitation and/or adsorption of these organic species onto minerals. 

As discussed above, space irradiation could have affected molecular diversity in extraterrestrial samples. Our result, showing the heterogeneous spatial distribution of SOM in A0080 Ryugu sample, would indicate that spatial distribution of the SOM formed by fluid activity during aqueous alteration has not disappeared by space weathering on asteroid Ryugu Furthermore, relationship between SOM and dolomite, which is a secondary mineral observed in A0080 indicate interaction of organic species and minerals via fluid activity rather than effect of space weathering.  However, heterogeneous space weathering could not be excluded completely as one of the possible mechanisms to explain the SOM spatial distribution. Effect of the space weathering on organic species and its heterogeneity on extraterrestrial samples should be investigated to reveal more detailed mechanism to produce heterogeneous distribution of SOM on Ryugu sample. 

Different spatial distribution of alkyl homologues of SOMs

Alkylated homologues were identified for CHN compounds in A0080 and were previously reported from Murchison (CM2), Murray (CM2), and Yamato 002540 (CR) (Naraoka et al., 2017; Naraoka and Hashiguchi 2018; Hashiguchi and Naraoka, 2019b; Isa et al., 2021). Different mass distribution patterns of SOM (CHN and CHNO) and different distributions of organic compounds extracted from Tagish Lake (C2_ungrouped) fragments vary with different degrees of aqueous alteration (Herd et al., 2011; Isa et al., 2021).

The reaction of aldehydes and ammonia through aldol condensation reaction process was proposed for the formation and growth of alkylated N-heterocycles including alkyl-pyridines in carbonaceous chondrite (Yamashita and Naraoka, 2014; Naraoka et al., 2017). On the other hand, Isa et al. (2021) suggested that the mass distribution of SOM in Tagish Lake sample cannot be explained by such a condensation process. The complex feature of the SOM is supposed to have been obtained before accretion on the parent body followed by simplification on the asteroid due to secondary processes such as aqueous alteration. 

The different spatial distribution of alkyl homologues of SOM in lithology 1 and lithology 2 in A0080 (Fig. 6) have probably been invoked by -CH₂ polymerization or simplification of mass distributions during aqueous alteration. However, the different degrees of aqueous alteration in the two lithologies was indistinct, although mineralogy (abundance of large carbonate, magnetite, and Fe-sulfide grains) was different. Thus, identification the detailed alteration mechanism for -CH₂ polymerization of CHN compounds in A0080 is unclear. 

In Murchison, carbonate-rich fragment 1 was enriched in C_nH_mN compounds with a larger C number, which is a different feature from A0080. Furthermore, a clearly different distribution of C_nH_mN and C_nH_mN₂ compounds implies either 1) different molecular growth of these compounds or 2) different interaction processes with minerals (e.g., carbonate grains). The specific relationships between minerals were not identified, therefore a detailed mechanism for the dislocation of C_nH_mN and C_nH_mN₂ compounds to produce this result cannot be entirely explained. And yet, that distinct feature between Murchison and A0080 is remarkable and implies that the formation or evolution (growth) mechanism for the alkylated homologues of CHN compounds may be different between Murchison and A0080. Carbonate grain might have played as catalyst for CH₂ growth of the CHN compounds, for example carbonate grains adsorbed CHN compounds (e.g., pyridine, imidazole, and pyrrole) onto its surface and play as reaction site for their sequential chemical evolution. Based on our result, such process may have been efficient on Ca carbonate (on parent body of Murchison meteorite). The growth of the CHN compounds by interaction with carbonates, or by fluid activity while carbonate grains have been precipitating from the aqueous fluid, but different characteristics of trend of molecular size of alkylated homologues of CHN vs. carbonate abundance is likely to be attributed from different adsorption effect and/or affinity of dolomite and Ca (-Na) carbonate for organic species, although adsorption effect of N-heterocyclic organic compounds are not clear (e.g., Thomas and Longo, 1993; Wada et al., 2001, Robert et al., 2013).  

A previous study suggested that Murchison meteorite appeared to be formed where the UV photon flux was negligible or it has been accreted and shielded from photolysis in a parent body quickly (Orthous-Daunay et al., 2019). However, some chemical processes during fluid activity appeared to be more likely than the effect of space weathering for a distinct feature of alkylated CHN homologues between Murchison and A0080, because the positive relationship of carbonate abundance of carbonate was observed in both two samples. 

Molecular distribution by ToF-SIMS

Organic-related ions detected by ToF-SIMS showed different abundance among lithology 1 (area C), and lithology 2 (area A) and around those boundaries (area B) (Fig.7). The ion intensities of these several ions from three regions was almost similar, and mostly rich in area C. DESI-HRMS imaging described abundant CHO compounds and poor CHN or CHNO compounds around area C than in area A and area B, which is inconsistent with the occurrence of CNO⁺, CHNO⁺, and CN^– ions by ToF-SIMS analysis. Carboxylate ions (CO₃^–, CO₄^– and probably CO₂^–) appeared to be consistent with the result of DESI-HRMS imaging result, even if they could be derived from CHO compounds or dolomite grains. 

Inconsistency of spatial distribution for organic species between ToF-SIMS data and DESI-HRMS imaging data for spatial distribution of organic species is probably result of different ionization mechanism between ToF-SIMS (spattering by ion beam) and DESI-HRMS imaging (desorption and ionization by charged solvent spray). Although soluble organic matter such as amino acids can be analyzed by ToF-SIMS (Noun et al., 2019), our result may indicate that ions detected from A0080 by ToF-SIMS would be mostly derived from methanol-insoluble organic compounds. Such organic compounds include IOM, which contains several functional groups including C=O and C=C (Yabuta et al., 2022). Further in-situ coordinated analysis using ToF-SIMS is expected to reveal the relationship between SOM, IOM, and minerals in asteroid Ryugu.

Summary And Conclusion

We performed in-situ analysis on a Ryugu sample A0080 using the DESI-HRMS imaging, ToF-SIMS, and mineralogical observation by FE-SEM-EDS, to investigate the relationship of organic compounds to minerals. Our result shows the following:

1) Different spatial distributions of SOM were observed on the surface of Ryugu grain A0080, which are assigned as CHN, CHO, CHONa, and CHNO in A0080 across a few hundred µm. The distribution could be due to different timing of precipitation of these SOM from the aqueous fluid during fluid flow and consumption on the asteroid Ryugu, or to different transportation by aqueous fluid.

2) Heterogeneous distribution among the same alkylated homologues of the SOMs. In particular, the CHN compounds with larger C number appeared to be enriched in lithology including large (> 10 µm) magnetite, Fe-sulfide, and carbonate (mainly dolomite) than in such minerals-poor lithology. On the other hand, the Murchison fragments showed that CHN compounds with larger C number were abundant in the carbonate-rich fragment. This result indicates different formation or growth mechanism for the alkylated homologues of CHN compounds during aqueous alteration between Murchison and A0080 would be different. Such difference seemed to be attributed to the presence of carbonate grains, particularly Ca carbonate, which may have played as catalyst for CH₂ growth of CHN compounds.

3) Spatial distribution of SOM in A0080 revealed by DESI-HRMS imaging was not consistent with the abundance of CN, CNO, or CHNO ions measured by ToF-SIMS, whereas carboxylate-ions detected by ToF-SIMS seemed to be consistent with DESI-HRMS data. According to that result, ToF-SIMS analysis has probably detected molecules mostly from methanol-insoluble organic matter, including IOM, on the surface of A0080.

Ryugu sample showed similar features to CI chondrites in chemical and isotopic composition, and in mineralogy. Analysis of CI chondrite using similar methods as this study is planned to compare with the result from A0080. This will provide more reliable constraints for history of soluble organic compounds in asteroid Ryugu.

Abbreviations

Carbonaceous chondrite (s) (CC (s))

SOM: soluble organic matter

IOM: insoluble organic matter

DESI: Desorption electrospray ionization

HRMS: high-resolution mass spectrometry

ToF-SIMS: time-of-flight secondary ion mass spectrometry

LC: liquid chromatograph (y)

FTICR-MS: Fourier transform ion cyclotron mass spectrometry

u-FTIR micro-Fourier transform infrared microspectroscopy

AFM-IR: atomic force microscope-based infrared spectroscopy

FE-SEM field emission secondary electron microscope

EDS: energy dispersive X-ray spectroscopy

BSE: backscattered electron

NUSS: Nagoya University Equipment Sharing System

Declarations

Availability of data and materials

It is omitted because it is necessary to discuss whether the whole of raw data can be shared and disclosed. 

Competing interests

The authors declare that they have no competing interests.

Funding 

This research is partly supported by the Japan Society for the Promotion of Science (JSPS) under KAKENHI grant numbers JP18K13605 and JP21K03641. 

Authors' contributions

MH contributed to sample preparation, DESI-HRMS analysis, and mineralogical observation by FE-SEM-EDS, 3D-laser microscope observation, data processing and

interpretation, and manuscript preparation. DA operated ToF-SIMS analysis and DA and KF contributed to ToF-SIMS data interpretation. HN contributed to data interpretation for whole data in this study. All authors read and approved the final manuscript. 

Acknowledgement

The Hayabusa2 project has been led by JAXA (Japan Aerospace Exploration Agency) in collaboration with DLR (German Space Center) and CNES (French Space Center) and supported by NASA and ASA (Australian Space Agency). We are grateful to all of the members of the Hayabusa2 project for their technical and scientific contributions.

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