Organic matter in extraterrestrial water-bearing salt crystals

INTRODUCTION

The study of the chemical and organic compositions of ancient [4.5 billion years old (1–3)] salt crystals in the Monahans and Zag ordinary chondrites provides key information about the raw materials present in the early solar system and clues for how solar system dynamics could have facilitated organic redistribution among various solar system bodies. Direct samples of early solar system fluids are present in these two ordinary chondrite regolith breccias [Monahans (1998) (H5), hereafter referred to as “Monahans,” and Zag (H3-6)], which were found to contain brine-bearing halite (NaCl) and sylvite (KCl) crystals (hereafter collectively called “halite”) that have been added to the regolith of an S-type asteroid following the latter’s thermal metamorphism (Fig. 1) (1, 4). Halite’s typical association with water as an evaporite mineral underscores its importance from the origin and detection of life perspective, in terms of the development of life via offering crystalline surfaces as adsorption sites for catalytic synthesis, concentration, polymerization, and organization of prebiotic molecules (5). Furthermore, inclusions in halite crystals raise the possibility of trapping life and/or biomolecules from the evaporating aqueous phase (6). The brine solutions in Zag and Monahans halite are samples of exogenous liquid water that record primitive aqueous processes on early planetesimals, and the halite hosts of the brines retain clues to the location and timing of the aqueous alteration event and capture an inventory of associated organic species.

Alongside the 1- to 10-μm-sized fluid inclusions in the halite are solid inclusions that comprise organic solids (7) and mineral components that are almost identical to the reported mineralogy of the Ceres regolith, having an affinity to the Mighei-type (CM)/Ivuna-type (CI) chondrites (Fig. 1) (8). Furthermore, interpretation of the Dawn mission data for Ceres also suggests the presence of a mixture of chloride salts and water ice (9). Ceres is a C-type asteroid located in the middle main asteroid belt [semimajor axis (a) = 2.767 astronomical units (AU), inclination (i) = 9.73°, eccentricity (e) = 0.097] (10). Asteroid 6 Hebe, a proposed parent body of H chondrites, is located in the inner asteroid belt (a = 2.426 AU, i = 14.8°, e = 0.203) close to the 3:l motion resonance with Jupiter at 2.50 AU (11), and similarities between the orbits of Hebe and Ceres permit exchange of material between these bodies today and possibly in the past (12). Solar system dynamics elucidate large-scale mixing of C- and S-type asteroidal bodies and facilitated material exchanges between different asteroidal bodies in the early solar system (13). Continuous dynamics influenced by a smaller-scale Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect (14, 15) permitted further fragmentation of smaller materials to ultimately deliver H chondrites to Earth.

Halite easily dissolves under humid conditions; hence, only the Monahans and Zag meteorite fragments that were carefully kept in desiccated environments, such as under dry nitrogen in a laboratory, have preserved abundant blue/purple halite crystals (Fig. 1). The abundance of surviving halite in Monahans and Zag suggests that the S-type asteroid was mostly anhydrous after the capture of the halite crystals, which is supported by the paucity of hydrous mineral phases in H chondrites (4, 16). The host lithologies are H3-6, suggesting significant thermal metamorphism up to 700°C. However, the presence of aqueous fluid inclusions indicates that the halite was formed and maintained at low temperatures (25° to 50°C) during its entire lifetime or else the fluids would have escaped from the halite (1), and thus, thermal metamorphism on the S-type asteroid ceased before the capture of the halite crystals. These halite crystals are the only available direct samples of a hydrovolcanically active, C-type asteroid, which previous studies have proposed to be asteroid 1 Ceres (7, 12). Because the S-type asteroid was unaltered and unheated after the deposition of halite, it preserves a mixture of organic material produced by distinctive synthetic processes on both asteroidal bodies. With the use of two-step laser desorption/laser ionization mass spectrometry (L²MS), Raman spectroscopy, scanning transmission x-ray microscopy (STXM) using x-ray absorption near-edge structure (XANES) spectroscopy, nanoscale secondary ion mass spectrometry (NanoSIMS), and ultra-performance liquid chromatography fluorescence detection and quadrupole time-of-flight hybrid mass spectrometry (UPLC-FD/QToF-MS) techniques, we analyzed the compositions of the organic solids and the amino acid content of millimeter-sized halite crystals hosted in the Monahans and Zag meteorites (<1 volume % of the meteorite) in detail.

RESULTS AND DISCUSSION

A Zag halite crystal was pressed flat onto annealed high-purity gold foils and analyzed with the μ-L²MS instrument located at NASA’s Johnson Space Center (JSC), which was optimized to detect aromatic/conjugated organic molecules at the micrometer scale and the subattomole level (1 amol = 10⁻¹⁸ mol) (17). The μ-L²MS uses separate laser sources to nonthermally desorb molecules from the sample surface as neutral species and to “soft”-ionize selective compounds. Energy in excess of that needed for ionization is transferred to the kinetic energy of the liberated photoelectron, thereby allowing us to detect intact positive molecular ions with virtually no fragmentation (18). Ionization with ~10-eV photons covers essentially all organic compounds because their first ionization potentials lie in the range of 5 to 10 eV. The μ-L²MS spectra show signatures of low-mass C₅ to C₁₀ hydrocarbons at around 70 to 200 atomic mass units (amu; Fig. 2). Each molecular ion in the μ-L²MS spectra can indicate the presence of different isomers or any molecules with the same molecular mass; hence, proper interpretation of the μ-L²MS spectra relies on the elucidation of perceivable structural patterns indicating functionalities. The sequence of peaks separated by 14 amu in the range of 70 to 140 amu due to successive addition of methylene (CH₂) groups indicates alkylated derivatives (19), suggesting the presence of monounsaturated alkenes. The high signal intensity at 112 amu suggests the presence of octene and other compounds, whereas chlorobenzene is a probable candidate, which can be formed by the reaction of oxidants (for example, Cl₂, SO₂) or brine with benzene. The low abundance of benzene (78 amu) indicates their consumption via chemical reactions to form larger polyaromatic hydrocarbons (PAHs), such as chlorobenzene, naphthalene (128 amu), acenaphthene (154 amu), and fluorene (166 amu). The presence of SO₂ (64 amu) is accompanied by a lower abundance of SO (48 amu; Fig. 2), suggesting a significant proportion of sulfur-containing material on the C-type asteroid. Both volatile sulfur and graphitized carbon were shown to be present on Ceres (20). The reaction of sulfur with benzene can produce diphenyl sulfide (186 amu) via synthesis at elevated or low temperatures with the presence of aluminum chloride as catalyst (21). Zag halite organics are composed predominantly of smaller molecules with an absence of larger PAHs that are common in chondritic material [for example, phenanthrene (178 amu) and pyrene (202 amu)] (22, 23).

We directly compared the insoluble organic contents of Zag halite, their solid inclusions, and an associated, carbonaceous, halite-bearing clast in Zag (24) using Raman spectroscopy. Raman spectra collected on the dark clast in the Zag meteorite show peaks around the ~1350 to 1600 cm⁻¹ spectral region, which are typical of the first-order defect (D) and graphite (G) bands of carbonaceous materials (Fig. 3A). Properties of the Raman bands describe the thermally induced crystalline ordering of macromolecular carbon (MMC) (25–27). We used a two-Gaussian peak-fitting model to decompose the peaks so that the results can be comparable to the published data (28).

We performed high-resolution Raman imaging on selected Monahans halite residues using a WITec α-scanning near-field optical microscope (SNOM), which has been customized to incorporate confocal Raman spectroscopy imaging. Although the Raman spectra of the halite crystals are featureless in the first-order spectral region, spectra of abundant micrometer-sized solid inclusions revealed that they consist largely of highly variable organic matter (OM) that includes a mixture of poorly ordered MMC and graphitic carbon. The Monahans MMC shows variability, indicating a complex formation and alteration history (Fig. 3B). Spectra collected from the Monahans halite show multiple populations of MMC. One subset (Fig. 3B, pink circle) shows an affinity with type 3 Vigarano-type (CV3)–like MMC. Other MMC inclusions in the halite residues and Zag matrix (cyan circles) exhibit structural similarity with carbon from CI, Renazzo-type (CR), CM, and/or Ornans-type (CO) chondrites. Several points lie along a line trending between crystalline graphite (Fig. 3B, lower left) and the other MMC that is best explained by partial amorphization of crystalline graphite, probably by shock (28). The Monahans halite incorporates MMC with a varied and complex formation and alteration history. Raman analysis of Monahans halites also revealed two instances of chloromethane dissolved in the halite matrix adjacent to the MMC inclusions (29), probably generated by evolution of methane from the MMC via ultraviolet (UV) photolysis (30) coupled with halogenation and partial dissolution in the halite.

We further analyzed the solid inclusions in halites by dissolving a 2-mm Monahans halite crystal in deionized water and concentrating the residues by filtering the solution through a 1-μm mesh filter membrane. We subsampled the halite residues by the focused ion beam (FIB) technique. Preliminary data from the ~100-nm-thick FIB sections obtained by transmission electron microcopy revealed that these grains include MMC similar in structure to CV3 chondrite matrix carbon, aliphatic carbon compounds, olivine of widely varying composition (Fo_99-59), high- and low-Ca pyroxene, feldspars, phyllosilicates (mainly saponite), magnetite, sulfides, metal, lepidocrocite (rust), carbonates, diamond, apatite, and zeolites (7). We further analyzed newly prepared FIB sections by STXM-XANES to locate C-rich areas and investigate the chemical structure of the carbonaceous material. Figure 4A shows an STXM image of the FIB section. C-rich areas were observed in the corresponding carbon map (Fig. 4B). On the basis of the method provided by Cody et al. (31), the atomic N/C ratio of the C-rich material is 0.076 ± 0.004, which lies between that of the insoluble OM (IOM) in primitive CI, CR, and CM chondrites (~0.04) (32) and organic cometary samples from the Stardust collection (~0.1) (31). The C-XANES spectrum of the C-rich areas (Fig. 4C) showed a peak at 285.0 eV (aromatic carbon) and 286.6 eV [ketone (C=O)], but the aliphatic feature was not present (at ~287.3 to 288.1 eV; Fig. 4D). N-XANES (Fig. 4E) showed a small peak at 398.7 eV [imine (C=N)] and 400.3 eV (protonated imine and/or imidazole). The C-XANES of the residue is dominated by aromatic structures with a feature indicating O-bearing functional groups, which is comparable to the IOM in typical primitive chondrites (CM/CI/CR) (33). There is no 1s-σ* exciton peak (at 291.7 eV) that is indicative of the development of a graphene structure (34), which suggests that most of the OM did not experience temperatures higher than ~200°C, and significant graphitization did not take place. An observation that marries well with the Raman data is shown in Fig. 3. Furthermore, some (but not all) organic nanoglobules found in the Murchison meteorite and cometary (Comet Wild 2) particles are known to be dominated by aromatic structure, but the ketone (C=O) feature at 286.6 eV is less prominent in these materials when compared to the halite residues (33). Hence, the organic structure can be explained by a significant abundance of bridging ketones, which indicates a highly primitive nature for the halite residue organics. The wide range of organic features picked up by Raman spectroscopy and STXM-XANES suggest that the organics hosted in the halites are compositionally diverse.

We used the isotopic compositions of the halite residues in Monahans to interpret the synthetic origin of the halite organic residue. We took isotopic images for C, N, H, and O with the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) NanoSIMS 50L ion microprobe at a spatial resolution of 100 nm (C, O, and N isotopes) and 200 nm (H isotopes; Fig. 5), and the isotopic ratios are listed in Table 1. NanoSIMS elemental images are shown in Fig. 5 (B, C, E, and F), and the O isotopic ratios are plotted on an oxygen three-isotope diagram (Fig. 5I). The NanoSIMS C elemental map shares similar features with the STXM-XANES data. The C-rich area (Fig. 5B) is depleted in ¹³C[δ¹³C = −37.6 per mil (‰)] and moderately enriched in ¹⁵N (δ¹⁵N = +164.5‰). These isotopic characteristics are broadly consistent with those of the IOM in unweathered CR chondrites and unequilibrated meteorites (32), which show typical enrichments in ¹⁵N that likely reflect sources of interstellar ¹⁵N such as ammonia (35) and not terrestrial contamination. High δ¹⁵N values suggest the presence of organic N compounds such as hydrocarbons and amino acids that are hosts to heavy N (for example, δ¹⁵N of polar hydrocarbons = +102‰ and δ¹⁵N of amino acids = +94‰) (36). The δD in the C-rich area shows a terrestrial value (+42.5 ± 54.3‰). The low δD values contrast with the high δD values of chondritic IOM [for example, ~600 to 1000‰ in CIs and CMs and ~3000‰ in CRs (32)]. The water on C-type parent bodies is typically D-poor (37); therefore, the OM synthesized in and/or processed by the D-poor water on Ceres would also be D-depleted. Assuming that Ceres had a larger water fraction than CI/CM, the associated OM would have low δD values provided that the time was sufficient for D/H exchange between OM and the D-poor water during a prolonged aqueous event. Alternatively, the low δD values might have been contributed by terrestrial water incorporated during the extraction of the halite residues in the laboratory. However, because halites rapidly dissolve in water, the organic residues were only in contact with the water for a few seconds; thus, this contamination is unlikely.

The Dawn spacecraft at Ceres reported extensive surficial carbonates accompanied by phyllosilicates and ammonium-bearing species, requiring substantial aqueous alteration (38). The organic analyses undertaken in this study indicate the presence of a wide range of highly primitive organic compounds with ketone features possibly contributed by organic solids originating from formaldehyde. IOM (or MMC) and amino acids can be synthesized from formaldehyde, glycolaldehyde, and ammonia under hydrous conditions at temperatures as low as 90°C (39). Kinetic experiments predict that organic solids could be synthesized on the order of 100 to 10⁴ years even at temperatures as low as 0°C (40). We envision that similar organic synthetic processes could have occurred on Ceres that synthesized organic solids and other crucial biomolecules including amino acids. To investigate the amino acid composition of the halite, we carefully opened a pristinely preserved Zag meteorite stone (~500 g) in a class 10 cleanroom, subsampled the newly revealed halite crystals (~3 mg; Fig. 1D) with handling tools that had been heated in air at 500°C for 24 hours, and studied the amino acid content of the halite and compared this to that of the matrix of the Zag meteorite using the UPLC-FD/QToF-MS technique.

The Zag meteorite is a matrix-supported regolith breccia composed of H3-4 matrix, H4-5 light-colored metamorphic lithologies, H5-6 silicate-darkened clasts, impact-melt clasts (4), and carbonaceous (CI-like) clasts (24, 41). The matrix is shocked [stage S3, weakly shocked up to 15 GPa (4, 42)] and thermally metamorphosed to 600° to 950°C (16), all predating incorporation of the halite and C chondrite clasts. Although amino acids (for example, isovaline) have been synthesized in the laboratory under simulated ice/rock impact conditions (43), the low shock level would not have provided sufficient energy for shock-driven organic reactions to occur. The total amino acid distribution and abundance of the matrix [~1940 parts per billion (ppb); table S1] are comparable to those of other ordinary chondrites (60 to 3330 ppb; figs. S1 and S2) (44, 45) and include amino acids (for example, glycine, α-alanine, and β-alanine; Fig. 6) that are typical products of the mineral-catalyzed Fischer-Tropsch–type (FTT)/Haber-Bosch–type gas-grain reactions at elevated temperatures (150° to 700°C) in the presence of CO, H₂, and NH₃ gases and mineral catalysts (46–48). The high enantiomeric ratio [d/l ≈ 0.95; that is, small l-enantiomeric excesses (l_ee) = −9.77 to 2.31%] of alanine suggest that it is indigenous to the meteorite (table S2). Racemic alanine accompanied by large l_ee for glutamic acid have been reported for meteorites exhibiting high degrees of aqueous alteration due to the differences in their solid-solution phase behaviors (49). These amino acids could have been synthesized on the S-type asteroid before the material exchange between the C- and S-type parent bodies because the latter was maintained at low temperatures (≤50°C) after the deposition of halite crystals, and hence, the physical conditions would then be unfavorable for FTT reactions to occur.

The abundances of the small straight-chain, amine-terminal (n-ω-amino) amino acids [for example, γ-amino-n-butyric acid (γ-ABA) and ε-amino-n-caproic acid (EACA)] in the Zag matrix are notably lower than those in the thermally altered meteorites (48, 50, 51). Whereas the Zag matrix is γ-ABA– and EACA-deficient, the halite is shown to exhibit an opposite trend and is enriched in γ-ABA and EACA (Fig. 6 and table S1). The total amino acid concentration in halite (~510 ppb) is also significantly lower than that in the Zag matrix. The marked difference in the amino acid contents between the halite and matrix indicates their separate synthetic origins (Fig. 6). This agrees with the Raman imaging and XANES analyses in this study, which indicate the presence of organic-rich solid inclusions that are composed of C chondrite–like MMC and other organic compounds with aromatic and ketone structures, which are incongruent with the organic content of the thermally metamorphosed H-type lithology. Also, the halite crystals are hosted as discrete grains (no reaction rims between the halite and surrounding silicate) within an H-chondrite matrix, and their mineral inclusions are incompatible with H chondrites (16). The continued presence of fluid inclusions in the halite is further evidence that the incorporation of the halites into the H chondrite postdates the metamorphic epoch (1, 2). These observations support the hypothesis that the halite is derived from an exogenic source, possibly a hydrovolcanically active C-type parent body (7, 52). Our coordinated organic analyses converge into the same conclusion that halite in Zag and Monahans is host to a wide range of OM, which probably facilitated amino acid synthesis through aqueous alteration on the C-type parent asteroid. The disparities in the amino acid contents between halite in H chondrites and matrix account for material mixing between the two asteroidal bodies and explain the complex suite of meteoritic organic compositions that can only be contributed by distinctive reaction pathways.

We propose the following sequence of events for Zag and Monahans and the halite. The halite originated from hydrovolcanism on a C-type parent body (probably asteroid 1 Ceres). IOM and amino acids were produced by aqueous alteration in the presence of formaldehyde and ammonia (39). The halite formed as surficial evaporate deposits at the end of the hydrothermal activity, approximately 4.5 billion years old (1–3), trapping brines as well as organic and inorganic solids and other soluble organic compounds. Subsequent large-scale hydrovolcanism had sufficient escape velocity (53) to expel surficial halite and some inorganic stones into space. It was probably during surface exposure or inter-asteroidal transit that the halite gained the blue-purple coloration from electrons trapped in anion vacancies through exposure to ionizing radiation, at which time some trapped carbon generated UV-photolysis–derived chloromethane. The halites and other stones were then deposited into the regolith of an S-type parent asteroid (possibly Hebe), which had already experienced thermal metamorphism that yielded its own distinctive suite of amino acids. Regolith evolution buried the halite and associated stones (the latter becoming the CI-like clasts found in Zag and other H chondrites), incorporating them into a fine-grained matrix. The meteorite was later stripped from the S-type parent body via a gentle process, possibly by the YORP effect (14, 15), and was eventually delivered to Earth.

CONCLUSION

The halite crystals and the organics contained within them provide a unique window into the early history of astromaterials and their mobilization across the early solar system. This model describes the brecciated nature of chondrites and elucidates the complex suite of organics, which could only be synthesized through individual processes with different physicochemical conditions. The extensive variety of organics hosted in the halite suggests that the original parent asteroid, where the halite precipitated, plausibly Ceres, contains a combination of precursor molecules for complex chemical reactions to occur. Furthermore, in the context of understanding moons such as Enceladus and Europa, halite crystals formed from cryovolcanism and ejected into space represent an ideal sample to study prebiotic and possibly biotic processes on these bodies.

MATERIALS AND METHODS

Supplementary Material