Primordial aqueous alteration recorded in water-soluble organic molecules from the carbonaceous asteroid (162173) Ryugu

Abstract

We report primordial aqueous alteration signatures in water-soluble organic molecules from the carbonaceous asteroid (162173) Ryugu by the Hayabusa2 spacecraft of JAXA. Newly identified low-molecular-weight hydroxy acids (HO-R-COOH) and dicarboxylic acids (HOOC-R-COOH), such as glycolic acid, lactic acid, glyceric acid, oxalic acid, and succinic acid, are predominant in samples from the two touchdown locations at Ryugu. The quantitative and qualitative profiles for the hydrophilic molecules between the two sampling locations shows similar trends within the order of ppb (parts per billion) to ppm (parts per million). A wide variety of structural isomers, including α- and β-hydroxy acids, are observed among the hydrophilic molecules. We also identify pyruvic acid and dihydroxy and tricarboxylic acids, which are biochemically important intermediates relevant to molecular evolution, such as the primordial TCA (tricarboxylic acid) cycle. Here, we find evidence that the asteroid Ryugu samples underwent substantial aqueous alteration, as revealed by the presence of malonic acid during keto-enol tautomerism in the dicarboxylic acid profile. The comprehensive data suggest the presence of a series for water-soluble organic molecules in the regolith of Ryugu and evidence of signatures in coevolutionary aqueous alteration between water and organics in this carbonaceous asteroid.

Fig. 1. Profiles of samples obtained from asteroid (162173) Ryugu and various observation photographs from kilometer to micrometer scales.

A Ryugu photograph taken with the Optical Navigation Camera Telescopic (ONC-T). The photo was taken on August 31, 2018. Credit: JAXA, Univ. Tokyo etc. B Thermal image of Ryugu from the thermal infrared imager (TIR). The observation indicates that the lowest temperature in the blue section is estimated to be below −50 °C, whereas the lowest temperature in the red section is estimated to be < 60 °C. Please see the onsite data acquisition and temperature dynamics^,. Credit: JAXA etc. The data were collected on August 31, 2018. C The surface of the asteroid Ryugu and the shadow of the Hayabusa2 spacecraft. The image was taken from ONC-W1 at an altitude of 70 m. Date taken: 21 September 2018. D The 1st touchdown operation on Ryugu with CAM-H imaging on 22 February 2019. The image was captured just before touchdown during descent at an altitude of approximately 4.1 m. Credit: JAXA. E Photograph of initial sample A0106 (38.4 mg) from the asteroid Ryugu during the 1st touchdown sampling^,. A photograph of C0107 (37.5 mg) from the 2nd touchdown sampling is shown in Supplementary Fig. S1. The scale bar represents 1 mm (red line). F Reference photograph of the discolored and altered cross-section of the Ryugu sample showing possible precipitates (e.g., C0041). G The isotopic compositions of C, N, H, and S of the Ryugu aggregate samples for A0106 and C0107 are shown after compilation^,,. The isotopic compositions of δ¹³C (‰ vs. VPDB), δ¹⁵N (‰ vs. Air), δD (‰ vs. VSMOW) and δ³⁴S (‰ vs. VCDT) are expressed as international standard scales. By comparing the classification of carbonaceous meteorites in the Solar System, the compiled data suggested that the Ryugu sample has isotopic characteristics most similar to the petrologic type of CI chondrite^,.

Fig. 2. Representative molecular structures of newly identified from Ryugu aggregate samples (A0106 and C0107).

The hot water-extractable molecular structures include α-hydroxy acids (e.g., glycolic acid, lactic acid, and 2-hydroxybutyric acid), β-hydroxy acids (e.g., glyceric acid, 3-hydroxybutyric acid, mevalonic acid, and hydroxybenzoic acid), dicarboxylic hydroxy acids (e.g., malic acid and citramalic acid), monocarboxylic acids (e.g., valeric acid, 4-oxovaleric acid, 5-oxohexanoic acid, tiglic acid, toluic acid, and cumic acid), dicarboxylic acids (e.g., oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, and maleic acid), tricarboxylic acid (e.g., citric acid), pyruvic acid and other nitrogen-bearing hydrophilic molecules (e.g., urea, methylurea, glycocyamine = guanidinoacetic acid, 6-hydroxynicotinic acid, isovalerylalanine, and dihydroxyindole). Notably, some hydroxy acids and carboxylic acids have chiral centers with left–right symmetry, but those enantiomers are not discussed in the present report. Newly identified cyclic sulfur compounds (S₆, S₇ in this study; Supplementary information) were also noted with the comparison of cyclic S₈ molecule.

Fig. 3. Representative hydrophilic molecular groups in hydroxy acid, dicarboxylic acid, and tricarboxylic acid in hot water extracts from Ryugu samples (A0106 and C0107) and a reference sample (Murchison).

A High-resolution mass electropherogram of capillary electrophoresis during the analysis of hot water extracts (#7-1). The blank was composed of ultrapure water before hot water extraction. Based on the migration time (min) and mass accuracy within ~1 ppm (μg/g) of the theoretical peak (m/z), we assigned each observed peak to the corresponding standard (Fig. S4). B Concentrations of representative hydroxy acids determined in Ryugu aggregate samples. In this graph, dark blue and light blue represent samples A0106 and C0107, respectively. These hydroxy acids and other related hydrophilic molecules from fraction #7-1 (hot water extracts) are in ppb. C The analytical accuracy for the concentration of short-chain α-hydroxy acids (i.e., glycolic acid, lactic acid, 2-hydroxybutyric acid, and 2-hydroxyvaleric acid) extracted from Murchison and Murray meteorites (glycolic acid as 100%) is shown for the same formulation.

Fig. 4. Evidence for aqueous alteration of the asteroid Ryugu revealed by dicarboxylic acids and molecular tautomerism of malonic acid.

A Dicarboxylic acid profiles (i.e., C₂, oxalic acid; C₃, malonic acid; C₄, succinic acid; C₅, glutaric acid; C₆, aspartic acid; C₇, pimelic acid; and C₈, suberic acid) for the Ryugu (A0106 and C0107) and CM types (Murchison and Murray) normalized by oxalic acid as 100%. B Mechanism underlying keto–enol tautomerism of malonic acid (MA), which converts a chemically stable keto form to an unstable enol- form in the aqueous alteration process. The two enol- forms of the unstable MA tautomer are symmetric and in fact identical molecule.

Fig. 5. Standardization to comparatively verify the elemental and hot water-extractable molecular properties of samples from the 1st touchdown site (TD1) and 2nd touchdown site (TD2) at Ryugu.

Notably, the Ryugu sample is of scientific value as a surface (TD1) and subsurface sample (TD2) from the carbonaceous asteroid^,. In this report, we evaluated the hydrophilic organic molecules in surface aggregate (A0106) and subsurface aggregate (C0107) samples as follows. A Light elements in IB samples for total C, N, H, and S and pyrolyzable O in wt%. Compilation after the references^,,. The error arc indicates the standard deviation (1σ). Here, we define IB as whole-rock bulk, which includes all inorganic matrices such as silicates and carbonates, and IOM as the fraction that does not contain silicates. B CNHOS contents in IOM (sample treatment and measurement by the present report) in wt% (Table S3). C Hydroxy acids, carboxylic acids and other newly identified N-bearing hydrophilic molecules in this study obtained from fraction #7-1 (hot water extracts) in ppb. The molecular assignments and raw data profiles are shown in Fig. 3 and Tables S1, S2, respectively. D Amino acids and amines from fraction #7-1 (hot water extraction) in ppb. The data were compiled after the references^,. Please see the error notation in the diagram. E Major inorganic cations and anions from fraction #7-1 (hot water extracts) on the ppm scale. Please see the report for ammonium ion detection (NH₄⁺, ~ 3 ppm; red diamond symbol) and other important molecules associated with organic and inorganic profiles. The error notations in the diagram indicate 2σ after the reference. F Concentrations of urea and alkyl-urea (i.e., methyl-urea, ethyl-urea, and other alkyl ureas up to C₆-urea) were measured in the present study.

Fig. 6. Summary of integrated observations of Ryugu with CI type (Ivuna) and CM type (Murchison and Murray) for aqueous alteration processes throughout their history.

A Hydroxy acids, dicarboxylic and tricarboxylic acids, and other newly identified hydrophilic molecules for comparison between Ryugu (this study) and CM (Murchison and Murray; this study) type at the ppb scale. The Ryugu values on the horizontal axis are shown as the average of A0106 and C0107 (Table S1). B Amino acids for the comparison between CI type (Ivuna) and CM type (Murchison and Murray) based on compilation. Please see the individual molecular information in the diagram with the following review and amino acid profiles for Ivuna and Orgueil (Fig. S14). The error arc indicates the standard deviation (1σ). C Principal component (PC) analysis between Ryugu and CM (Murchison and Murray) regarding hydroxy acids and dicarboxylic and tricarboxylic acids with other hydrophilic molecules based on the panel (A) raw data profile (this study). The PC2 scores between the CM type (Murchison, Murray for sample description) and CI type of Ryugu (A0106 and C0107; Tables S1, S2) are shown, suggesting a different history of indigenous organic molecules.

Fig. 7. Carbon, nitrogen, hydrogen and sulfur abundances and their isotopic profiles before and after the solvent extraction processes from the organic matter facies.

A The ¹⁵N-nitrogen isotopic depletion between the supernatant and IOM residue during sequential solvent extraction for the Ryugu (A0106 and C0107) and Orgueil samples. The pinkish color originates from formic acid extract #9, and the yellowish color originates from HCl extract #10. The other chemical profiles are shown in Figs. S6, S7, and S8. Please also see the residue of IOM (black color) on the bottom of the vial. Unique brownish colloidal-colored fractions (#4 MeOH extract, #5 water extract) were observed for A0106 and C0107 (cf. Figs. S5, S9). B Carbon, nitrogen, hydrogen, and sulfur profiles (wt%) and their isotopic shifts observed from IB, ΣSOM and ΣIOM. The data were compared with previous references^,, and this study. The abundance of carbon (wt%), nitrogen (wt%), hydrogen (wt%) and sulfur (wt%) in IOM increased by one order of magnitude because of the dissolution of silicates and other mineral structures. The error arc indicates the standard deviation (1σ). Note the previous reports regarding volatile components^, and inorganic profiles^,. The isotopic profiles of ΣSOM from the sequential extractions are shown in Table S4. Please see the IOM treatment and C-N-S isotopic variations of the Solar System^,,,.

Fig. 8. Aqueous alteration of primordial hydrophilic organic molecules and minerals during parent body processing of asteroid (162173) Ryugu.

The left panel represents the initial primary mineral assemblage and fluid veins in the early stage of interaction between water, organics, and rock within the bedrock. The right panel represents altered secondary mineral assemblages (i.e., porous and physically fragile), desiccated veins, and precipitates in the late stage and ongoing stage with dehydration processes at Ryugu^,. Within cold hydrothermalism, thermal history in the asteroid^,, and temperature constraints, this figure conceptualizes aqueous alteration, and the sizes of regolith particles and bedrock are arbitrary scales. Amino acids and other hydrophilic molecules with “salt” formation are overviewed in the illustration diagram of chemical evolution. The organic analysis of the asteroid Bennu is a valuable opportunity to consider the scientific consequences of this study.

Fig. 9. Photographs showing representative altered aqueous signatures, desiccated veins, and spatial cross-sections of Ryugu samples.

A Representative photographs of the chamber A sample series and (B) chamber C sample series, showing the cross-sectional areas that are past fluid veins and/or hydrothermally produced precipitates, which are indicated by blue arrows. For a description of the sample properties of organic homogeneity or heterogeneity^,, please see the details in a previous report^,,.