Carbonaceous asteroids, like Ryugu, are remnants from the early Solar System and hold clues to its formation and the origins of organic molecules. The Hayabusa2 mission successfully returned pristine samples from Ryugu, offering a unique opportunity to study these ancient materials. Key questions surrounding carbonaceous asteroids include their role in Solar System history, the origins of light elements (C, N, H, O, S), their isotopic compositions, and how they record primordial organic evolution. Understanding molecular chirality and the interaction between water, organic matter, and minerals are also crucial. This study focuses on the water-soluble organic molecules (SOM) extracted from Ryugu samples to investigate the aqueous alteration processes experienced by the asteroid.

Previous studies of Ryugu samples have revealed diverse organic molecules, including amino acids and N-heterocycles. Naraoka et al. (2023) reported on the organic molecular diversity from initial bulk (IB) to insoluble organic matter (IOM) using sequential extraction. Other research has focused on isotopic compositions of elements, primordial salts, aliphatic hydrocarbons, PAHs, and sub-mm scale spatial imaging of organic matter. These studies provide a foundation for understanding the complexity of Ryugu’s organic inventory and its geological history, informing the current work on water-soluble organic molecules and their significance in aqueous alteration.

Samples A0106 and C0107 (∼10 mg each) from Ryugu were subjected to hot water extraction at 105 °C for 20 h. The resulting water-extractable compounds were analyzed using capillary electrophoresis (CE) coupled with high-resolution mass spectrometry (HRMS). This method enabled the identification and quantification of various hydrophilic molecules, including hydroxy acids, dicarboxylic acids, and N-bearing molecules. Reference standards (Murchison meteorite) were used for identification. Additional analyses included tracing CNHOS contents and their isotopic compositions to the IOM fraction using ultrasensitive nano-EA/IRMS and EA/IRMS methods. Surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS), FTIR, and UV-vis spectroscopy were also employed for complementary analyses. GC/MS was used to analyze hexane extracts from the IOM fraction to identify cyclic sulfur molecules.

The study identified a wide range of water-soluble organic molecules, including low-molecular-weight hydroxy acids (glycolic acid, lactic acid, glyceric acid) and dicarboxylic acids (oxalic acid, succinic acid). The concentrations of these molecules ranged from parts per billion (ppb) to parts per million (ppm). Various structural isomers, including α- and β-hydroxy acids, were observed. Biochemically important intermediates such as pyruvic acid and citric acid were also identified. The presence of malonic acid, and its potential decomposition via keto-enol tautomerism to form acetic acid and CO2, is presented as strong evidence for aqueous alteration. The relative abundance of malonic acid in Ryugu is significantly lower than in CM meteorites, indicating a different aqueous history. Systematics of elemental and organic chemical surveys from two Ryugu sampling locations (TD1 and TD2) revealed similarities in the profiles of hydrophilic molecules, amino acids, amines and inorganic cations/anions, suggesting potential organic homogeneity. Stepwise 15N depletion and 13C depletion during sequential solvent extraction was observed, suggesting isotopic fractionation during the formation of meteoritic organic matter. Comparisons with CI-type (Ivuna) and CM-type (Murchison, Murray) meteorites showed that Ryugu has lower abundances of amino acids, reflecting its different aqueous alteration history.

The presence of various hydroxy acids and dicarboxylic acids, along with the malonic acid signature of aqueous alteration, supports the hypothesis that Ryugu experienced significant aqueous alteration in its history. The similarities in hydrophilic molecule profiles between TD1 and TD2 suggest relatively homogeneous organic distribution across the sampled areas. The isotopic fractionation during solvent extraction indicates complex processes during organic matter formation and alteration. The comparison with CI and CM meteorites highlights the unique aqueous alteration history of Ryugu, suggesting different parent-body conditions. These observations provide valuable insights into the evolution of organic matter in the early Solar System and the role of aqueous alteration in shaping the composition of carbonaceous asteroids.

This study provides compelling evidence for primordial aqueous alteration on asteroid Ryugu, as recorded in its water-soluble organic molecules. The findings expand our understanding of the chemical processes on carbonaceous asteroids and their potential contribution to the delivery of prebiotic molecules to early Earth. Future research should focus on further exploring the relationship between aqueous alteration, organic matter evolution and the isotopic compositions of the Ryugu samples. Comparative analysis with samples from other asteroids (such as Bennu) will be crucial for understanding the diversity of processes that have shaped these bodies.

The study focuses on water-extractable organic molecules and may not fully represent the total organic inventory of Ryugu. The interpretation of malonic acid as a definitive indicator of aqueous alteration relies on the assumed keto-enol tautomerism and subsequent decomposition. Further research is needed to validate this interpretation. The sample size may limit the generalizations made about the broader composition of Ryugu.