Soluble organic matter Molecular atlas of Ryugu reveals cold hydrothermalism on C-type asteroid parent body

This research investigates the soluble organic matter (SOM) in a sample from the near-Earth carbonaceous asteroid (162173) Ryugu, collected by the Hayabusa2 spacecraft. The study aims to characterize the molecular composition of Ryugu's SOM and to understand the processes that shaped its organic inventory. The research is crucial because Ryugu is considered a primitive Solar System body, and its composition can provide insights into the early stages of solar system formation and the potential delivery of prebiotic molecules to early Earth. The analysis of Ryugu's SOM is particularly important for comparing its characteristics with those of carbonaceous chondrites (CCs), meteorites believed to originate from similar parent bodies. Previous studies on CCs have revealed a rich diversity of organic molecules, suggesting a complex history of aqueous alteration and thermal processing on their parent bodies. However, these studies are limited by terrestrial contamination and weathering of the meteorite samples. The direct sampling of Ryugu by Hayabusa2 provides a unique opportunity to study extraterrestrial organic matter without such complications. This study focuses on understanding the processes that created Ryugu's diverse organic inventory, whether these processes occurred during the formation of the asteroid's parent body or during subsequent aqueous alteration. The research will also shed light on the possible presence of prebiotic molecules on Ryugu, informing our understanding of the origins of life on Earth.

Previous research on carbonaceous chondrites has highlighted the chemical diversity of their soluble organic matter (SOM). Studies have used various analytical techniques like mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy to characterize the molecular composition of SOM. These studies have revealed the presence of various organic compounds, including amino acids, polycyclic aromatic hydrocarbons (PAHs), and other heterocyclic molecules. The presence of these molecules indicates a complex history of chemical processing, including aqueous alteration and thermal events. The abundance and types of organic compounds vary depending on the specific type of carbonaceous chondrite, reflecting differences in the thermal and aqueous alteration history of their parent bodies. For example, CI chondrites are known to have experienced extensive aqueous alteration, resulting in a high abundance of polar organic molecules. In contrast, other types of CCs show evidence of higher temperature processing. The detailed analyses of these SOM profiles have led to various hypotheses regarding the formation processes and origin of these organic molecules, including interstellar synthesis, alteration on the parent body, and potential contributions from various sources. However, the lack of pristine samples has hampered a complete understanding of the processes that shaped the organic diversity observed in these meteorites. The study of Ryugu's SOM provides a unique opportunity to test these hypotheses and refine our understanding of the formation and evolution of organic molecules in asteroids.

The Ryugu sample (A0106) was subjected to sequential solvent extraction using solvents of increasing polarity: hexane, dichloromethane, methanol, and water. This approach allowed for the fractionation of SOM based on polarity. The extracts were analyzed using ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR/MS) with multiple ionization techniques: electrospray ionization (ESI) in both positive and negative modes, and atmospheric pressure photoionization (APPI) in positive mode. These different ionization methods enhance the detection of a wider range of molecules. The high mass accuracy of FT-ICR/MS enables the precise determination of elemental compositions, providing insights into the molecular formulas of the detected compounds. In addition to FT-ICR/MS, nuclear magnetic resonance (NMR) spectroscopy was employed to obtain structural information on the organic molecules in the methanol extract. This complementary technique provides insights into the chemical functionalities and connectivity of the detected compounds. The data from FT-ICR/MS were processed using advanced data analysis techniques, including the construction of mass difference networks (MDNs). MDNs help to visualize the structural relationships between the detected molecules, revealing potential pathways of molecular transformation and the presence of homologous series. This comprehensive analytical approach combines the high molecular specificity of FT-ICR/MS with the structural insights of NMR to provide a detailed characterization of Ryugu's SOM. Specifically, the researchers optimized injection flow rates to maximize the signal-to-noise response, fully utilizing the limited sample amount (5 mg). They compared their data to a dataset of 36 carbonaceous chondrites (CCs) to place Ryugu’s organic diversity in context. The selection of 36 CCs was made in order to represent a range of thermal and aqueous alteration histories. Statistical analyses, including Principal Component Analysis (PCA), were performed to compare Ryugu's SOM with that of known carbonaceous chondrites and to understand its formation processes in the context of hydrothermal activity.

The analysis of Ryugu's SOM revealed an exceptionally high molecular diversity, with over 200,000 signals detected at a signal-to-noise ratio (S/N) ≥ 3. This diversity was observed across all solvent extracts and ionization modes, showcasing a wide range of molecular polarities and sizes. The dominant chemical families were N-containing molecules, followed by CHO/Na, CHOS/Na, CH, and CHS compounds. The abundance of nitrogen- and sulfur-containing molecules indicated the involvement of these elements in diverse functional groups, with sulfur being significantly more oxygenated in the polar extracts (indicative of sulfonates), while nitrogen increased in the polar extracts (indicative of amines). Mass difference networks (MDNs) constructed from the FT-ICR/MS data illustrated a structural continuum of complex, kerogen-like organic matter, indicating a complex interplay of chemical processes. The most abundant mass differences corresponded to methylation (CH2), hydration (H2O), hydrogenation (H2), and oxygenation (O), along with various sulfur and nitrogen transformations. The absence of organomagnesium compounds suggests a low-temperature, water-rich environment during the formation of Ryugu's organic matter. ¹H NMR spectroscopy revealed a prevalence of alkyl branching and termination by methyl and carboxylic groups, with lower abundances of other structural features such as aliphatic OCH units, olefins, and aromatics. Notably, a significant abundance of ammonium (NH4) ions was observed. Comparison of Ryugu's SOM with that of Murchison CM2 and Orgueil CI meteorites revealed similarities and differences in molecular compositions. Ryugu's SOM showed a closer resemblance to Orgueil CI in terms of functionalization, but shared similarities with Murchison CM2 in heteroatom abundance. Principal Component Analysis (PCA) of the FT-ICR/MS data revealed that Ryugu's SOM clustered with CCs that experienced low-temperature hydrothermal alteration, demonstrating a high degree of water processing at low temperatures. The identification of homologous chemical series, particularly nitrogen-rich compounds, suggests potential origins from organic ices inherited from the solar system's dense molecular cloud, consistent with the interpretation that the organic diversity of Ryugu's SOM results from a complex interplay of processes, including aqueous alteration, carbonation, and serpentinization.

The findings of this study strongly suggest that Ryugu's SOM underwent extensive low-temperature hydrothermal alteration on its parent body. The high abundance of water-processed molecules like ammonium ions and the lack of organomagnesium compounds support this conclusion. The observed molecular diversity and the presence of homologous series of nitrogen- and sulfur-containing molecules indicate a complex interplay of abiotic chemical processes, possibly including serpentinization and carbonation, that contributed to the formation of Ryugu's complex organic matter. These processes are consistent with those found in terrestrial environments, suggesting that similar chemical processes may have been prevalent in the early solar system. The observed similarities between Ryugu's SOM and those of low-temperature, water-altered meteorites underscore the significance of aqueous alteration in shaping the organic compositions of asteroids. This research contributes significantly to our understanding of the origin and evolution of organic molecules in the early solar system and has implications for the study of prebiotic chemistry and the potential delivery of prebiotic molecules to Earth.

This study provides a detailed molecular atlas of the soluble organic matter (SOM) in the Ryugu asteroid sample A0106. The findings demonstrate an extraordinary level of molecular diversity reflecting a complex history of abiotic synthesis and alteration at low temperatures. The absence of organomagnesium compounds and the presence of numerous water-processed molecules, particularly nitrogen- and sulfur-containing compounds, provide strong evidence for extensive low-temperature hydrothermal alteration. The results indicate that Ryugu's SOM may represent a snapshot of ongoing abiotic processes and a potential source of prebiotic molecules in the early solar system. Further research should focus on analyzing additional Ryugu samples and exploring potential links between the organic molecules identified in Ryugu and the building blocks of life.

The primary limitation of this study is the limited amount of Ryugu sample available for analysis. Although advanced analytical techniques were used to maximize the information extracted, the small sample size might have prevented the detection of some low-abundance molecules. Future studies with larger samples may reveal a more complete picture of Ryugu's SOM. Additionally, while the study provides strong evidence for low-temperature hydrothermal alteration, the exact conditions and timing of these processes remain uncertain. Further research incorporating other datasets and modeling approaches is needed to refine our understanding of the specific environments and processes involved. Finally, the study focuses on soluble organic matter. Future work should investigate insoluble organic matter to gain a more holistic understanding of the organic inventory of Ryugu.