The study of prebiotic evolution and the origin of life is a central theme in astrobiology. While terrestrial planets like early Venus, Earth, and Mars are prime candidates for the emergence of life, other celestial bodies, including comets, asteroids, Kuiper Belt Objects, and satellites, hold valuable records of early planetary formation and prebiotic chemistry. These extraterrestrial bodies serve as time capsules, preserving physicochemical organic processes predating the appearance of life on Earth. Understanding the prebiotic chemistry on early planetesimals is crucial, even if it didn't directly lead to biology, as it helps us understand the conditions that either fostered or prevented the emergence of life. Asteroids, the building blocks of planets, are remnants of the early Solar System, recording events from supernovae and interstellar space to the formation of the solar nebula and planetary bodies. Their impacts on early planetary surfaces significantly shaped the environment where life eventually arose. Investigating the hydrothermal and organic physicochemical evolution across interstellar, nebular, and early planetary stages will provide a more comprehensive understanding of prebiotic evolution. The Hayabusa2 mission returned about 5 grams of samples from the carbonaceous asteroid 162173 Ryugu, offering a unique opportunity to study early Solar System organic evolution. Carbonaceous asteroids are the most abundant type in the Solar System, characterized by low albedo and infrared hydration spectral features, suggesting rich water content. Samples from Ryugu's equator, collected from both regolith and deeper crater material, exhibit a CI (Ivuna-type) carbonaceous chondrite composition, rich in water and organic matter. The CI chondrites are petrologic type 1, largely composed of secondary minerals like phyllosilicates, indicating extensive aqueous alteration. The scarcity of CI chondrites among Earth's meteorite collection might be due to their relative fragility hindering atmospheric entry survival. The OSIRIS-REx mission's sample return from asteroid Bennu also shows preliminary indications of similar CI-like material.

Prior to the sample returns from Ryugu and Bennu, carbonaceous chondrites (CCs) provided the best record of early Solar System organic evolution. Studies on soluble organic matter (SOM) and insoluble organic matter (IOM) in bulk meteorite separates and in situ characterizations have significantly contributed to our understanding. CCs have the highest concentrations of both SOM and IOM, with up to 4 wt% total organic carbon (TOC). Hydration in CCs generally correlates positively with TOC abundance. In highly hydrated CCs such as CI, CR, and CM chondrites, micron to submicron organic particles (OPs), consistent with IOM, are ubiquitous in their matrices at nanoscales. IOM is acid-insoluble, macromolecular organic material mostly composed of small aromatic groups with short, highly branched aliphatic moieties. OPs often appear as rounded submicron objects (nanoglobules) or as dendrites/veins. X-ray Absorption Near Edge Structure (XANES) analysis reveals two distinct OP spectral types: aromatic/olefinic (C=C)-carbonyl (C=O)-carboxylic/ester (COOR) ‘3-peak’ OPs characteristic of most CC IOM, and (C=C)-(COOR) ‘2-peak’ OPs, which are aromatic-rich and lack the carbonyl peak. The origin of chondritic IOM is debated, with suggestions including interstellar space, protoplanetary disks, and asteroids themselves. Hydrated silicates in CCs also contain diffuse OM, a widespread, aromatic-poor and carboxylic-rich material found within phyllosilicates and hydrated amorphous silicates. Soluble organic molecules like amino acids are well-documented in CCs, likely residing within hydrated phases like phyllosilicate layers. Diffuse OM is more concentrated in type 1 CCs. Studies using AFM-IR and FT-IR have provided further insight into the functional chemical variations in different phyllosilicate domains.

This study employed a novel approach to minimize mechanical damage during sample preparation for detailed analysis. Grain A0083 (Radegast) from the Hayabusa-2 mission's first TAG site was meticulously prepared in a class 100 cleanroom. A unique mounting method using an Au divot and Al foil secured the grain without adhesives or coatings, allowing for manipulation while preserving integrity. FIB-SEM tomography was performed on a 60x70x25 µm volume to analyze the grain's internal structure. Lamellae were extracted for STXM-TEM analysis of organic matter and surrounding petrofabric. A Ga+ source and Xe PFIB were used for lamella preparation, allowing a comparison of their effects on the organic matter's functional chemistry. The preparation methodology involved precise milling and platinum deposition to minimize beam damage. STXM measurements, using the Photon Factory KEK Beamline BL 19A, generated C, Fe, and O maps, and XANES stacks (280-320 eV) were analyzed to determine the functional chemistry of organic matter. TEM analysis, using a Thermo Fisher Talos F200C, provided high-resolution images and EDX mapping to correlate with STXM data. Ivuna samples were also prepared using ultramicrotomy for comparison. Data analysis included OD image calculations, XANES spectral extraction, and background subtraction.

The study's analysis of grain A0083 revealed a mineralogy consistent with CI chondrites, showing abundant Fe-sulfides, Fe-oxides, carbonates, and phosphates within a phyllosilicate groundmass. FIB-SEM tomography revealed ubiquitous micron to submicron OPs with morphologies consistent with IOM from CCs. A 5-µm coarse OP displayed vermicular shapes, with terminal hollow and compounded nanoglobule morphologies. The rounded and hollow morphologies suggest surface tension-controlled formation. Some OPs were found to completely encapsulate silicate material, likely phyllosilicate. STXM-TEM analysis of lamellae prepared with both Ga+ and Xe FIB sources showed that both methods preserved the functional chemistry of OPs and diffuse OM. The OPs displayed aromatic-rich 3-peak characteristics similar to 'aromatic' particles reported in previous Ryugu studies, while diffuse OM was aromatic-poor and aliphatic/carboxylic-rich. Comparing Ryugu's organic particles with other CCs showed a closer similarity to type 1 CR and CI chondrites. The carbonyl peak absorption in Ryugu and type 1 CCs was significantly lower than in type 2/3 CCs, suggesting an alteration process enriching the aromatic fraction. SEM and TEM revealed coarse and finer domains of phyllosilicate, with diffuse OM present in both but less concentrated in the coarse domains. HRTEM showed the coarse phyllosilicate as mixed layers of 1:1 and 2:1 phyllosilicate. Analysis of additional OPs encapsulating matrix material in A0083 and Ivuna further confirmed this phenomenon.

Ryugu and Bennu, initially classified as Cb-type carbonaceous asteroids, were revealed to have CI chondrite composition. This suggests extensive aqueous alteration, erasing primary chondritic components. Although traces of primary mineralogy were identified, their relatively small size and the lack of evidence for sufficient internal heating suggest they may have undergone alteration on larger parent bodies before becoming rubble piles due to impacts. Hydrothermal alteration on Ryugu led to organic matter compositions more similar to type 1 CCs. Type 1 CCs have higher abundances of aromatic-rich OPs and diffuse OM with higher carboxyl/aromatic ratios than type 2/3 CCs. The high aromatic content in Ryugu and type 1 CCs could be linked to maturation of organic matter, akin to terrestrial shale formation. The presence of OPs encapsulating phyllosilicate suggests that aqueous alteration led to the incorporation of soluble organic molecules into these particles. This implies that significant delivery of biologically relevant molecules, within phyllosilicate and amorphous silicates, occurred to early Earth.

This study provides strong evidence for the aqueous alteration of organic matter on asteroid Ryugu, resulting in the formation of aromatic-rich organic particles and aromatic-poor, carboxylic-rich diffuse organic matter. The discovery of organic particles encapsulating phyllosilicate highlights a novel mechanism for the preservation and delivery of biologically relevant molecules to Earth. The findings challenge existing models of organic matter formation and evolution in the early Solar System. Future research should focus on more detailed characterizations of soluble organic molecules within these hydrated phases and a broader comparative study across various carbonaceous asteroids to establish better constraints on the prevalence and diversity of these processes.

The study focused primarily on a single grain (A0083), limiting the statistical representation of the entire Ryugu sample set. While the sample preparation method minimized damage, potential alterations during FIB milling cannot be entirely ruled out. The interpretation of the organic matter's evolution relies on inferences from current understanding of alteration processes in carbonaceous chondrites, and more sophisticated models accounting for the unique conditions of Ryugu's parent body would improve accuracy.