Polycyclic aromatic hydrocarbons in samples of Ryugu formed in the interstellar medium.

Polycyclic aromatic hydrocarbons (PAHs) contain ≲20% of the carbon in the interstellar medium. They are potentially produced in circumstellar environments (at temperatures ≳1000 kelvin), by reactions within cold (~10 kelvin) interstellar clouds, or by processing of carbon-rich dust grains. We report isotopic properties of PAHs extracted from samples of the asteroid Ryugu and the meteorite Murchison. The doubly-13C substituted compositions (Δ2×13C values) of the PAHs naphthalene, fluoranthene, and pyrene are 9 to 51‰ higher than values expected for a stochastic distribution of isotopes. The Δ2×13C values are higher than expected if the PAHs formed in a circumstellar environment, but consistent with formation in the interstellar medium. By contrast, the PAHs phenanthrene and anthracene in Ryugu samples have Δ2×13C values consistent with formation by higher-temperature reactions.

Chemical evolution of primordial salts and organic sulfur molecules in the asteroid 162173 Ryugu.

Samples from the carbonaceous asteroid (162173) Ryugu provide information on the chemical evolution of organic molecules in the early solar system. Here we show the element partitioning of the major component ions by sequential extractions of salts, carbonates, and phyllosilicate-bearing fractions to reveal primordial brine composition of the primitive asteroid. Sodium is the dominant electrolyte of the salt fraction extract. Anions and NH4+ are more abundant in the salt fraction than in the carbonate and phyllosilicate fractions, with molar concentrations in the order SO42- > Cl- > S2O32- > NO3- > NH4+. The salt fraction extracts contain anionic soluble sulfur-bearing species such as Sn-polythionic acids (n < 6), Cn-alkylsulfonates, alkylthiosulfonates, hydroxyalkylsulfonates, and hydroxyalkylthiosulfonates (n < 7). The sulfur-bearing soluble compounds may have driven the molecular evolution of prebiotic organic material transforming simple organic molecules into hydrophilic, amphiphilic, and refractory S allotropes.

Macromolecular organic matter in samples of the asteroid (162173) Ryugu.

Samples of the carbonaceous asteroid (162173) Ryugu were collected and brought to Earth by the Hayabusa2 spacecraft. We investigated the macromolecular organic matter in Ryugu samples and found that it contains aromatic and aliphatic carbon, ketone, and carboxyl functional groups. The spectroscopic features of the organic matter are consistent with those in chemically primitive carbonaceous chondrite meteorites that experienced parent-body aqueous alteration (reactions with liquid water). The morphology of the organic carbon includes nanoglobules and diffuse carbon associated with phyllosilicate and carbonate minerals. Deuterium and/or nitrogen-15 enrichments indicate that the organic matter formed in a cold molecular cloud or the presolar nebula. The diversity of the organic matter indicates variable levels of aqueous alteration on Ryugu's parent body.

Soluble organic molecules in samples of the carbonaceous asteroid (162173) Ryugu.

The Hayabusa2 spacecraft collected samples from the surface of the carbonaceous near-Earth asteroid (162173) Ryugu and brought them to Earth. The samples were expected to contain organic molecules, which record processes that occurred in the early Solar System. We analyzed organic molecules extracted from the Ryugu surface samples. We identified a variety of molecules containing the atoms CHNOS, formed by methylation, hydration, hydroxylation, and sulfurization reactions. Amino acids, aliphatic amines, carboxylic acids, polycyclic aromatic hydrocarbons, and nitrogen-heterocyclic compounds were detected, which had properties consistent with an abiotic origin. These compounds likely arose from an aqueous reaction on Ryugu's parent body and are similar to the organics in Ivuna-type meteorites. These molecules can survive on the surfaces of asteroids and be transported throughout the Solar System.

New Applications of High-Resolution Analytical Methods to Study Trace Organic Compounds in Extraterrestrial Materials.

Organic compounds are present as complex mixtures in extraterrestrial materials including meteorites, which may have played important roles in the origin of life on the primitive Earth. However, the distribution and formation mechanisms of meteoritic organic compounds are not well understood, because conventional analytical methods have limited resolution and sensitivity to resolve their molecular complexity. In this study, advanced instrumental development and analyses are proposed in order to study the trace organic compounds of extraterrestrial materials: (1) a clean room environment to avoid organic contamination during analysis; (2) high-mass-resolution analysis (up to ~150,000 m/m) coupled with high-performance liquid chromatography (HPLC) in order to determine the elemental composition using exact mass for inferring the chemical structure; (3) superior chromatographic separation using a two-dimensional system in order to determine the structural and optical isomers of amino acids; and (4) in situ organic compound analysis and molecular imaging of the sample surface. This approach revealed a higher complexity of organic compounds with a heterogeneous distribution in meteorites. These new methods can be applied to study the chemical evolution of meteoritic organic compounds as well as the molecular occurrence in very-low-mass extraterrestrial materials such as asteroid-returned samples.

In situ organic compound analysis on a meteorite surface by desorption electrospray ionization coupled with an Orbitrap mass spectrometer.

RATIONALE: Since extraterrestrial organic matter in meteorites is a very complex mixture that is hard to ionize due to its association with minerals, in situ analysis of polar organic compounds has never been performed. In addition, when studying powdered samples, spatial information of organic compounds is lost. METHODS: In situ molecular analysis and chemical imaging of polar organic compounds were performed on a meteorite surface by desorption electrospray ionization coupled with high-resolution mass spectrometry (DESI-HRMS) using an Orbitrap mass spectrometer. RESULTS: Many CHN compounds, including alkylated pyridine and imidazole homologues, were identified from the complex peaks by HRMS using a spray of electrically charged methanol with a spatial resolution of approximately 50 µm. The same alkylated homologues have the same spatial distribution in the meteorite matrix, while alkylpyridines occur in a different location from alkylimidazoles. CONCLUSIONS: The compound distribution suggests a different source for each compound series or a chromatographic separation effect associated with fluid movement in the meteorite parent body. The DESI-HRMS imaging will further our understanding of organic compound distribution with respect to mineral and water interactions in meteorites.

Oxygen isotopic compositions of asteroidal materials returned from Itokawa by the Hayabusa mission.

Meteorite studies suggest that each solar system object has a unique oxygen isotopic composition. Chondrites, the most primitive of meteorites, have been believed to be derived from asteroids, but oxygen isotopic compositions of asteroids themselves have not been established. We measured, using secondary ion mass spectrometry, oxygen isotopic compositions of rock particles from asteroid 25143 Itokawa returned by the Hayabusa spacecraft. Compositions of the particles are depleted in (16)O relative to terrestrial materials and indicate that Itokawa, an S-type asteroid, is one of the sources of the LL or L group of equilibrated ordinary chondrites. This is a direct oxygen-isotope link between chondrites and their parent asteroid.