Soluble organic molecules in samples of the carbonaceous asteroid (162173) Ryugu
Hiroshi Naraoka, Yoshinori Takano, Jason P. Dworkin, Yasuhiro Oba, Kenji Hamase, Aogu Furusho, Nanako O. Ogawa, Minako Hashiguchi, Kazuhiko Fukushima, […], and Yuichi Tsuda
24 Feb 2023
Vol 379, Issue 6634
Surface material from the near-Earth carbonaceous (C-type) asteroid (162173) Ryugu was collected and brought to Earth by the Hayabusa2 spacecraft. Ryugu is a dark, primitive asteroid containing hydrous minerals that are similar to the most hydrated carbonaceous meteorites. C-type asteroids are common in the asteroid belt and have been proposed as the parent bodies of carbonaceous meteorites. The samples of Ryugu provide an opportunity to investigate organic compounds for comparison with those from carbonaceous meteorites. Unlike meteorites, the Ryugu samples were collected and delivered for study under controlled conditions, reducing terrestrial contamination and the effects of atmospheric entry.
Primitive carbonaceous chondrite meteorites are known to contain a variety of soluble organic molecules (SOMs), including prebiotic molecules such as amino acids. Meteorites might have delivered amino acids and other prebiotic organic molecules to the early Earth and other rocky planets. Organic matter in the Ryugu samples is the product of physical and chemical processes that occurred in the interstellar medium, the protosolar nebula, and/or on the planetesimal that became Ryugu’s parent body. We investigated SOMs in Ryugu samples principally using mass spectrometry coupled with liquid or gas chromatography.
We identified numerous organic molecules in the Ryugu samples. Mass spectroscopy detected hundreds of thousands of ion signals, which we assigned to ~20,000 elementary compositions consisting of carbon, hydrogen, nitrogen, oxygen, and/or sulfur. Fifteen amino acids, including glycine, alanine, and α-aminobutyric acid, were identified. These were present as racemic mixtures (equal right- and left-handed abundances), consistent with an abiotic origin. Aliphatic amines (such as methylamine) and carboxylic acids (such as acetic acid) were also detected, likely retained on Ryugu as organic salts.
The presence of aromatic hydrocarbons, including alkylbenzenes, fluoranthene, and pyrene, implies hydrothermal processing on Ryugu’s parent body and/or presolar synthesis in the interstellar medium. Nitrogen-containing heterocyclic compounds were identified as their alkylated homologs, which could have been synthesized from simple aldehydes and ammonia. In situ analysis of a grain surface showed heterogeneous spatial distribution of alkylated homologs of nitrogen- and/or oxygen-containing compounds.
The wide variety of molecules identified indicates that prolonged chemical processes contributed to the synthesis of soluble organics on Ryugu or its parent body. The highly diverse mixture of SOMs in the samples resembles that seen in some carbonaceous chondrites. However, the SOM concentration in Ryugu is less than that in moderately aqueously altered CM (Mighei-type) chondrites, being more similar to that seen in warm aqueously altered CI (Ivuna-type) chondrites. The chemical diversity with low SOM concentration in Ryugu is consistent with aqueous organic chemistry at modest temperatures on Ryugu’s parent asteroid.
The samples collected from the surface of Ryugu were exposed to the hard vacuum of space, energetic particle irradiation, heating by sunlight, and micrometeoroid impacts, but the SOM is still preserved, likely by being associated with minerals. The presence of prebiotic molecules on the asteroid surface suggests that these molecules can be transported throughout the Solar System.”