NASA probe brings back samples from asteroid Bennu, discovers diverse amino acids: International research group confirmed the discovery of "Materials responsible for life"

An international research group encompassing Associate Professor Yasuhiro Oba of the Institute of Low Temperature Science, Hokkaido University, Principal Scientist Yoshinori Takano (Project Associate Professor at Keio University), and Postdoctoral Fellow Toshiki Koga of the Japan Agency for Marine-Earth Science and Technology, Associate Professor Yoshihiro Furukawa of the Graduate School of Science, Tohoku University, and Professor Hiroshi Naraoka of the Department of Earth and Planetary Sciences at Kyushu University, and their colleagues successfully detected various organic compounds, including amino acids, nucleobases, carboxylic acids, and amines, in samples returned by NASA from the carbonaceous B-type asteroid Bennu. These organic compounds were produced by reactions in the ammonia solution under a low-temperature environment, providing unprecedented insights into the synthesis of extraterrestrial organic matter. The results were published in Nature Astronomy.

In the asteroid exploration mission, the OSIRIS-REx spacecraft brought back 121.6 g carbonaceous asteroid Bennu sample to Earth on September 24, 2023. This sample was allocated for initial analysis to SOAWG (Sample Organic Analysis Working Group) led by Dr. Daniel Glavin of NASA. The group analyzed the Bennu sample and performed a comprehensive analysis of organic compounds contained in the sample. The Japanese research term analyzed nitrogen heterocyclic compounds, such as nucleobases, contained in the genes of life on Earth. The first results of the analysis have been published.

The approximately 100 mg Bennu sample was divided into smaller portions to analyze for various targets and then assigned to small analytical teams, each studying a specific compound, such as amino acids or carboxylic acids. A total of 25.6 mg sample was heated in a glass ampoule tube with water (1 mL) to extract the water-soluble components, and the dissolved components, such as amino acids, in the extract solution were analyzed via liquid chromatography-ultrahigh-resolution-mass spectrometer. Nucleobases were extracted in a clean room at Kyushu University by heating 17.75 mg sample with 20% hydrochloric acid in a glass ampoule, which were then analyzed in the aforementioned manner after removing inorganic salts from the extract.

A total of 33 amino acids, including 14 contained in proteins associated with life, were detected in the hot water extract. Among these amino acids, many molecules had enantiomers, observing almost equal amounts of right- and left-handed structures (racemic mixture). In particular, the presence of proteinaceous amino acids, such as alanine and aspartic acid, in the racemic mixture indicated that the samples did not contain terrestrial biogenic contamination and did not match the excess of left-handed amino acids seen in terrestrial life. These results suggest that asteroids may have provided Earth with a diverse range of amino acids, further deepening the mystery of the origin of homochirality in extraterrestrial life.

In addition, all five nucleobases (cytosine, uracil, thymine, adenine, and guanine) contained in the genetic DNA and RNA of life were detected in the hydrochloric acid extract. Although these nucleobases were previously detected in carbonaceous meteorites, this is the first time they were detected in an asteroid sample, except for uracil. In addition, the concentration of ammonia, which is an extremely important nitrogen source for synthesizing organic compounds such as amino acids, was extremely higher in the asteroid sample than in extraterrestrial materials analyzed so far. The composition of these nitrogen-rich Bennu samples was in contrast to the sulfur-rich Ryugu samples. Because ammonia is highly volatile and only exists stably in low-temperature environments, the presence of high concentrations of ammonia indicates that organic compound synthesis in the Bennu parent body was dominated by reactions in ammonia solutions at low temperatures.

When and how did life on Earth originate? To solve this mystery, it is necessary to understand what type and how much materials responsible for life existed on the Earth before the origin of life. Understanding this aspect requires the analysis of extraterrestrial materials, such as the Bennu sample, which is a major source of material. Because the Bennu sample brought back under the OSIRIS-REx project is one of the least contaminated extraterrestrial samples ever analyzed on the Earth, a further detailed analysis of organic compounds contained in the Bennu sample will reveal more about the inventory of biological materials. There are also high hopes not only for detailed comparative verification with organic molecules contained in the asteroid Ryugu sample, but also for discovering potential materials responsible for life other than amino acids and nucleobase, including the history of water-mineral-organic interactions due to aqueous alteration.

Currently, large-scale extraterrestrial sample return programs are underway, such as the Japan-led MMX (Martian Moons eXploration) sample return program from the Martian satellite Phobos. The technology developed and knowledge gained via this project will surely contribute to their success.

Journal Information
Publication: Nature Astronomy
Title: Abundant ammonia and nitrogen-rich soluble organic matter in samples from asteroid (101955) Bennu
DOI: 10.1038/s41550-024-02472-9