A mosaic image of asteroid Bennu, composed of 12 PolyCam images collected by the OSIRIS-REx spacecraft from a range of 24 kilometres. (NASA/Goddard/University of Arizona). Credit: NASA/ Goddard/ University of Arizona.
All five nucleobases — the building blocks of DNA and RNA — have been found in samples returned from asteroid Bennu.
Asteroids, small airless bodies within the inner Solar System, are theorised to have contributed water and chemical building blocks of life to Earth billions of years ago.
Although meteorites on Earth come from asteroids, the combination of exposure to moisture in the atmosphere and to an uncontrolled biosphere means that interpreting the data from them is challenging.
Pristine samples collected from asteroids in space would be the ideal candidates, and successful sample collection missions have only been achieved by two countries: Japan (Hayabusa and Hayabusa2) and the United States (OSIRIS-REx).
NASA’s OSIRIS-REx mission returned 121.6 grams of sample from asteroid (101955) Bennu in September 2023 – the largest sample ever returned to Earth.
Now, an international team of OSIRIS-REx sample analysis team scientists, led by Dr Daniel Glavin and Dr Jason Dworkin at the NASA Goddard Space Flight Center, has reported the discovery of ammonia and nitrogen-rich soluble organic matter in these samples.
Among the findings, the Japanese contributors detected all five nitrogenous bases, molecules required for building DNA and RNA, supporting the theory that asteroids could have brought the building blocks of life to Earth.
OREX-800044-101, the sample that was analysed by the Japanese members of the OSIRIS-REx sample analysis team. Credit: Photo provided by Yasuhiro Oba.
Analysis
The Bennu samples from NASA were handled under nitrogen to prevent contamination by Earth’s atmosphere.
A 17.75 mg sample was processed and analysed for N-heterocycles – organic molecules with a ring structure containing carbon and nitrogen – using high-resolution mass spectrometry at Kyushu University.
The analysis revealed that the concentration of N-heterocycles is approximately 5 nmol/g, 5–10 times higher than that reported from Ryugu.
In addition to the five nitrogenous bases—adenine, guanine, cytosine, thymine and uracil—required for building DNA and RNA, the researchers also found xanthine, hypoxanthine, and nicotinic acid (vitamin B3).
Dr Toshiki Koga of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), said: “In previous research, uracil and nicotinic acid were detected in the samples from asteroid Ryugu, but the other four nucleobases were absent.
“The difference in abundance and complexity of N-heterocycles between Bennu and Ryugu could reflect the differences in the environment to which these asteroids have been exposed in space.”
Comparison
Samples from the meteorites Murchison and Orgueil were also processed and analysed previously under identical conditions for comparison.
The research team observed that the ratio of purines (adenine and guanine) to pyrimidines (cytosine, thymine and uracil) was much lower in the Bennu samples compared to both Murchison and Orgueil.
Associate Professor Yasuhiro Oba of Hokkaido University, said: “There are multiple possible reasons for this observed difference.
“They may be due to differences in parent bodies or formation pathways, or the Bennu asteroid was exposed to a cold molecular cloud environment where pyrimidine formation is more likely to occur.”
Kyushu University’s Professor Hiroshi Naraoka added: “Our findings, which contribute to the larger picture painted by all the authors of the paper, indicate that nucleobase chemistry in the Bennu samples must be further studied.”
Another important result of this study is that, by comparing meteorites with Bennu samples, a reference for the reanalysis of other meteorites in collections across the globe has been created.
The findings are published in the journal Nature Astronomy.
