Recent analyses of samples returned from asteroids Ryugu and Bennu have identified key organic molecules essential for the formation of life, including all five nucleobases found in DNA and RNA, and numerous amino acids. These discoveries, stemming from Japan's Hayabusa2 mission to Ryugu and NASA's OSIRIS-REx mission to Bennu, provide insights into the non-biological formation of life's building blocks in space and support the theory that asteroids may have delivered these crucial ingredients to early Earth.
These discoveries, stemming from Japan's Hayabusa2 mission to Ryugu and NASA's OSIRIS-REx mission to Bennu, provide insights into the non-biological formation of life's building blocks in space and support the theory that asteroids may have delivered these crucial ingredients to early Earth.
Nucleobases Discovered on Asteroid Ryugu
Scientists have confirmed the presence of all five nucleobases—adenine, guanine, cytosine, thymine, and uracil—in samples collected from the asteroid Ryugu. These molecules are the fundamental components of DNA and RNA. The findings, building upon a 2023 report that identified uracil, were published by a Japanese research group in Nature Astronomy.
The samples, totaling 5.4 grams of material, were retrieved by the Japan Aerospace Exploration Agency’s (JAXA) Hayabusa2 mission, which returned them to Earth in December 2020 after traveling over 300 million kilometers. Analysis was conducted under ultra-clean laboratory conditions to prevent Earth-based contamination, distinguishing them from typical meteorite samples. The nucleobases were found in similar amounts across the two analyzed samples.
Researchers have noted that the presence of these nucleobases on Ryugu does not indicate the existence of life on the asteroid itself.
Instead, it suggests that primitive asteroids are capable of synthesizing and preserving molecules crucial for the chemistry associated with the origin of life through non-biological pathways.
Beyond nucleobases, the Ryugu samples also contained other organic molecules such as vitamin B3, amino acids, urea, and ethanolamine.
A correlation was observed between the ratios of nucleobases and ammonia concentrations in the samples, leading researchers to suggest potential unknown formation pathways for these molecules in early Solar System materials. Ryugu is a carbonaceous asteroid, considered an ancient remnant from the early Solar System, capable of preserving materials largely unchanged for approximately 4.5 to 4.6 billion years.
Amino Acids Found in Asteroid Bennu Samples
NASA's OSIRIS-REx mission returned samples from the asteroid Bennu to Earth in September 2023. Subsequent analysis identified at least 14 of the 20 amino acids used by life on Earth, alongside 19 other amino acids not found in terrestrial life.
New isotopic analysis of these amino acids from Bennu indicates a colder origin for their formation, occurring in the presence of ice in environments far from the young sun. This finding contrasts with earlier theories that proposed these amino acids formed in warm, watery conditions nearer to the infant sun approximately 4.5 billion years ago. For example, glycine, a common amino acid, exhibited isotopic compositions consistent with formation in a chemically distinct, frozen environment that was still exposed to solar ultraviolet radiation.
The early solar system was characterized by a "snow line," a boundary beyond which water remained frozen. Data from the OSIRIS-REx mission and isotopic analyses suggest that Bennu's parent body likely formed beyond this snow line.
A specific observation from the Bennu amino acid analysis revealed that glutamic acid exists in both left- and right-handed forms in equal proportions within the sample. However, the nitrogen isotope values differed between these two forms, presenting a new area of research, particularly concerning why life on Earth predominantly utilizes left-handed amino acids.
Broader Implications and Comparative Analysis
These discoveries from both Ryugu and Bennu support the hypothesis that prebiotic chemicals, capable of forming non-biologically and remaining stable for billions of years, were widely distributed throughout the solar system.
This widespread presence suggests that such molecular ingredients could have been delivered to early Earth via comet and asteroid impacts approximately 4 billion years ago, contributing to the planet's prebiotic chemical inventory.
Similar organic molecules, including nucleobases and amino acids, have also been found in other extraterrestrial samples such as the Murchison meteorite, which fell in Australia in 1969, and the Orgueil meteorite, which fell in France in 1864.
Comparative analysis of nucleobase concentrations among Ryugu, Bennu, Murchison, and Orgueil samples has revealed variations. Ryugu showed roughly equal amounts of purine nucleobases (adenine, guanine) and pyrimidine nucleobases (cytosine, thymine, uracil). In contrast, the Murchison meteorite was found to be richer in purines, while samples from Orgueil and Bennu exhibited higher concentrations of pyrimidines. These differences may reflect distinct chemical evolutionary paths and origins of their respective parent bodies.
The finding that life's building blocks can form under a wider range of conditions than previously understood enhances the prospects for potentially finding life beyond Earth. Researchers continue to explore these non-biological chemical pathways and their role in the genesis of life on Earth.