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A new analysis of samples collected from asteroid Ryugu has yielded all five canonical nucleobases that make up RNA and DNA.
It's not the first time all five have been isolated from asteroid material – asteroid Bennu also made the grade. But that's what makes this new discovery even more exciting: We're now two for two for a full set of nucleobases on carbonaceous asteroids.
This discovery suggests that the ingredients for life may not be rare in the Solar System after all.
"The detection of diverse nucleobases in asteroid and meteorite materials demonstrates their widespread presence throughout the Solar System," writes a team led by biogeochemist Toshiki Koga of the Japan Agency for Marine-Earth Science and Technology, "and reinforces the hypothesis that carbonaceous asteroids contributed to the prebiotic chemical inventory of early Earth."
The two tiny samples analyzed by the researchers. (JAXA/JAMSTEC)All life on Earth relies upon two basic molecules to store and transfer genetic information: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). In turn, these are made up of five basic molecular building blocks: adenine, cytosine, guanine, thymine, and uracil.
Understanding the abundance of these ingredients on early Earth, and where they could have come from, is crucial to understanding how life emerged in the first place.
Asteroids like Ryugu and Bennu may have played an important role in supplying these ingredients. Carbon-rich asteroids are known to contain a wide range of organic molecules that formed during the early history of the Solar System.
In recent years, two daredevil missions have delivered samples collected right from the surfaces of asteroids into scientists' eager gloved hands: JAXA's Ryugu mission, Hayabusa2, and NASA's Bennu mission, OSIRIS-REx.
The discovery of all five nucleobases on Bennu was announced in January 2025. At the time, however, Ryugu had yielded just one of them: uracil.
Ryugu is a 'rubble pile' asteroid around 1 kilometer (0.6 miles) in diameter. (JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, and AIST)Now, the new research has completed the set. Koga and his colleagues analyzed two separate samples of Ryugu material and identified all five nucleobases in both.
Asteroids aren't the only space rocks in which scientists have found nucleobases. There are two carbon-rich meteorites in particular – rocks that fell to Earth from space – that also fit the bill, Murchison and Orgueil.
To gain a better snapshot of nucleobase content and distribution in the Solar System, the researchers compared the contents of Ryugu with those of Bennu, Murchison, and Orgueil, revealing some interesting differences.
The five nucleobases fall into two families: The purines, which are adenine and guanine; and the pyrimidines, which are cytosine, thymine, and uracil.
Ryugu had roughly equal amounts of purines and pyrimidines. Bennu and Orgueil were richer in pyrimidines, while Murchison was richer in purines.
The researchers found that these differences relate to ammonia levels in the samples, suggesting the chemical environment inside asteroid parent bodies may influence which nucleobases form.
The discovery of thymine is also quite intriguing. The components of DNA and RNA are subtly different. DNA is made up of adenine, cytosine, guanine, and thymine – the source of the famous ACGT acronym. RNA is made up of adenine, cytosine, guanine, and uracil.
One theory of how life formed is the so-called RNA World hypothesis, which suggests that RNA emerged first. Thymine is a chemically modified form of uracil, and uracil is generally considered easier to form in prebiotic chemistry, leading scientists to suppose that uracil was more available for the early Earth chemistry that gave rise to life.
Related: Molecule Vital to Happiness Found in Material From Asteroid Bennu
The previous discovery of only uracil on Ryugu slotted nicely into this hypothesis. The new discovery of thymine suggests that asteroid chemistry can produce both nucleobases, rather than strongly selecting for one or the other.
The results indicate that nucleobase synthesis may be common on carbon-rich bodies in the Solar System, which may have borne the full set of ingredients to Earth via bombardment early in the planet's history.
"The universal detection of all five canonical nucleobases in samples from the carbonaceous asteroids Ryugu and Bennu highlights the potential contribution of these exogenous molecules to the organic inventory that supported prebiotic molecular evolution and ultimately enabled the emergence of RNA and DNA on the early Earth," the researchers write.
The findings have been published in Nature Astronomy.
