A “potentially hazardous” asteroid contains all the “letters” that make up DNA, suggesting that these key ingredients for life may be common in the solar system.
Scientists made the discovery after analyzing samples collected from the asteroid Ryugu, a 3,000-foot-wide (900-meter) space rock shaped like a spinning top.
The researchers discovered a complete set of canonical nucleobases, which are the building blocks of DNA – the genetic basis of all life on Earth – and its lesser-known cousin RNAaccording to a new study published Monday (March 16) in the journal Natural astronomy.
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This “does not mean that life existed on Ryugu,” study lead author Toshiki Kogaa biogeochemist at the Japan Agency for Marine-Earth Science and Technology, told AFP, per Phys.org. “Instead, their presence indicates that primitive asteroids can produce and preserve molecules important to the chemistry associated with the origin of life.”
This is not the first time an asteroid has been found to carry all five nucleobases. NASA recovered the same set of nucleobases from the asteroid Bennu in 2023, courtesy of OSIRIS-REx spacecraft. Scientists have also discovered nucleobases on meteorites. Taken together, these findings suggest that nucleobases may be widespread in the Solar System.
Cosmic origin of life?
Scientists aren’t sure how life began on earth. Some theories suggest that it originated here, for example in deep sea vents. However, there is also the possibility that life – or the building blocks of life – did not form on Earth at all, but was carried here on comets or asteroids.
César Menor Salvánan astrobiologist at the University of Alcalá in Spain who was not involved in the study stressed in an interview with AFP that the new results “do not suggest that the origin of life took place in space”.
But “with this and the results from Bennu, we have a very clear idea of what organic materials can form under prebiotic conditions anywhere in the universe,” Salván added.
The Japan Aerospace Exploration Agency (JAXA) collected the Ryugu samples as part of its Hayabusa2 mission, which launched in 2014. The unmanned Hayabusa2 spacecraft landed on the asteroid in 2019, before collecting two dust samples from the asteroid’s surface and returning them to Earth in 2020.
The samples weigh just 5.4 grams (0.19 ounces) each, less than the weight of a quarter, but have excited scientists for years. Preliminary analysis of a small fraction of the sample material in 2023 showed that the asteroid contained many of the building blocks for lifeincluding a nucleobase (uracil) and a number of other organic materials, including 15 amino acids, which are the basis of proteins. These are prebiotic molecules, and although they are not life, they are found in all life.
A study also revealed microorganisms crawling over an asteroid Ryugu sample. But these microorganisms closely matched soil bacteria, and their presence was almost certainly the result of contamination after the sample returned to Earth. (Even NASA has had problems keeping soil bacteria away from its interplanetary spacecraft in apparently sterile space).
The closest look yet
For the new study, the researchers did a much more extensive analysis of nucleobases than was done during the preliminary research, using more sample material and optimized analytical techniques.
This time, the researchers found all the nucleobases – adenine, guanine, cytosine, thymine and uracil. These natural compounds mix with ribose and phosphate to form DNA and RNA. The researchers also looked at the ratios of nucleobases and compared them to those discovered on Bennu and on two meteorites (Murchison and Orgueil) that had fallen to Earth.
Nucleobases are divided into two groups based on their chemical structure. Adenine and guanine are purines, which are known for their double ring structure, while cytosine, thymine and uracil belong to the single ring structured pyrimidines.
The researchers found that Ryugu had equal amounts of purines and pyrimidines, while Bennu and Orgueil were more enriched in pyrimidines and Murchison was more enriched in purines. Notably, the researchers also identified a strong correlation between the purine-pyrimidine ratio and the concentrations of ammonia in Ryugu, Bennu and Orgueil, suggesting that ammonia, another life-friendly moleculemay have been a key factor driving similar nucleobase formation pathways in the rocks’ distinct environments, according to the study.
“Because no known formation mechanism predicts such a relationship, this finding may point to a previously unknown pathway for nucleobase formation in early solar system materials,” Koga said.
Ryugu and Bennu are a common type of asteroid known as carbonaceous asteroids, which make up 75% of all asteroids in our solar system. James Webb Space Telescope (JWST) observations suggest so both asteroids may originate from the same parent asteroid which broke apart billions of years ago. The Orgueil meteorite also originates from a carbonaceous asteroid.
These ancient rocks are left over from when the solar system was still forming about 4.5 billion years ago, when the Earth was also formed. The detection of nucleobases therefore suggests that carbonaceous asteroids may have helped Earth obtain its life-forming chemicals.
“The detection of different nucleobases in asteroid and meteorite materials demonstrates their widespread presence throughout the Solar System and reinforces the hypothesis that carbonaceous asteroids contributed to the prebiotic chemical inventory of the early Earth,” the researchers wrote in the study.
