Summary:

"This paper reports the definitive detection of all five canonical nucleobases (adenine, guanine, cytosine, thymine and uracil) in samples returned from the asteroid Ryugu by JAXA’s Hayabusa 2 mission. Nucleobases are biomolecules that are important for life on Earth because they form the building blocks of the information-carrying molecules DNA and RNA. Some of them also combine with sugars and phosphates to form other bioessential molecules such as adenosine triphosphate (ATP), the energy currency of all known life. Nucleobases have also been invoked as being critical in the origin of life, whether through the ‘RNA world’ hypothesis or other
mechanisms.

"While this is not the first time nucleobases have been detected in a meteorite or asteroid, this report is unique because the authors were able to identify a correlation between nucleobase ratios and the concentration of another nitrogen-containing molecule – ammonia – that is consistent across three similar asteroids/meteorites, suggesting a shared chemical pathway. This discovery has important implications for how biologically-important molecules may have originally formed and promoted the genesis of life on Earth. Such meaningful work has only been possible due to the return of multiple pristine samples from primitive asteroids via OSIRIS-REx and Hayabusa 2."

Additional points:

"The authors of this work compared nucleobase levels in Ryugu to previous studies of Bennu (a carbonaceous asteroid sampled by the NASA OSIRIS-REx mission in 2020), as well as the Orgueil and Murchison meteorites. Based on this comparison the authors suggest that the purine/pyrimidine ratio can yield insights into different nucleobase chemistry among different types of asteroids and meteorites. In the asteroid Bennu samples, the pyrimidines (cytosine, thymine and uracil) were found to be more abundant than purines (adenine and guanine). In the Murchison meteorite, the opposite was found – more purines. The authors posit that the lower purine/pyrimidine ratios in the Ryugu, Bennu and Orgueil samples may indicate alternative synthetic pathways to the standard HCN polymerization route, which is known to form purines under prebiotic reaction conditions. Given that Bennu likely experienced more alkaline and ammonia-rich aqueous alteration than Ryugu, analysing both purines and pyrimidines in Ryugu samples is crucial for understanding how nucleobase distributions reflect the chemical history and formation processes in different primitive parent bodies.

"Ryugu is very similar to most CI chondrites in terms of its elemental, chemical and mineralogical properties. CI chondrites are, water-rich, stony meteorites that are a rare form of carbonaceous chondrites. They represent the most chemically primitive meteorites, meaning they likely have a similar elemental composition to that of the early solar system. Hence, by studying CI chondrites, we can learn about what was around during the time life might have been forming on Earth.

"The authors hypothesize that variations in ammonia availability—delivered via accreted ices from an outer Solar System reservoir—combined with differences in reactant abundances, modulated nucleobase synthesis on the parent bodies of Ryugu, Bennu and Orgueil.

"Overall, the universal detection of all five 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."