Nucleobase uracil found in Ryugu asteroid samples

An international team of researchers have analysed samples of the asteroid Ryugu, collected by the Japan Aerospace Exploration Agency’s (JAXA) Hayabusa2 spacecraft, and found uracil — one of the informational units that make up RNA. Nicotinic acid, also known as vitamin B3 or niacin, was also detected in the same samples. Described in the journal Nature Communications, the discovery adds to the evidence that important building blocks for life are created in space and could have been delivered to Earth by meteorites.

“Scientists have previously found nucleobases and vitamins in certain carbon-rich meteorites, but there was always the question of contamination by exposure to the Earth’s environment,” said research leader Associate Professor Yasuhiro Oba, from Hokkaido University. “Since the Hayabusa2 spacecraft collected two samples directly from asteroid Ryugu and delivered them to Earth in sealed capsules, contamination can be ruled out.”

The researchers extracted these molecules by soaking the Ryugu particles in hot water, followed by analyses using liquid chromatography coupled with high-resolution mass spectrometry. This revealed the presence of uracil and nicotinic acid, as well as other nitrogen-containing organic compounds.

“We found uracil in the samples in small amounts, in the range of 6–32 parts per billion (ppb), while vitamin B3 was more abundant, in the range of 49–99 ppb,” Oba said. “Other biological molecules were found in the sample as well, including a selection of amino acids, amines and carboxylic acids, which are found in proteins and metabolism, respectively.” The compounds detected are similar but not identical to those previously discovered in carbon-rich meteorites.

The team hypothesises that the difference in concentrations in the two samples, collected from different locations on Ryugu, is likely due to the exposure to the extreme environments of space. They also hypothesised that the nitrogen-containing compounds were, at least in part, formed from the simpler molecules such as ammonia, formaldehyde and hydrogen cyanide. While these were not detected in the Ryugu samples, they are known to be present in cometary ice — and Ryugu could have originated as a comet or another parent body which had been present in low-temperature environments.

“The discovery of uracil in the samples from Ryugu lends strength to current theories regarding the source of nucleobases in the early Earth,” Oba said. “The OSIRIS-REx mission by NASA will be returning samples from asteroid Bennu this year, and a comparative study of the composition of these asteroids will provide further data to build on these theories.”

Image caption: Asteroid Ryugu photographed from a distance of around 22 km by the ONC-T on 26 June 2018 at around 12.50 JST. Image credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, Aizu University, AIST.