Asteroid Ryugu provides clues about beginnings of life on Earth

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Samples taken from the near-Earth asteroid 162173 Ryugu continue to provide scientists with important insights, this time about the potential beginnings of life on our planet.

In December 2014, the Japanese Aerospace Exploration Agency (JAXA) launched Hayabusa2 on a return mission to collect samples from the carbon-rich asteroid, Ryugu. After rendezvousing with the asteroid in June 2018, Hayabusa2 surveyed it for 16 months, taking samples.

Hayabusa2 delivered its extraterrestrial payload to Earth in December 2020, a total of 0.2 oz (5.4 g) of pristine samples collected during two touchdown operations on the asteroid. Previous analysis of the samples uncovered the oldest material ever identified. Now, the space rock has given up more of its secrets.

An international team of researchers led by Hokkaido University in Japan analyzed the samples using high-performance liquid chromatography coupled with electrospray ionization high-resolution mass spectrometry (HPLC/ESI-HRMS). This powerful tool allows for molecular-level analysis of complex substances, particularly nucleobases, which is what the researchers were looking for in particular.

A nucleobase is a compound containing nitrogen that may form nucleosides, a component of nucleotides that, in turn, are part of DNA and RNA. The nucleobases in DNA are cytosine, guanine, adenine, and thymine. Uracil is found in RNA, which provides instructions for building living organisms. These five nucleobases are called "canonical" since they are the fundamental units of the genetic code.

"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 Yasuhiro Oba, first author of the study. "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 considered that, if found, nucleobases would provide a better understanding of their role in prebiotic evolution, the stage of evolution that is assumed to have taken place before the emergence of life on Earth.

Analysis of the asteroid samples showed the presence of uracil, in addition to nicotinic acid - otherwise known as niacin, a form of vitamin B3 - and other nitrogen-containing organic compounds.

"We found uracil in the samples in small amounts, in the range of 6 to 32 parts per billion (ppb), while vitamin B3 was more abundant, in the range of 49 to 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 discovered by the researchers are similar, but not identical, to compounds discovered previously in meteorites.

The researchers hypothesize that the nitrogen-containing compounds were likely formed from simpler molecules such as ammonia, formaldehyde, and hydrogen cyanide. While these molecules weren't detected in the Ryugu samples, they do exist in ice found on comets. Researchers think Ryugu may have originated in a low-temperature environment, as a comet or other planetary body. The findings shed more light on the evolutionary history of life on Earth, especially the role that RNA played in it.

"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 research team hopes to compare their results with those obtained from samples collected by NASA's explorer craft OSIRIS-REx, which are due to be delivered to Earth in September 2023. OSIRIS-REx was sent to collect samples from another carbonaceous asteroid, 101955 Bennu, in 2020.

"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," Oba said.

The study was published in the journal Nature Communications.

Source: Hokkaido University