A six-year round trip to an asteroid and back has yielded unprecedented insights into the formation of the solar system. Scientists have conducted the first comprehensive analysis of samples returned from asteroid Ryugu, finding a detailed history starting with the oldest material ever found.
The solar system started out as basically a giant disc of dust and gas, which gradually clumped together to form the Sun, then the planets and moons. The crumbs of this cake were left to drift around in space as asteroids and comets, and as such, they’re basically time capsules that preserve pristine material from the formation of the solar system.
We can study them when they fall to Earth as meteorites, but unfortunately these samples will be contaminated by Earthly processes. The best samples come straight from the source, which have been locked in space largely unchanged for billions of years. And that was the goal of the Hayabusa2 mission, launched by the Japan Aerospace Exploration Agency (JAXA) in 2014.
Hayabusa2 successfully made contact with the asteroid Ryugu in 2019, swooping down and scooping up a sample of rocky material from the surface, and bringing it back to Earth in December 2020. Now, the first comprehensive analysis of this pristine sample has been completed.
The scientists studied 16 particles from Ryugu, and found that it contains the oldest material ever identified, with some grains older than the Sun. While the current asteroid is only about 1 km (0.6 miles) wide, this is just a fragment of a parent body (a planetesimal) that was likely a few dozen kilometers in diameter, which formed in the outer solar system.
Intriguingly, the material showed signs of having been soaked in water in the distant past. Radioisotope dating revealed that Ryugu’s parent body had been altered by water circulation a few million years after the birth of the solar system.
“One must picture an aggregate of ice and dust floating in space, that turned into a giant mudball when ice was melted by nuclear energy from the decay of radioactive elements that were present in the asteroid when it formed,” said Nicolas Dauphas, an author of the study.
Those radioactive elements would have all decayed away after five million years, and the planetesimal would have frozen over again. An impact by another object would have thrown off fragments of this planetesimal, including one that became Ryugu. Eventually, Ryugu was shunted by the gravity of planets into the inner solar system, where its icy layers would have vaporized away, leaving the dry ball of rocks and dust seen today.
The scientists also detected primitive organic material in the sample, including protein-forming amino acids. That lends weight to the idea that the ingredients of life can begin forming in space, before being delivered to Earth by asteroids like Ryugu. The planetesimal’s wet history also adds evidence for the hypothesis that water could have been deposited here the same way.
As insightful as this analysis was, it’s only the beginning. The material examined in this study is just a fragment of the total sample recovered from Ryugu, with some of it being put away into storage for future analysis with more advanced technology. NASA did much the same thing with lunar samples brought back from the Apollo missions that have only recently been opened and studied, 50 years later.
Other missions are currently in the works to bring back pristine samples from another asteroid, Bennu, while the Perseverance rover is stashing samples of Martian rock for an eventual return trip to Earth.
The research was published in the journal Science.
Sources: Okayama University, University of Chicago
