Asteroid Ryugu contains material older than the planets
Ryugu contains some of the most primitive material ever studied on Earth, dating back to just 5 million years after the formation of the Solar System according to analyses of samples retrieved by Japan’s Hayabusa2 mission. Because it’s so old, it’s made of the same material that formed the planets. “Ryugu is one of the building blocks of Earth,” Hisayoshi Yurimoto, a professor at Hokkaido University in Japan, said.
The Japan Aerospace Exploration Agency’s Hayabusa2 launched in 2014 and arrived at Ryugu in 2019. It retrieved two small samples of regolith from the asteroid. These samples then landed on Earth in a capsule equipped with a parachute in December 2020. Upon their return, the samples were distributed among scientific groups, including one team led by Tetsuya Yokoyama, a professor at the Tokyo Institute of Technology. The team’s newly published results suggest that the composition of the samples is the closest match to the solar nebula – the gas cloud that condensed to form the Sun and planets – ever found, made up of the ingredients that formed the Solar System 4.5 billion years ago. The findings support previous research that also concluded that Ryugu was made of primitive material, but until now it wasn’t known just how old it was.
Ryugu is a carbonaceous chondrite, meaning it’s made from carbon-rich stony material. Remote observations by Hayabusa2 found some unexplained discrepancies, including a darker surface colour, a larger abundance of phyllosilicate materials and a more porous composition than expected. Laboratory analysis was required to better understand it’s true nature. Ryugu is somewhat similar to the Ivuna meteorite which fell in Tanzania in 1938 and was loaned to Yokoyama’s team.
Utilising a range of techniques, including electron microscopy, X-ray fluorescence, inductively coupled plasma mass spectrometry and thermal ionisation, the team found that the samples had formed within liquid water at a temperature of about 27 to 47 degrees Celsius (81 to 117 degrees Fahrenheit) roughly 5 million years after the Solar System began to form. With a diameter of 900 metres (3,000 feet), Ryugu is too small to have generated enough heat to melt water ice. Ryugu itself must have originated from a larger parent body that formed just 2 to 4 million years after the birth of the Solar System. At some point after 5 million years, a powerful impact with another asteroid smashed Ryugu’s parent body apart, with some of the fragments forming Ryugu. This idea is supported by the presence of large boulders on Ryugu’s surface, which appear to have originated as debris from a giant impact.
One key question that needs to be answered to fully explain the origin of the planets is where minor bodies such as asteroids and comets, some of which became planetary building blocks, formed. Their compositions suggest that many of these bodies did not form in their current orbits and that in the chaotic early solar system, with its turbulent protoplanetary disc and migrating planets, the minor bodies were pushed around and moved away from where they formed.