A Japanese spacecraft brought less than 6 grams of dark particles from an asteroid to Earth, the most pristine part of the infant solar system ever studied, scientists announced Thursday.
This fact should help planetary scientists refine their understanding of the composition of the disk of dust and gas that circled the Sun some 4.6 billion years ago before it combined with planets and smaller bodies.
“We need to rewrite the chemistry of the solar system,” said Hisayoshi Yurimoto, a professor of Earth and planetary sciences at Hokkaido University in Japan and lead analysis of the study described in a paper published Thursday in the journal Science.
The Hayabusa2 probe arrived at the carbon-rich asteroid Ryugu in 2018. The mission was carried out by Jaxa, the Japanese space agency, and spent more than a year studying Ryugu. This included several brief descents to the surface to collect soil samples from the asteroid, and even using explosives to blast a new crater on its surface.
In December 2020, Hayabusa2 passed Earth again, leaving behind a small capsule containing Ryugu debris inside the Australian desert.
For the past year, mission scientists have been studying what Hayabusa2 brought back. “It’s a pile of rocks, pebbles and sand,” said lead researcher Shogo Tachibana, a planetary scientist at the University of Tokyo who analyzed the samples. The largest piece was about a centimeter long, he said. Many particles are only 1 mm wide.
Yurimoto’s team received a small sample of the asteroid — less than 0.15 grams.
The biggest surprise in his analysis was that the Ryugu fragment resembled a meteorite weighing about 700 grams that fell in Tanzania in 1938. The Ivuna meteorite, named after the fall region, is a rare type. Of the more than 1,000 space rocks found on Earth’s surface, only five are of this type, known as CI chondrites.
(The “C” stands for carbonaceous, meaning it contains carbon compounds, and the “I” stands for Ivuna. Chondrites are stony meteorites.)
“It’s very similar,” said Sarah Russell, head of the Planetary Materials Group at the Natural History Museum in London, who was part of the Hayabusa2 mission and the science team for NASA’s OSIRIS-REX mission, which visited discovered a carbon-rich asteroid, Bennu… She is one of the authors of the work published in the journal Science.
The Osiris-Rex Bennu sample will arrive on Earth next year.
Dating of the Ryugu sample shows that this material formed about 5.2 million years after the birth of the solar system.
Carbonaceous chondrites are thought to have formed in the outer solar system, farther out than most asteroids currently orbit, Russell said. She describes them as “basic frozen remnants of the early solar system”.
CI meteorites have compositions similar to those of heavier elements measured at the surface of the sun, such as sodium and the ratio of sulfur to calcium. So planetary scientists think they are a good indication of the elements in the early solar system. This provides key parameters for computer models aimed at understanding how planets form.
Analysis showed that the material was heated early in its history, melting the ice into water, causing a chemical reaction that changed the minerals. But scientists say the relative amounts of the various elements remain virtually unchanged.
This matches the image formed by Ryugu, which was formed by debris from a much larger asteroid several miles in diameter. (CI meteorites may also have come from larger parent asteroids other than Ryugu.)
The results are “very important,” said Victoria Hamilton, a scientist at the Southwest Research Institute in Boulder, Colorado, who was not involved in the study. “While we learn a lot about the early solar system from meteorites on Earth, they don’t have any context.”
In this case, planetary scientists know exactly where the sample came from.
The combination of Ryugu with the CI meteorite was unexpected because the CI meteorite contained a large amount of water, while the remote measurements of Hayabusa2 on Ryugu indicated the presence of a small amount of water and the surface was almost dry. However, laboratory measurements showed about 7 percent water, said Tachibana, co-author of the new scientific study. That’s a huge amount for this kind of element.
Scientists are working to understand the difference, Tachibana said.
The scientists also found some differences between the samples from the Ryugu and Ivuna meteorites. The Ivuna meteorite contains more water and minerals called sulfates, which are not present in Ryugu.
These differences may indicate how the mineralogy of the meteorite changed over the decades as it landed on Earth, absorbing moisture from the atmosphere and undergoing chemical reactions. This, in turn, could help scientists figure out what the solar system formed 4.6 billion years ago, and what happened to CI meteorites on Earth in recent decades.
“It shows why it’s important to do space missions and explore and bring back materials in a really controlled way,” Russell said.
It also raised expectations for the Bennu sample from Osiris-Rex, which will land in the Utah desert (central US) on September 24, 2023. The mission’s principal investigator, Dante Lauretta, chose the asteroid primarily because it looked like a CI meteorite, while Osiris-Rex measurements at Bennu showed more water than Hayabusa2 observed at Ryugu. But if Ryugu is already comparable to the CI meteorite, it suggests that Bennu may be made of something different.
“So now I ask myself, ‘What are we going to bring back?'” said Lauretta, who also wrote the Science paper. “It’s exciting, but also intellectually challenging.”
Luiz Roberto M. Gonçalves Translator