Less than 6 grams of dark motes brought to Earth from an asteroid by a Japanese spacecraft are some of the most pristine pieces of a baby solar system ever studied, scientists announced Thursday.
That fact should help planetary scientists refine their knowledge of the ingredients in the disk of dust and gas that circled the sun some 4.6 billion years ago, before it coalesced with planets and smaller bodies.
“We need to rewrite the chemistry of the solar system,” said Hisayoshi Yurimoto, a professor of terrestrial and planetary science at Hokkaido University in Japan and head of the research analysis described in a paper published in the journal Science on Thursday.
The Hayabusa2 probe arrived at Ryugu, a carbon-rich asteroid, in 2018. The mission was operated by Jaxa, the Japanese space agency, and spent more than a year studying Ryugu. This included briefly descending to the surface a few times to collect soil samples from the asteroid and even using an explosive to blast a new crater on its surface.
In December 2020, Hayabusa2 passed by Earth again, leaving a small capsule with Ryugu’s pieces in Australia’s desert interior.
Mission scientists have spent the last year studying what Hayabusa2 has brought back. “It’s a pile of rocks, pebbles and sand,” said Shogo Tachibana, a planetary scientist at the University of Tokyo and principal investigator responsible for analyzing the samples. The biggest piece was about 1 centimeter long, he said. Many of the particles were only 1 millimeter wide.
Yurimoto’s team received a small sample of the asteroid — less than 0.15 grams.
The biggest surprise of his analysis was that the Ryugu pieces resemble a meteorite weighing approximately 700 grams that fell in Tanzania in 1938. The Ivuna meteorite, named after the region of the fall, was of a rare type. Of the more than 1,000 space rocks that have been found on the Earth’s surface, only five are of this type, known as CI chondrite.
(The “C” stands for carbonaceous, meaning it contains carbon compounds, and the “I” stands for Ivuna. A chondrite is a stony meteorite.)
“It’s super similar,” said Sara Russell, planetary materials group leader at the Natural History Museum in London, who was a member of the science team on the Hayabusa2 mission and the NASA Osiris-Rex mission, which visited a carbon-rich asteroid, Bennu. . She is one of the authors of the work published in Science.
Osiris-Rex Bennu samples will arrive on Earth next year.
The dating of the Ryugu samples indicated that the material formed about 5.2 million years after the birth of the solar system.
Russell said that carbonaceous chondrites were thought to form in the outer part of the solar system, farther away than the current orbits of most asteroids. She described them as “basically frozen relics of the early solar system”.
CI meteorites have a composition of heavier elements similar to what is measured at the surface of the sun — such as the ratios of sodium and sulfur to calcium. So planetary scientists thought they were a good indication of the elements that filled the early solar system. This provides key parameters for computer models that aim to understand how planets formed.
The analysis indicated that the material was heated early in its history, melting ice into water, which caused chemical reactions that altered the minerals. But the relative amounts of various elements remained nearly the same, the scientists said.
This fits the image Ryugu formed from the debris that was knocked off a much larger asteroid, miles in diameter. (CI meteorites probably also came from the larger mother asteroid, not Ryugu.)
The results were “very important,” said Victoria Hamilton, a scientist at the Southwest Research Institute in Boulder, Colorado, who was not involved in the research. “Although we learned a lot about the early solar system from meteorites here on Earth, they don’t have any kind of context.”
In this case, planetary scientists know exactly where the samples came from.
The combination of Ryugu with CI meteorites was unexpected because CI meteorites contain a lot of water, and remote measurements of Hayabusa2 while on Ryugu indicated the presence of little water, the surface was almost dry. Laboratory measurements, however, revealed about 7% water, said Tachibana, a co-author of the new Science study. This is a significant amount for such an element.
Tachibana said scientists are working to understand the discrepancy.
Scientists also found some differences between samples from Ryugu and the Ivuna meteorite. The Ivuna meteorite included even greater amounts of water and minerals known as sulfates that were not present in Ryugu.
The differences could indicate how the meteorite’s mineralogy has changed over decades as it lands on Earth, absorbing water from the atmosphere and undergoing chemical reactions. That, in turn, could help scientists figure out what formed as part of the solar system 4.6 billion years ago and what has recently changed in CI meteorites over the course of a few decades on Earth.
“It shows why it’s important to do space missions and explore and bring back material in a really controlled way,” Russell said.
It also raises expectations about Bennu samples from Osiris-Rex, which will land in the Utah desert (Central US) on September 24, 2023. Dante Lauretta, the principal investigator for this mission, chose this asteroid primarily because it appeared to resemble CI meteorites, and Osiris-Rex measurements at Bennu indicated more water than Hayabusa2 observed at Ryugu. But if Ryugu is already comparable to a CI meteorite, it suggests that Bennu might be made of something different.
“So now I ask myself, ‘What are we bringing back?'” said Lauretta, who also authored the Science paper. “It’s exciting, but also intellectually challenging.”
Translated by Luiz Roberto M. Gonçalves
