One-fifth of an ounce of dark specks brought to Earth from an asteroid by a Japanese spacecraft are some of the most pristine bits 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 about 4.6 billion years ago before coalescing into the planets and smaller bodies.
“We must rewrite the chemistry of the solar system,” said Hisayoshi Yurimoto, a professor of Earth and planetary sciences at Hokkaido University in Japan and the head of the research analysis described in a paper published by the journal Science on Thursday.
The Hayabusa2 spacecraft 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. That included briefly descending to the surface a couple of times to pick up samples of dirt from the asteroid and even using an explosive to blast a new crater in its surface.
In December 2020, Hayabusa2 flew past Earth again, dropping off a small capsule containing the bits of Ryugu in the Australian Outback.
The mission scientists spent last year studying what Hayabusa2 had brought back. “It’s a pile of rocks, pebbles and sand,” said Shogo Tachibana, a planetary scientist at the University of Tokyo and the principal investigator in charge of the analysis of the samples. The largest piece was about 4/10 of 1 inch, in size, he said. Many of the particles were about .04 inches wide.
Yurimoto’s team received just a smidgen of the asteroid – less than 1/200th of an ounce.
The biggest surprise from their analysis is that the bits of Ryugu are a close match to a 1.5-pound meteorite that landed in Tanzania in 1938. The Ivuna meteorite, named after the region it fell in, was of a rare type. Of the more than 1,000 space rocks that have been found on Earth’s surface, only five are of the type known as a CI chondrite.
(The “C” stands for carbonaceous, which means containing carbon compounds, and the “I” stands for Ivuna. A chondrite is a stony meteorite.)
“It’s super similar,” said Sara Russell, lead of the planetary materials group at the Natural History Museum in London who was a member of the science team on the Hayabusa2 mission as well as a NASA mission, OSIRIS-Rex, that visited a different carbon-rich asteroid, Bennu. She was an author on the Science paper.
OSIRIS-Rex’s samples from Bennu will arrive back on Earth next year.
Dating of the Ryugu samples indicated that the material formed about 5.2 million years after the birth of the solar system.
Russell said carbonaceous chondrites were thought to have formed in the outer part of the solar system, farther out than the current orbits of most asteroids. She described them as “basically deep frozen relics from the early solar system.”
CI meteorites possess a makeup of heavier elements similar to what is measured at the sun’s surface – like the ratios of sodium and sulfur to calcium. Thus, planetary scientists thought these were a good indication of building blocks that filled the early solar system. That provides key parameters for computer models aiming to understand how the planets formed.
The analysis indicated that the material was heated early in its history, melting ice to water, which led to chemical reactions altering the minerals. But the relative amounts of various elements remained almost unchanged, the scientists said.
That fits in with the picture the Ryugu formed out of the rubble that was knocked off a much larger asteroid miles in diameter. (The CI meteorites probably also came from the larger parent 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 with the research. “Even though we’ve learned a lot about the early solar system from meteorites here on Earth, they lack any kind of context.”
In this case, planetary scientists know exactly where the samples came from.
The match of Ryugu with CI meteorites was unexpected because CI meteorites contain a lot of water, and Hayabusa2’s remote measurements while at Ryugu indicated the presence of some water but that the surface was mostly dry. The laboratory measurements, however, revealed about 7% water, said Tachibana, a co-author of the new Science study. That is a significant amount for such a mineral.
Tachibana said scientists were working on understanding the discrepancy.
The scientists also found some differences between the Ryugu samples and the Ivuna meteorite. The Ivuna meteorite included even higher amounts of water and contained minerals known as sulfates that were absent from Ryugu.
The differences could indicate how the mineralogy of the meteorite changed over decades sitting 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 changed recently in CI meteorites over a few decades on Earth.
“This shows why it’s important to go and have space missions, to go out and explore and bring back material in a really controlled way,” Russell said.
This also raises expectations for OSIRIS-Rex’s Bennu samples, which will land in the Utah desert Sept. 24, 2023. Dante Lauretta, principal investigator of that mission, chose that asteroid in large part because it looked like it could be similar to CI meteorites, and OSIRIS-Rex’s measurements at Bennu indicated more water than what Hayabusa2 observed at Ryugu. But if Ryugu is already a match for a CI meteorite, that suggests Bennu might be made of something different.
“So now I’m wondering, ‘What are we bringing back?’ ” said Lauretta, who was also an author on the Science paper. “It’s kind of exciting, but it’s also intellectually challenging.”
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