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One-fifth of an ounce of black spot brought to Earth from an asteroid by a Japanese spacecraft is some of the most pristine parts of a baby solar system ever studied, scientists said on Thursday.
This fact should help planetary scientists improve their knowledge of the components in the disk of dust and gas that surrounded the sun about 4.6 billion years ago, before they merged into planets and smaller bodies.
“We have to rewrite the chemistry of the solar system,” said Hisayoshi Yurimoto, a professor of earth and planetary sciences at Hokkaido University in Japan. Article published in the journal Science Thursday.
The Hayabusa2 spacecraft reached Ryugu, a carbon-rich asteroid, in 2018. The mission was led by the Japanese space agency JAXA and studied Ryugu for over a year. including briefly to the surface several times to collect dirt samples from the asteroid, and even using an explosive blowing up a new crater on its surface.
In December 2020, Hayabusa2 flew past Earth again, Leaving a small capsule containing pieces of Ryugu in the Australian outback.
Mission scientists have spent the past year studying what Hayabusa2 brings back. “It’s a pile of rocks, gravel and sand,” said Shogo Tachibana, a planetary scientist at the University of Tokyo and principal investigator for the analysis of the samples. He said the largest piece was about a centimeter, or four-tenths of an inch. Most of the particles were about one millimeter wide.
Dr. Yurimoto’s team only took a small amount from the asteroid – less than one-200th of an ounce.
The biggest surprise from their analysis is that the Ryugu fragments are a close match to a 1.5-pound meteorite that crashed in Tanzania in 1938. The Ivuna meteorite, named after the region where it fell, was a very rare species. Of the more than 1,000 space rocks found on the Earth’s surface, only five are of this type, known as CI chondrite.
(C stands for carbonaceous, meaning containing carbon compounds, and I stands for Ivuna. Chondrite is a stony meteorite.)
“It’s very similar,” said Sara Russell, leader of the planetary materials group at the Natural History Museum in London and a member of the science team on the Hayabusa2 mission alongside the NASA mission. OSIRIS-REX visited a different carbon-rich asteroid, Bennu.. He wrote for the journal Science.
Samples that OSIRIS-REX retrieved from Bennu will return to Earth next year.
Dating the Ryugu samples showed that the material formed about 5.2 million years after the birth of the solar system.
Dr. Carbonaceous chondrites are thought to form in the outer part of the solar system, farther away from the current orbits of most asteroids, Russell said. He described them as “basically deep frozen remnants from the ancient solar system”.
CI meteorites have a structure made up of heavier elements very similar to those measured at the sun’s surface—like the ratios of sodium and sulfur to calcium. Thus, planetary scientists thought they were a good indication of the building blocks that populated the early solar system. This provides important parameters for computer models aimed at understanding how planets form.
The analysis showed that the material was heated early in its history, melting the ice into water, which led to chemical reactions that changed the minerals. But the scientists said the relative amounts of the various elements remained virtually unchanged.
This fits Ryugu’s picture of debris the diameter of a much larger asteroid shaft. (The CI meteorites likely came from the larger parent asteroid, not Ryugu.)
Victoria Hamilton, a scientist at the Southwest Research Institute in Boulder, Colo., who was not involved in the research, said the results were “very significant.” “While we’ve learned a lot about the early solar system from meteorites on Earth, they lack any context.”
In this case, planetary scientists know exactly where the samples came from.
Matching Ryugu’s CI meteorites was unexpected because the CI meteorites contained so much water, and Hayabusa2’s remote measurements while on Ryugu showed some water, but the surface was mostly dry. But co-author of the new Science study, Dr. Laboratory measurements revealed about 7 percent water, Tachibana said. This is a significant amount for such a mineral.
Dr. Tachibana said scientists are working to understand the contradiction.
The scientists also found some differences between the Ryugu specimens and the Ivuna meteorite. The Ivuna meteorite contained even higher amounts of water and minerals known as sulfates not found in Ryugu.
The differences may indicate how the meteorite’s mineralogy has changed over the decades by sitting on Earth, absorbing water from the atmosphere and undergoing chemical reactions. This, in turn, could help scientists figure out what formed as part of the solar system 4.6 billion years ago, and what has changed in CI meteorites over the last few decades on Earth.
Dr. “This shows why it’s important to go on space missions, get out and explore, and really bring back material in a controlled way,” Russell said.
This also raises expectations for OSIRIS-REX’s Bennu samples, which will land in the Utah desert on September 24, 2023. This mission’s principal investigator, Dante Lauretta, chose this asteroid in large part because it looked like it might. CI meteorites and OSIRIS-REX’s measurements at Bennu showed more water than Hayabusa2 observed at Ryugu. But if Ryugu already matches a CI meteorite, this indicates that Bennu may have been made of something different.
Also co-author of Science newspaper, Dr. “Now I’m wondering, ‘What are we bringing back?’” Lauretta said. “It’s somewhat exciting, but also intellectually challenging.”
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