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Analysis of samples collected from Ryugu ― New solar system reference materials reveal conventional rare meteorites contaminated on Earth

2022.07.12

Samples from the asteroid Ryugu are likely to have a major impact on space science, astronomy, and even geochemistry as a new reference material for the materials that make up the solar system. The chemical analysis team of the Hayabusa2 Initial Analysis Team (led by Professor Shogo Tachibana, School of Science, The University of Tokyo) revealed that the asteroid Ryugu is composed of Ivuna-type carbonaceous meteorites (CI chondrites). These findings were published in the journal Science. The team from the Institute for Planetary Materials at Okayama University conducting Phase 2 curation (initial description) also revealed that the sample contains 23 amino acids linked to the origin of life (published in the Proceedings of the Japan Academy).

There are still many questions remaining about the Earth, for example, where did the Earth's water come from? Where did the organic matter that makes up life originate? How did the first planetesimals, thought to have first formed in the solar system repeatedly collide, be destroyed, and merge to form planets? It is believed that the C-type asteroid Ryugu still contains water and organic matter from the formation of the solar system dating back some 4.6 billion years.

The asteroid probe Hayabusa2 brought back samples from the asteroid Ryugu in December 2020, and the samples were subjected to basic analysis (initial description) at the Institute of Space and Astronautical Science (ISAS) Phase 1 Curation Facility. An initial analysis team (consisting of six sub-teams) with 384 participants from 134 universities and research institutions in 14 countries was launched in June 2008 in parallel with two teams of the Phase 2 curation.

The chemical analysis team examined Ryugu's chemical composition, isotopic composition, constituent composition, age of the constituents, and relationship to meteorites. They found that the 66 elements that make up Ryugu have the same concentration ratios as the CI chondrites, which are the same as the Ivuna-type carbonaceous meteorites.

Ivuna-type meteorites, of which only nine have been found out of approximately identified 70,000 meteorites, have a chemical composition (excluding hydrogen, noble gases, carbon, nitrogen, and oxygen) similar to that of the entire solar system and are therefore used as the standard material of the solar system. The most recent one was found 50 years ago, making Ryugu the newest and least contaminated.

Ryugu contains about 7% water and 5% carbon, and Ivuna-type meteorites with their 15-20% water content are thought to have been contaminated while on the ground. Ryugu is composed mainly of hydrous clay minerals, with other carbonate minerals, iron sulfides and iron oxides.

These minerals are precipitates formed when the original minerals dissolved in the aqueous solution that occurred on the parent planetesimal that spawned Ryugu. The age at which aqueous alteration occurred, determined by manganese-chromium dating and oxygen isotope thermometry, was about 5 million years after the birth of the solar system, when the temperature was about 40°C and the lower limit of pressure was 0.06 atm.

The parent body was later destroyed, and fine fragments gathered to form the asteroid Ryugu, and some of the water from the clay minerals evaporated into space. "Since aqueous alteration occurred, Ryugu does not seem to have been heated above 100°C," says Professor Tachibana. "Most of the interlayer water contained between the layers of layered silicate has evaporated into space. This is a major difference between the Ryugu sample and Ivuna-type meteorites. The Ryugu sample is a primordial sample that is chemically closer to the average composition of the solar system than any natural sample available to mankind. Although Ivuna-type meteorites have been used as the standard, they were actually contaminated on Earth and may have distorted our understanding of the origin and evolution of the solar system. Moving forward, using Ryugu samples as standards may update the chemical composition of the Sun itself, and this may have a significant impact not only on space science but also on geoscience and other fields."

The Okayama University team performed a comprehensive geochemical analysis using 16 particles from the Ryugu sample. They reached the same conclusions as the chemical analysis team, such as the fact that the particles have the same composition as CI chondrites. However, they had major differences in that they found 23 amino acids and nitrogen-containing heterocyclic compounds and that the carbonate minerals were formed by aqueous alteration about 2.6 million years after the formation of the solar system at a temperature between 0-30℃.

The team estimates that the amino acids and nitrogen-containing heterocyclic compounds formed from a mixture of simple organic molecules originating from interstellar clouds and primordial solar nebulae as the Ryugu's parent body's internal temperature increased and repeated aqueous alteration occurred. Research institutes around the world will now verify the different timing of aqueous alteration from the chemical analysis team.

The prevailing theory about the formation process of Ryugu is that it was re-accumulated after celestial bodies collided with each other, but the Okayama University team has proposed that it may have been created based on a comet nucleus.

Journal Information
Publication: Science
Title: Samples returned from the asteroid Ryugu are similar to Ivuna-type carbonaceous meteorites
DOI: 10.1126/science.abn785

This article has been translated by JST with permission from The Science News Ltd.(https://sci-news.co.jp/). Unauthorized reproduction of the article and photographs is prohibited.

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