Japan Aerospace Exploration Agency

Kyushu University

Ibaraki University

Hokkaido University

Tohoku University

Kyoto University

Hiroshima University

The University of Tokyo

Summary of Presentation

The isotopic compositions of noble gases and nitrogen were measured in surface and subsurface samples from the near-Earth-orbiting asteroid Ryuguu, which was brought back to Earth by the asteroid probe Hayabusa2. It was found that Ryuguu contains noble gases from the formation of the solar system, and that the amount of noble gases is greater than that of any other meteorite reported to date. The nitrogen isotopic composition of each sample was different, indicating that a variety of nitrogen-bearing materials are still preserved in the Ryuguu samples. In addition to primordial gases from the formation of the solar system, there are also rare gases produced by galactic

cosmic ray-produced noble gases and two types of noble gases of solar wind origin were also found in the samples. The solar wind gas in most of the Ryuguu samples was in small quantities. Ten samples from the first touchdown recovery and six samples from the second touchdown recovery were analyzed. Most of the samples did not contain significant amounts of solar wind noble gases, and only two samples contained solar wind equivalent to 3,500 years and 250 years of irradiation in their current orbits, respectively. Since solar wind can only strike the topmost material of a celestial body, this means that these samples have existed in the topmost layer of the celestial body for 3,500 years and 250 years, respectively. The second touchdown sample was recovered from near an artificial crater and is expected to contain subsurface material. The second touchdown sample did not contain much solar wind noble gases, indicating that the subsurface material at a depth of 1-2 m was not well stirred. Based on the amount of galactic cosmic-ray originated neon, the duration of galactic cosmic-ray irradiation of the Ryuguu sample is about 5 million years. Two ages have been proposed for the craters on the surface of Ryuguu: one is assumed to have been created by impacts in near-Earth orbit ( 2 to 8 million years), and the other is assumed to have been created by frequent impacts in the asteroid belt ( 100,000 to 300,000 years). The period of Galactic cosmic ray irradiation obtained from noble gas analysis is consistent with the former age, suggesting that Ryuguu moved from asteroidal orbit to near-Earth orbit, where meteorite impacts on the celestial surface are less frequent, about 5 million years ago (Figure).

Figure: Evolution of the Ryuguu: 1. formation of the Ryuguu parent body and acquisition of pre-solar and primordial gas; 2. water alteration in the Ryuguu parent body (about 4. 56 billion years ago); 3. formation of the Ryuguu by accumulation of parent body debris; 4. migration to near-Earth orbit (about 5 million years ago); 5. reddening by heating (over 1 million years ago); 6. 6. present-day Ryuguu.

When the Ryuguu sample was heated to 100°C in a vacuum apparatus, gas of galactic cosmic-ray origin corresponding to an irradiation period of 1 million years was detected. This means that Ryuguu surface materials have not experienced high temperatures above 100°C for the past million years. Substances that appear red in visible spectroscopy have been found in the mid-latitude region of the Ryuguu surface layer. Research has suggested that the red material may be the result of strong heating due to Ryuguu's temporary proximity to the sun. If the reddening was caused by heating in the vicinity of the sun, it would have occurred more than 1 million years ago (Figure).

The Graduate School of Science and Earthquake Research Institute of The University of Tokyo are participating in this research project. The following members of the Science Department are participating in this research.

Shogo Tachibana	Professor, Department of Earth and Planetary Science, UTokyo Organization for Planetary and Space Science
Akihiro Kano	Professor, Department of Earth and Planetary Science
Seiji Sugita	Professor, Department of Earth and Planetary Science
Tomokatsu Morota	Associate Professor, Department of Earth and Planetary Science
Shizumoe Furuya	Project Specialist, UTokyo Organization for Planetary and Space Science

For more information, please visit the website of the Japan Aerospace Exploration Agency.

Journals

Journal name	Science
Title of paper	Noble gases and nitrogen in samples of asteroid Ryugu record its volatile sources and recent surface evolution
DOI number	10.1126/science.abo0431