SPring-8, the large synchrotron radiation facility

Tomoki Nakamura (Associate Professor) at the Department of Earth and Planetary Material Sciences, Faculty of Science, Tohoku University, and his colleagues, in cooperation with scientists from KEK (Director General, Atsuto Suzuki), analyzed granular samples of the asteroid Itokawa, returned to Earth in a capsule by the asteroid explorer Hayabusa developed by the Japan Aerospace Exploration Agency (JAXA), using a high-resolution electron microscope and synchrotron X-rays at SPring-8. They clarified, for the first time in the world, the material composition of Itokawa and its formation history.

It is considered that the dust constituting the primordial solar nebula accumulated in the early stages of solar system formation to form near-Earth asteroids, such as Itokawa, which were the first small asteroids created in the solar system. Therefore, we can determine the mineral composition of asteroids when the solar system was formed by analyzing samples collected from asteroids. In addition, the history of thermal metamorphism reveals the formation history of asteroids as well as information on the early stages of planet formation. It is considered that most of the meteorites that fall to Earth originate from asteroids, which is also confirmed by the results of this study.

For the 38 granular samples (with diameters in the range of 30-150 µm) recovered from the surface of the asteroid Itokawa by the asteroid explorer Hayabusa, the mineral content and chemistry of the samples were examined in detail using synchrotron radiation X-ray diffraction (Fig. 1) and high-resolution electron microscopy (Fig. 2). The results indicate that the 38 granular samples are pieces of LL chondrites^*2 (Fig. 3). It was found that some samples had been heated to a high temperature inside the parent asteroid, whereas others had not (Fig. 2). From these findings, the scientists concluded the following about the origin and formation process of Itokawa (Fig. 4). It is considered that the parent body of Itokawa was initially approximately 20 km in diameter. The temperature at the core of the parent asteroid reached 800^oC, then slowly decreased. After that, another asteroid struck and smashed Itokawa; the scattered pieces of the original interior reassembled to form the present asteroid Itokawa.

The achievements of this study were obtained through an initial analysis of samples recovered from a near-Earth asteroid for the first time in human history. It is expected that results of the future detailed analysis will help clarify the formation process of small asteroids in the primordial solar system and the metamorphic process of surface materials after asteroid formation.

The results were published online in the special issue "Hayabusa at Asteroid Itokawa" of the American scientific journal Science on 25 August 2011 along with five other papers reporting the results of the analysis of Itokawa granular samples by other methods.

Publication:
"Itokawa Dust Particles: A Direct Link Between S-Type Asteroids and Ordinary Chondrites"
Tomoki Nakamura, Takaaki Noguchi, Masahiko Tanaka, Michael E. Zolensky, Makoto Kimura, Akira Tsuchiyama, Aiko Nakato, Toshihiro Ogami, Hatsumi Ishida, Masayuki Uesugi, Toru Yada, Kei Shirai, Akio Fujimura, Ryuji Okazaki, Scott A. Sandford, Yukihiro Ishibashi, Masanao Abe, Tatsuaki Okada, Munetaka Ueno, Toshifumi Mukai, Makoto Yoshikawa and Junichiro Kawaguchi
Science 333 (6046), 1113-1116 (2011), pulished online 26 August 2011