SPring-8, the large synchrotron radiation facility

Release Date

26 Aug, 2011

Osaka University

<<Key research findings>>
• It was demonstrated that the materials constituting the asteroid Itokawa are similar to LL5 or LL6 chondrite materials.
• Particle samples had the same features as the regolith of Itokawa, which has only a low gravity.
• The above findings revealed the active state of the asteroid surface.

<<Background>>
The explorer Hayabusa, launched by Japan Aerospace Exploration Agency (JAXA) in May 2003, arrived at the asteroid Itokawa in September 2005. Hayabusa carried out remote-sensing observation over approximately two months and collected samples from a smooth terrain of the regolith (fine sand particles formed by the impact of meteorites) on Itokawa called MUSES-C Regio. Hayabusa returned to Earth in June 2010 and the collected samples were subjected to curation processes by JAXA, including the collection of particles from the sample capsule. In these processes, at least 1,534 particles with diameters of 200-300 µm or less were confirmed to be present in the capsule, although most particles had a diameter of 10 µm or less. In January 2011, the full-scale initial analysis of the samples was started.

The extraterrestrial samples collected in the Hayabusa project are the first asteroid materials to be collected and brought back to Earth and also the second regolith samples following the lunar regolith samples collected in the US Apollo program and the former Soviet Lunar program. According to the analysis of their trajectories as they fall to Earth, most meteorites are considered to originate from asteroids. In addition, a comparison of the reflection spectra of asteroid materials obtained by astronomical observation and those of meteorites measured in laboratories has led to speculation on the association between various asteroid materials and meteorites. Not only terrestrial microscopic observation but also the close-up observation by the explorer Hayabusa has indicated that the materials of the asteroid Itokawa, which exhibit an S(IV)-type reflection spectrum, are similar to LL5 or LL6 ordinary chondrites, the truth of which will be determined by the analysis of the samples. Moreover, the close-up observation by the explorer Hayabusa indicated that the surface of the small asteroid Itokawa, having the dimensions of 535 m × 294 m × 209 m, consists of a smooth terrain of millimeter- or centimeter-size regolith particles and a terrain covered with boulders with a length of about 10 m. However, the origin and evolution of the regolith on the Itokawa surface remains unclear. It is important to determine the association between the millimeter-size or smaller particles collected from the smooth terrain of Itokawa and the observation results and to obtain clues for clarifying the origin and evolution of the regolith.

Meanwhile, the mean density of Itokawa determined by remote-sensing observation is approximately 1.9 g/cm³, which is lower than that of LL chondrites (3.54 g/cm³), with measurments indicating that 40% of the interior of Itokawa consists of voids. From the presence of these internal voids and the above-mentioned boulders, a rubble pile model has been proposed in which the asteroid Itokawa was originally larger than its present size, but collided with another asteroid which smashed the original Itokawa. Then some of the scattered pieces reassembled to form the present Itokawa. However, the densities of the materials constituting Itokawa have not yet been measured.

<<Contents of Research>>
As the first research group carrying out the initial analysis of Itokawa samples, scientists of Osaka University nondestructively examined the three-dimensional (3D) structure of the Itokawa particles by synchrotron radiation X-ray microcomputed tomography (microCT) (Fig. 1). They determined, with a spatial resolution of approximately 0.2 or 0.5 µm, the 3D external and internal structures of 40 particle samples (30-180 µm in diameter) using a projection-type CT system installed at the hard X-ray photoelectron spectroscopy and microCT beamline (BL47XU) at SPring-8 (Fig. 2). In addition, CT images were obtained with two different X-ray energies, 7 and 8 keV, enabling the identification of minerals, which was impossible with conventional techniques, and the obtainment of the 3D distribution of each mineral (Fig. 3).

The obtained composition ratio of minerals in the 40 particles was similar to that in LL chondrites (Fig. 4), more specifically, LL5 or LL6 chondrites subjected to thermal metamorphism, on the basis of the internal structure (rock texture) of the samples determined in this study. This result was in agreement with the results of the analyses of the same particle samples by other research groups: a chemical composition analysis of minerals by a research group led by Tomoki Nakamura, an associate professor of Tohoku University, and an oxygen isotope analysis by a research group led by Hisayoshi Yurimoto, a professor of Hokkaido University. As some particles had an internal structure that had not been subjected to strong thermal metamorphism, particles with different degrees of thermal metamorphism coexisted in the samples, indicating that the particles might have originally been breccias. Because chemical composition analysis enables the calculation of mineral density, the bulk density of the entire set of particles was calculated from the mineral composition ratio obtained by CT and the percentage of voids and was determined to be 3.4 g/cm³. If the materials obtained from the Itokawa regolith are assumed to be representative of the entire asteroid, the rubble pile model would be valid.

CT images provided quantitative information on the 3D exterior of the samples. The scientists calculated the diameter of each particle from its volume and obtained the distribution of particle diameter. When the cumulative number of particles was plotted logarithmically against particle diameter, the log slope was approximately -2, which is smaller than that obtained for boulders on Itokawa (5-30 m in diameter), i.e., -3.1. Assuming that, on the Itokawa surface, particles greater and smaller than millimeter or centimeter size have log slopes of approximately -3 and -2, respectively, the above result is in agreement with the observation that the smooth terrain of the Itokawa surface mainly consists of millimeter- or centimeter-size regolith particles. The assumption is also reasonable considering the fact that the log slope for lunar regolith particles (20-500 µm in diameter) is approximately -3 and that fine regolith particles are distributed on the lunar surface. The small number of particles of millimeter or smaller diameter on the Itokawa surface may be because small particles with a high release rate were selectively lost from its weak gravitational field upon the impact of other asteroids, as well as because such small particles electrostatically floated and were selectively lost. It may also be because moderately large particles selectively assembled on the Itokawa surface as a result of the vibration of granular materials (the Brazil nut effect). When the 3D exterior of the particles was approximated by an ellipsoid, the mean axial ratio of the ellipsoid was similar to that of fragments obtained in a laboratory impact experiment (longest: middle: shortest axial diameters = 2:√ 2:1), and these ratios were statistically indistinguishable. This indicates that the Itokawa particles are fragments formed by impact. Although the lunar regolith is considered to have been formed by impact, the mean axial ratio of its particles is close to that of a sphere; scientists have considered that the particles on the large satellite, moon, became increasingly spherical while existing as regolith particles for hundreds of millions of years. When the exterior of the Itokawa particles was closely observed, not only sharp but also round edges were found on the particles. This indicates that fragments of the original interior formed by impact were abraded for some reason (Fig. 5). For small asteroids such as Itokawa, their particles are considered to move as a result of vibration generated by the impact of meteoroids (which negligibly attenuates owing to reflection at the surface); at this time, particles were considered to rub against each other, causing abrasion. No textures, such as agglutinates that indicate large-scale melting, as seen on the lunar regolith, were observed for the Itokawa particles, implying a difference in the impact speeds of meteoroids with Itokawa (approximately 5 km/s) and with the moon (approximately 10 km/s or higher).

As discussed above, the Itokawa particles analyzed in this research are different from the regolith particles of large celestial bodies with strong gravity, such as the moon, and have the same features as the regolith on asteroids with low gravity. Such regolith particles are considered to have been formed by the impact of another asteroid on the Itokawa surface and were abraded because of the particle motion by the vibration excited by impact with another asteroid (Fig. 6). Thus, the active state of the asteroid surface was clarified.

<<Significance of Research Achievements>>
The analysis of the Itokawa particles in this research proved the conventionally predicted association between asteroid materials and meteorites. This leads to the clarification of the association between the approximately 250,000 asteroids and 60,000 meteorites discovered to date and can be considered, from the viewpoint of scientific history, to be a significant achievement that will contribute to a greater understanding of the formation of the solar system.

In addition, the features of the asteroid Itokawa, which had not previously been clarified by conventional astronomical observation or meteorite analysis, were revealed from the features of the regolith particles determined by analyzing the 3D exterior of the Itokawa particles, and the origin and evolution of Itokawa were discussed on the basis of these features. We hope that a clear picture of Itokawa will be revealed by more detailed analysis in the future.

Publication:
"Three-Dimensional Structure of Hayabusa Samples: Origin and Evolution of Itokawa Regolith"
Akira Tsuchiyama et al.
Science 333 (6046), 1125-1128 (2011), published online 26 August 2011

<<Figures>>