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内容記述 |
This thesis reports results on the major elemental composition ratios of asteroid <br />25143 Itokawa through remote X-ray fluorescence spectroscopy using the X-ray <br />fluorescence spectrometer (XRS) onboard the Japanese asteroid spacecraft Hayabusa. <br />Much more detailed and plausible results are provided than in the previous preliminary<br />study (Okada <i>et al</i>. Science 312, 1338-1341, 2006), utilizing newly released spacecraft's <br />position and attitude data. With these results, a relationship between asteroids and <br />meteorites is discussed as well as the origin and evolution ofthe asteroid. <br /> Hayabusa successfully carried out <i> in situ</i> observations of 25143 Itokawa, an S <br />(IV)-class near-Earth asteroid. The primary scientific goal of the mission is to unveil an <br />asteroid-meteorite connection and consequently apply the evidence of meteorites to the <br />solar system evolution. A fundamental question remains unsolved for decades whether <br />asteroids are truly parent bodies of meteorites or not. There has been a "paradox" in <br />asteroid science that reflectance spectra of S-class asteroids, most abundantly discovered in the solar system, have features more reddened, darkened, and shallower in absorption band depth than those of ordinary chondrite meteorites, most frequently found on earth. On the other hand, these spectral features rather resemble those of less frequently found iron-rich stony iron meteorites. Some explanations were proposed that S-class asteroids are substantially enriched in iron or they are intrinsically like ordinary chondrites but only the uppermost surface has been altered by some processes like space weathering. <br /> The XRS observed X-ray fluorescence emission from the uppermost surface materials of Itokawa, excited by solar X-rays, in order to determine major elemental composition of surface of the asteroid. This is the second <i>in situ</i> asteroid X-ray fiuorescence observation in history next to the Near-Earth Asteroid Rendezvous Shoemaker mission (Trombka <i>et al</i>., 2000; Nittler <i>et al</i>., 2001), and the first one with the advanced instruments based on charge-coupled device (CCD). In this study, composition of major elements such as Mg, Al, Si, S, Ca, and Fe was estimated and the relation of the asteroid to some possible candidates of meteorite-analogues was investigated. <br /> For that purpose, analyses of the XRS data have been carefully carried out. The failure of two reaction wheels of the spacecraft caused severe disturbance of the attitude control and significantly reduced duration to view the asteroid by the XRS. Many observational data include cosmic X-ray backgrounds and solar wind particles other than instrumental noises. Effective energy resolution was also degraded due to frequent thermal changes of the XRS. Thus, XRF spectra were extracted by subtraction of all these effects. Only a few percents of useful data out of three month observation were finally extracted for further detailed analysis under appropriate conditions. Considering the instrumental characteristics, intensities of XRF line spectra were determined by fitting with the response function of each CCD chip that has been evaluated in the laboratory. Temporal fluctuation of incident solar X-rays as well as regional variation of surface geomorphology of Itokawa were also the important points considered. Geometric corrections were carried out using the shape model developed by Aizu University (Demura<i> et al</i>., 2006) and the updated spacecraft's position and attitude data relative to the asteroid. <br /> A key result of analysis indicates that major elemental composition of the surface materials of Itokawa resembles that of ordinary chondrite meteorite-analogues. In particular, the results show that (1) elemental mass ratio of Fe/Si is somewhat larger than that of the average composition of ordinary chondrites, but its corrected result with mineral mixing effect indicates the ratio is considered almost equal to it, (2) elemental mass ratio of Mg/Si is globally homogeneous on the surface of Itokawa, neglecting the occurrence of substantial chemical differentiation, (3) in spite of large uncertainties, elemental mass ratios of best-fit Al/Si and S/Si may slightly vary in local areas, and the corrected results with mineral mixing effect suggest that elemental mass ratio of S/Si is less than, or almost the same as that of average composition of ordinary chondrites, indicating slight processes underwent on the surface. <br /> Asteroid 25143 Itokawa, an S (IV)-class of the taxonomy by Gaffey<i>et al</i>. (1993), is attempted to be classified as meteorite-analogues referring to the elemental mass ratios of Mg/Si, Al/Si, S/Si, Ca/Si and Fe/Si. The results indicate that iron and stony iron meteorites such as IAB, pallasites, and mesosiderites are ruled out due to its Fe/Si. This fact strongly supports the idea that the reddened and darkened features as well as shallower absorption band depth found in visible and near infrared reflectance spectra is not by substantial amount of iron but by the alteration due to uppermost surface processes such as space weathering. Additionally, these elemental composition ratios suggest that most probable candidates for Itokawa are ordinary chondrite meteorites, especially L- or LL-chondrites. But other types of meteorites including EL-chondrites, or primitive achondrites such as Brachinites, Acapulcoites or Winonaites still cannot be ruled out when Mg/Si vs. Fe/Si are compared. <br /> These results imply that Itokawa is like a primitive meteorite. Regional distribution of S/Si found on local areas implies that some surface alteration might occur on Itokawa. However, sulfur does not seem globally depleted and has regional distribution. Only the surface material on Itokawa might be altered but the inner materials are expected to remain fresh and primordial. Resurfacing on Itokawa due to seismic destruction by impacts and/or tidal destruction by planet encounters (Miyamoto, <i>et al</i>., 2007) could cause such surface heterogeneity. Low average density and global compositional homogeneity of Itokawa implies that the asteroid is a rubble-pile (Fujiwara <i>et al</i>., 2006) and assembled by compositiomally homogeneous rocks originated from the parent bodies. |
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