EXPERIMENTAL RESEARCH ON REFLECTANCE SPECTRA CHANGE OF SOLAR SYSTEM BODIES BY SPACE WEATERHING : A NEW CHRONOLOGY

s型小行星的反射光谱与普通球粒陨石不同。月球岩石和相似成分的土壤之间也存在光谱差异。这些光谱不匹配可以用所谓的太空风化来解释。与普通球粒陨石相比，s型小行星表现出更多的整体耗损和光谱变红，吸收带更弱。为了解释月球岩石和土壤的光谱差异以及月球土壤的磁共振特性，提出了硅酸亚铁蒸汽沉积在土壤涂层中形成纳米相金属铁颗粒的理论。高速尘埃粒子的撞击以及太阳风的溅射将导致蒸汽的形成。我们的研究小组利用纳秒脉冲激光照射模拟高速尘埃撞击，成功地再现了太空风化过程中预期的光谱变化。我们利用透射电镜证实了激光照射橄榄石和辉石颗粒气相沉积边缘内形成纳米相铁颗粒。在本研究中，为了获得从紫外到近红外区域的连续双向反射光谱，我们安装了一台新的反射光谱仪。采用自动控制系统，可实现大范围反射光谱的连续测量。为了减少不同探测器之间光谱间隙的影响，我们可以测量重叠的光谱。我们得到了激光照射前后陨石碎片和颗粒颗粒的反射光谱。我们选择了L和LL普通球粒陨石，以便与日本“隼鸟”号航天器上的多波段相机和红外光谱仪观测到的岩石碎石堆小行星Itokawa的光谱进行比较。通过与隼鸟号观测到的光谱数据进行比较，我们证实了即使在相对年轻的小行星（小于10^7年）上，空间风化也在持续进行。

Reflectance spectra of S-type asteroids are different from those of ordinary chondrites. There are also spectral differences between lunar rocks and soils of the similar composition. These spectral mismatches are explained by the so-called space weathering. S-type asteroids exhibit more overall depletion and reddening of spectra, and more weakening of absorption bands relative to ordinary chondrites. To explain spectral differences between lunar rocks and soils as well as magnetic resonance properties of lunar soils, it was proposed the formation of nanophase metallic iron particles in soil coatings from the deposition of ferrous silicate vapor. High-velocity dust particle impacts, as well as sputtering by solar wind would be responsible for vapor formation. Our group succeeded in reproducing the spectral change expected in space weathering, using nanosecond pulse laser irradiation simulating highvelocity dust impacts. We confirmed the formation of nanophase iron particles within the vapor-deposited rim of laser-irradiated olivine and pyroxene grains using TEM.In the present study, in order to obtain continuous bi-directional reflectance spectra from UV to NIR region, we installed a new reflectance spectrometer. Using the auto-control system, wide range of reflectance spectra can be measured continuously. To minimize the effect of spectral gaps between different detectors, we can measure the overlapped spectra. We obtained reflectance spectra of meteorite fragments as well as particulate pellets before and after laser irradiations. We selected L and LL ordinary chondrites in order to compare the measured spectra with those of asteroid Itokawa, which is a rock rubble pile observed by a multiband camera and an infrared spectrometer on board Japanese spacecraft Hayabusa. Compared with the observed spectral data of Itokawa by Hayabusa spacecraft, we confirmed that the space weathering is ongoing even on relatively young (<a few 10^7yr) asteroid Itokawa.