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

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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.