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S Sasaki (Univ. Tokyo), K Nakamura (Kobe Univ.), Y Hamabe, E Kurahashi (Univ. Tokyo), T Hiroi (Brown Univ.)
Space weathering is the process where fast dust impacts as well as solar wind sputtering changes the optical properties of asteroid surfaces. It would explain the missing spectral link between ordinary chondrites and S-type asteroids. Formation of nanophase iron particles, which were found in the rim of lunar soils, may be responsible for the reflectance changes. Here we succeeded in forming nanophase iron particles in the vapor-deposited rim of pulse-laser treated olivine grain samples, showing significant spectral reddening.
To simulate space weathering by impact heating of dust particles, we irradiated pellet samples of olivine and pyroxene grains (< 75 micron) with a pulse laser (1064 nm) with pulse duration 6-8 ns. . Reflectance spectra were reduced and reddened: reflectance of olivine changed more significantly than that of pyroxene, which is compatible with compiled asteroid spectral data where olivine-rich asteroids have more reddened spectra than pyroxene-rich asteroids .
From irradiated pellet samples, altered (colored) surface grains were picked up and their thin sections (70-100nm thickness) were observed by TEM. In the grain rim region, nanophase particles were ubiquitous. EDS analysis showed the rim region is relatively rich in iron. Size of those particles, which were not observed in un-irradiated samples, is between several to 30nm. These nanophase particles are responsible for the change of optical properties such as darkening and reddening. Different from olivine grains, olivine crystal samples were hardly changed optically and no structural change was observed by TEM, although evaporation was confirmed during laser irradiation. Evaporated materials should escape quickly from flat crystal surfaces, whereas they may recondense and deposit on nearby grains or pellet samples. This suggests that the presence of regolith-like surface should be preferable for effective space weathering on asteroids.
 Yamada, M. et al. (1999) Earth Planets Space 51, 1255-1265.
 Hiroi, T. and Sasaki, S. (1999) LPSC XXX, 1444.
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