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S. Sasaki, E. Kurahashi (Univ. Tokyo), K. Nakamura (Kobe Univ.), T. Hiroi (Brown Univ.), C. Yamanaka (Osaka Univ.)
The space weathering process, which can explain spectral mismatch between S-type asteroids and ordinary chondrites and between lunar soils and rocks, is simulated by nanosecond pulse laser irradiation. In the space weathering process, high-velocity dust impacts should change optical properties of the uppermost regolith of airless bodies. Asteroids show overall depletion and reddening of spectra, and weakening of absorption bands relative to meteorites.
Spectral darkening and reddening are produced clearly by pulse laser irradiation. Through evaporation and recondensation, nanophase iron particles would be formed on the irradiated sample. Previously we found nanophase iron particles are distributed on laser-irradiated olivine . In the present study, we confirmed nanophase iron particles not only on olivine but also on pyroxene. Although sizes of nanophase particles are similar, the presence of nanophase iron particles on pyroxene is different from that on olivine. Nanophase iron particle are embedded within aggregates of amorphous enstatite.
ESR (electron spin resonance) signals of the surface of laser-irradiated samples are measured. Strong ESR spectra, characteristic to nanophase iron particles, are observed on irradiated samples. ESR results show that quantity of nanophase metallic iron particles in olivine samples increases at higher space weathering degree. Darkening trend of the reflectance spectra relative to the quantities of nanophase metallic iron particles becomes moderate at higher space weathering degree.
Instead of the crater density, we will be able to estimate relative ages of asteroids using the relation between optical effects and quantities of produced nanophase iron particles from dust impacts except for high space weathering degree.
 Sasaki S., et al., Nature, 410, 555-557, 2001.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.