A STUDY ON CHANGES OF REFLECTANCE SPECTRA OF PLANET-FORMING MINERALS : DEVELOPMENT OF SPACE WEATHERING SIMULATOR

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"Space weathering" is a proposed process responsible for the darkening and reddening of planetary surface materials and for decreasing depths of their optical absorption bands. It may explain the spectral mismatches between lunar soils and rocks as well as between asteroids and meteorites. The formation of nanophase iron particles on regolith grains due to micrometeorite impacts and/or solar wind irradiation is considered as the cause of the optical properties changes. Various laboratory simulations of space weathering have been performed. Nano-second pulse laser irradiation reproduced the optical changes associated with the space weathering. whereby compositional dependence of weathering of asteroids was also explained. However, the existence of nanophase iron particles has been a conjecture. Here we report observations of olivine samples after pulse-laser irradiation by transmission electron microscopy (TEM). Vapor-deposited amorphous rims of olivine grains have nanophase iron (aFe) particles (several to 30 nm in size) similar to those observed in lunar space weathering. Reduction by hydrogen atoms is not necessary to form nanophase iron particles. Our results should give additional support to the idea that ordinary chondrites have come from S-type asteroids, providing constraints on the surface exposure age of asteroids.