Study on the evolution of the surface of solar system body: Effect of iron

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S-type asteroids show more overall depletion and reddening of spectra, and more weakening of absorption bands than ordinary chondrites. These spectral mismatches are explained by the so-called "space weathering". High-velocity dust particle impacts as well as sputtering by solar wind would be responsible for the production of nano-iron particlesOur group succeeded in reproducing the spectral change expected as space weathering, using nanosecond pulse laser irradiation simulating high-velocity 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, we developed the comminution method of iron-bearing meteorites and succeeded in grinding the iron particles. We confirmed the optical change of ordinary chondrite powder samples.Hayabusa spacecraft observed the S-type asteroid (25143) Itokawa. Optically, the surface of Itokawa is divided into brighter (and bluer) areas and darker (and redder) areas. In rough zones, dark boulder-rich surfaces usually superpose on bright materials. We can interpret that removal of dark space-weathered boulder-rich surface materials by shaking caused by impacts or planetary encounters should lead to exposure of underlying relatively fresh bright area. High resolution images indicate that boulders are firmly covered with weathered fine particles or boulders' surface are optically weathered. We irradiate pulse laser on meteorite fragments with cut flat surface under vacuum. As expected, significant darkening and reddening are observed under the irradiation of pellet samples. This is the first confirmation of optical change of rock surface by space weathering. Small less-regolith asteroids with silicate composition may also change its brightness and color by space weathering, although their weathering degree would be weaker than regolith-covered asteroids.