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Space weathering: spectral change and formation of nanophase iron due to pulse laser irradiation simulating impact heating of interplantary dust flux

S. Sasaki", T. Hiroib, K. Nakamura0, Y. Hamabea, E. Kurahashia, and M. Yamada3

"Department of Earth and Planetary Science, University of Tokyo, Tokyo 113-0033, Japan bDepartment of Geological Sciences, Brown University, Providence, RI 02912, USA

department of Earth and Planetary Sciences, Kobe University, Kobe 657-0013, Japan

The spectral mismatch between S-type asteroids and ordinary chondrites, is explained by the so-called "space weathering" process where impacts of interplanetary dust should change the optical properties of asteroid surfaces. To simulate the heating by dust impacts, comminuted olivine and pyroxene samples were irradiated by a nanosecond pulse laser. Laser-irradiated samples show significant depletion and reddening of the reflectance. Some asteroid spectra such as 349 Dembowska and 446 Aeternitas are reproduced by mixing of irradiated spectra. Changes of pyroxene spectra are much smaller than those of olivine. This is consistent with the statistical data of asteroids where olivine-rich asteroids have more reddened spectra. If energy efficiencies for changing the optical properties are the same between dust impact and laser irradiation, olivine-rich surfaces should be darkened and reddened in 108 yrs.

According to TEM analysis, nanophase particles (several to 30 nm in size) probably of iron are widely spread in the rim region of irradiated olivine grains. In contrast, no trace of structural change was observed on irradiated olivine crystal samples. The presence of a regolith-like surface would be essential for effective space weathering on asteroids.

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