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S. Sasaki, E. Kurahashi (Univ. Tokyo)
Space weathering is proposed to explain spectral mismatch between asteroids and meteorites, especially between S-type asteroids and ordinary chondrites, as well as spectral difference between lunar soils and rocks. Recently it is established that the space weathering is caused by formation of nanophase iron particles around the surface regolith grains; iron nanoparticles are formed through deposition of silicate vapor from micrometeorite impact heating and/or solar wind sputtering [1, 2, 3, 4]. Using nanosecond pulse laser simulating micrometeorite impacts, we reproduced darkening and reddening of reflectance spectra of various silicate minerals [5]. From TEM analysis, we found that iron nanoparticles are ubiquitous in the amorphous rim around laser-irradiated olivine grains [3]. The change of reflectance is much more significant in olivine than in pyroxene even under similar iron contents; olivine-rich asteroids would be more easily weathered than pyroxene-rich asteroids [6].
Since the vapor-deposited rim around regolith grains is responsible for the reflectance change, there is a possibility that the space weathering degree does not directly reflect mineral composition in the surface regolith. For example, if surface regolith contains some amounts of olivine, the amorphous rim with iron nanoparticles would be also formed around pyroxene grains. Here, we simulated space weathering on samples of various mixing ratio between olivine and enstatite and between olivine and hypersthene. We examine overall spectra as well as continuum slope of infrared reflectance and 1 micron Ð 2 micron band area ratio. In our results, small amounts of olivine should enhance the reflectance change at olivine-enstatite mixture, whereas hypersthene mainly controls the reflectance spectra at olivine-hypersthene mixture.
[1] Pieters, C. M. et al., MAPS 35, 1101, 2000. [2] Hapke, B. JGR 106, 10039, 2001. [3] Sasaki, S. et al., Nature 410, 555, 2001. [4] Clark, B. et al., in Asteroid III, 2002. [5] Yamada, M. et al., Earth Planets Space 51, 1255, 1999. [6] Hiroi, T. and Sasaki, S., MAPS in press, 2002.