Laboratory simulation of space weathering: Changes of optical properties and TEM/ESR confirmation of nanophase metallic iron

Abstract

Reflectance spectra of S-type asteroids are different from those of ordinary chondrites. This spectral mismatch is explained by space weathering processes, where high-velocity dust particle impacts change the optical properties of the uppermost regolith surface of asteroids. S-type asteroids exhibit more overall depletion and reddening of spectra, and more weakening of absorption bands relative to ordinary chondrites. Nanophase metallic iron particles, which are formed through vapor deposition from dust impact heating, are considered as the most essential cause of space weathering. In this study, we describe the spectral changes of olivine and pyroxene using nanosecond pulse laser irradiation and the presence of nanophase metallic iron particles in laser-irradiated materials by transmission electron microscopy (TEM) and electron spin resonance (ESR). The irradiated spectra of the samples show a reduction of the overall spectra (250–2600 nm) and a reddening with weakened absorption bands. Nanophase metallic iron particles were found not only in laser-irradiated olivine samples, but also in laser-irradiated pyroxene samples by TEM. Strong ESR signals, which derive from nanophase iron particles, are observed in the irradiated olivine samples. Moreover, ESR intensities increase with the space weathering degree simulated as laser irradiation time. One possible application of space weathering is the estimation of the relative age of asteroids using the relation between optical effects and quantities of produced nanophase iron particles.