Change of Asteroid Reflectance Spectra By Space Weathering: Pulse Laser Irradiation on Meteorite Samples

There are spectral mismatches between S-type asteroids and ordinary chondrites. S-type asteroids exhibit more overall depletion and reddening of spectra, and more weakening of absorption bands relative to ordinary chondrites. These spectral mismatches are explained by the so-called “space weathering”. Hapke et al. proposed that the spectral change should be caused by formation of nanophase metallic iron particles. High-velocity dust particle impacts as well as sputtering by solar wind would be responsible for vapor formation. Nanophase iron particles are confirmed in the rim of lunar soil grains. Recent ground-based and spacecraft observations confirmed the strong link between S-type asteroids and ordinary chondrites.

Our group succeeded in reproducing the optical property change expected as space weathering, using nano-second pulse laser irradiation simulating high-velocity dust impacts. Using a transmission electron microscope (TEM), we confirmed the formation of nanophase iron particles within the vapor-deposited rim of laser-irradiated olivine and pyroxene grains. ESR (Electron Spin Resonance) measurements confirmed that the olivine reflectance should decrease more with the increasing amount of nanophase iron particles.

Here we show that reflectance spectra of ordinary chondrites are darkened and reddened by pulse laser irradiation simulating the space weathering. Reflectance spectra of carbonaceous meteorite Allende is also changed by pulse laser irradiation.