Simulation of space weathering by nanosecond pulse laser heating: dependence on mineral composition, weathering trend of asteroids and discovery of nanophase iron particles

The spectral mismatch between asteroids and ordinary chondrites, is explained by a so-called "space weathering" process where impacts of interplanetary dust altered the optical properties of asteroid surfaces. To simulate the heating by micrometeorite impacts, pellet samples of olivine, pyroxene, and anorthite are irradiated by a pulse laser beam (1064nm) with a pulse duration of 6-8 nanoseconds, which is comparable with a micron-sized particle impact. After the laser irradiation, bidirectional reflectance spectra between 250 and 2600nm of samples are measured by step of 10nm. Laser-irradiated samples show significant reddening: the reduction of spectra is much larger in the visible region than in the near-infrared region. Changes of pyroxene spectra are much smaller than those of olivine. Some asteroid spectra such as 349 Dembowska and 446 Aeternitas can be reproduced by the mixing of spectra of irradiated samples. To clarify the microscopic process and cause of reflectance change, we observed the irradiated samples using transmission electron microscopy (TEM). In rim regions of irradiated olivine grains from pellet samples, nanophase iron particles (several to 30 nm in size) are widely spread. In contrast, no trace of structural change is found on irradiated olivine crystal samples. This suggests that the presence of regoligh-like surface would be essential for the effective space weathering on asteroids. Very small asteroids without regolith could be hardly weathered.