Spectral feature variations of low-iron olivine under intense pulse-laser irradiations

Abstract

Space weathering processes, including micrometeoroid impact and solar wind irradiation typically redden, darken, and attenuate the fingerprint absorption features in the visible and near-infrared (VNIR) reflectance spectra of planetary surface materials. The so-called lunar style space weathering typically produces nanophase metallic iron (npFe⁰) and amorphous mineral layers. This paradigm has been known to be inadequate in describing the weathering processes on many other airless bodies and many open questions are waiting to be answered. For example, the greater flux of micrometeorite impacts or higher surface temperature on Mercury may produce larger npFe⁰ particles; the gardening effects on space weathering remain largely unknown; on asteroids such as Vesta, random regolith mixing and contamination by exogenic material from impacts are believed to be the dominant space weathering processes. To understand these and other questions in non-lunar style space weathering, we conducted pulsed laser irradiations on low-iron olivine grains in powders and pellets at various energy levels. By performing transmission electron microscope and reflectance spectroscopic measurements, we found that progressive irradiation caused continuous darkening. Meanwhile, the VNIR spectral slope changed from reddening to bluing after reaching a “saturation point”, and the absorption band depth transitioned from weakening to stabilization. At the same time, repeated irradiations led to limited growth of npFe⁰ particles in low-iron olivine. In all simulated irradiations, significant spectral alterations occurred in early stages, implying that fresh surfaces are more sensitive to space weathering. The rates of spectral modification of powder samples were found to be remarkably lower than those of the pellet samples. We also observed that exogenous metal contaminants could evaporate and condense into an opaque layer during simulated bombardments, obscuring the original spectral features of regolith.