Space weathering by simulated micrometeorite bombardment on natural olivine and pyroxene: A coordinated IR and TEM study

We studied space-weathering effects caused by micrometeorite bombardment simulated by pulsed intense infrared laser, generating similar to 15 mJ per pulse in high vacuum. For our investigation, we selected a natural olivine (San Carlos olivine (Fo(91))) and a natural pyroxene (Bamble orthopyroxene (En(87))) as important rock forming minerals of the Earth upper mantle as well as key planetary minerals. Irradiated areas of powdered pressed samples were examined by optical reflection spectroscopy in a broad optical and infrared wavelength range (visible-, near-, and mid-infrared) and transmission electron microscopy to identify changes due to micrometeorite impacts. The present study aims to investigate especially the effects of micrometeorite bombardment on reflectance spectra in the mid-IR in preparation for future space missions, as well as for the MERITS experiment onboard the BepiColombo mission. For both irradiated samples, we found a reduction in albedo and in the reflectance of characteristic Reststrahlen bands and an increase of the transparency feature. VIS and NIR spectra of both minerals show the typical darkening and reddening as described for other space-weathered samples. TEM studies revealed that space-weathered layers in olivine and pyroxene differ in their respective thickness, similar to 450 nm in olivine, 100-250 nm in pyroxene, as well as in developed nanostratigraphy of laser-ablated material, like nanophase iron (npFe). In conclusion, our spectral and structural findings were compared to samples in which space weathering was caused by different processes. A comparison with these data demonstrates that there is no difference in optical reflectance spectroscopy, but a significant difference in the microstructure of minerals due to the weathering source in space, as there are solar wind and solar flares cause other structural and chemical changes as the bombardment with micrometeorites. (C) 2019 Elsevier B.V. All rights reserved.