Sub-surface alteration and related change in reflectance spectra of space-weathered materials

¹ Department of Geosciences and Geography, PO Box 64, 00014 University of Helsinki, Helsinki, Finland
e-mail: katerina.chrbolkova@helsinki.fi
² Astronomical Institute, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, 18000 Prague, Czech Republic
³ Czech Academy of Sciences, Institute of Geology, Rozvojová 269, 16500 Prague, Czech Republic
⁴ Research Centre Řež, Hlavní 130, 250 68 Husinec–Řež, Czech Republic
⁵ Department of Physics, PO Box 43, 00014 University of Helsinki, Helsinki, Finland
⁶ Department of Chemical Physics and Optics, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 12116 Prague, Czech Republic
⁷ Institute of Physics, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 12116 Prague, Czech Republic
⁸ Department of Physics, PO Box 64, 00014 University of Helsinki, Helsinki, Finland

Received: 8 February 2022
Accepted: 22 June 2022

Abstract

Context. Airless planetary bodies are studied mainly by remote sensing methods. Reflectance spectroscopy is often used to derive their compositions. One of the main complications for the interpretation of reflectance spectra is surface alteration by space weathering caused by irradiation by solar wind and micrometeoroid particles.

Aims. We aim to evaluate the damage to the samples from H⁺ and laser irradiation and relate it to the observed alteration in the spectra.

Methods. We used olivine (OL) and pyroxene (OPX) pellets irradiated by 5 keV H⁺ ions and individual femtosecond laser pulses and measured their visible (VIS) and near-infrared (NIR) spectra. We observed the pellets with scanning and transmission electron microscopy. We studied structural, mineralogical, and chemical modifications in the samples. Finally, we connected the material observations to changes in the reflectance spectra.

Results. In both minerals, H⁺ irradiation induces partially amorphous sub-surface layers containing small vesicles. In OL pellets, these vesicles are more tightly packed than in OPX ones. Any related spectral change is mainly in the VIS spectral slope. Changes due to laser irradiation are mostly dependent on the material’s melting temperature. Of all the samples, only the laser-irradiated OL contains nanophase Fe particles, which induce detectable spectral slope change throughout the measured spectral range. Our results suggest that spectral changes at VIS-NIR wavelengths are mainly dependent on the thickness of (partially) amorphous sub-surface layers. Furthermore, amorphisation smooths micro-roughness, increasing the contribution of volume scattering and absorption over surface scattering.

Conclusions. Soon after exposure to the space environment, the appearance of partially amorphous sub-surface layers results in rapid changes in the VIS spectral slope. In later stages (onset of micrometeoroid bombardment), we expect an emergence of nanoparticles to also mildly affect the NIR spectral slope. An increase in the dimensions of amorphous layers and vesicles in the more space-weathered material will only cause band-depth variation and darkening.