  One of the main complications for the interpretation of reflectance spectra
of airless planetary bodies is surface alteration by space weathering caused by
irradiation by solar wind and micrometeoroid particles. We aim to evaluate the
damage to the samples from H and laser irradiation and relate it to the
observed alteration in the spectra. We used olivine (OL) and pyroxene (OPX)
pellets irradiated by 5 keV H ions and individual fs 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 and connected them to
changes in the reflectance spectra. 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. Related
spectral change is mainly in the VIS spectral slope. Changes due to laser
irradiation are mostly dependent on material's melting temperature. 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
thickness of (partially) amorphous sub-surface layers. Amorphisation smooths
microroughness, increasing the contribution of volume scattering and absorption
over surface scattering. 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 dimensions of amorphous layers and vesicles
in the more space-weathered material will only cause band-depth variation and
darkening.
