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

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.

Simulation of Space Weathering on Asteroid Spectra through Hydrogen Ion Irradiation of Meteorites

“…Comparable studies observed that H + ion irradiation resulted in similar spectral changes (Chrbolková et al 2021) as observed in our samples, but these were caused predominantly by a thin, partially amorphous layer (up to 200 nm) and small vesicles with no npFe 0 (Takigawa et al 2019;Chrbolková et al 2022).…”

“…We cannot rule out the presence of npFe 0 in our samples, but its abundance is most likely lower compared to those generated during laser irradiation experiments. While Chrbolková et al (2022) did not observe npFe 0 on He + -irradiated samples, they did observe it on identical samples irradiated by a femtosecond laser, pointing to a more likely npFe 0 appearance during spot heating compared to ion irradiation associated with milder thermal conditions. Loeffer et al ( 2009) also observed during 4 keV He ion irradiation that, due to partial oxidation of the redeposited sputter, npFe 0 formed more slowly on a powder (as in our case) than on a flat surface of olivine.…”

“…The simulation only imitates individual events and does not take into account the cumulative damage. The damaged zone in our case will be slightly thinner than that observed by Chrbolková et al (2022) for 5 keV H + experiments, but the observed features (partial amorphization, vesicles) may be similar. Hence, we attribute the significant reddening and albedo reduction in the 1 μm region to be predominantly caused by a thin, partially amorphous layer and the presence of vesicles, rather than the presence of npFe 0 .…”

“…According to Chrbolková et al (2022), thickness of the amorphous layer plays a major role in absorption spectra, as the amorphous layer absorbs the wavelengths that are equivalent to its thickness. We carried out H + ion penetration depth and associated damage simulations using SRIM software (Ziegler & Biersack 1985) for energies of 1-5 keV, and we obtained a penetration depth and thickness of the damaged area of <40 nm for our case of 1 keV (see the "Supplementary Figure S13.pdf" file in the Zenodo repository).…”

“…This is supported by, for example, simulating timescales of 10 7 yr with 60 keV Ar ++ on Epinal H5 chondrite and observing reddening in the whole NIR region. Laser irradiation experiments simulating microimpacts, typically on timescales larger than 10 4 yr, also show spectral changes in the whole NIR region (e.g., Moroz et al 1996;Yamada et al 1999;Sasaki et al 2002;Chrbolková et al 2021Chrbolková et al , 2022. The dominant spectrally significant agent in these experiments is identified as npFe 0 embedded in surficial amorphous rims, which was also observed in lunar soil samples (e.g., Keller & McKay 1997;Hapke 2001).…”