Spectral effects of regolith porosity in the mid-IR – Forsteritic olivine

Abstract

Studies have long utilized laboratory derived spectra to understand the surface composition and other properties of planetary bodies. One variable that is believed to affect remotely acquired spectra in the mid-infrared (MIR; 5–35 μm) is the surface porosity of the airless body, yet there have been few laboratory studies to quantify this effect. Thus, here we report systematic laboratory experiments aimed at quantifying the effect that porosity has on the MIR spectra of silicate regoliths. To simulate the effects of regolith porosity, we mixed olivine powder with KBr powder of the same size range (< 20 μm, 20–45 μm, and 45–63 μm). Olivine was mixed with KBr from 0% - 90% with 10% intervals by weight. Finally, we measured spectra with a Fourier transform infrared (FTIR) spectrometer in the MIR. Our results indicate a transition from a primarily surface scattering regime to a primarily volume scattering regime with increasing regolith porosity. Evidence of the dominating regime transition includes: the primary Christiansen Feature at ~8.8 μm decreases in spectral contrast, and shifts slightly to longer wavelengths as regolith porosity increases, reststrahlen bands in the 10-μm region do not shift significantly in wavelength but do decrease in spectral contrast, vibrational bands in the volume scattering regime (i.e., peaks in emissivity) increase in spectral contrast with increasing regolith porosity, and the spectral contrast of 10-μm plateau increases exponentially with increasing regolith porosity. The MIR spectral analysis of asteroids, such as Hektor, suggests a highly porous surface regolith (at least 81% void space) of fine-particulate silicates. These results demonstrate that some asteroids support highly porous regoliths whose spectra are not well-matched by standard (low porosity) laboratory spectra of powders. The spectra presented here enable analysis of both the porosity and mineralogy of olivine-rich, low porosity asteroid surfaces.