Spectral effects of regolith porosity in the Mid-IR – Pyroxene

The regolith porosity on airless bodies in the Solar System has a known effect on remotely acquired spectra in the mid-infrared (MIR; 5–35 μm). Previous experiments quantifying this effect have focused on olivines. Here we report systematic laboratory experiments designed to quantify the effect of regolith porosity on the MIR spectra of pyroxene, an important rock forming mineral on Earth, the Moon, and throughout the Solar System. Specifically, we have measured MIR spectra of eight natural pyroxene minerals and mixtures over three particle size fractions (< 20 μm, 20–45 μm, and 45–63 μm) with varying degrees of porosity. Our results indicate that 0% regolith porosity spectra are dominated by surface scattering, 90% regolith porosity spectra are dominated by volume scattering, and the transition between the two regimes is gradual. Comparing selected spectra to the Trojan asteroid (624) Hektor, we estimate the regolith porosity of its surface to be >80%, and that the surface contains a significant fraction of amorphous material. Overall, these results are quite similar to our previous olivine findings, supporting the conclusion that regolith porosity strongly affects the shape and subsequent interpretation of MIR spectra of silicate-rich surfaces.