Mid-Infrared Spectral Effects of Regolith Porosity: Phyllosilicates

Regolith porosity on airless Solar System bodies affects mid-infrared (MIR; 5–35 μm) spectra, which can, in turn, affect spectral interpretation. Phyllosilicates are a major mineral component of some groups of carbonaceous chondrites and, presumably, their analog asteroid spectral groups (e.g., B-, C-, D-, and P-type asteroids) (e.g., McSween 1979; Gaffey et al., 1992; Hiroi et al., 2001).  These asteroids are typically featureless in the visible and near-infrared (VNIR; 0.5 – 2.5 µm), making it difficult to determine mineralogy from spectral observations at those wavelengths. As such, identifying phyllosilicates with MIR spectra is important for understanding the complex mineralogy on asteroid surfaces. We will present systematic laboratory experiments designed to quantify the effect of regolith porosity on the MIR spectra of the phyllosilicate, serpentine.

The serpentine was ground and sieved in a controlled environment in to three particle size fractions (0-20 μm, 20-45 μm, and 45-63 μm). We then mixed the powders with different percentages of potassium bromide (KBr; a MIR transparent salt that is used as a proxy for regolith porosity). Samples were mixed with KBr in ratios from 0% to 90% by weight, in 10 wt.% intervals. We made reflectance measurements of all samples using a Thermo-Nicolet iS50 Fourier Transform infrared (FTIR) spectrometer, under ambient conditions, with the PIKE Technologies EasiDiff diffuse reflectance accessory. Previous work showed that with increasing the regolith porosity of anhydrous silicate-rich sample (e.g., olivine), spectra transition from surface scattering dominant (at low regolith porosities) to volume scattering dominant (at high regolith porosities) (Martin et al., 2022; 2023; Dausend et al., 2025). We observe the same trend in the phyllosilicate spectral suite. Indicators of the scattering regime transition include decreasing spectral contrast of the Christiansen Feature (CF) and growth of the 10-μm plateau with increasing regolith porosity. 

While phyllosilicates, olivine, and pyroxene are common minerals found on many extraterrestrial surfaces, it is unlikely for a surface to be comprised entirely of these three silicate constituents. Thus, in future studies, we plan to explore how the porosity affects the MIR spectra of sample mixtures.

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