Spectral effects of regolith porosity in the Mid-IR: Properties and mineralogy of Trojan asteroid surfaces

Jupiter’s Trojan asteroids (hereafter Trojans) are a substantial group of primitive bodies that carry insight into the building blocks that make the larger bodies in our Solar System as well as the formation mechanisms that shaped it. The surface composition of asteroids can be indicative of formation region. By constraining non-compositional parameters that affect mid-infrared (MIR; 5-35 μm) spectra, Trojan surface mineralogy can be more accurately interpreted, leading to a deeper understanding of formation. MIR spectra of Trojans exhibit silicate emission features, not unlike a comet comae. The resemblance was initially perplexing, because asteroid regoliths are optically thick and comet comae are optically thin. To explain this resemblance, researchers hypothesized that Trojan surfaces may consist of a fine grained, ‘fluffy’ regolith of silicates (hereafter ‘regolith porosity’ will refer to how fluffy a regolith is). To understand the MIR spectral region with respect to regolith porosity, and transitively the Trojan surfaces, we test the following hypothesis: Porosity in regoliths of fine-grained silicates have a systematic and quantifiable effect on the band position, shape, and spectral contrast of MIR spectra. To simulate the effects of regolith porosity, we mixed silicate (olivine for this study) powder with KBr powder with the same size range. Each olivine powder sample was mixed with KBr from 0%-90% with 10% intervals by weight. To control for a grain size, we ground and sieved olivine into the following grain sizes: < 20 μm, 20-45 μm, and 45-63 μm. Finally, we measured spectra with a Fourier transform infrared (FTIR) spectrometer in the MIR. Our results show the following effects of porosity: the Christiansen Feature at ~ 8.8 μm decreases in spectral contrast, and shifts slightly to longer wavelengths as regolith porosity increases, restrahlen bands near 10 & 12 μm don’t shift wavelengths but do decrease in spectral contrast, and the broad 13 μm transparency feature deepens with increased regolith porosity. Additionally, the effect of regolith porosity may be grain size dependent. These results support the hypothesis that regolith porosity has a systematic effect on silicate features in the MIR.