Pyroxene regolith porosity in the Mid-IR: Spectral effects and implications for Trojan asteroid surfaces

Asteroid surface composition can be indicative of formation region, a particularly useful parameter to constrain for Jupiter’s Trojan asteroids (hereafter Trojans). Trojans, a large group of primitive asteroids, carry insight into the formation mechanisms that shaped the Solar System. 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 comets’ optically thin comae, even though Trojans do not have comae. Researchers hypothesize that Trojan surfaces may consist of a fine grained, ‘fluffy’ regolith of silicates (hereafter ‘regolith porosity’). To understand the MIR spectral region with respect to regolith porosity, and transitively Trojan surfaces, we test the following hypothesis: Porosity in fine-grained silicate regoliths have a systematic and quantifiable effect on the band position, shape, and spectral contrast of MIR spectra. To simulate regolith porosity effects with minerals potentially appropriate for Trojan surfaces, we mixed pyroxene powder with KBr, a MIR transparent salt we use as a proxy for regolith porosity. Pyroxene samples were chosen to have a wide range of compositional variation to isolate general effects of porosity and illuminate compositionally unique effects. The samples include two hedenbergites, two diopsides, aegirine, augite, and enstatite. We ground and sieved the pyroxenes into the following grain sizes: < 20 μm, 20-45 μm, and 45-63 μm, and mixed them with KBr from 0%-90% with 10% intervals by weight. Using a Fourier transform infrared spectrometer we measured MIR spectra of each pyroxene sample at every regolith porosity step. Our presentation will focus on quantifiable effects dependent on regolith porosity and grain size relevant to remote sensing of Trojan asteroids, such as variation in the Christiansen feature, transparency features, as well as the broad 10-μm absorption feature.