Theoretical Radiative Transfer Emissivity Modeling of Quartz Fines At Nadir Incidence: Sensitivity Studies With Applications to Planetary Regolith

When scattering grains are in near or close contact, as is the case for planetary fines measured by thermal IR spectrometers aboard orbiters and rovers, the grains no longer scatter light like single particles. This is problematic for radiative transfer (RT) analyses of the photometric properties of surfaces such as the Martian regolith. After 10 years of advances within the planetary community, RT models are still inadequately predicting nadir emissivity values measured for laboratory geologic samples, calling even the most fundamental of theoretical assumptions into question. Here we present a continuation of sensitivity studies performed to quantify the effects of single scattering albedo and asymmetry parameter on emissivity for packed and unpacked model alpha quartz fines. Using hybrid computational solutions (Mie theory + discrete ordinates RT algorithm) to calculate theoretical nadir emissivity values in the thermal IR, we upgrade numerical implementation of previous works and explore assumptions at different stages in the transfer theory to show that assumptions about the scattering grain itself, rather than the machinery of the computational method used, result in the mismatch between model and lab data. To facilitate connections between laboratory geology and theoretical physics studies, we also present ``the theorist's checklist'' of information most useful to RT modelers to incorporate real world geology into model parameters, as well as specific methods to quantitatively determine particle size distribution that are beneficial to both camps. This work is supported through NASA MDAP (MJW, KMP).