Full‐Field Modeling of Heat Transfer in Asteroid Regolith: Radiative Thermal Conductivity of Polydisperse Particulates

Abstract

Characterizing the surface material of an asteroid is important for understanding its geology and for informing mission decisions, such as the selection of a sample site. Diurnal surface temperature amplitudes are directly related to the thermal properties of the materials on the surface. We describe a numerical model for studying the thermal conductivity of particulate regolith in vacuum. Heat diffusion and surface-to-surface radiation calculations are performed using the finite element (FE) method in three-dimensional meshed geometries of randomly packed spherical particles. We validate the model for test cases where the total solid and radiative conductivity values of particulates with monodisperse particle size frequency distributions (SFDs) are determined at steady-state thermal conditions. Then, we use the model to study the bulk radiative thermal conductivity of particulates with polydisperse, cumulative power law particle SFDs. We show that for each polydisperse particulate geometry tested, there is a corresponding monodisperse geometry with some effective particle diameter that has an identical radiative thermal conductivity. These effective diameters are found to correspond very well to the Sauter mean particle diameter, which is essentially the surface area-weighted mean. Next, we show that the thermal conductivity of the particle material can have an important effect on the radiative component of the thermal conductivity of particulates, especially if the particle material conductivity is very low or the spheres are relatively large, owing to non-isothermality in each particle. We provide an empirical correlation to predict the effects of non-isothermality on radiative thermal conductivity in both monodisperse and polydisperse particulates. Plain Language Summary The thermal conductivity of asteroid regolith is related to the properties of the particulate assemblage (e.g., size distribution). Spacecraft missions that measure the surface temperature of asteroids, like OSIRIS-REx at asteroid Bennu, can take advantage of this by relating the observed temperatures to the physical properties of the regolith. We present a 3D model for studying the thermal conductivity of regolith, where heat flow is simulated in randomly packed spheres. We found that for cases where the particle sizes are monodisperse, our model reproduces the thermal conductivity values predicted by simpler theoretical models. However, this is only true if the particles themselves are made of a material that itself has relatively high thermal conductivity, which may not be the case for the regolith on Bennu. We determined the values for a correction factor to account for these cases. Neglecting it could cause one to appreciably underestimate particle sizes on asteroid surfaces, which could pose a risk for sample collection. Finally, we found that regoliths with particle size mixtures can have radiative thermal conductivities that are identical to monodisperse regoliths. We found that the surface area-weighted mean particle size of the mixed regoliths is representative of the bulk radiative thermal conductivity.