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The thermal conductivity of granular planetary regolith is strongly dependent on the porosity, or packing density, of the regolith particles. However, existing models for regolith thermal conductivity predict different dependencies on porosity. Here, we use a full-field model of planetary regolith to study the relationship between regolith radiative thermal conductivity, porosity, and the particle non-isothermality. The model approximates regolith as regular and random packings of spherical particles in a 3D finite element mesh framework. Our model results, which are in good agreement with previous numerical and experimental datasets, show that random packings have a consistently higher radiative thermal conductivity than ordered packings. From our random packing results, we present a new empirical model relating regolith thermal conductivity, porosity, temperature, particle size, and the thermal conductivity of individual particles. This model shows that regolith particle size predictions from thermal inertia are largely independent of assumptions of regolith porosity, except for when the non-isothermality effect is large, as is the case when the regolith is particularly coarse and/or is composed of low thermal conductivity material.Plain Language SummaryThe temperature of a planetary surface is strongly controlled by the thermal inertia of the surface materials. Specifically, if the surface is covered in a granular regolith, then the size, thermal conductivity, and packing density of the regolith particles strongly affects the surface thermal inertia, which in turn controls surface temperatures. In this work, we use 3D numerical simulations of heat transfer through beds of spherical particles, representing a planetary regolith, to investigate how thermal conductivity and thermal inertia are controlled by the packing density and thermal conductivity of the spheres. Our results are presented in the form of a new empirical model, which could be used to calculate regolith thermal conductivity from knowledge of particle size, composition, and packing density. The use of this model is demonstrated in the typical reverse fashion, where an observed planetary thermal inertia is converted into a predicted regolith particle size. Our model shows that the predicted particle size is largely independent of regolith particle packing density, in contrast to other common regolith models.

International audience; Thermal stress weathering is now recognized to be an active and significant geomorphological process on airless bodies. This study aims to understand the key factors governing thermal stresses in rocks on airless bodies through extensive numerical calculations and analytic analyses. Some of the key factors governing thermal stresses, are found to be the diurnal surface temperature variation, the second-order spatial gradient of the temperature field, the thermal skin depth, and the rock size of interest. Microscopic (grain-scale) thermal stresses are driven primarily by the amplitude of the magnitude of the maximum diurnal temperature variation at said depth. Macroscopic (rock-scale) thermal stresses are more complex, and their nature fundamentally depends on two length scales: the thermal skin depth and the rock size of interests. For rock sizes larger than the thermal skin depth, macroscopic thermal stresses are driven primarily by second (and higher) order spatial gradients of temperature. For rock sizes smaller than the thermal skin depth, macroscopic thermal stresses are primarily driven by the ratio of rock size to thermal skin depth with macroscopic thermal stresses being greatest when this ratio is 1/2. Additionally, scaling relations for diurnal surface temperature variation, time-rateof-change of surface temperature, as well as peak microscopic (grain-scale) and macroscopic (rock-scale) thermal stresses are derived to provide a more accessible modeling tool. These scaling relations are remarkably accurate when compared to both the numerical calculations as well as three-dimensional finite element calculations. The model formulation, results, and scaling relations provided here allow the estimation of diurnal temperatures and thermal stresses on rocks of various size and materials on airless bodies at any orbital distance with a broad spectrum of spin rates. Lastly, we postulate and confirm that there is a critical spin rate where macroscopic thermal stresses will be greatest.

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 temperatur...