Temperature-dependent thermal inertia of homogeneous Martian regolith

Past studies of the thermophysical properties of the Martian surface layer have assumed temperature-independent thermal inertia, which is a function of the material density, specific heat, and bulk conductivity. In this paper, we evaluate the temperature-driven variations of these quantities for particulated and cemented material under Martian conditions of atmospheric pressure and temperature. Temperature-driven density variations are negligible. The specific heat of a basaltic material is strongly influenced by the temperature (∼75% increase from 150 to 315 K), inducing significant variations of the thermal inertia. The thermal conductivity of uncemented Martian regolith is weakly controlled by the solid phase conductivity and strongly controlled by the gaseous phase conductivity. As a result, the conductivity of the solid phase (i.e., composition, temperature) is unimportant, whereas medium to large variations (30-50%) of the bulk conductivity are associated with temperature-induced fluctuations of the pore-filling gas conductivity. Overall, the thermal inertia of uncemented Martian soils is predicted to vary significantly (∼80%) throughout the range of the observed surface temperatures. In the case of cemented soils, the contribution of the gas conductivity is generally small, and the solid phase (i.e., grains and cement) conductivity (i.e., composition, temperature) becomes more important. Consequently, the magnitude of the thermal inertia change for cemented soils is variable, and smaller than that predicted for uncemented materials (10-50%). Large diurnal and seasonal temperature variations only occur within the top material, and most of the near-surface regolith does not experience large thermal inertia variations. The shapes of modeled diurnal temperature curves are not significantly modified (e.g., the 0200 LT (Martian local time) apparent thermal inertia of uncemented regolith is up to ∼15% lower than the average daily inertia of the top material). Thermally derived grain sizes are usually based on conductivity measurements operated at room temperature or above, where the thermal inertia and specific heat are higher than on Mars, implying that grain size predictions for Mars are currently underestimated. However, in situ observations by the Mars Exploration Rovers and thermal conductivity modeling suggest that this underestimation is not significant.