A Model for the Thermophysical Properties of Lunar Regolith at Low Temperatures

The thermophysical properties of lunar regolith have been thoroughly investigated for temperatures higher than 100 K. For the near-equatorial thermal measurements of the Apollo era, this temperature range was sufficient to generate appropriate models. However, recent measurements from the Lunar Reconnaissance Orbiter Diviner Lunar Radiometer Experiment have revealed polar temperatures as low as 20 K, with apparently lower thermal inertia than explainable by existing theory. In the absence of comprehensive laboratory measurements of regolith thermal properties at low temperatures (<100 K), we investigate solid state theory and fits to lunar simulant materials to derive a semiempirical model of specific heat and thermal conductivity in lunar regolith in the full range 20-400 K. The primary distinctions between these previous models are (1) the temperature dependence of the solid conduction component of thermal conductivity at low temperatures, (2) the focus on regolith bulk density as the primary variable, and (3) the concept that the composition and modal petrology of grains could significantly influence thermal properties of the bulk regolith. This model predicts that at low temperatures, thermal conductivity is as much as an order of magnitude lower and specific heat is likely higher than expected from current models. The thermal conductivity at low temperature should vary depending on the constituent grain materials, their crystallinity, contributions from phonon scattering modes, bulk porosity, and density. To demonstrate the impact of our finding, we extrapolate the effects of our conductivity model on temperature variations in permanently shadowed regions on the Moon. This work motivates experimental confirmation of thermophysical properties of lunar regolith at low temperature.