THERMAL TRANSPORT MECHANISMS IN LUNAR REGOLITH AT LOW AND HIGH TEMPERATURES

Many planetary bodies, such as the Moon, comet, asteroids, and meteoroids, are covered by regolith and exhibit a wide range of temperature variations from daytime to nighttime and across different altitudes. The thermal conductivity of regolith is a critical parameter in determining surface heat flow and temperature variation. In this study, we develop a comprehensive model, combining a semi-empirical approach based on Watson's equation and an effective medium theory approach based on morphology and mineral composition, to predict the temperature-dependent thermal conductivity of lunar regolith. We study the solid thermal conductivity (k(s)) induced by the conductive interactions among solid particles and the radiative thermal conductivity (k(r)) caused by radiative heat transfer through the void spaces in porous media. We find the conductive contribution to be a complex function of morphology, particle size, packing, and the efficiency of contacts between adjacent particles, whereas the radiative contribution strongly depends on the emissivity, particle size, and porosity. More specifically, we estimate the elastic-wave-driven crystalline thermal conductivity (k(cr)) and the random-walk-driven amorphous thermal conductivity (k(am)) and evaluate the impact of crystallinity on k(s). We further consider the electron contribution to k(cr) and the radiative influence on k(r) when metal-rich meteorites are present in the regolith. Considering all the relative contributions to the effective thermal conductivity (k(eff)), we find that k(eff) at low temperatures is dominated by the solid contribution (i.e., k(s) proportional to x[(1 - y)k(cr,p) + y center dot k(cr,e)] + (1 - x)k(am)) and exhibits substantial variations depending on the degree of crystallinity (x) and metallic composition (y). We observe three distinct thermal transport behaviors for the corresponding solids: (1) k(cr,p) first increases rapidly to a peak at around 10 K and then decreases monotonically with increasing temperature due to substantial phonon-phonon scattering; (2) k(cr,e) increases linearly with temperature and reaches a plateau for T > 100 K due to the increase of electronphonon scattering; (3) k(am) exhibits a unique temperature dependence due to its random walk nature. Furthermore, we find that k(eff) at high temperatures is dominated by the radiative contribution, where k(r) proportional to T-3. This work identifies various thermal transport mechanisms in lunar regolith with respect to microstructures, morphology, composition, and temperature. It provides valuable insights into the fundamental thermophysical properties of regolith concerning future planetary exploration.