Thermal conductivity of surficial lunar regolith estimated from Lunar Reconnaissance Orbiter Diviner Radiometer data

Abstract

The thermal conductivity of lunar regolith is a critical parameter in the determination of surface heat flow and understanding the nature of surface material. As a refinement on this thermophysical parameter, this study aims to present an estimation on the solid thermal conductivity of surficial lunar regolith based on the data of Lunar Reconnaissance Orbiter (LRO) Diviner radiometer. Theoretically, the thermal conductivity of lunar regolith is composed of solid thermal conductivity (i.e. the conductive contribution induced by the interactions among solid grains) and radiative thermal conductivity (i.e. the conductive contribution induced by the inter-granular radiation). The solid thermal conductivity depends on the thermal conductivity of solid grains as well as the effective contact areas among grains, which reflects the nature of lunar surface materials. It is well known that the surface temperature variation in the nighttime is dominated by the thermophysical parameter of surficial lunar regolith. A temperature model based upon heat conductive equation is adopted to correlate the thermophysical parameters of lunar regolith with the surface temperature. Diviner Level-3 bolometric temperature dataset is adopted as the proxy of lunar surface temperature. The estimation is carried out by searching for the optimal surficial solid component that makes the root-mean-square error (RMSE) between Diviner data and simulated temperatures minimum.The estimation result shows that the solid thermal conductivity of surficial lunar regolith has a wide variation range (0.0001–0.0300Wm−1K−1) between 60°N and 60°S. The solid thermal conductivity on the lunar surface has a prominent latitude dependence, which is doubted as the consequence of the specific surface energy variation versus latitude. In addition, the surface solid thermal conductivity in the older regions is ubiquitously higher than younger regions. This phenomenon would be relative with the variation of grain radius across the entire lunar surface.In addition, the enrichment of rock abundance at young impact craters makes the estimated solid thermal conductivity abnormally high. We also find a strong correlation between the absolute ages and the estimated surficial solid thermal conductivity of these craters. Based upon the fitted relation between them, the exact absolute ages of Crookes and Ohm are determined as 268 and 252Myr respectively.