Retrieval of the bulk density of lunar subsurface layer from diurnal differences of Chang′E-2 microwave and LRO infrared brightness temperature data

Comparisons of microwave brightness temperature (TB) observed by Chang′E-1 and Chang′E-2 with FeO + TiO2 content observed by Clementine indicate a correspondence of diurnal difference of microwave TB to FeO + TiO2 content in distribution, especially for 37 GHz. The correlation of FeO + TiO2 content and 37 GHz TB is discussed. Consequently, we choose the regions, e.g. (30°N, 25°W) in Mare Imbrium and (30°N, 55°W) in Oceanus Procellarum of uniform FeO + TiO2 content for further studying the bulk density of lunar subsurface layer, avoiding the interference of FeO and TiO2. The lunar surface is regarded as a two-layer model. According to the infrared TB observed by LRO which is approximately equal to lunar surface temperature, it is found that the thermophysical properties of the lunar upper surface layer appear uniform for the sampled regions. In that way, distinct diurnal differences of microwave TB in both regions are related to the bulk density of lunar subsurface layer. A theoretical analysis based on a two-layer microwave TB model confirms the correlation between diurnal differences of microwave TB and bulk density of lunar subsurface layer. Finally, the bulk density of lunar subsurface layer is retrieved from microwave TB observed by Chang′E-2, ranging from 1.90 to 2.04 g/cm3 for a profile at 33°N extending from 120°W to 6°E.