Lunar thermal emission and remote determination of surface properties

Abstract

Grain size, bulk density (or porosity), thermal conductivity, and rock abundance all play an important role in the thermal behavior of the lunar surface. Direct investigation of these properties of the lunar surface layer is not presently possible, with the exception of the samples returned from the Apollo landing sites. An indirect measurement of lunar surface properties may be possible using remote thermal infrared observations. In order to better understand the interplay between these properties, a diurnal thermal model for the lunar surface and near subsurface with temperature‐dependent specific heat and thermal conductivity was developed. The inclusion of the temperature dependence of thermal conductivity and specific heat was found to be essential when attempting to derive regolith properties of the Moon due to the large difference in surface temperatures between day and night. Although particle size, bulk density, and thermal conductivity cannot be investigated completely independently, a clear relationship between these parameters, and their effects on lunar surface temperatures, is determined. An increase in the bulk density of the regolith is found to correspond to an increase in the nighttime temperature of the surface. Similarly an increase in the rock fraction also raises the predicted nighttime temperatures. Increasing grain sizes correspond to decreasing nighttime temperatures. No unique set of surface properties can be determined from thermal remote sensing measurements alone. Grain size is the most difficult regolith property to determine remotely, and rock abundance is by far the strongest contributor to the derived thermal inertia of the bulk surface.