Low-Temperature Thermal Properties of Lunar Materials

Up to now, thermal property measurements of lunar materials have been limited to temperatures over 100 K. However, thermal emission data from the LRO Diviner Lunar Radiometer Experiment indicate that permanently shadowed regions of the lunar surface, e.g. near crater rims in the extreme polar regions – candidates for reservoirs of permanently frozen water – may experience temperatures as low as 20 K.

We report here heat capacity, thermal conductivity, coefficient of thermal expansion, thermal inertia, and thermal diffusivity data from 5–300 K for six lunar meteorites: four feldspathic breccias (NWA 5000, NWA 10678, NWA 11421, and NWA 11474), a gabbro (NWA 6950) and a troctolite (NWA 8687). (In addition we have measured heat capacities for a further 15 African dry desert lunar meteorites.) All thermal measurements were conducted using a Quantum Design Physical Properties Measurement System (QD-PPMS) at Boston College on specimens cut to parallelapipeds a few mm per side.

Heat capacity is a strong function of temperature, rising steadily over the range 5–300 K. Thermal conductivity is a weak function of temperature above ~100 K, but in the range 5–100 K it drops rapidly toward zero.

Thermal diffusivity and thermal inertia are functions of heat capacity, thermal conductivity, and meteorite bulk density. The thermal diffusivity for all samples increases rapidly as temperature drops below 100 K; below 25 K the temperature dependence of this property can vary by as much as two orders of magnitude. The thermal inertia falls off rapidly toward zero as the temperature falls below 100 K. Because thermal conductivity and bulk density, factors in thermal inertia, depend on porosity, we expect to see thermal inertia for loose, highly porous regolith to be approximately one order of magnitude less than measured for these meteorites.

All six of the lunar meteorites in this study include a zone of negative thermal expansion (NTE) centered at various temperatures ranging from 60–100 K. This NTE behavior differs from what we have reported in CM2 meteorites; the effect is smaller and occurs at a much lower temperature, thus likely caused by a different mineral component of the lunar silicate mineralogy.