Apollo 15 measurement of lunar surface brightness temperatures thermal conductivity of the upper 1 1/2 meters of regolith

Abstract

In situ measurements of lunar surface brightness temperatures made as a part of the Apollo Lunar Surface Experiments Package at the Apollo 15 Hadley Rille landing site are reported. Data derived from 5 thermocouples of the Heat Flow Experiment, which are lying on or just above the surface, are used to examine the thermal properties of the upper 15 cm of the lunar regolith using eclipse and nighttime cool-down temperatures. Application of finite-difference techniques in modeling the lunar soil shows the thermocouple data are best fit by a model consisting of a low-density and low-thermal conductivity surface layer approximately 2 cm thick overlying a region increasing in conductivity and density with depth. Conductivities on the order of 1 × 10 −5 W/cm-°K are postulated for the upper layer, with conductivity increasing to the order of 1 × 10 −4 W/cm-°K at depths exceeding 20 cm. An increase in mean temperature with depth indicates that the ratio of radiative to conductive transfer at 350°K is 2.7 for at least the upper few centimeters of lunar soil; this value is nearly twice that measured for returned lunar fines. The thermal properties model deduced from Apollo 15 surface temperatures is consistent with earth-based microwave observations if electrical properties measured on returned lunar fines are assumed.