The effect of interstitial gaseous pressure on the thermal conductivity of a simulated Apollo 12 lunar soil sample

Abstract

The thermal conductivity of a simulated Apollo 12 lunar soil sample was measured with a needle probe under vacuum. The result showed that the sample, with bulk densities of 1.70–1.85 g cm −3 held in a vertical cylinder (2.54 cm in diameter and 6.99 cm long) has a thermal conductivity ranging from 8.8 to 10.9 mW m −1 K −1. This is comparable to the lunar regolith's thermal conductivity as determined in situ. Besides the dense packing of the soil particles, an enhanced intergranular thermal contact, due to the self-compression of the sample, is necessary to raise the sample's thermal conductivity from the level of loose soil (< 5 mW m −1 K −1) to that of the lunar regolith deeper than 35 cm (∼ 10 mW m −1 K −1). A model of the lunar regolith, a thin layer of loose soil resting on a compacted self-compressed substratum, is consistent with the lunar regolith's surface structure as deduced from an observation of the lunar surface's brightness temperature. Martian regolith surface structure is similar, except that its surface layer may be missing in places because of aeolian activity. Measurements of thermal conductivity under simulated martian surface conditions showed that the thermal properties of loose and compacted soils agreed with the two peak values of the martian surface's thermal inertia as observed from “Viking” orbiters, suggesting that drifted loose soil and exposed compacted soil are responsible for the bimodal distribution of the martian surface's thermal inertia near zero elevation. For compacted soil exposed to the martian surface to have the same thermal conductivity as that buried under the surface layer, a cohesion of the soil particles must be assumed.