Thermal conductivity of planetary regoliths: The effects of grain-size distribution

Highlights

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Thermal conductivity data is presented for analog planetary regoliths.

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Bimodal grain size mixtures reflect the pressure dependence of the fine grains.

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The grain size of the coarse component plays no role in bulk conductivity.

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A physically based predictive model of regolith thermal conductivity is given.

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Mars thermal inertia may reflect interstitial dust and sand-or-cobble mixtures.

Abstract

The thermal properties of a planet's regolith are of primary importance in solar system exploration. Thermal conductivity and the related parameter thermal inertia are often used to decipher the regolith's structure, grain size, and areal distribution. In this work we utilize a guarded-heat-flow apparatus to measure the thermal conductivity of mono-dispersed and bimodal size populations of regolith analogs (borosilicate-glass beads and terrestrial soils) under a range of interstitial gas pressures from 10⁻⁵ to 10³ mb, at 20 °C. From these measurements we further develop a physically based analytical model for use as a predictive tool for planetary research.

While our results for mono-dispersed grains agree well with previous studies, our findings for bimodal grain-size mixtures do not. Our results demonstrate that the functional dependence of thermal conductivity on interstitial gas pressure closely follows that of the fine-grained component of the mixture, but uniformly offset to higher conductivity values depending only on the volume fraction of the coarse component. The grain size of the coarse fraction plays no role. The reason for the difference with previous studies appears to be related to limitations with the transient-heated-wire method in previous work. Our results suggest that, on Mars, large quantities of coarse grain material, such as sand or cobbles, could be hidden in regional dust deposits. Additionally, variations in the thermal properties of aeolian dune fields may result from differences between age-related dust infiltration and self-cleansing sand migration, rather than any real differences in local grain size. On Earth, at high gas pressures, grain size mixtures result in higher thermal conductivity than any component alone, consistent with field observations.