Thermal Conductivity of Lunar Simulant With Varying Water-Ice Saturation and Morphology

Abstract

Water ice in lunar polar regolith is both a scientific target and a potential resource for sustained exploration, but its abundance cannot be inferred reliably unless its physical morphology is also constrained. Here we measure the thermal conductivity of a lunar regolith simulant under low-temperature and low-pressure conditions relevant to the lunar polar environment, with systematic variations in dry density, ambient pressure, temperature, ice saturation, and pore-ice morphology. Two contrasting specimen-preparation routes were used to generate ice-cemented (IC) and inter-mixed fabrics. Low-field nuclear magnetic resonance measurements and soil-freezing characteristic analysis show that unfrozen water is negligible under the thermal conductivity test conditions, so the observed response can be interpreted primarily in terms of solid ice content and ice morphology. The results show that the thermal conductivity of the dry simulant is governed jointly by density, pressure, and temperature through changing contributions from particle-contact conduction, pore-gas conduction, and radiative transfer. For ice-bearing specimens, thermal conductivity increases with ice saturation in both fabrics, but the conductivity-saturation trajectories differ fundamentally because the pore ice is distributed differently. Inter-mixed specimens, in which ice occurs mainly as discrete pore-filling particles, show a gradual increase close to lower-bound composite predictions, whereas IC specimens, in which ice bridges and cements particle contacts, show a much faster sigmoidal increase with saturation. These results demonstrate that thermal conductivity is not simply a proxy for ice abundance, but a joint indicator of ice content and pore-ice morphology, with direct implications for interpreting future thermophysical measurements in the lunar south polar region.