^1,2Xinxin Yan, ³Xinzhuan Guo, ³Yun Zhou, ^1,2Yuping Song, ⁴Qingshan Zhang, ^1,2Meng Lv
Earth and Planetary Science Letters 684, 120009 Link to Article [https://doi.org/10.1016/j.epsl.2026.120009]
¹Key Laboratory of High-Temperature and High-Pressure Study of the Earth’s Interior, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, Guizhou 550081, China
²University of Chinese Academy of Sciences, Beijing 100049, China
³State Key Laboratory of Critical Mineral Research and Exploration, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, Guizhou 550081, China
⁴China University of Mining and Technology, Xuzhou 221116, China
Copyright Elsevier

The thermal conductivity and diffusivity of mantle minerals fundamentally control planetary cooling rates. Orthopyroxene is a major constituent of the lunar mantle, yet the influence of trace water on its thermophysical properties under high-pressure and high-temperature conditions relevant to the lunar interior has remained unquantified. Here, we present high P–T measurements of these properties for synthetic enstatite containing 0–427 ppm H2O using an enhanced transient plane source method. Our results demonstrate that even trace water drastically reduces thermal transport efficiency by enhancing phonon scattering. Incorporating these data into lunar thermal evolution models reveals that a hydrated mantle maintains significantly higher internal temperatures than an anhydrous system over geologic time. By reconciling our model geotherms with the solidus of various lunar mantle constituents and with seismic constraints on the largely solid modern mantle, we constrain the bulk water content of the lunar mantle to around 300 ppm. This work redefines the thermal state of the Moon and provides a critical mechanism for explaining its prolonged magmatic evolution.