A Global Thermal Conductivity Model for Lunar Regolith at Low Temperatures.

Although some of the coldest surface temperatures in the entire Solar System are found near the poles of our own Moon, the thermophysical properties of lunar regolith at these ultracold temperatures (i.e., below ∼150 K) are not well understood. Standard lunar thermal models generally match the surface temperatures observed by global orbital remote sensing data but are inconsistent with infrared data collected from ultracold polar terrain. We build upon previous theoretical work on the low‐temperature physics of lunar regolith to introduce a global thermal conductivity model consistent with the temperature trends observed by the Diviner Lunar Radiometer Experiment (Diviner). This updated thermophysical model primarily affects nighttime surface temperatures, subsurface temperatures at high latitudes, and permanently shadowed regions (PSRs). An additional outcome of this thermophysical model is the ability to accommodate the surface temperature trends observed by Diviner both in warm low latitudes and cold high latitudes. Subsurface temperatures in near‐polar craters are ∼5–10 K warmer than previous thermal models, and cooler nighttime surface temperatures are observed globally. Model results of PSRs reveal larger surface temperature amplitudes (as observed by Diviner) and steeper geothermal gradients. A comprehensive understanding of lunar regolith's low‐temperature thermal behavior is an essential step in modeling the potential location and quantity of cold trapped volatiles in the lunar south pole. Here, we hope to provide theoretical support and motivation for more complete low‐temperature thermal conductivity laboratory measurements. Plain Language Summary: A theory for describing the low‐temperature thermal properties of lunar soil is developed and applied to the top outer layer of the Moon. We compare model results with global surface temperature data collected by the Diviner Lunar Radiometer Experiment (Diviner) onboard the Lunar Reconnaissance Orbiter (LRO). The new thermal model differs from previous standard models in several ways. The model produces cooler nighttime surface temperatures at low and high latitudes, and we observe warmer subsurface temperatures with increasing latitude. Additionally, the updated model provides larger surface temperature amplitudes in polar terrain, specifically in shadowed near‐polar craters. Overall, a novel outcome of this new model is the ability to accommodate surface temperature trends observed by Diviner both at low and high latitudes, and the ability to predict warmer subsurface temperatures in very cold polar regions. By updating the low‐temperature thermal properties of lunar soil, we hope to better understand the stability of important organic volatiles, such as water ice, in the polar regions of the Moon. Key Points: We develop a low‐temperature thermal conductivity model of lunar regolith that agrees with in situ and orbital remote sensing dataA temperature‐dependent solid conduction term accommodates the large surface temperature amplitudes observed in low‐temperature regionsAverage subsurface temperatures in near‐polar terrain may be ∼5–10 K warmer than previous thermal model calculations [ABSTRACT FROM AUTHOR]