A Model for the Thermophysical Properties of Lunar Regolith at Low Temperatures

Abstract

The thermophysical properties of lunar regolith have been thoroughly investigated for temperatures higher than 100 K. For the near‐equatorial thermal measurements of the Apollo era, this temperature range was sufficient to generate appropriate models. However, recent measurements from the Lunar Reconnaissance Orbiter Diviner Lunar Radiometer Experiment have revealed polar temperatures as low as 20 K, with apparently lower thermal inertia than explainable by existing theory. In the absence of comprehensive laboratory measurements of regolith thermal properties at low temperatures (<100 K), we investigate solid state theory and fits to lunar simulant materials to derive a semiempirical model of specific heat and thermal conductivity in lunar regolith in the full range 20–400 K. The primary distinctions between these previous models are (1) the temperature dependence of the solid conduction component of thermal conductivity at low temperatures, (2) the focus on regolith bulk density as the primary variable, and (3) the concept that the composition and modal petrology of grains could significantly influence thermal properties of the bulk regolith. This model predicts that at low temperatures, thermal conductivity is as much as an order of magnitude lower and specific heat is likely higher than expected from current models. The thermal conductivity at low temperature should vary depending on the constituent grain materials, their crystallinity, contributions from phonon scattering modes, bulk porosity, and density. To demonstrate the impact of our finding, we extrapolate the effects of our conductivity model on temperature variations in permanently shadowed regions on the Moon. This work motivates experimental confirmation of thermophysical properties of lunar regolith at low temperature.

Lunar Regolith Temperature Variation in the Rümker Region Based on the Real-Time Illumination

“…This result confirms that the lunar regolith tends to slow down thermal conduction [13,16]. Such thermal insulation could be ascribed to the porous, pulverized, and granular attributes of the regolith, as investigated by the study [23]. Its improvement is the prediction of a lower thermal conductivity but a higher specific heat than previous research [9−13].…”

“…This research found potential landing sites for future explorations but did not further study subsurface temperature variation. The recent studies [13,23] explored the lunar regolith thermophysical properties. They used a theoretical formula of the lunar orbital elements to estimate the solar hour angle and the incident solar flux for one lunar day.…”

An Exogenic Origin for the Volatiles Sampled by the LCROSS Impact

“…The long-term stability of each species depends on how the temperature varies diurnally with depth (12). Thermal modeling shows that temperatures are stable below ~ 0.2 m depth (13). The LCROSS impactor was estimated to have excavated material from 1-3 meters deep in the PSR (14), so the volatiles observed in the plume originated below the depth of thermal stability.…”

Study of Chang’E-2 Microwave Radiometer Data in the Lunar Polar Region

“…Other speculations of such correlation include the fact that the presence of water ice deposits changes the average physical properties of lunar regolith, e.g., thermal inertia, dielectric constant, which results in large TB variations at different depths. Simulation of TB with physical parameters of lunar regolith and accurate solar flux in PSRs is necessary to provide proof for such speculation; however, the difficulty in obtaining the thermophysical parameters of lunar regolith at extreme low temperature and unknown rock abundance, etc., would result in the uncertainties of results [40]. Whether the temperature resolution of MRM data is enough to detect the presence of ice deposits is also questionable.…”