Knowledge Gaps in our Understanding of the Mechanisms Governing Heat Transport in Regolith at Low Temperatures

Abstract

Permanently shadowed areas (PSAs) on, e.g., the Moon or Mercury, can show surface temperatures down to 30 K (Martinez & Siegler 2021) and can act as cold traps for water ice. At such cryogenic temperatures, water ice will be stable over geological timescales and PSAs are therefore of high interest for in-situ resource utilization (ISRU) efforts due to the potential of finding stable ice in the shallow subsurface. To better understand the ice deposition process and to better predict water ice stability, reliable models using realistic material parameters are needed. At cryogenic temperatures (<80 or 90 K), the conventional assumptions on thermophysical properties of regolith for the description of thermal transport break down. Notably specific heat capacity cp and effective thermal conductivity, keff will differ significantly from their values at 250 K. In addition, the thermal conductivity of the matrix material can change very significantly. However, at low temperatures, the heat transport process through this mixture of crystalline and amorphous components is not well understood. For example, crystalline grains at low T can have a solid thermal conductivity orders of magnitude higher than amorphous (glassy) particles; it is not clear how to combine the components in a mix of crystalline and amorphous grains.