Thermal Properties of the Mojave Mars Regolith Simulant in Mars-Like Atmospheric Conditions

Kurzfassung

On Mars, thermal conductivity of regolith may vary with atmospheric conditions. When atmospheric pressure rises, the regolith’s pore gas pressure also increases, and that results in greater thermal conductivity, and vice versa. Atmospheric temperature fluctuation may also influence thermal properties of the regolith. In recent robotic Mars-landing missions, in-situ measurements of regolith thermal properties and heat flow have been attempted. The challenges for the in-situ measurements on these missions is that they may be able to capture only a snapshot of the regolith properties while they would continuously fluctuate with the atmospheric conditions. To support these in-situ measurement efforts, we attempted to characterize the possible annual variation of thermal properties of Mars regolith due to seasonal pore gas pressure and temperature fluctuations, using the Mojave Mars simulant (MMS) in Mars-like atmospheric conditions. Two batches of samples with differing densities (1540 kg·m−3 and 1660 kg·m−3) were placed in a thermal vacuum chamber, one at a time, filled with CO2 gas, and their thermal conductivity was measured by the hotwire and the dual-probe heat pulse (DPHP) methods. Their specific heat was measured by the DPHP method. Because these two methods were originally developed for Earth’s soils that have much greater thermal conductivity than the regolith on Mars, we modified their standard data reductions schemes, aided by finite-element thermal modeling of the individual measurement experiments. We found that thermal conductivity of MMS increased by~20 % from 600 Pa to 1000 Pa, the range of atmospheric pressure observed by the current and the previous Mars missions. We also found that specific heat of MMS increased by ~16 % from~− 30 °C (243 K) to 30 °C (303 K). Thermal diffusivity, which is the primary control factor of the thermal skin depth, could vary by 20 % to 25 % in the range of CO2 pressures and temperatures the MMS samples were tested .