Thermal Properties of the Mojave Mars Regolith Simulant and Their Sensitivity to Ambient CO2 Gas Pressure and Temperature

Abstract

Determination of the endogenic heat flow of Mars is one of the InSight mission’s objectives, and that will be done based on in-situ temperature and thermal conductivity measurements made in shallow (< 5-m depth) subsurface regolith. Previous studies on soil-like materials on Earth suggest that their bulk thermal properties can be sensitive to changes in temperature and atmospheric pressure in the ranges observed/expected at the InSight landing site. There, surface temperature varies diurnally from 170 K to 290 K. Atmospheric pressure would vary from 600 Pa to 1000 Pa seasonally. Thermal properties of Mars’ shallow regolith may fluctuate diurnally and seasonally with large enough amplitudes to significantly affect the heat flow through it. In order to quantify the possible effects of temperature and atmospheric pressure variations on Mars regolith, we carried out thermal conductivity and volumetric heat capacity measurements on the Mojave Mars Simulant (MMS) in a CO2-filled, thermal vacuum chamber. The needle probe (also known as the hot wire) method was used for the former, and the dual-probe heat-pulse method was use for the latter. The simulant was placed in a 1-gallon bin and vibratory compacted to a density of 1500 kg/m3. The two probes were then inserted to the simulant, and the entire assembly was placed in a thermal vacuum chamber of 0.7 m x 0.7 m x 0.7 m. While the temperature inside the chamber was fixed, a series of thermal measurements were taken at 400-, 600-, 800-, and 1000-Pa chamber CO2 pressures. Such series of measurements were made at chamber temperatures 241 K, 294 K and 303 K. The data show that thermal conductivity of the simulant increased by 30% in each run from 600 Pa to 1000 Pa. Volumetric heat capacity of the simulant increased by more than 10% from 241 K to 294 K. The pressure-induced variation of this magnitude should be detectable by periodically repeating in-situ thermal conductivity measurements over the duration of the mission (2 earth years) at the final depth the heat flow probe reaches. The knowledge of the temperature-induced variation in thermal properties will help us determine the regolith depth influenced by the insolation cycles of the landing site.