Comparison among the in-situ thermal properties measurements on InSight and Phoenix and laboratory measurements on Mars regolith simulant

Abstract

The Phoenix mission was the first to measure the thermal properties of regolith of Mars in situ. The Thermal and Electrical Conductivity Probe (TECP) was inserted to the surface regolith by the robotic arm of the spacecraft at 7 locations andobtained several repeated measurements of thermal conductivity and volumetric heat capacity for a duration of 1 to 2 sols at each location. Volumetric heat capacity is a product of density and specific heat. Nearly a decade later, the first heat flow measurement on Mars was attempted on the InSight mission. The Heat flow and Physical Properties Package (HP3), unfortunately, penetrated only ~35 cm into the regolith, not deep enough to obtain the thermal gradient representative of the heat flow from the martian interior. However, HP3 repeatedly measured in-situ thermal conductivity of the regolith over a duration of a Martian year and observed its seasonal variation. The average of the thermal conductivities reported from Phoenix (0.085 W/m/K) is more than twice as that of InSight (0.038 W/m/K). Here we discuss possible implications of these results by also comparing them to laboratory measurements on Mars regolith simulant conducted under Mars-like atmospheric conditions.