Thermal Conductivity of the Martian Regolith at the InSight Landing site from HP3 Active Heating Experiments

The heat flow and physical properties package (HP3) of the InSight Mars mission is an instrument package designed to determine the martian planetary heat flow. To this end, the package was designed to emplace sensors into the martian regolith and measure the regolith thermal conductivity as well as the geothermal gradient in the 0-5 m depth range. After emplacing the probe to a tip depth of 0.37 m, a rst reliable measurement of the average regolith thermal conductivity in the 0.03 to 0.37 m depth range was performed. Using the HP3 mole as a modied line heat source, we determined a regolith thermal conductivity of 0.039 0.002 W m􀀀1 K􀀀1, consistent with the results of orbital and in-situ thermal inertia measurements. This low thermal conductivity implies that 85 to 95% of all particles are smaller than 125-160 m and suggests that any cement contributing to soil cohesion cannot significantly increase grain-to-grain contact areas by forming cementing necks, but could be distributed in the form of grain coatings instead. Regolith densities compatible with the measurements are 1211+149-113 kg m􀀀3, indicating regolith porosities of 61 %.