Thermal Conductivity of the Martian Soil at the InSight Landing Site From HP3 Active Heating Experiments
Creators: name: Grott, M.
name: Spohn, T.
name: Knollenberg, J.
name: Krause, C.
name: Hudson, T. L.
name: Piqueux, S.
name: Müller, N.
name: Golombek, M.
name: Vrettos, C.
name: Marteau, E.
name: Nagihara, S.
name: Morgan, P.
name: Murphy, J. P.
name: Siegler, M.
name: King, S. D.
name: Smrekar, S. E.
name: Banerdt, W. B.
Publisher: name: FID GEO
Year: 2021

description: 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 subsurface and measure the 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 first reliable measurement of the average soil thermal conductivity in the 0.03–0.37 m depth range was performed. Using the HP3 mole as a modified line heat source, we determined a soil thermal conductivity of 0.039 ± 0.002 W m−1 K−1, consistent with the results of orbital and in‐situ thermal inertia estimates. This low thermal conductivity implies that 85%–95% of all particles are smaller than 104–173 μm and suggests that soil cementation is minimal, contrary to the considerable degree of cementation suggested by image data. Rather, cementing agents like salts could be distributed in the form of grain coatings instead. Soil densities compatible with the measurements are 1211−113+149 kg m−3, indicating soil porosities of 63−9+4%.
description: Plain Language Summary: The heat flow and physical properties package (HP3) of the InSight Mars mission is an instrument package that was designed to measure soil temperature as well as the soil's ability to transport heat, the so called thermal conductivity. After the probe was inserted to a depth of 0.37 m a first measurement of the soil's thermal conductivity was performed. The soil was found to be a poor thermal conductor with average conductivity close to 0.039 W m−1 K−1. As thermal transport properties in sands are related to grain size, the latter can be estimated based on the performed measurement. We find that particles must be smaller than about 150 μm, corresponding to fine sand that may be intermixed with dust. Further, salts in the soil can act as cementing agents, which connect individual sand grains and thus increase the strength of grain‐to‐grain contacts and therefore thermal conductivity. However, given the low thermal conductivity determined here, the amount of such cement must be minimal, contrary to what is suggested by image data. Finally, we find that the soil must have significant porosity of about 60% to be compatible with our measurements.
description: Key Points: The Heat Flow and Physical Properties Package (HP3) measured the average thermal conductivity of the martian soil. Average soil thermal conductivity in the 0.03–0.37 m depth range is 0.039 ± 0.002 W m−1 K−1. This implies that 85%–95% of all particles are smaller than 104–173 μm.
description: Deutsches Zentrum für Luft‐ und Raumfahrt http://dx.doi.org/10.13039/501100002946
description: National Aeronautics and Space Administration http://dx.doi.org/10.13039/100000104
description: Österreichischen Akademie der Wissenschaften http://dx.doi.org/10.13039/501100001822
description: Polish Academy of Science