Journal Article

Mars Soil Temperature and Thermal Properties From InSight HP3 ${\\mathrm{H}\\mathrm{P}}^{3}$ Data

Knollenberg, J., Banerdt, W. B., Grott, M., Plesa, A.‐C., Morgan, P., Krause, C., Spohn, T., Breuer, D., Müller, N., Smrekar, S. E., Bickel, V. T., Golombek, M. P.

2024

Overview

Diurnal and seasonal variations in soil and surface temperature measured with the HP3 ${\\mathrm{H}\\mathrm{P}}^{3}$ thermal probe and radiometer of NASA's InSight Mars mission are reported. At a representative depth of 10–20 cm, an average temperature of 217.5 K was found, varying by 5.3–6.7 K during a sol and by 13.3 K during the seasons. From the damping of the temperature variation with depth and the phase shift, a thermal diffusivity of (3.93 ± $\\pm $ 0.39) × 10−8 ${10}^{-8}$ m2 ${\\mathrm{m}}^{2}$/s was derived for the upper ∼ ${\\sim} $10 cm from the diurnal temperature variation and of (3.63 ± $\\pm $ 0.53) × 10−8 ${10}^{-8}$ m2 ${\\mathrm{m}}^{2}$/s for the ∼ ${\\sim} $40 cm depth range of the mole from the annual temperature variation. Using published thermal conductivity and inertia values together with the diffusivities, soil densities of 1,470 and 1,730 kg/m3 ${m}^{3}$ were derived for these depths. The temperatures allow the deliquescence of thin films of brine, the efflorescence of which may explain the cemented duricrust observed. Plain Language Summary Temperature is an important factor in understanding the physical properties of Martian soil. It determines how quickly physical processes and chemical reactions occur, including the transport of heat and materials. Temperature is crucial to astrobiology because it affects the habitability of the soil and the potential for water or brine to support microbial life. We measured the temperature in the soil during several Martian days and over a Martian year using the NASA InSight Mars mission's Heat Flow and Physical Properties Package. The average temperature was −56°C (217.5 K) over the depth extent of the thermal probe, which was about 40 cm. The temperature varied by 5–7° during the day, which is only a tenth of the daily surface temperature variation. It varied by 13° during the seasons. The temperature is subfreezing for water, but it allows the formation of thin films of salty brine for 10 hr or more during a Martian day. The solidification of the brine is a likely explanation for the observed few tens of centimeters thick duricrust, a layer of consolidated, cohesive sand, which is thought to have hampered the penetration to greater depth of the mission's thermal probe. Key Points We measured the temperature and its diurnal and annual variations in the top 40 cm of the Martian soil at the InSight landing site The soil thermal diffusivity was calculated from the diurnal and seasonal surface and soil temperature variations The soil temperature allows the formation of thin films of brine; their efflorescence may explain the formation of the observed duricrust

• Publisher: John Wiley & Sons, Inc
• Subject: Annual temperatures / Astrobiology / Brines / Chemical reactions / Daily temperatures / Damping / deliquescence/efflorescence of brine / Depth / Diurnal / duricrust / Efflorescence / Habitability / Heat flow / Heat transfer / Heat transmission / Inertia / Mars / Mars missions / Mars soil / Mars soil temperature / Mars soil thermal diffusivity / Mars surface / Mars surface temperature / Microorganisms / Physical properties / Radiometers / Seasonal variation / Seasonal variations / Soil / Soil chemistry / Soil density / Soil properties / Soil temperature / Soils / Solidification / Surface temperature / Temperature / Temperature probes / Temperature variations / Thermal conductivity / Thermal diffusivity / Thermal properties / thermal skin depth / Thermodynamic properties / Thick films / Thin films