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Mars Soil Temperature and Thermal Properties as Constrained by InSight HP³ Data

Presentation #217.03 in the session Mars’ and Martian Moons’ Surface Properties and Composition (Oral Presentation)

Published onOct 23, 2023
Mars Soil Temperature and Thermal Properties as Constrained by InSight HP³ Data

Soil temperature is of primary importance for the values of physical properties such as e.g., elasticity, thermal conductivity and heat capacity, which are temperature dependent. Its value and the manner in which it varies in time and space determines the rates and directions of soil physical processes and of energy and mass exchange with the atmosphere. Moreover, temperature governs the rates of chemical reactions that take place in the soil, including biological processes and is of particular interest to astrobiology.

Diurnal and annual surface and soil temperature variations have been measured using sensors on the NASA InSight Mars mission’s HP3 mole and radiometer. The diurnally averaged surface brightness temperature has been recorded for almost two Martian years (more specifically, 1225 sols) and was found to vary by ±7 K through the seasons around an average value of 222.4 K. The peak-to-peak diurnal brightness temperature varies through the seasons by 85 K for the coldest sol sampled by a 24h recording to 105 K for the warmest sol. At the depth of the mole extending from 1 – 36 cm temperature recordings are available from sol 680 on, when the mole was fully buried. Temperature was recorded by the TEM-A sensors with an accuracy of <1K for 24h on 6 sols and by a H/K sensor mounted at the mole motor twice a sol for 550 sols during the second Martian year of the mission. The mole H/K sensor was recalibrated using the TEM-A recordings and the recordings of the temperature sensors on the HP3 science tether, originally planned to record the temperature gradient in the soil. An average soil temperature of 219 K was measured varying by 5.3 for the coldest sol 827 (peak-to-peak) and 6.7 K during the warmest sol 1202 (of the 6 sols of the 2nd Martian year sampled for 24h). The daily averaged temperature varies by 12.8 K (peak-to-peak) during the seasons. Considering the damping and the phase shift of the thermal wave, a representative depth of 7 cm was found for the diurnal signal and a representative depth of about 20 cm for the seasonal variation. The damping and the phase shifts of the diurnal and seasonal waves have been used to derive a depth to the center of the back-cap of the mole of 1.2 cm and a thermal diffusivity increasing from of 2.5 x 10-8 m2/s for the diurnal wave with a skin depth of 2.6 cm to 8 x 10-8 m2/s for the annual wave with a skin depth of about 1.2 m. These values compare with a thermal diffusivity derived from the HP3 thermal conductivity measurement (k = 0.039 W/m K) of 5 x 10-8 m2/s. The latter would be representative of a depth to the mole center of 17.5 cm.

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