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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 siteThe soil thermal diffusivity was calculated from the diurnal and seasonal surface and soil temperature variationsThe soil temperature allows the formation of thin films of brine; their efflorescence may explain the formation of the observed duricrust [ABSTRACT FROM AUTHOR]

The heat flow and physical properties package measured soil thermal conductivity at the landing site in the 0.03–0.37 m depth range. Six measurements spanning solar longitudes from 8.0° to 210.0° were made and atmospheric pressure at the site was simultaneously measured using InSight's Pressure Sensor. We find that soil thermal conductivity strongly correlates with atmospheric pressure. This trend is compatible with predictions of the pressure dependence of thermal conductivity for unconsolidated soils under martian atmospheric conditions, indicating that heat transport through the pore filling gas is a major contributor to the total heat transport. Therefore, any cementation or induration of the soil sampled by the experiments must be minimal and soil surrounding the mole at depths below the duricrust is likely unconsolidated. Thermal conductivity data presented here are the first direct evidence that the atmosphere interacts with the top most meter of material on Mars. Plain Language Summary: A soil's ability to transport heat is a fundamental parameter that holds information on quantities like soil bulk porosity, composition, grain size, and the state of cementation or induration. In the soil, heat is transported through grain‐to‐grain contacts as well as through the pore filling CO2 gas. The heat flow and physical properties package (HP3) of the InSight Mars mission measured soil thermal conductivity at the landing site repeatedly over the course of a martian year. As atmospheric pressure changes between seasons due to the redistribution of CO2 across the planet, we found that soil thermal conductivity also changes. Thermal conductivity increased for increased atmospheric pressure, a behavior typical for unconsolidated material. This implies that the amount of cement or induration of the sampled soil must be minimal. Key Points: We measured thermal conductivity of the martian soil and found that its conductivity strongly correlates with atmospheric pressureWe conclude that heat conduction through the pore‐filling gas is significant and that cementation of the soil must be minimalOur data show that the atmosphere directly interacts with the top most meter of material on Mars [ABSTRACT FROM AUTHOR]

We use the Martian surface temperature response to Phobos transits observed next to the InSight lander in Elysium Planitia to constrain the thermal properties of the uppermost layer of regolith. Modeled transit lightcurves validated by solar panel current measurements are used to modify the boundary conditions of a 1D heat conduction model. We test several model parameter sets, varying the thickness and thermal conductivity of the top layer to explore the range of parameters that match the observed temperature response within its uncertainty both during the eclipse as well as the full diurnal cycle. The measurements indicate a thermal inertia (TI) of 103−16+22Jm−2K−1s−1/2 in the uppermost layer of 0.2–4 mm, significantly smaller than the TI of 200Jm−2K−1s−1/2 derived from the diurnal temperature curve. This could be explained by larger particles, higher density, or some or slightly higher amount of cementation in the lower layers. Plain Language Summary: The Mars moon Phobos passed in front of the Sun from the perspective of the InSight lander on several occasions. The Mars surface temperatures measured by the lander became slightly colder during these transits due to the lower amount of sunlight the surface received at this time. The transits only last 20–35 s and therefore only the very top layer, about 0.3–0.8 mm, of the ground has time to cool significantly. The top layer cools and heats up faster than we expected based on the temperature changes of the day‐night cycle, which affects about 4 cm of the ground. Based on this observation we conclude that the material in the top mm of the ground is different from that below. A possible explanation would be an increase of density with depth, a larger fraction of smaller particles such as dust at the top, or a layer where particles are slightly cemented together beginning at 0.2–4 mm below the surface. Key Points: The Martian surface temperature response to Phobos transits at the InSight landing site is interpretedThe thermal inertia of the uppermost layer of soil is 103−16+22Jm−2K−1s−1/2The thermal conductivity or density of the top 0.2–4 mm is significantly less than that of the top 4 cm [ABSTRACT FROM AUTHOR]

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%. 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. Key Points: The Heat Flow and Physical Properties Package (HP3) measured the average thermal conductivity of the martian soilAverage soil thermal conductivity in the 0.03–0.37 m depth range is 0.039 ± 0.002 W m−1 K−1This implies that 85%–95% of all particles are smaller than 104–173 μm [ABSTRACT FROM AUTHOR]

The (HP 3) of the InSight Mars mission is an instrument package to measure the Martian geothermal heat flow. The instrument's platinum resistance temperature sensors have been calibrated between −75 and +55 °C, and the absolute temperature uncertainty of the calibration is 10 mK. Temperature differences can be measured with an uncertainty of better than 5 mK. HP 3 determines regolith thermal conductivity by using the HP 3 mole as a modified line heat source, and thermal conductivity can be measured with an uncertainty of better than 4%. Tilt sensors used to determine the attitude of the mole during penetration measure tilt with an uncertainty of better than 0.25°. Key Points: The Heat Flow and Physical Properties Package (HP 3) of the InSight Mars mission is a heat flow probe designed to measure the temperature gradient and thermal conductivity of the Martian regolith to a depth of 5 mTo reconstruct sensor depth, tiltmeters measure the emplacement angle with an uncertainty of better than 0.25°Temperature sensors have been calibrated to measure temperature difference with an uncertainty of better than 5 mK; thermal conductivity can be measured with an uncertainty of better than 4%; all stated uncertainties are 1 σ confidence limits [ABSTRACT FROM AUTHOR]

The Heat Flow and Physical Properties Package HP3 for the InSight mission will attempt the first measurement of the planetary heat flow of Mars. The data will be taken at the InSight landing site in Elysium planitia (136 ∘E, 5 ∘N) and the uncertainty of the measurement aimed for shall be better than ±5 mW m−2. The package consists of a mechanical hammering device called the “Mole” for penetrating into the regolith, an instrumented tether which the Mole pulls into the ground, a fixed radiometer to determine the surface brightness temperature and an electronic box. The Mole and the tether are housed in a support structure before being deployed. The tether is equipped with 14 platinum resistance temperature sensors to measure temperature differences with a 1-σ uncertainty of 6.5 mK. Depth is determined by a tether length measurement device that monitors the amount of tether extracted from the support structure and a tiltmeter that measures the angle of the Mole axis to the local gravity vector. The Mole includes temperature sensors and heaters to measure the regolith thermal conductivity to better than 3.5% (1-σ) using the Mole as a modified line heat source. The Mole is planned to advance at least 3 m—sufficiently deep to reduce errors from daily surface temperature forcings—and up to 5 m into the martian regolith. After landing, HP3 will be deployed onto the martian surface by a robotic arm after choosing an instrument placement site that minimizes disturbances from shadows caused by the lander and the seismometer. The Mole will then execute hammering cycles, advancing 50 cm into the subsurface at a time, followed by a cooldown period of at least 48 h to allow heat built up during hammering to dissipate. After an equilibrated thermal state has been reached, a thermal conductivity measurement is executed for 24 h. This cycle is repeated until the final depth of 5 m is reached or further progress becomes impossible. The subsequent monitoring phase consists of hourly temperature measurements and lasts until the end of the mission. Model calculations show that the duration of temperature measurement required to sufficiently reduce the error introduced by annual surface temperature forcings is 0.6 martian years for a final depth of 3 m and 0.1 martian years for the target depth of 5 m. [ABSTRACT FROM AUTHOR]