The heat flow and physical properties package measured soil thermal conductivity at the landing site in the 0.03 to 0.37 m depth range. Six measurements spanning solar longitudes from 8.0$^\circ$ to 210.0$^\circ$ 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. This implies that any cementation or induration of the soil sampled by the experiments must be minimal and that the soil surrounding the mole at depths below the duricrust is unconsolidated. Thermal conductivity data presented here are the first direct evidence that the atmosphere interacts with the top most meter of material on Mars.

Observations of the South Polar Residual Cap suggest a possible erosion of the cap, leading to an increase of the global mass of the atmosphere. We test this assumption by making the first comparison between Viking 1 and InSight surface pressure data that have been recorded with ~40 years of difference. Such a comparison also allows us to determine changes in the dynamics of the seasonal ice caps between these two periods. To do so, we first had to recalibrate the InSight pressure data because of their unexpected sensitivity to the sensor temperature. Then, we had to design a procedure to compare distant pressure measurements. We propose two surface pressure interpolation methods at the local and global scale to do the comparison. The comparison of Viking and InSight seasonal surface pressure variations does not show major changes in the CO2 cycle. Such conclusions are also supported by an analysis of the Mars Science Laboratory (MSL) pressure data. Further comparisons with images of the south seasonal cap taken by the Viking 2 orbiter and MARCI camera do not display significant changes in the dynamic of this cap within ~40 years. Only a possible larger extension of the North Cap after the global storm of MY 34 is observed, but the physical mechanisms behind this anomaly are not well determined. Finally, the first comparison of MSL and InSight pressure data suggests a pressure deficit at Gale crater during southern summer, possibly resulting from a large presence of dust suspended within the crater.

The heat flow and physical properties package (HP$^3$) 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 to 0.37 m depth range was performed. Using the HP$^3$ mole as a modified line heat source, we determined a soil thermal conductivity of 0.039 $\pm$ 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 104-173 $\mu$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. Soil densities compatible with the measurements are 1211$_{-113}^{+149}$ kg m$^{-3}$, indicating soil porosities of 61 \%.

Despite nearly complete coverage of the Martian surface with thermal infrared datasets, uncertainty remains over a wide range of observed thermal trends. Combinations of grain sizes, packing geometry, cementation, volatile abundances, subsurface heterogeneity, and sub-pixel horizontal mixing lead to multiple scenarios that would produce a given thermal response at the surface. Sedimentary environments on Earth provide a useful natural laboratory for studying how the interplay of these traits control diurnal temperature curves and identifying the depositional contexts those traits appear in, which can be difficult to model or simulate indoors. However, thermophysical studies at Mars-analog sites are challenged by distinct controls present on Earth, such as soil moisture and atmospheric density. In this work, as part of a broader thermophysical analog study, we developed a model for determining thermal properties of in-place sediments on Earth from thermal imagery that considers those additional controls. The model uses Monte Carlo simulations to fit calibrated surface temperatures and identify the most probable dry thermal conductivity as well as any potential subsurface layering. The program iterates through a one-dimensional surface energy balance on the upper boundary of a soil column and calculates subsurface heat transfer with temperature-dependent parameters. The greatest sources of uncertainty stem from the complexity of how thermal conductivity scales with water abundance and from surface-atmosphere heat exchange, or sensible heat. Using data from a 72-hr campaign at a basaltic eolian site in the San Francisco Volcanic Field, we tested multiple models for how dry soil components and water contribute to thermal conductivity and multiple approaches to estimating sensible heat from field measurements. Field measurements include: upwelling and downwelling radiation, air temperature, relative humidity, wind speed, and soil moisture, all collected from a ground station, as well as UAV-derived surface geometries. By mitigating Earth-specific uncertainty and isolating the controls that are most relevant to Martian sediments, we can then validate those controls with in situ thermophysical probe measurements and ultimately improve interpretations of thermal data for the Martian surface.

We use the surface temperature response to Phobos transits as observed by a radiometer on board of the InSight lander 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 of 103+48-24 Jm-2K-1s-1/2 in the uppermost layer of 0.2 to 4 mm, significantly smaller than the thermal inertia of 200 Jm-2K-1s-1/2 derived from the diurnal temperature curve. This could be explained by larger particles, higher density, or a very small amount of cementation in the lower layers.

Before dawn on the dustiest regions of Mars, surfaces measured at or below ∼ 148 K are common. Thermodynamics principles indicate that these terrains must be associated with the presence of CO2 frost, yet visible wavelength imagery does not display any ice signature. We interpret this systematic absence as an indication of CO2 crystal growth within the surficial regolith, not on top of it, forming hard-to-distinguish intimate mixtures of frost and dust, i.e., dirty frost. This particular ice/regolith relationship unique to the low thermal inertia regions is enabled by the large difference in size between individual dust grains and the peak thermal emission wavelength of any material nearing 148 K (1-2 μm vs. 18 μm), allowing radiative loss (and therefore ice formation) to occur deep within the pores of the ground, below several layers of grains. After sunrise, sublimation-driven winds promoted by direct insolation and conduction create an upward drag within the surficial regolith that can be comparable in strength to gravity and friction forces combined. This drag displaces individual grains, possibly preventing their agglomeration, induration, and compaction, and can potentially initiate or sustain downslope mass movement such as slope streaks. If confirmed, this hypothesis introduces a new form of CO2-driven geomorphological activity occurring near the equator on Mars and explains how large units of mobile dust are currently maintained at the surface in an otherwise soil-encrusting world.