Soil Thermophysical Properties Near the InSight Lander Derived From 50 Sols of Radiometer Measurements

Abstract

The Interior Exploration using Seismic Investigations, Geodesy and Heat Transport mission (InSight; Banerdt et al., 2020) landed on Mars in Elysium Planitia, on November 26, 2018, with the goal of understanding the formation and evolution of terrestrial planets through the investigation of the interior structure of Mars. To meet these objectives, the lander carries multiple instruments, including the Heat Flow and Physical Properties Package (HP 3 ;Spohn et al., 2018) that consisted of a penetrator, colloquially referred to as "the mole," a tether equipped with platinum resistance temperature sensors (TEM-P), and a suite of Abstract Measurements from the InSight lander radiometer acquired after landing are used to characterize the thermophysical properties of the Martian soil in Homestead hollow. This data set is unique as it stems from a high measurement cadence fixed platform studying a simple well-characterized surface, and it benefits from the environmental characterization provided by other instruments. We focus on observations acquired before the arrival of a regional dust storm (near Sol 50), on the furthest observed patch of soil (i.e., ∼3.5 m away from the edge of the lander deck) where temperatures are least impacted by the presence of the lander and where the soil has been least disrupted during landing. Diurnal temperature cycles are fit using a homogenous soil configuration with a thermal inertia of 183 ± 25 J m −2 K −1 s −1/2 and an albedo of 0.16, corresponding to very fine to fine sand with the vast majority of particles smaller than 140 μm. A pre-landing assessment leveraging orbital thermal infrared data is consistent with these results, but our analysis of the full diurnal temperature cycle acquired from the ground further indicates that near surface layers with different thermophysical properties must be thin (i.e., typically within the top few mm) and deep layering with different thermophysical properties must be at least below ∼4 cm. The low thermal inertia value indicates limited soil cementation within the upper one or two skin depths (i.e., ∼4-8 cm and more), with cement volumes <<1%, which is challenging to reconcile with visible images of overhangs in pits.Plain Language Summary InSight carried a six-channel radiometer used to measure diurnal surface temperatures over the duration of the mission. Surface temperatures are controlled by insolation and the microscopic physical properties of the soil (typical grain size, density, degree of soil cementation, or internal layering) that could not be resolved without a microscope or other instruments. Because the InSight lander does not have any systematic way to interrogate the soil, we have instead analyzed these temperature data and characterized the near-surface soil properties. We found that the soil structure near the lander is homogeneous within the top few cm, and is made of loose sandy material with very little to no cementation. These properties are consistent with those derived from orbit before landing using remote sensing technique...

Near Surface Properties of Martian Regolith Derived From InSight HP3‐RAD Temperature Observations During Phobos Transits

“…The diurnal curves in this study can be fitted with a thermal conductivity of 50 and the more detailed study of the data up to sol 50 by Piqueux et al (2021) (Grott et al, 2021). The reason why the bulk TI appears to be changing over time is not clear yet.…”

“…Thus there is no evidence for unexpected properties of the regolith at the landing site; however, the Heat Flow and Physical Properties Package ( 3 HP ) mole (Spohn et al, 2018) failed to deploy its instrumented tether to the subsurface (Figure 1). The mole is designed to measure thermal conductivity and the results at its current position in the top 40 cm of regolith are consistent with the observed TI (Grott et al, 2021;Piqueux et al, 2021). The steep wall of the pit created by the penetration attempt, and clasts embedded therein, have been interpreted as evidence for a duricrust , that is, that the bonds between grains are strengthened by a cementing material.…”

“…Increasing the top layer thickness further, or decreasing the lower half-space thermal conductivity, does not significantly affect the transit temperature minimum, and only slightly reduces the post-transit temperatures. The temperatures recorded after the transit on sol 501 (Figure 3a) seem to indicate a top layer thicker than 4 mm, but this can be ruled out by comparison to the diurnal data (see also Piqueux et al, 2021). The 4 mm top layer already cools too fast in the afternoon and evening just so that model temperatures fall outside of the total measurement error bar around 19 h local true solar time (LTST) (Figure 3b).…”

“…The most consequential input variable is the visible wavelength atmospheric dust opacity  (Table 1) derived from sky imaging (Banfield et al, 2020). The dust optical properties are the same as in the work of Piqueux et al (2021) based on the work of Vasavada et al (2017). The visible flux is modified with the relative transit light curves, as shown in Figure 2.…”

Specific Heat Capacity Measurements of Selected Meteorites for Planetary Surface Temperature Modeling

“…We also ignore the impact of experimental Cp(T) measurements uncertainties (i.e., ∼4-5%) because they only yield an equivalent square root uncertainty (see Equation 2) on thermal inertia values, which is about one order of magnitude smaller than remotely-derived thermal inertia values uncertainties reported in the literature from robotic missions (Edwards et al, 2018;R. L. Fergason, Christensen, Bell, et al, 2006;Piqueux, Müller, et al, 2021).…”