Thermal Conductivity of the Martian Soil at the InSight Landing Site From HP³ Active Heating Experiments

Abstract

The martian near surface layer consists of sand-sized as well as dust-sized particles (Christensen & Moore, 1992) interspersed with larger rocks, and its detailed structure depends on the deposition process as well as subsequent surface modifications by eolian and fluvial activity. Under present martian atmospheric conditions sand-sized particles in the 100-600 μm size range can be moved by winds through saltation (Kok et al., 2012), and dust particles of typical sizes around 1.5 μm are suspended in the atmosphere and can reach the ground in the form of airfall (Lemmon et al., 2019), such that aeolian processes are generally recognized to be the prevalent surface modification process on Mars today.

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

“…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.…”

“…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.…”

The InSight HP$^3$ Penetrator (Mole) on Mars: Soil Properties Derived From the Penetration Attempts and Related Activities

“…To measure thermal conductivity, the mole is used as a modified line-heat source (e.g., Carslaw and Jaeger, 1959;Banaskiewicz et al, 1997) when these foils (called TEM-A, short for Thermal Excitation and Measurement-Active) are supplied with a constant power, generating heat that is then conducted to the surrounding soil. The change in mole temperature is recorded during the heating and cooling phases (24 h and 48 h, respectively) to derive thermal conductivity (Grott et al, 2021).…”

“…Images of the pit wall suggest that there may be hollows in the duricrust but there is no way to prove or disprove their existence. An alternative proposal is based on the high porosity of the crust concluded from the interpretation of the TEM-A data that suggest a bulk density of 1211 +149 −113 kg/m 3 and a bulk porosity of 63 +9 −4 % (Grott et al, 2021). Assuming that the mole upon penetrating destroyed the fabric of the duricrust and reduced the porosity of the material, it is proposed that the pit formed as a consequence of the mole grinding duricrust to sand.…”

“…Heating the sensors and measuring the temperature rise (24 h heating) and fall (48 h cooling) as a function of time, would have allowed a measurement of the thermal conductivity (Spohn et al, 2018). Grott et al (2021) have recently published a measurement of the thermal conductivity by the mole at a depth of 40 cm, its final depth after unsuccessfully trying to dig deeper.…”