Mole configuration during the heating experiment after scraping soil into the mole pit.
Mole configuration during the heating experiment after scraping soil into the mole pit. Credit: Grott et al. [2021], Figure 1b
Source: Journal of Geophysical Research: Planets

The InSight lading spacecraft, which successfully landed on Mars on 26 November 2018, contained a heat flow and physical properties (HP3) package. One component of this was a heating experiment designed to measure the Martian soil’s ability to transport heat. Grott et al. [2021] report on results of this experiment. Although HP3 was designed to be deployed in a vertical configuration below the ground up to 5 meters (16.4 feet) deep via a self-hammering penetrator dubbed the “mole”, a depth of only 30 centimeters was achieved due to issues with the deployment. However, that depth was sufficient to conduct the heating experiment (see image above).

Numerical modeling of the heating data resulted in soil thermal conductivity of 0.039 W/mK. This value is more than a factor of two smaller than determined at the Phoenix Mars mission, the only direct measurement of this quantity prior to InSight (Zent et al., 2010). The presence of cementing agents at Phoenix such as shallow subsurface water ice and perchlorate salts likely explain this difference. Overall, both values show that the Martian soil is a poor thermal conductor.

Comparisons with results from laboratory experiments were established to estimate the soil grain size, suggesting that the overwhelming majority of the soil particles are less than 200 microns or 0.2 mm (just over 1/16 inch) across, corresponding to a fine sand. This is consistent with the surface geology of the landing site, named Homestead hollow, being a depression filled with eolian or wind-blown deposits (Grant et al., 2020; Weitz et al., 2020). The heat flow data also suggested that the porosity values are high (>60%) and the degree of cementation is inferred to be low. This latter inference is a somewhat surprising result since visual observations strongly suggest the presence of a duricrust or partially cemented layer (Golombek et al., 2020; Marteau et al., 2021). The surface of Mars continues to yield mysteries, even when we dig beneath the surface.

Citation: Grott, M., Spohn, T., Knollenberg, J., Krause, C., Hudson, T. L., Piqueux, S., et al. [2021]. Thermal conductivity of the martian soil at the Insight landing site from HP3 active heating experiments. Journal of Geophysical Research: Planets, 126, e2021JE006861.

—Germán Martinez, Associate Editor, and Bradley J. Thomson, Editor, JGR: Planets

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