Using the HP3 mole on InSight to probe the thermal and mechanical properties of the Martian regolith

The NASA InSight Lander on Mars includes the Heat Flow and Physical Properties Package HP3 (see Spohn et al. (2018) for a description of the package) to measure the surface heat flow of the planet. The package uses temperature sensors that would have been brought to the target depth of 3-5 m by a small penetrator, nicknamed the mole. The mole requiring friction on its hull to balance remaining recoil from its hammer mechanism did not penetrate to the targeted depth. Instead, it reached a depth of 40 cm, bringing the mole body 1-2 cm below the surface. A discussion of the lessons learned from the penetration failure and suggestions for an improved mole have been given by Spohn et al. (2022). The root cause of the failure - as was determined through an extensive almost two years long campaign - was a lack of friction in an unexpectedly thick cohesive duricrust. (compare Figure 1)Figure 1. The HP3 mole before complete burial and the properties of the hole that the mole had punched in the duricrustDuring the campaign the mole penetrated further aided by friction applied using the scoop at the end of the robotic Instrument Deployment Arm and direct support by the latter. The mole reversed its downward motion twice during attempts to provide friction through pressure on the regolith instead of directly with the scoop to the hull. The penetration record of the mole and its thermal sensors were used to measure thermal and mechanical soil parameters such as the penetration resistance of the duricrust. These parameter values are summarized in Table 1 below. The combined data suggest a model of the regolith that has an about 20 cm thick duricrust underneath a 1 cm thick sand layer and above another 10 cm of sand. Underneath the latter, a layer more resistant to penetration and possibly consisting of debris from a small impact crater was found. The thermal conductivity increases from 14 mW/m K in the 1 cm sand layer to 34 mW/m K in the duricrust and the sand layer underneath the duricrust to 64 mW/m K in the gravel layer below. Applying cone penetration theory, the resistance of the duricrust was used to estimate a cohesion of the latter of 4 - 25 kPa depending on the friction angle of the crust. Pushing the scoop with its blade into the surface and chopping of a piece of crust provided another estimate of the cohesion of 5.8 kPa.The hammerings of the mole were recorded by the seismometer SEIS and the signals could be used to derive a P-wave velocity and an S-wave as listed (see also Brinkman et al., 2022) representative of the topmost tens of cm of the regolith. Together with a density provided by a thermal conductivity and diffusivity measurement using the mole thermal sensors of about 1211 (Grott et al., 2021), the elastic moduli could be calculated from the seismic velocities.Table 1. Model of the InSight landing site regolith After burial, the mole was used to measure the thermal conductivity of the regolith as a function of the solar longitude (the seasons on Mars). The variations of the thermal conductivity are consequences of the variations in atmosphere pressure with the seasons and the contribution of atmosphere gas in the porous regolith contributing to the thermal conductivity. Figure 2 Thermal conductivity in the regolith top 40 cm as measured by sensors on the HP3 mole as a function of the solar longitude (seasons on Mars). Brinkmann et al. (2022), submitted to J. Geophys. Res. PlanetsGrott et al. (2021), Planet and Space Sci, DOI: 10.1029/2019EA000670Spohn et al. (2018), Space Sci Rev, DOI:10.1007/s11214-018-0531-4 Spohn et al. (2022), Advances in Space Research, DOI: 10.1016/j.asr.2022.02.00

Thermal Conductivity of the Martian Soil at the InSight Landing site from HP3 Active Heating Experiments

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 to 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/mK, 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 micrometer 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/m3, indicating soil porosities of 61

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

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.Additional co-authors: Axel Hagermann, Matthew Siegler, and W Bruce Banerd

Molecular and Historical Aspects of Corn Belt Dent Diversity

Tens-of-thousands of open-pollinated cultivars of corn (Zea mays L.) are being maintained in germplasm banks. Knowledge of the amount and distribution of genetic variation within and among accessions can aid end users in choosing among them. We estimated molecular genetic variation and looked for influences of pedigree, adaptation, and migration in the genetic makeup of conserved Corn-Belt Dent-related germplasm. Plants sampled from 57 accessions representing Corn-Belt Dents, Northern Flints, Southern Dents, plus 12 public inbreds, were genotyped at 20 simple sequence repeat (SSR) loci. For 47 of the accessions, between 5 and 23 plants per accession were genotyped (mean = 9.3). Mean number of alleles per locus was 6.5 overall, 3.17 within accessions, and 3.20 within pooled inbreds. Mean gene diversity was 0.53 within accessions and 0.61 within pooled inbreds. Open-pollinated accessions showed a tendency toward inbreeding (FIS = 0.09), and 85% of genetic variation was shared among them. A Fitch-Margoliash tree strongly supported the distinctiveness of flint from dent germplasm but did not otherwise reveal evidence of genetic structure. Mantel tests revealed significant correlations between genetic distance and geographical (r = 0.54, P= 0.04) or maturity zone (r = 0.33, P = 0.03) distance only if flint germplasm was included in the analyses. A significant correlation (r = 0.76, P \u3c 0.01) was found between days to pollen shed and maturity zone of accession origin. Pedigree, rather than migration or selection, has most influenced the genetic structure of the extant representatives of the open-pollinated cultivars at these SSR loci

Mars Soil Properties from Phobos Eclipse Observations by InSight HP³ RAD

Mars surface temperature response to insolation variations constrains soil properties and indicates layering consistent with cementation at depth

The InSight HP3 Penetrator (Mole) on Mars: Soil Properties Derived from the Penetration Attempts and Related Activities

The NASA InSight Lander on Mars includes the Heat Flow and Physical Properties Package HP3 to measure the surface heat flow of the planet. The package uses temperature sensors that would have been brought to the target depth of 3–5 m by a small penetrator, nicknamed the mole. The mole requiring friction on its hull to balance remaining recoil from its hammer mechanism did not penetrate to the targeted depth. Instead, by precessing about a point midway along its hull, it carved a 7 cm deep and 5–6 cm wide pit and reached a depth of initially 31 cm. The root cause of the failure – as was determined through an extensive, almost two years long campaign – was a lack of friction in an unexpectedly thick cohesive duricrust. During the campaign – described in detail in this paper – the mole penetrated further aided by friction applied using the scoop at the end of the robotic Instrument Deployment Arm and by direct support by the latter. The mole tip finally reached a depth of about 37 cm, bringing the mole back-end 1–2 cm below the surface. It reversed its downward motion twice during attempts to provide friction through pressure on the regolith instead of directly with the scoop to the mole hull. The penetration record of the mole was used to infer mechanical soil parameters such as the penetration resistance of the duricrust of 0.3–0.7 MPa and a penetration resistance of a deeper layer (> 30 cm depth) of 4.9±0.4 MPa. Using the mole’s thermal sensors, thermal conductivity and diffusivity were measured. Applying cone penetration theory, the resistance of the duricrust was used to estimate a cohesion of the latter of 2–15 kPa depending on the internal friction angle of the duricrust. Pushing the scoop with its blade into the surface and chopping off a piece of duricrust provided another estimate of the cohesion of 5.8 kPa. The hammerings of the mole were recorded by the seismometer SEIS and the signals were used to derive P-wave and S-wave velocities representative of the topmost tens of cm of the regolith. Together with the density provided by a thermal conductivity and diffusivity measurement using the mole’s thermal sensors, the elastic moduli were calculated from the seismic velocities. Using empirical correlations from terrestrial soil studies between the shear modulus and cohesion, the previous cohesion estimates were found to be consistent with the elastic moduli. The combined data were used to derive a model of the regolith that has an about 20 cm thick duricrust underneath a 1 cm thick unconsolidated layer of sand mixed with dust and above another 10 cm of unconsolidated sand. Underneath the latter, a layer more resistant to penetration and possibly containing debris from a small impact crater is inferred. The thermal conductivity increases from 14 mW/m K to 34 mW/m K through the 1 cm sand/dust layer, keeps the latter value in the duricrust and the sand layer underneath and then increases to 64 mW/m K in the sand/gravel layer below