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

Abstract

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 ($>30~\text{cm}$ > 30 cm depth) of $4.9\pm0.4~\text{MPa}$ 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.

In Situ Regolith Seismic Velocity Measurement at the InSight Landing Site on Mars

“…Assuming a density of 1,200 kg/m 3 (Spohn et al., 2021 ), the v P and v S values with calculated uncertainties of m/s and m/s translate into a bulk, shear, and Young's modulus and a Poisson's ratio of MPa, MPa, MPa, and , respectively (Table 4 ). When interpreting these values, one should keep in mind that they reflect values for a homogeneous volume and were derived from elastic waves with a dominant frequency of around 60 Hz.…”

“…A total stroke energy of around 0.7 J was measured in the laboratory for a hammer strike with regular mole operation (Wippermann et al., 2020 ). Estimates of the seismic energy radiated by the mole during one of the first hammering sessions (sol 158) are around 1.3 mJ (Spohn et al., 2021 ). Hence, only a small portion of the stroke energy was partitioned into seismic energy that reached SEIS.…”

“…During the hammering sessions, the motion of the mole was tracked using the tilt meters incorporated in the mole and images from the two cameras. The depth of the mole was determined with an accuracy of ±0.5 cm for the hammer sessions on sols 308 to 458 (sessions 5–18), when the mole could be seen by both the cameras on the robotic arm and the lander and later with an accuracy of ±1.0 cm, when the mole could only be imaged from the lander (Spohn et al., 2021 ). Since the back cap of the mole was flush with the surface after sol 536, the depth of the mole could no longer be determined from camera images and no other means were available to measure the depth of the mole.…”

“…Hence, no depth readings are available for sessions recorded after sol 536. But, the analysis of images taken during subsequent hammerings indicates that the mole did not significantly move after sol 536 (Spohn et al., 2021 ).…”

“…For the sessions of interest for this study, the mole penetrated from being about halfway buried in the subsurface to a stage where the back cap was completely flush with the regolith. This motion resulted in a reduction of the distance between the mole tip and SEIS s from 1.17 to 1.08 m. However, most of the mole motion took place during seven sessions (i.e., sessions on sols 325, 349, 373, 380, 407, 458, and 472) when the mole moved on average >0.16 mm/stroke (Spohn et al., 2021 ).…”