The InSight HP$^3$ 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 HP 3 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 finally reached a depth of 40 cm, bringing the mole body 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 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 of 0.5 -1.2 MPa and a penetration resistance of a deeper layer (> 30 cm depth) of 5.3 MPa. 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 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 could be used to derive a P-wave velocity of 114 +40 −19 m/s and a S-wave velocity of 60 +10 −7 m/s (Brinkman et al., 2021) representative of the topmost tens of cm of the regolith. Together with a density of 1211 +149 −113 kg/m 3 (Grott et al., 2021) provided by a thermal conductivity and diffusivity measurement using the mole thermal sensors, the

Seismology in the Solar System

“…This made initial experimentation much more difficult Knapmeyer, Fischer, Knollenberg, Seidensticker, Thiel, Arnold, Faber, et al 2018. The Rosetta mission was launched in 2004, the problem was encountered in 2014, but surprisingly, the same problem occurred again during the InSight HP3 seismic experiment, launched in 2018, when the seismometer SEIS was listening for seismic waves produced by the hammer of the HP 3 heat flow probe (Spohn, Hudson, et al 2021;Sollberger, Schmelzbach, Andersson, et al 2020). The lack of a joint time signal between the two instruments HP 3 and SEIS meant that a convoluted process was necessary to infer the exact time of each hammer blow from the seismic signal itself, significantly increasing the uncertainty of the observation.…”