Sol 128 image taken April 7 by the Instrument Deployment Camera (IDC) shows the Heat and Physical Properties Package (HP3). There remains uncertainty as to why the “mole” — the nickname for the self-hammering spike that is part of HP3 — is not performing as expected.
Credit: NASA/JPL-Caltech



Engineers on the NASA InSight Mars lander mission are still trouble-shooting the Heat and Physical Properties Package (HP3). There remains uncertainty as to why the “mole” — the nickname for the self-hammering spike that is part of HP3 — is not performing as expected.

“The discussion about the reasons of the mole not penetrating further have settled to three hypotheses of similar credibility but differing likelihood of occurring,” reports Tilman Spohn of the German Aerospace Center’s (DLR) Institute of Planetary Research in Berlin.

Components of the HP3 heat flow probe. Top left: the radiometer (RAD), which is used to measure the radiation temperature (roughly equivalent to the ground temperature) of the surface. Right: the casing with the mole penetrometer, the temperature measuring cable (TEM-P) and the data cable (ET) connected to the lander. In addition, the casing contains an optical length meter for determining the length of the temperature measuring cable that has been pulled from the casing. The mole contains the TEM-A active thermal conductivity sensor and the STATIL tiltmeter. Bottom left: the electronic control unit, known as the back end electronics (BEE), which remains on the lander and is connected to the probe via the ET.
Credit: DLR

  • The mole or the tether that it is trailing behind may be snagged in the Support Structure. While this hypothesis is credible it so far lacks a clear mechanism of how this may have happened. Tests at the DLR Institute of Space Systems have shown, that the tether may get snagged but only in very special circumstances.
  • The mole may have encountered a sufficiently large rock or stone at 30cm depth. The size of the rock would have to be 10cm or larger for the mole not being likely to push it aside or to go around it. This explanation is so simple that everybody would be ready to believe it. But, the likelihood of such a stone blocking the moles’ way is only a few percent judging from the well-established (surface) rock-size-frequency distribution for the landing site.
  • The mole may not have enough friction on the hull to balance the recoil. Geologists have seen that the topmost centimeters on Mars is formed by what is called a “duricrust”. Here, chemical reactions between grains of sand have made them stick together, providing cohesion in that layer. The duricrust is usually thin and not a problem. But at the InSight landing site, it seems to be 20 or so centimeters thick! If the mole is sitting in the duricrust, its hull may very well have lost friction and upon time, the mole may have widened the hole in the duricust as is suggested by the data from our accelerometer and by our thermal data.

InSight’s Instrument Deployment Camera (IDC) acquired this image showing the HP3 experiment and SEIS seismometer (Seismic Experiment for Interior Structures) on Sol 99, March 8, 2019.
Credit: NASA/JPL-Caltech


“At the moment it seems that the discussion is leaning towards hypothesis #3, not the least as we have seen such a behavior in tests on Earth in cohesive sand and low atmosphere pressure,” Spohn says. “But we have not settled yet on this explanation.”

In any case, if hypothesis #3 were found to be the best explanation, Spohn adds, “it may also offer the simplest remedy. All we would need to do is help the mole to balance the recoil.”

InSight’s robotic arm may be of help here but that needs to be assessed further by engineers.

“Otherwise, the mole is entirely healthy,” Spohn concludes. “Stay tuned.”

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