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The InSight HP^3 mole on Mars: Lessons learned from attempts to penetrate to depth in the Martian soil
The NASA InSight mission payload includes the Heat Flow and Physical
Properties Package HP^3 to measure the surface heat flow. The package was
designed to use a small penetrator - nicknamed the mole - to implement a string
of temperature sensors in the soil to a depth of 5m. The mole itself is
equipped with sensors to measure a thermal conductivity as it proceeds to
depth. The heat flow would be calculated from the product of the temperature
gradient and the thermal conductivity. To avoid the perturbation caused by
annual surface temperature variations, the measurements would be taken at a
depth between 3 m and 5 m. The mole was designed to penetrate cohesionless soil
similar to Quartz sand which was expected to provide a good analogue material
for Martian sand. The sand would provide friction to the buried mole hull to
balance the remaining recoil of the mole hammer mechanism that drives the mole
forward. Unfortunately, the mole did not penetrate more than a mole length of
40 cm. The failure to penetrate deeper was largely due to a few tens of
centimeter thick cohesive duricrust that failed to provide the required
friction. Although a suppressor mass and spring in the hammer mechanism
absorbed much of the recoil, the available mass did not allow a system that
would have eliminated the recoil. The mole penetrated to 40 cm depth benefiting
from friction provided by springs in the support structure from which it was
deployed. It was found in addition that the Martian soil provided unexpected
levels of penetration resistance that would have motivated to designing a more
powerful mole. It is concluded that more mass would have allowed to design a
more robust system with little or no recoil, more energy of the mole hammer
mechanism and a more massive support structure.Comment: 34 pages, 15 figures, submitted to Adnaves in Space Researc