Thermal probe for measuring surface heat flow

Planetary heat flow

The surface planetary heat flow is one of the key quantities characterizing the thermal state of a planet and significantly influences tectonic, magmatic and geological processes present on the surface. Its value is intimately connected to the bulk concentration of radioactive, heat producing elements in the planetary interior and its spatial variability gives clues to the style of mantle heat transport.

While heat flow measurements are routinely done on Earth, the only successful heat flow measurements on other planetary bodies have so far been performed by astronauts during the Apollo 15 and 17 missions to the Moon. A measurement of the planetary heat flow requires two independent measurements to be made, i.e., the thermal conductivity and the temperature gradient need to be known. This requires access to the planetary sub-surface, which usually requires drilling and is in general a task demanding huge amounts of resources in terms of payload mass and power.

Heat flow probes

Figure 1: Schematics of the probe consisting of the tractor mole, payload compartment, instrumented tether and the support system

The Institute of Planetary Research is involved in the development of in-situ heat flow probes, one of which – MUPUS, the Multi-Purpose Sensor - is currently onboard ESA’s Rosetta mission on its way to the comet 67P/Churyumov-Gerasimenko. Active development is currently ongoing for HP3, the Heat flow and Physical Properties Package, which was originally designed for Mercury, but is now being adapted for measurements on Mars and the Moon. HP3 consists of a tractor mole, which houses a hammering mechanism that achieves the soil advancement and pulls behind a payload compartment, and a flat print cable. Both elements can be equipped with different sensors (Figure 1) and it is currently foreseen to measure the soil’s thermo-physical and electrical properties. The system has flight-heritage from the subsurface soil sampling tool PLUTO, which was part of the lost ESA-Lander Beagle 2, and was pre-developed during ESA funded precursor studies. Because soil penetration is achieved by means of a hammering mechanism no drilling is required, making this a relatively light-weight solution which can be operated purely robotically.

Theoretical modeling

The department of planetary physics is involved in the HP3 laboratory program and theoretical studies of the planetary near surface layers. Among other things, Finite Element Models are used to study the thermal environment at possible landing sites and to help in the experiment design and data analysis (Figures 2a and b). The current emphasis lies on modeling the Martian regolith in preparation for ESA’s ExoMars mission, but with the upcoming International Lunar Network (ILN), the Moon is also moving into focus.

Contact: Dr. Matthias Grott

Figures 2a and b: Finite element model used to study the effects of lander-shadowing on the subsurface temperatures. Left: Schematic model setup. Right: Resulting temperature perturbation two Martian years after landing.

Selected publications

  • Spohn, T., A.J. Ball, K. Seiferlin, V. Conzelmann, A. Hagermann, N.I. Kömle, G. Kargl, 2001. A heat flow and physical properties package for the surface of Mercury, Plan. Space Sci., 49, 14-15, 1571-1577.
  • Spohn, T., K. Seiferlin, A. Hagermann, J. Knollenberg, A.J. Ball, M. Banaszkiewicz, J. Benkhoff, S. Gadomski, W. Gregorczyk, J. Grygorczuk, M. Hlond, G. Kargl, E. Kührt, N. Kömle, J. Krasowski, W. Marczewski, J.C. Zarnecki, 2007. Mupus – A Thermal and Mechanical Properties Probe for the Rosetta Lander Philae Space Sci. Rev., 128, 1-4, 339-362, doi:10.1007/s11214-006-9081-2.
  • Grott, M., J. Helbert and R. Nadalini, 2007. The thermal structure of Martian soil and the measurability of the planetary heat flow. J. Geophys. Res., 112, E09004, doi:10.1029/2007JE002905.
  • Grott, M., 2008. Thermal disturbances caused by lander shadowing and the measurability of the martian planetary heat flow. Planetary and Space Science (57), Elsevier, S. 71 - 77, DOI 10.1016/j.pss.2008.11.005

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