Institute of Planetary Research
Berlin-Adlershof

Department of
Planetary Physics

Thermal Probe for measuring surface heat flow

Regoliths, the soil-like materials characteristic of the surface of various planets, moons and other minor bodies in the solar system, store in their physical properties a record of the physical and chemical processes that have taken place during and after their deposition. The study of these properties, especially below the surface, allows the exploration of the bodies' past for periods of up to billions of years (i.e. Lunar of Hermean (mercurian) regoliths).

At present, there are many missions, which analyzed or will analyze the solar systems surfaces from orbit. Just a very few of them plan a landing, with only minor investigations of the subsurface (i.e. Rosetta). Access to depths up to a few meters still remains to be accomplished and is a relevant scientific objective, especially if complementary to the recovery of samples.

To reach these goals, many efforts have been made in the last decade by different institutions to create and enhance a new class of investigative tools, able to self-penetrate the regolith: the Moles. An example is PLUTO, the subsurface soil sampling systems placed on board the unsuccessful Beagle 2 (ESA Mars Express landed element). With funding from the ESA scientific development programme, and with the aim of creating the first prototype of an ?instrumented Mole?, our team is developing the Heat flow and Physical Properties Package (HP³), is a combination of various sensors whose final purpose is the measurement of the intrinsic heat flow.

HP³ is an integrated instrument, designed to be attached to, and deployed by a dedicated Mole (under development in a parallel project). Once in place, it will perform in-situ measurements of the regolith status and properties. Weighing only 300g (of which only 210 attached to the Mole), it is composed by three different sensor sets. A cluster of thermometers and heaters measures the thermal status at different depths and the thermal properties; a set of accelerometers, tiltmeters and displacement sensors derives the mechanical characteristics and the precise position of the tool, while a gamma-ray Compton backscattering densitometer measures the density.

By combining the data, HP³ derives the heat flow, which provides data on the internal structure and thermal evolution of the measured body. In the chosen configuration, HP³ is housed in a trailed compartment, connected to the deployment Mole (the 'tractor') and, by a flat copper-kapton tether, to the surface element. Most of the sensors and the electronics are accommodated into the trailed body, while the thermometers and heaters are integrated in the tether, with an extremely lightweight construction.