Planetary heat flow is a key quantity characterizing the thermal state of a planet. It has significant influence on processes like plate tectonics, magmatism, and the geological activity we observe at the planetary surface. The amount of heat radiated from the surface is closely connected to the concentration of heat producing elements in the interior, and its spatial variability is indicative of heat transport mechanisms operating in the planetary mantle. To determine planetary heat flow, two independent quantities need to be known: The thermal conductivity and the thermal gradient in the subsurface need to be determined. To perform these measurements, drilling is usually required, making the measurements technically challenging.
While the heat flow of the Earth has been determined at more than 20.000 field sites, heat flow on other planetary bodies has only been measured in-situ on the Moon. Measurements have been performed by the Apollo 15 and 17 astronauts, and heat flow probes were inserted into holes manually drilled using a rotary percussion drill system
The DLR Institute for Planetary Research is building the Heat Flow and Physical Properties Package (HP3) for NASA’s InSight mission to Mars. InSight will arrive on the red planet in 2016 and will perform measurements for an initial period of 2 years. Apart from instrument development and building the actual flight hardware, the Institute for Planetary Research is responsible for the scientific data analysis and interpretation. In this context, algorithms for data inversion as well as numerical models of the Martian regolith are being developed (Figure 1).
Figure 1: Finite element model used to study the effects of lander-shadowing on subsurface temperatures. Top: Schematic model setup and finite element grid used for the computations. Bottom: Resulting temperature perturbation for different soil models two Martian years after landing.
Radiometers are used to remotely determine the surface temperature of planetary bodies. Surface temperatures give insight into the thermophysical properties of the regolith, which are connected to the compaction state of the soil, as well as to the fraction of coarsely grained material and rocks. The latter may be interpreted in terms of the regolith’s formation history. For small solar system objects like asteroids the thermophysical properties like conductivity and heat capacity are of particular interest, as the asymmetric re-radiation of heat to space can have an impact on the asteroid’s orbit (Poynting-Robertson effect). Radiometers built by the DLR Institute for Planetary Research are being used on different space missions. Apart from the MUPUS-TM (Thermal Mapper) and the MERTIS Radiometer, DLR is building the HP3 radiometer for the InSight mission as well as the MASCOT radiometer MARA for the Hayabusa II mission. Data analysis and interpretation is also being prepared at the Institute for Planetary Research.
Figure 2: CAD model of the Hayabusa II MASCOT lander. The radiometer is mounted in between the spectrometer MicOmega (left) and the MASCOT camera (right). The field of view of the six MARA sensors as well as the field of view of the camera are indicated.