The DLR Institute for Planetary Research builds the in-situ heat flow probe HP3, which will determine the heat flow of Mars as part of NASA’s discovery class mission InSight (Figure 1). InSight will land a geophysical station in the southern Elysium region of Mars in 2016 to investigate the deep interior of the planet. Apart from the heat flow probe, the scientific payload consists of a seismometer and a geodetic experiment to accurately determine the rotational state of the planet. The combination of seismic, geodetic, and thermal investigations will allow for a characterization of the Martian crust and mantle, as well as the properties of the Martian core.
Figure 1: Artists impression of the InSight lander on the Martian surface. The HP3 instrument (left) and seismometer (right) have been deployed. The geodetic experiment as well as a camera are mounted on the lander. An additional camera is mounted on the robotic arm.
The HP3 instrument design is based on the MUPUS sensor (Multi-Purpose Sensor), currently flying onboard ESA’s Rosetta mission on its way to rendezvous the comet 67p/ Churyumov-Gerasimenko. Its design is similar to the PLUTO (Planetary Underground Tool) probing device, which would have been used on ESA’s MarsExpress lander Beagle 2. In order to perform measurements in the subsurface, HP3 consists of a so called “Mole”, which will hammer itself into the subsurface. The mole pulls an instrumented tether behind it, which is equipped with temperature sensors to determine the thermal gradient in the ground. The mole is targeted for a depth of 5 m below the surface. In addition to the temperature sensors, the mole is equipped with heating foils, which will be used to determine the thermal conductivity of the regolith by operating the mole as a modified line heat source (Figure 2).
Figure 2: HP3 system consisting of the Mole, which houses the hammering mechanism and provides the means to emplace the sensors in the ground, as well as the scientific tether (yellow), which is equipped with temperature sensors to determine the thermal gradient in the subsurface. The thermal conductivity of the regolith is determined by heating the mole body and measuring its self-heating curve. A second cable (green) provides the electrical connection to the lander.
In order to determine the surface temperature at the landing site, the HP3 system has been augmented with a radiometer, which is mounted on the lander. Knowledge of the surface temperature is important to quantify perturbations to the subsurface temperature field, which can be caused by shadows cast by the lander, as well as by albedo variations caused by dust lifting during landing.
The HP3 radiometer determines the radiative flux in 3 spectral bands to measure the surface brightness temperature. In addition, the observed spectral bands are sensitive to the amount of dust coverage on the surface. Surface temperatures furthermore constrain the thermophysical properties of the upper few centimeters of the regolith, thus allowing us to estimate the compaction state of the soil at the landing site.
Figure 3: CAD model of the HP3 radiometer. The 6 sensors observe two spots on the surface in 3 different infrared channels.