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It stands vertically on flat ground, ready for its historic mission. At 19:18 CET on 12 February 2019, the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) Heat Flow and Physical Properties Package (HP³) or 'Mole' was deployed on the Martian surface using the NASA InSight mission's robotic arm. In the coming weeks, the remote controlled penetrometer is expected to make space history by becoming the first probe to reach a depth of up to five metres in the Martian subsurface. Its goal is to measure the temperature and thermal conductivity of the subsurface and thus determine the heat flow from the interior of Mars. The heat flow gives researchers indications about the thermal activity of the Red Planet. This can provide insights into the evolution of the Martian interior, whether it still has a hot liquid core, and what makes Earth so special in comparison.

"We are pleased that the deployment of our HP³ experiment onto the Martian surface went so smoothly," says Principal Investigator Tilman Spohn from the DLR Institute of Planetary Research in Berlin. HP³ is now in a stable position approximately 1.5 metres from the lander. "We hope that the Mole will not encounter any large rocks on its way into the subsurface," Spohn says. The Seismic Experiment for Interior Structure (SEIS) was deployed previously – complete with an additional cover to protect it against wind and temperature fluctuations – at a similar distance from the InSight lander. SEIS and HP³ are approximately one metre apart.

Thermal evolution of the planets and life on Earth

"From a thermophysical perspective, planets can be considered heat engines that generate volcanism, tectonics and magnetism," explains Spohn. Heat flow measurements are important boundary conditions for modelling the thermal evolution of Earth, Mars and other planets. While the seismometer and the observation of perturbations of the planet's rotation axis as part of the InSight Rotation and Interior Structure Experiment (RISE) shed light on Mars' interior structure, the measured heat flow constrains hypotheses about its evolution.

For the most part, scientists are convinced that a planet's geological evolution has great significance for its ability to host life and for the events that allow life to emerge at all. In the course of Earth's evolution, continents and oceans formed that are constantly subject to change and tectonic shifts. The shallow continental seas or the chains of volcanoes in the oceans could be the places where life emerged. Mars lacks these tectonic elements, probably because it is smaller, and also because it does not have enough water to facilitate the process of plate tectonics – as happens on Earth – over a longer time period or permanently. Mars had more water and ice in its early days than it does today and could have been hospitable to life, at least intermittently. With the help of InSight's measurements, researchers aim to gain a better understanding of the planetary-physical aspects of these complex interrelationships.

Into the depths in intervals

The Mole will pull a five-metre-long tether equipped with temperature sensors behind it into the Martian soil. After the target depth is reached, the tether's sensors will measure the temperature distribution at different depths and as it changes over time. In addition, the thermal infrared radiometer RAD on the InSight lander will measure the temperature of the Martian soil at the surface. Operational planning for the DLR instrument is currently underway.

The HP³ instrument on NASA's InSight mission

The InSight mission is being conducted by the Jet Propulsion Laboratory (JPL) in Pasadena, California, on behalf of NASA's Science Mission Directorate. InSight is part of NASA's Discovery Program. DLR contributed the HP³ experiment to the mission. Scientific leadership lies with the DLR Institute of Planetary Research, which was also in charge of developing the experiment in collaboration with the DLR institutes of Space Systems, Optical Sensor Systems, Space Operations and Astronaut Training, Composite Structures and Adaptive Systems, System Dynamics and Control, as well as the Institute of Robotics and Mechatronics. Industrial partners are Astronika and the CBK Space Research Centre, Magson GmbH and Sonaca SA, the Institute of Photonic Technology (IPHT) and Astro- und Feinwerktechnik Adlershof GmbH. Scientific partners are the ÖAW Space Research Institute at the Austrian Academy of Sciences and the University of Kaiserslautern. The DLR Microgravity User Support Center (MUSC) in Cologne is responsible for HP³ operations. In addition, the DLR Space Administration, with funding from the Federal Ministry for Economic Affairs and Energy, supported a contribution by the Max Planck Institute for Solar System Research to the French main instrument SEIS (Seismic Experiment for Interior Structure).

Detailed information on the InSight mission and the HP³ experiment can be found on DLR's dedicated mission site, with extensive background articles and the mission brochure. For mission updates, follow the hashtag #MarsMaulwurf on the DLR Twitter feed. Principal Investigator Tilman Spohn reports on the status of the mission in a blog.