The Mars Mole and the Challenging Ground of the Red Planet

DLR ‘Mole’ after having moved backward. (Credit NASA/JPL)

COLOGNE, Germany (DLR PR) — NASA’s InSight mission landed on Mars in November 2018. The German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) is involved in the Heat Flow and Physical Properties Package (HP3) experiment. In addition to a radiometer for measuring the surface temperature, the core component of the experiment is the Mars ‘Mole’ – a 40-centimetre-long penetrometer designed to measure the heat flow from the Martian interior at a depth of several metres.

In February 2019, the Mole began hammering. It got stuck at first, but with the help of InSight’s robotic arm it was able to penetrate almost completely into the Martian surface in October 2019. Without the additional support from the arm’s scoop, it then began a sudden retreat which has led to almost half of the Mole protruding from the Martian regolith.

With the help of the robotic arm, the plan now is to return the Mars Mole to its position deeper in the ground. This procedure will again require detailed planning and prior checks and tests on Earth. In the following Q & A, the HP3/InSight mission team answers frequently asked questions about the Mars Mole and the current situation.

Why can you not pick up the Mole and move it to another spot?

The Mole is part of the instrument called the Heat Flow and Physical Properties Package, or HP3, and was designed to be housed within HP3‘s support structure. The support structure of HP3 was outfitted with a knob, or ‘grapple point’, that the robotic arm can grasp in order to move it from the lander’s deck onto the Martian surface. Designed to be housed within the support structure, the Mole itself has no grapple point and was not intended to be grasped or moved.

Even if the Mole could be moved, relocating it would be an unlikely solution. The team is confident that the probe has been unable to dig because the soil does not provide enough friction. Anywhere you moved the Mole to around the lander would likely pose the same problem. The strategy of ‘pinning’ – pressing the robotic arm’s scoop against the side of the Mole – compensates for that lack of friction and helped the Mole progress downward in early October.

Hammer mechanism of the DLR Mars ‘Mole’ (Credit DLR)

Why does the Mole not include a drill?

A drill would require a much bigger, more powerful motor than the InSight lander can accommodate. It would also require more power than the solar-powered lander can practically provide. A drill would also require rigging to stabilise it as the motor spins, just as with a drill press. Rigging cancels out the force of a drill’s spin, which would otherwise spin the motor in the other direction.

Approximately 2.7 centimetres in diameter and about 40 centimetres long, the Mole was designed to be light enough and small enough to suit the constraints of the lander deck. The Mole functions somewhat like a pile driver. A motor attached to a gearbox inside the Mole slowly compresses and then quickly releases a spring that drives a tungsten hammer against the interior of the Mole’s tip, at a pace of one stroke every 3.7 seconds.

Are you sure the Mole did not hit a rock?

Most of the team remains confident that a rock did not cause the Mole to rebound. The landing site, Elysium Planitia, was selected partly because it has so few visible rocks, implying few, large subsurface rocks. The Mole is strong enough to nudge small rocks out of its way and was designed to go around medium-sized rocks – anything less than about 10 centimetres in diameter – once it is fully buried.

Further analysis will be required to determine why the Mole backed out of the hole once there was too little of it exposed above the Martian surface to make pinning possible.

How much of a factor is Mars’ reduced gravity when trying to replicate the Mole’s behaviour on Earth?

The robotic arm testbed at NASA’s Jet Propulsion Laboratory in Pasadena, California, uses weight models (non-functional replicas of the instruments that are the weight they would be on Mars) to simulate the effects of the Red Planet’s reduced gravity. But certain experiments, like ones involving the Mole digging, cannot be fully re-created. The team cannot simulate low gravity at the Earth’s surface. It would not be possible to make a lightweight version of the Mole for testing that has the same digging power as the Mole on Mars.

Is there any chance the hammering mechanism is damaged?

The InSight science team has not seen any noticeable difference between the performance of the Mole on Mars and what has been seen in testing on Earth. InSight’s seismometer can detect the vibrations from each of the Mole’s hammer strokes and is being used to analyse tiny (millisecond-level) variations in each stroke for signs of any problems. To date, the team has not seen evidence that the hammering mechanism is damaged.

Despite the challenges it poses to the Mole, is the soil teaching us anything new about Mars?

Absolutely! The Mole was designed with the loose, sandy soils seen by the Spirit and Opportunity rovers in mind. The soil under InSight is mechanically different from what we have seen at other landing sites. The formation of the pit around the probe, the resistance of the pit to collapsing and the very interesting self-extraction of the Mole are all surprising and will be investigated by the InSight science team to revise our understanding of the formation and variations of the Martian soil.

Why push the Mole from the side rather than applying pressure from above?

Pushing from the side allowed the InSight team to almost fully bury the Mole in October of 2019, so it is a technique that appears to work well. The team wants to avoid pressing on top of the Mole, where a sensitive tether supplying power and relaying data sticks out. It is still possible that the team will press down on the top of the Mole if there are no other options, but has been pressing from the side until now because it is safer for all the hardware, including InSight’s robotic arm.

The HP3 instrument on NASA’s InSight mission

The InSight mission is being carried out by NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, on behalf of the agency’s Science Mission Directorate. InSight is part of NASA’s Discovery Program. DLR is contributing the Heat Flow and Physical Properties Package (HP3) experiment to the mission. The scientific leadership lies with the DLR Institute of Planetary Research, which was also in charge of developing and implementing the experiment in collaboration with the DLR Institutes of Space Systems, Optical Sensor Systems, Space Operations and Astronaut Training, Composite Structures and Adaptive Systems, and System Dynamics and Control, as well as the Institute of Robotics and Mechatronics. Participating industrial partners are Astronika and the CBK Space Research Centre, Magson GmbH and Sonaca SA, the Leibniz Institute of Photonic Technology (IPHT) as well as 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 German 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 is available on DLR’s dedicated mission site with extensive background articles. You can also find information in the animation and brochure about the mission or via the hashtag #MarsMaulwurf on the DLR Twitter channel. Tilman Spohn, the Principal Investigator for the HP3 experiment, is also providing updates in the DLR Blog portal about the activities of the Mars ‘Mole’.