The heat probe developed and built by the German Aerospace Center (DLR) and deployed on Mars by NASA’s InSight lander has ended its portion of the mission. Since 28 February, 2019, the probe, called the “mole,” has been attempting to burrow into the Martian surface to take the planet’s internal temperature, providing details about the interior heat engine that drives the Mars’ evolution and geology. But the soil’s unexpected tendency to clump deprived the spike-like mole of the friction it needs to hammer itself to a sufficient depth.
After getting the top of the mole about 2 or 3 centimeters under the surface, the team tried one last time to use a scoop on InSight’s robotic arm to scrape soil onto the probe and tamp it down to provide added friction. After the probe conducted 500 additional hammer strokes on Saturday, Jan. 9, with no progress, the team called an end to their efforts.
Part of an instrument called the Heat Flow and Physical Properties Package (HP3), the mole is a 40-centimetre-long pile driver connected to the lander by a tether with embedded temperature sensors. These sensors are designed to measure heat flowing from the planet once the mole has dug at least 3 metres deep.
“We’ve given it everything we’ve got, but Mars and our heroic mole remain incompatible,” said HP3’s principal investigator, Tilman Spohn of (DLR). “Fortunately, we’ve learned a lot that will benefit future missions that attempt to dig into the subsurface.”
While NASA’s Phoenix lander scraped the top layer of the Martian surface, no mission before InSight has tried to burrow into the soil. Doing so is important for a variety of reasons: Future astronauts may need to dig through soil to access water ice, while scientists want to study the subsurface’s potential to support microbial life.
“We are so proud of our team who worked hard to get InSight’s mole deeper into the planet. It was amazing to see them troubleshoot from millions of miles away,” said Thomas Zurbuchen, associate administrator for science at the agency’s headquarters in Washington. “This is why we take risks at NASA – we have to push the limits of technology to learn what works and what doesn’t. In that sense, we’ve been successful: We’ve learned a lot that will benefit future missions to Mars and elsewhere, and we thank our German partners from DLR for providing this instrument and for their collaboration.”
The unexpected properties of the soil near the surface next to InSight will be puzzled over by scientists for years to come. The mole’s design was based on soil seen by previous Mars missions – soil that proved very different from what the mole encountered. For two years, the team worked to adapt the unique and innovative instrument to these new circumstances.
“The mole is a device with no heritage. What we attempted to do – to dig so deep with a device so small – is unprecedented,” said Troy Hudson, a scientist and engineer at NASA’s Jet Propulsion Laboratory in Southern California who has led efforts to get the mole deeper into the Martian crust. “Having had the opportunity to take this all the way to the end is the greatest reward.”
Besides learning about the soil at this location, engineers have gained invaluable experience operating the robotic arm. In fact, they used the arm and scoop in ways they never intended to at the outset of the mission, including pressing against and down on the mole. Planning the moves and getting them just right with the commands they were sending up to InSight pushed the team to grow.
They’ll put their hard-earned wisdom to use in the future. The mission intends to employ the robotic arm in burying the tether that conveys data and power between the lander and InSight’s seismometer, which has recorded more than 480 marsquakes. Burying it will help reduce temperature changes that have created cracking and popping sounds in seismic data.
There’s much more science to come from InSight, short for Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport. NASA recently extended the mission for two more years, to December 2022. Along with hunting for quakes, the lander hosts a radio experiment that is collecting data to reveal whether the planet’s core is liquid or solid. And InSight’s weather sensors are capable of providing some of the most detailed meteorological data ever collected on Mars. Together with weather instruments aboard NASA's Curiosity rover and its new Perseverance rover, which lands on 18 February, the three spacecraft will create the first meteorological network on another planet.