Mars "Mole" Is in the Ground (Again)

The "mole" of NASA’s Insight mission is back in business on Mars. The team scientists have been quiet on the progress of the mole, which has had difficulty digging into the Red Planet's surface to measure the heat it emits. But the DLR's mission blog finally released word of progress on June 3rd.

Insight is a complete dedicated geodesy mission, the first on Mars designed specifically to study planetary geology — and the first altogether to directly probe the interior of another world.

To this end, Insight deployed two tethered instrument packages on the Martian surface shortly after landing in 2018: the Seismic Experiment for Interior Structure (SEIS) and the Heat and Physical Properties Package (HP³). Designed by the German Aerospace Center (DLR), HP³ includes the mole, a mechanism designed to burrow into the surface Mars using a repetitive hammering motion.

Drilling Dilemmas

When the mole was first deployed on February 12, 2019, it began hammering away into the Martian surface. All seemed well until the mole suddenly seemed to stop after drilling only 12 inches (30 centimeters) down, far shy of its target depth of 3 meters needed to obtain useful science measurements. Had it struck a rock? The Elysium Planitia site was specifically chosen as it was presumed to have soft soil, ideal for digging.

NASA decided to use Insight’s robotic arm to gently lift up the HP³ surface structure and see what was going on underneath. This was a delicate maneuver: If the mole was pulled out of the ground in the process, it wouldn't be able to be placed back in. With the surface structure lifted, cameras showed that the mole seemed to be hopping in place, a sign that it wasn’t gaining traction. The regolith is unexpectedly compressed, and as a result it doesn't provide friction against the hammer's recoil.

After an failed attempt to pin the mole in place as it digs — the mole was only able to dig a bit before popping out again — engineers hit upon another idea. They would use the scoop to push down on the mole as it digs. This approach isn't without its risks; not only does this force exert some horizontal pressure on the mole, but it could also cause damage to the mole's backcap and tether. The scoop arm would have to be placed on the backcap within a precision of a few millimeters.

"After each placement, the situation was checked through imaging and recordings of the arm motor current data before a number of hammer strokes were commanded," says Tilman Spohn (DLR) on the DLR mission blog. "We started the procedure with only a few (25) hammer strokes. Only after the team had gained some confidence in its ability to carefully place the scoop and in the rate of progress of the mole did we increase the number of hammer strokes per session to 150 strokes."

The team started the backcap push in early March, when the mole was 7 cm above the surface. The backcap reached flush with the surface — with the mole fully back in the ground — on May 30th.

Now that the mole is fully in the ground, the team will see if it can gain traction on its own as it resumes hammering. The scoop will continue to press the surface to provide some additional traction.

"Should the mole move into the subsurface on its own, friction will increase and improve the situation as the mole moves deeper," says Spohn on the DLR mission blog. "When the mole back cap is at a depth of approximately 20 centimetres, loading the surface will have become ineffective and the regolith push should be no longer necessary."

These operations have moved exceedingly slowly because only one cycle of operations can be commanded each week. And Martian winter is approaching with the ensuing dust storms, which could affect power supply for the solar-powered lander. For now, InSight’s mole has avoided falling prey to the "Great Galactic Ghoul" of Mars, so we'll see what the future holds.