It’s been a long way for InSight’s Mole. InSight landed on Mars nearly two years ago in November 2018. While the lander’s other instruments are working fine and returning scientific data, the Mole has struggled to make its way to the planet’s surface.
After a lot of hard work and a lot of patience, the Mole has finally managed to bury herself fully in the Marian regolith.
But the drama is not yet over.
The mole is a 16-inch long thermal probe that hammers deep into the surface. Its maximum depth is 5 meters (16 feet) below the surface, and this is its ideal operating depth. But it can also gather useful scientific data at depths as shallow as about 3 meters (1
But after two years, this is still the most profound it has been.
The actual name of the mole is Heat Flow and Physical Properties Package, or HP3. It is designed to measure the heat coming from inside Mars. The cable connecting it to the InSight lander contains heat sensors along its entire length. InSight stands for Interior Exploration using Seismic Investigations, Geodesy and Heat Transport. The heat-carrying part of the mission is the work of the mole.
Since the tool has been used, it has faced problems. The mole penetrates by slowly hammering into the ground. But that pounding motion is based on the friction between the mole and the sides of its hole. Without that friction, the tool bounces out of the hole.
The problem is what is called duricrust. It is a hardened surface layer that forms in arid areas. And Mars is very dry. The duricrust around the mole prevents the soil from falling into the mole hole as it hammers and deprives the tool of the friction needed to hammer into Mars.
While InSight is primarily a NASA mission, the Mole was designed and built by the DLR (German Aerospace Center). They worked with NASA’s JPL, which has an engineering version of the Mole on a test bed. That’s where they tried to overcome these challenges.
They tried using the vane at the end of the InSight’s tool arm to apply lateral pressure to the mole, hoping to provide the necessary friction. They also tried to push the Mole down, carefully avoiding the sensitive cable. And they tried to scoop up loose material and deposit it in the mole hole.
Today, NASA announced that the Mole has finally been completely buried in dust. This is a kind of victory, but there is still a long way to go. Now that it’s buried, the InSight team will continue to collect more soil on top of the tool and compress it before resuming hammering.
But all of this takes time.
“I am very pleased that we were able to recover from the unexpected ‘pop-out’ event we experienced and take the mole deeper than it has ever been,” said Troy Hudson, scientist and engineer at Jet Propulsion. NASA laboratory that led the work to have the mole excavated. “But we’re not done yet. We want to make sure there is enough ground above the mole to allow it to dig on its own without any assistance from the arm,” Hudson said in a press release.
Harvesting the soil and tamping it will take months. NASA says the hammering operation is unlikely to resume until January 2021. Part of what hinders the operation is the accumulation of dust on InSight’s solar panels. This reduces the power available for the entire mission.
Tilman Spohn is the scientific director for the mole at DLR. He wrote a blog about the effort to make the mole work. In today’s October 16, 2020 entry, Spohn talked about the next steps and how they are working on another “free mole test”. The free mole test is when they let the mole try to make its way under the surface without the assistance of the scoop.
“After some discussion on the next steps, we have decided that two parallel movements of scoop should be conducted on Saturday 17 October (Sol 659),” he wrote.
“Next, a measurement of thermal conductivity will be performed, which should also give us indirect indications about the filling,” writes Spohn. “Then, the filling will be pressed to compress the sand and press on the mole. Depending on the result of the filling, further actions will be planned to fill the pit before further hammering and afterwards another free mole test will take place. “
On Earth, it would be easy to use a drill to penetrate below the surface. But drills are heavy, require a lot of power, and need stability to keep them from turning instead of drilling. It just isn’t possible on Mars. A drill would weigh too much and require much more power than the mole. The mole is only 1 inch (2.7 centimeters) in diameter and approximately 16 inches (40 centimeters) long. It had to be light enough and small enough to fit the mission constraints.
Hopefully the mole will eventually reach its working depth. Meanwhile, the other InSight tools are working and returning data. Thanks to the SEIS (Seismic Experiment for Interior Structure) we know that Mars is a seismically active planet.
But without the mole and its heat transport readings, the InSight lander will never live up to its mission.