It’s been a long road for InSight’s Mole. InSight landed on Mars almost two years ago, in November 2018. While the lander’s other instruments work fine and return scientific data, Mole has struggled to hammer itself into the planet’s surface.
After a lot of hard work and a lot of patience, Mole has finally managed to bury herself completely into the Marian regolith.
But the drama is not over yet.
Mole is a 16-inch long heat probe that hammers deep into the surface. Its maximum depth is 5 meters (16 feet) below the surface and that is its ideal operating depth. But it can also collect useful scientific data at lower depths of approx. 3 meters (1
But after two years, this is still the deepest it has been.
The actual name of the pier is Heat Flow and Physical Properties Package or HP3. It is designed to measure the heat from Mars’ interior. The tether that connects it to the InSight lander includes longitudinal heat sensors. InSight stands for Interior Exploration using seismic surveys, geodesy and heat transport. The heat transport part of the mission is the job of the mole.
Ever since the instrument was implemented, it has faced problems. The mole penetrates by hammering itself slowly into the ground. But the hammering motion depends on friction between the mole and the sides of its hole. Without this friction, the instrument just jumps out of the hole.
The problem is what is called duricrust. It is a hardened surface layer that forms in dry areas. And Mars is certainly dry. Duricrust around the mole prevents soil from falling into the hole of the pier when it hammers, depriving the instrument of the necessary friction to hammer itself into Mars.
While InSight is primarily a NASA mission, the Mole was designed and built by DLR (German Aerospace Center). They have worked with NASA’s JPL, which has a technical version of the mole in a test bed. This is where they have tried to overcome these challenges.
They have tried to use the scoop on the end of InSight’s instrument arm to put Mole on its side, hoping to provide the necessary friction. They have also tried to push down on the mole, all the while carefully avoiding the sensitive tie. And they have tried to collect loose material with the scoop and deposit it in the mole hole.
Today, NASA announced that the mole was finally completely buried in dirt. It’s a kind of victory, but there’s still a long way to go. Now that it’s buried, the InSight team will continue to increase more soil on top of the instrument and stomp it down before resuming hammer operations.
But all this takes time.
“I’m very happy that we were able to recover from the unexpected ‘pop-out’ event we experienced and get the mole deeper than it has ever been,” said Troy Hudson, a scientist and engineer at NASA. “But we are not quite done. We want to make sure there is enough soil on top of the mole so that it can dig alone without the help of the arm,” said Jet Propulsion Laboratory. Hudson in a press release.
Cutting the ground and stomping it down will take months. NASA says the hammer operation is unlikely to resume until January 2021. Part of what is hampering the operation is the accumulation of dust on InSight’s solar panels. It reduces the power available to the entire mission.
Tilman Spohn is the scientific director of mole at DLR. He has written a blog about the efforts to make the mole work. On today’s October 16, 2020, Spohn spoke about the next steps and how they are working toward another “Free Mole Test.” The free mole test is when they let the mole try to hammer under the surface without the help of the shovel.
“After some discussion about the next steps, we decided that two parallel scoop movements should be carried out on Saturday, October 17 (Sol 659),” he wrote.
“Afterwards, a thermal conductivity measurement is performed, which should also give us indirect indications of refilling,” writes Spohn. Then press the filling to compress the sand and press on the mole. Depending on the result of the backfill, further actions are planned to fill the well before further hammering, and a new Free Mole Test will take place later. ”
On the ground, it would be simple to use a drill to penetrate below the surface. But drills are heavy, require a lot of power and need stability to prevent them from turning instead of drilling. It is simply not possible on Mars. A drill would weigh way too much and would require far more power than the mole. The mole is only 1 inch (2.7 centimeters) in diameter and approx. 16 centimeters (40 centimeters) long. It should be both light enough and small enough to fit within the constraints of the mission.
Hopefully, the mole will eventually reach its working depth. Meanwhile, InSight’s other instruments work and return data. Thanks to SEIS (Seismic Experiment for Inner Structure), we know that Mars is a seismically active planet.
But without the mole and its heat transfer measurements, the InSight lander will never live up to its mission.