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 are working fine and returning scientific data, the Mole has been struggling to hammer its way into the surface of the planet.
After much hard work and a lot of patience, the Mole has finally succeeded in burying itself all the way into the Marian regolith.
But the drama hasn’t concluded yet.
The Mole is a 16-inch long heat probe that hammers itself deep into the surface. Its maximum depth is 5 meters (16 ft) below the surface, and that’s its ideal operating depth. But it can also gather useful scientific data at shallower depths of about 3 meters (10 ft). As it is now, the mole is nowhere deep enough to do any science.
But after two years, this is still the deepest it’s been.
The mole’s actual name is the Heat Flow and Physical Properties Package, or HP3. It’s designed to measure the heat coming from Mars’ interior. The tether connecting it to the InSight lander contains heat sensors along its length. InSight stands for Interior Exploration using Seismic Investigations, Geodesy and Heat Transport. The heat transport part of the mission is the mole’s job.
Ever since the instrument was deployed, it’s faced problems. The mole penetrates by hammering itself slowly into the ground. But that hammering motion relies on friction between the mole and sides of its hole. Without that friction, the instrument just bounces back out of the hole.
The problem is what’s called duricrust. It’s a hardened surface layer that forms in arid areas. And Mars is definitely arid. The duricrust around the mole prevents soil from falling into the mole’s hole as it hammers, and deprives the instrument from the necessary friction for hammering its way into Mars.
While InSight is primarily a NASA mission, the Mole was designed and built by the DLR (German Aerospace Center). They’ve been working with NASA’s JPL, which has an engineering version of the Mole in a test-bed. It’s there that they’ve tried to overcome these challenges.
They’ve tried using the scoop on the end of InSight’s instrument arm to apply sideway pressure to the Mole, hoping to provide the necessary friction. They’ve tried pushing down on the Mole, too, all while carefully avoiding the sensitive tether. And they’ve tried scooping up loose material with the scoop and depositing it into the Mole’s hole.
Today, NASA announced that the Mole was finally completely buried in the dirt. That’s a victory of sorts, but there’s still a long way to go. Now that it’s buried, the InSight team will continue to scoop more soil on top of the instrument and tamp it down before resuming hammering operations.
But all of this is taking time.
“I’m very glad we were able to recover from the unexpected ‘pop-out’ event we experienced and get the mole deeper than it’s ever been,” said Troy Hudson, the scientist and engineer at NASA’s Jet Propulsion Laboratory who led the work to get the mole digging. “But we’re not quite done. We want to make sure there’s enough soil on top of the mole to enable it to dig on its own without any assistance from the arm,” Hudson said in a press release.
Scooping the soil and tamping it down will take months. NASA says that it’s unlikely that the hammering operation will resume until January 2021. Part of what’s hampering the operation is the accumulation of dust on InSight’s solar panels. That’s reducing the power available to the entire mission.
Tilman Spohn is the Scientific Director for the Mole at the DLR. He’s been writing a blog about the effort to get the mole working. In today’s October 16th, 2020 entry, Spohn talked about the next steps, and how they’re working towards another “Free Mole Test.” The free mole test is when they let the mole try to hammer its way below the surface without assistance from the scoop.
“After some discussion about the next steps, we decided that two parallel scoop movements should be conducted on Saturday 17 October (Sol 659),” he wrote.
“Afterwards, a thermal conductivity measurement will be carried out, which should also give us indirect indications about the backfilling,” writes Spohn. “Then, the filling will be pressed to compress the sand and press on the Mole. Depending on the result of the backfilling, further actions to fill the pit will be planned before further hammering and another Free Mole Test will take place later on.”
On Earth, 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 spinning instead of drilling. That’s simply not possible on Mars. A drill would weigh far too much and would require far more power than the mole does. The mole is only 1 inch (2.7 centimeters) in diameter and about 16 inches (40 centimeters) long. It had to be both light enough and small enough to fit within the constraints of the mission.
Hopefully, the mole will eventually get to its working depth. In the meantime, InSight’s other instruments are functioning 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.
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