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After its first attempt to penetrate the rust-colored Martian surface in 2019, NASA's “mole” sent a signal back to Earth.
It was not good news.
The mole is part of the Heat Flow and Physical Properties Package(HP³) on NASA's InSight lander, which touched down on Mars in 2018.
Built by the German Aerospace Center, the mole works by hammering itself into the ground. Data sent from the mole showed hammer strokes. But measurements from an optical ruler showed that it hadn't moved much.
Troy Hudson, instrument system engineer for InSight, and his coworkers at NASA's Jet Propulsion Laboratory in Pasadena, Calif., were eagerly awaiting the data. When they realized what happened, it felt like the air was sucked out of the room, said Hudson.
The mole was supposed to hammer itself 16 feet into the crust of Mars to take the planet’s internal temperature, which could help unravel the mystery of how the planet was formed and how it evolved.
Figuring out what went wrong took extensive testing.
“For a long time, I would say about six months, we didn't really know what the problem was,” Hudson said. “One of the ideas we came up with was lack of friction. That was the problem.”
As the mole drove down, the regolith — the mix of dust, dirt, and rocks just below the surface — clumped up instead of filling out the gap around the probe. To help it along, the team on Earth attempted to press the probe to one side of the soil using InSight’s robotic arm, hoping there wasn’t a Martian rock in the way.
It seemed to work.
“It actually went downlike a centimeter,” Hudson said, recalling that he was almost in tears. “I was ecstatic.”
A shot of InSight's robotic arm pinning the heat probe to the side of the soil on Oct. 3, 2019. Credit: nasa / jpl-caltech The team adjusted the arm to protect a tether on the mole's left side. The mole then lost friction, causing it to back out of the hole as it tried to hammer itself deeper.
“That was crushing,” Hudson said. “I was beside myself with frustration, and grief even, because it backed out of the ground a lot.”
Another adjustment and another hammer attempt later, it went deeper again.
“More joy,” Hudson said. “And then it backed out again. More anger and frustration.”
The Mars mole sticks out of the surface of Mars to the left of InSight's arm. It came out of the ground after trying to dig further down on Oct. 27, 2019. Credit: nasa / jpl-caltech After almost two years of trying, the team called off the digging in January. The top of the mole now sits one inch below the surface, its 16-inch-long body just barely buried in the soil.
Exploring the unknown
So what went wrong? Using their knowledge of planetary evolution, as well as image, temperature, spectrometry, and other data collected by past Mars missions, scientists predicted what the regolith would be like at InSight’s landing zone.
"Exploration always carries an element of risk."
“All of these things led us to expect that we would be landing in a place where the surface was broken up, unconsolidated, loose, regolith material,” said Hudson. “But what we found when we arrived there, which still surprises me, is this thick, cohesive, dirt crust layer.”
Imagine sticking your finger into granulated sugar, the individual grains falling and filling in toward the hole your finger makes. Now imagine sticking your finger into wet sand and wiggling it around a bit. The sand would press outward and create steep walls around your finger instead of filling in around it.
The team built the mole for the former, but found itself in the latter. Designed like a self-hammering nail, the mole hits itself with its own weight to drive downward.
Via GiphyThe design worked in terrestrial tests that simulated what they predicted the Martian ground would be like, Hudson said. But the actual cohesiveness of Mars soil proved troublesome.
Hudson had been working on the project that turned into InSight for 12 years. InSight’s principal investigator, JPL’s Bruce Banerdt, has been attempting to get a seismometer on Mars for three decades, Hudson said. To have one part of the project fall short is gutting. But that kind of thing happens when you go places where nobody has gone before.
"Exploration always carries an element of risk," Hudson said. “We do our best at JPL and NASA to minimize that risk, and do our best to ensure success. But there are always what we call 'unknown unknowns.'"
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Scientists can try to predict risks and mitigate them, but you can’t know what you don’t know. Now they know what they’re working with under InSight, and that can better inform future missions to Mars and beyond.
Far from a failure
While it's easy to point at the Mars mole’s shallow hole and say "failure," getting that probe down 16 feet was just one facet of the broader InSight mission.
More than 500 Marsquakes have been detected by InSight’s seismometer, including a handful with a magnitude stronger than 3. Scientists can now extrapolate how and where these quakes are happening.
"We've been able to localize them to the Cerberus Fossae region, which is one of the younger formerly volcanically active regions on Mars," Hudson said. "It makes very good sense that we would be detecting seismic signals from there."
The seismometer and a radio instrument have uncovered previous unknowns like the size and density of Mars’s core. Meanwhile, above ground, InSight has a microbarometer measuring things like wind temperature and air pressure, giving atmospheric scientists the longest standing high-resolution weather monitor on Mars.
HP³ is alive and collecting data even though the probe isn’t going any deeper. The mole itself is still measuring temperatures and heat conductivity near the surface.
The knowledge gathered from the digging attempt is being compiled by Hudson and others, colloquially known as the Mole Saga.
SEE ALSO: How NASA's Perseverance is making oxygen on Mars“I'm working with colleagues at DLR and other places to write the scientific papers about what we've learned about Mars and what we've learned about the design of the mole,” he said. “There will be two separate papers that hopefully will come out later this year that describe everything in detail and provide suggestions for future subsurface explorers.”
Right now, there aren't plans for another probe to dig on Mars. There are plans for the moon, however. A surface-penetrating instrument dubbed LISTER is in development by Honeybee Robotics with Texas Tech University professor Seiichi Nagihara, Hudson said. The project will launch through NASA’s Commercial Lunar Payload Services initiative.
“It uses pneumatics — air — to blow away regolith material from an unspooling tape that turns into a cylinder,” he said. “It's a very clever way to do it and probably a very good idea for the moon because the regolith on the moon is much denser than the stuff on Mars.”
Even though the Mars mole wasn’t entirely successful, it’s still paving the way for future space exploration.