•             NASA has given up on its InSight lander's "Mars mole" after two years of trying to burrow into the planet's surface.

•             The Martian soil turned out to be too thick, and dust accumulating on the lander's solar panels was causing the robot to generate less and less power.

•             No Mars mission in the foreseeable future will take the planet's temperature — a measurement crucial to understanding the planet's history and potential underground water.

NASA sent its InSight lander to Mars with an ambitious mission: to study the planet's deep internal structure. A crucial piece of that effort - the "mole" - has failed despite two years of attempts to salvage it.

The mole is a revolutionary heat probe designed to burrow 16 feet into the Martian soil and take the planet's temperature. Its measurements would have revealed clues about how the planet formed and has changed over the last 4.6 billion years - a history that would help scientists track down Martian water, and possibly life.

But the mole has made little progress in the unexpectedly thick soil. Now the InSight team must ration the lander's solar power. NASA announced Thursday that the mole won't be able to dig its hole.

"It's a bit of a personal tragedy," Sue Smrekar, a lead scientist on the InSight team who has spent 10 years working on the mole, told Insider. "Everyone tried as hard as they could make it work. So I can't ask for anything more than that."

No other Mars mission in NASA's foreseeable can take the internal temperature measurements for which the mole was designed.

"This has been our best attempt to get that data," Smrekar added. "From my personal standpoint, it's super disappointing, and scientifically it's also a very significant loss. So it feels really like a huge letdown."

An unexpected energy crisis

The InSight team spent two years maneuvering the lander's robotic arm to see if it could help the mole burrow further. The probe, a 16-inch-long pile driver, is designed to leverage the loose dirt that other Mars missions have encountered. The soil would flow around the mole's outer hull and provide friction to keep hammering deeper.

But in February 2019, the mole found itself bouncing in place on a foundation of firm soil called "duracrust." The next two years were spent troubleshooting, beaming new software to InSight to teach its robotic arm new maneuvers to assist the mole, and anxiously waiting for photos that might show progress.

"It's just been a huge effort across the board, and one that we never anticipated," Smrekar said. "We thought that we were going to punch the hole down."

The InSight team first instructed the robotic arm to push on the mole, but that just caused it to pop out of the hole. Once they got the probe back in the ground, a year later, they instructed the arm to pile dirt on top of it, hoping that would provide enough friction for the probe to dig deeper.

But the mole made no progress with 500 hammer strokes last Saturday. The top of it was just 2 or 3 centimeters below the surface.

By then, InSight's problems were compounding. Unlike other sites where NASA has sent rovers and landers, the open plain where InSight sits wasn't having powerful gusts of wind. Smrekar calls such gusts "cleaning events," since they blow the planet's pervasive red dust off any robots in the area. Without them, InSight's solar panels have accumulated a significant layer of dust.

At the same time, the seasons were changing and InSight's home on a flat plain near Mars' equator was getting colder. In the chill, InSight will require more energy just to stay functional, even while its solar panels are absorbing less sunlight than they should.

"Power is decreasing and so we're coming up on a time period where, for probably two or three months, we're probably going to have to stand down from doing instrument operations for awhile and just kind of go into survival mode until it gets warmer on Mars," Smrekar said.

With this new time constraint, Saturday's hammering attempt was the mole's last chance to burrow.

Over the next two years, InSight will still listen for quakes on Mars and collect data on the planet's rumblings with its seismometer. This can provide some insight about the planet's interior. Already, Mars quakes have revealed that the Martian crust is drier and more broken up than scientists had thought ⁠- more like the moon than like Earth.

A planet's internal temperature reveals its history

If the mole had hammered down to 16 feet below, it would have measured temperatures all the way down its hole. That would allow scientists to calculate how much heat is leaving Mars - a metric called "heat flow."

"It's a single number, the heat flow, but it has ramifications for all kinds of aspects of understanding Mars," Smrekar said.

Heat leaving a planet is, in part, warmth left over from its formation, but it also comes from decaying radioactive elements. Measuring the heat flow would tell scientists how much radioactive material is inside the Martian crust - the outer layer of the planet - versus the mantle beneath.

That would reveal not only how material was distributed when the planet formed (and whether it's made of the same stuff as Earth), but also how the planet's internal structure has changed over time.

"That goes back to understanding the early evolution of Mars, that time period when there was a lot of liquid water on the surface," Smrekar said.

A higher concentration of radioactive material in the mantle would make that layer more active. More radioactive material in the crust could keep the planet's upper layers warm.

Heat flow could also indicate how deep you'd have to drill into Mars to reach liquid water today. Underground water on the planet could still host microbial life. Future humans traveling to Mars will likely need to harvest water there.

Now there is no possibility of measuring the planet's heat flow in the foreseeable future.

"I was hoping to get the data and be able to understand what that means for Mars," Smrekar said.

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