On December 12, 1972, Gene Cernan parked his moon buggy in the southeastern edge of the Sea of Serenity, in a valley named Taurus-Littrow. A gray hill called the North Massif loomed in the distance. On its western side was a slumping escarpment, nicknamed the Lee-Lincoln scarp. It was a landslide, forming a low wall that seemed to cross the valley, like a shrug in a shoulder of the moon. Cernan and his seatmate, fellow astronaut Harrison “Jack” Schmitt, stared at it and snapped some pictures.

“Hey, look at how that scarp goes up the side there,” said Schmitt, a geologist. “There's a distinct change in texture. Look over by Hanover [crater].”

“Okay. Oh, man; yeah, I can see what you’re talking about now. It looks like the scarp overlays the North Massif, doesn’t it?” Cernan said.

“Yeah,” Schmitt said. To Houston, he narrated what he saw: “The appearance you have of the scarp–North Massif contact is one of the scarp being smoother-textured, less cratered, and certainly less lineated. And I wouldn't be a bit surprised if it's, as Gene says, younger.”

Schmitt meant that the scarp had formed after the mountain was raised. Something moved the mountain, in other words. Something deep within the moon had stirred, and its surface shifted.

This could have happened tens of millions of years prior. But at other locations on the moon, it is happening now. Scientists have for the first time connected seismic data to the changing lunar landscape, showing that the moon is tectonically active today and is building new outcrops and calving boulders in response. Regular moonquakes along a network of faults are energetic enough to rattle a moonwalking astronaut, and to shake the foundations of any future habitats, according to the new study, which appears in Nature Geoscience.

“If you’re interested in an outpost, and you expect to be there for some time, you need to be mindful that if you are too close to one of these faults, your structure is going to be shaking,” says Tom Watters, a geophysicist at the Smithsonian Institution’s National Air and Space Museum.

Moonquakes, Revisited

During the Apollo era, seismic instruments left by Cernan, Schmitt and their predecessors showed that the moon experienced occasional quakes. These are mostly related to internal cooling and the tidal forces of Earth’s gravitational tug. And they were mostly deep within the moon, and not powerful enough to affect changes on the moon’s surface.

When NASA’s Lunar Reconnaissance Orbiter (LRO) started taking high-resolution images in 2009, scientists realized the moon had numerous scarps like Lee-Lincoln, including many smaller ones just a few meters high. The latter are small cliff-like features, almost impossible to see from orbit, Watters says, and only LRO’s high-resolution cameras could make them out.

“You can think of them as like stairsteps when you are coming up on them,” he says. “You’re not going to walk over one and not know it’s there, but if you don’t have the right resolution and the right lighting conditions, you’re not going to see them.”

With LRO’s high-resolution pictures, scientists started scrutinizing numerous scarps and counting craters, a proxy for figuring out a scarp’s age. In 2012, Watters figured out that the scarps and related features—long, thin valleys called grabens—formed recently, maybe as recently as 50 million years ago.

“We were all converging on the idea that these things are really young. That always leads you to the intriguing possibility that maybe these things are still active, and maybe these faults are showing us current tectonic activity on the moon,” Watters says. “But we were sort of out of ways to really refine the ages.” And there was no way to connect moonquakes to changing surface features.

Tectonic activity on the moon—and Mars, for that matter—is not the same as it is on Earth. Our planet has plate tectonics, in which the Earth’s cracked crust dives down, melts and recycles itself. Collisions at the boundaries of Earth’s plates are the main cause of earthquakes, volcanoes, and seafloor spreading. The moon and Mars have no mobile plates, so the internal tremors are a result of interior heat. In the case of the moon, quakes happen because it is losing its primordial heat—literally, the warmth of its creation—and contracting as it cools.

The four Apollo seismic stations, which operated from 1969 until 1977, counted several deep quakes and just 28 shallower ones, some of which were equivalent to about a magnitude 5 tremor on the Richter scale. Renee Weber, a seismologist at NASA’s Marshall Space Flight Center, says the shallow quakes are more like those we experience on Earth.

Once LRO images showed multiple scarps—plus rock piles, landslides and other features that suggest the moon is shaking—she and colleagues decided to go back through the Apollo records, trying to tie specific moonquakes to features on its face.

The team had to perform a series of complicated calculations and simulations, in part because the Apollo records are so difficult to use. Apollo was the first time anyone tried to digitize seismological data, Weber says. What is more, the moon’s crumbly regolith dampens seismic waves, making them harder to trace back to their origin.

After several years of analysis, the team was able to determine the epicenters of eight Apollo moonquakes, and tie them to specific faults observed with LRO. They were even able to correlate them to the moon’s orbital position around Earth, finding that more moonquakes occur during apogee, when the moon is at its most distant and Earth’s gravitational pull wanes.

While the Apollo seismic data showed the moon was shaking and stirring, the LRO cameras were necessary to show how this internal turmoil was altering the surface, says Patrick McGovern, a lunar scientist at the Lunar and Planetary Institute in Houston who was not involved in the work.

“Before something like LRO, we were blind to exactly how that was manifesting itself. The thing with LRO is that we can see all these fault scarps, and we can tie that together to the data from the seventies, to say how much activity there is and how it is distributed,” he says. “It was sort of like we had half the picture, and now we have the whole picture.”

Interplanetary Seismology

The quakes happen because the moon is contracting as it cools, Watters and Weber say. Imagine an overripe piece of fruit: As the inside shrinks and dries out, the skin ripples and sags.

“When you have a solid surface that is brittle, and you radiate heat, the planet will shrink. Cooling things shrink,” Weber says. “The surface kind of buckles, like a grape turning into a raisin. The surface area of the skin isn’t changing; it just gets convoluted and folded up on itself.” In other words, the shrugging scarp of Lee-Lincoln glimpsed by Cernan and Schmitt (and later by LRO) is a wrinkled grape skin, the result of one of these faults.

Charting these and other recent moonquakes could shed more light on lunar history and structure, says Francis Nimmo, a geophysicist at the University of California, Santa Cruz, who was not involved in the new research. It could even aid scientists attempting to understand the origins of temblors elsewhere—namely, Mars.

The moonquake history comes as scientists are downloading the first sets of data from the InSight lander on the Red Planet, a seismology mission tasked with probing that world’s deep interior. So far, Mars seems to be slightly less seismically active than people expected. “One of the big differences between the moon and Mars is that tides are more important on the moon. The fact that the moon is seismically active may have a lot to do with the fact that it is being squeezed and stretched with these tides,” Nimmo says.

Weber, who is an investigator on the InSight team, says she hopes future crewed or robotic landings could deploy a broader seismic network for long-term monitoring. Such a network could ensure the safety of any astronauts who follow in Cernan and Schmitt’s footsteps, according to McGovern.

“It may turn out that it’s not the hugest deal, or it may turn out to be some places are safer than others,” McGovern says. Maybe Taurus-Littrow and the Lee-Lincoln scarp wouldn’t be the best place to revisit.

On December 13, 1972, the day after they studied the scarp, Cernan and Schmitt parked closer to the base of North Massif. At Geology Station 6, the duo clambered out near a great gray boulder, broken in half during its roll from the massif. It towered over Schmitt.

“Hey,” he called to Cernan. “I'm standing on a boulder track. How does that make you feel?"

“Like I'm coming over to do some sampling,” Cernan said. He paused for a beat. “Think how it would have been if you were standing there before that boulder came by.”

“I’d rather not think about it,” Schmitt replied.

That particular boulder had come by some 22 million years ago, according to later geochemical analysis. But Watters’ study shows that others like it could come by tomorrow—or anytime, shaken loose by the interior stirrings of a world that is still active, and in a way, still alive.