Mysterious tremors on the moon have been traced back to an unexpected source

• Data showed a mysterious quake happening every lunar morning, and scientists weren't sure why. 
• A new study has found morning tremors are coming from the Apollo 17 lunar lander base.
• The lander base expands and vibrates every morning as it gets heated by the sun.

Scientists baffled by mysterious daily quakes on the surface of the moon have traced back their origin to an unexpected source. 

Researchers from NASA and the California Institute of Technology reanalyzed data on tiny moonquakes, which was gathered in the 1970s by astronauts from the Apollo 17  mission.

They discovered that some of the tremors were being produced by the descent stage of the Apollo 17 lunar module, left behind on the lunar surface 51 years ago. 

"Every lunar morning when the sun hits the lander, it starts popping off," says Allen Husker, research professor of geophysics and coauthor of the new study, in a press release.

The moon doesn't have tectonic activity, but it's still shaking

Part of the goal of the Apollo mission — which put the first humans on the moon and ran from 1961 to 1972 — was to collect information about the moon's seismic activity. 

To do so, astronauts placed seismic detectors on the moon over the course of Apollo missions 11 to 17. The Apollo mission seismometers stopped being operational in 1977, but scientists have continued to re-analyze the data to gain crucial clues about how our satellite cracks and stretches. 

It's thanks to these readings that scientists first gained an understanding of moonquakes. Unlike on Earth, tremors on the moon have nothing to do with tectonic plate shifting.

When a quake happens on the moon, it's due to one of three factors: a meteor hits the surface, the Earth's gravity stretches or squeezes the moon's interior, or the moon's surface bakes under the intense sunlight hitting it during the day, a post from the European commission explained. 

That's why scientists were so surprised when they saw the unusual readings when reanalyzing the data from the Apollo 17 seismic detectors: they saw quakes that had nothing to do with these three causes. 

The lunar lander is creaking in the hot sun

These quakes had never been spotted before, even though the data was captured between October 1976 to May 1977.

That's because the readings from the Apollo 17 mission were very messy and difficult to interpret, the Caltech and NASA scientists said in a study presenting their findings, which was published in the peer-reviewed journal JGR Planets on Sept 5. 

Using machine learning they were able to clean up the data. As they expected, they picked up readings from so-called thermal quakes that happen in the afternoon when the moon starts to cool. 

But each morning, scientists also saw bizarre waves popping up every few minutes. By triangulating the signal, they were able to figure out the source of the mysterious tremors.

"We found that impulsive moonquakes are not due to natural processes, but are vibrations generated from the lunar module descent vehicle left by the astronauts in 1972," Husker said in the study with co-authors Francesco Civilini, from Caltech and Renee Weber of the NASA Marshall Space Flight Center.

When Apollo 17 astronauts lifted off from the moon in 1972, they left behind part of the lunar lander — you can see this in the video below. It turns out that hunk of metal is what was causing these mysterious quakes, as it boils and creaks as temperatures range from +250 to -208° F, per the press release.

This is important information for upcoming missions 

Going to the moon always comes with a certain number of unknowns, for a very simple reason: we haven't been to the moon very many times. 

We also don't have a very good idea of what happens to space gear if it's left on the moon for a very long time.

But that information is quite important because NASA is aiming to go back to the moon for the first time in 50 years, and this time, it wants astronauts to stay there for an extended period of time. 

The data provides rare information on what happens to a spaceship that's left to bake in the sun, which can be incorporated when designing new tech, per the CalTech press release.

It's also important because a lot of future planned missions are going toward the south pole of the moon, which never sees sunlight, Husker said in the press release.