Thousands of Moonquakes Rocked the Apollo Landing Sites in Less Than a Decade

About 1 hour and 40 minutes after Neil Armstrong first stepped onto the moon on July 20, 1969, a message from Earth came over his radio. The team in Houston wanted to let the astronauts on the lunar surface know that one of the three pieces of scientific equipment they’d deployed, a passive seismometer designed to detect vibrations reverberating through the moon, was successfully transmitting data.

“Tranquility Base, this is Houston,” said astronaut Bruce McCandless, whose job it was to communicate with Armstrong and Buzz Aldrin during their lunar expedition, “the passive seismic experiment has been uncaged, and we’re observing short-period oscillations in it. Over.”

Armstrong didn’t answer—he was too busy running around near the lunar lander. In fact, the “short-period oscillations” the seismometer was picking up were Armstrong’s footsteps.

Now, 55 years after the Apollo 11 seismometer recorded Armstrong’s run, scientists are still learning more about the moon from seismic data collected between 1969 and 1977. A study published in July in the Journal of Geophysical Research: Planets re-analyzed the decades-old data and found thousands of moonquakes. These include newly discovered moonquakes big enough to worry future visitors to the moon.

“It’s so thrilling,” says Teasel Muir-Harmony, a historian of lunar exploration and curator of the Apollo collection at the Smithsonian’s National Air and Space Museum, which has a lunar seismometer on display in the “Destination Moon” exhibition. “We’re celebrating the 55th anniversary of Apollo 11,” she says. “A program that ended over 50 years ago is still informing science.”

When President John F. Kennedy declared that “we choose to go to the moon” in 1962, scientists had very little idea what astronauts would encounter once they left their spacecraft. Some at NASA worried that the surface was covered in such a thick layer of dust it would be like stepping into quicksand. “They didn’t know whether that meant that [you would] just sink in and never be seen again,” says Ceri Nunn, a lunar seismologist who wasn’t involved in the new study but recompiled the Apollo data the paper uses into a standardized, modern format and fixed time-stamp issues associated with the original compilation.

But for as little as the astronauts knew about the moon’s surface, they knew even less about its interior­. Was it cold, was it solid rock, or did it move around like Earth’s tectonic plates in a way that could cause shaking?

That’s where the Apollo missions’ seismometers come in.

Moonquakes are the lunar equivalent of earthquakes. With a seismometer, scientists can decipher what types of materials the waves of energy produced by a quake pass through on their way to the instrument. In other words, seismometers allow geologists to peek beneath the lunar surface and learn about what’s happening further down.

“We know much about the surface of the moon,” says Keisuke Onodera, a lunar scientist at the University of Tokyo and author of the new study, “but on the other hand, the deep interior is still unclear. And seismology is one way to reveal the internal structure.”

The Apollo missions used two types of moonquake detectors, called short-period and long-period seismometers. The short-period devices were more sensitive to events that occur close to the instrument, but they were also more susceptible to picking up errant signals that obscured moonquakes. This made the short-period data harder to analyze. Even so, researchers immediately began searching for moonquakes in both data sets by hand, using pen and paper. This effort found about 13,000 moonquakes, and the short-period data was primarily used to get more information about a quake after it had already been positively identified in the long-period data.

But, since the last lunar seismometer stopped transmitting data back to Earth in 1977, a large portion of the short-period record hadn’t been thoroughly reviewed in the way modern computer-based methods enable. This meant that there were likely moonquakes sitting undiscovered in the hard-to-work-with short-period data. In the new study, Onodera uses the data set recompiled by Nunn to find them.

Onodera created a computer program to automatically detect seismic events in the short-period data, creating a catalog of potential moonquakes he could then check one by one. “I think it took three months to check all the 30,000 signals, and based on that I started to categorize,” he says.

Onodera found 22,000 previously overlooked moonquakes that fall into three main categories. The vast majority, all but 46, fall into the first two categories—due to either small meteorite impacts or the expansion and contraction of the moon as its surface temperature changes over 500 degrees Fahrenheit between day and night.

The remaining moonquakes, in the third category, really caught Onodera’s eye as he parsed the data. Called “shallow” moonquakes, these events appear to be the result of movement up to 155 miles beneath the surface of the moon. Interestingly, Onodera found that the shallow moonquakes seemed to be concentrated in the moon’s northern hemisphere.

“To explain this,” he says, “you need to go back in time three billion years ago.”

Back then, the moon was hotter and more active than it is now. Onodera says that in the region near the Apollo 15 landing site in the lunar north, molten magma from deep in the moon intruded toward the surface, causing cracks in the brittle crust as it expanded. Now, billions of years later, the moon is cooling off and shrinking. Onodera thinks the moon’s crust on either side of the ancient cracks might now be moving in the opposite direction to accommodate the contraction.

According to Onodera, this causes moonquakes large enough, around magnitude five or six, to be dangerous to astronauts or any potential structures built on the moon in the future. “The typical moonquakes are just magnitude one or two, those are just negligible,” he says. “But on the other hand, magnitude six cannot be ignored.”

Nunn says she’s excited by the new work. “It’s actually very impressive that this paper has been written,” she says, “and that they’re still using the data from 50 years ago that was always very hard to work with.”

And while some secrets might still be lurking in the old Apollo seismic data, lunar scientists are forging ahead with plans to put new, sensitive seismometers on the moon for the first time since the Apollo missions in the 1970s. Nunn is working on the team developing the Farside Seismic Suite that is planned to be placed on the far side of the moon by commercial spaceflight in the next few years. Additionally, NASA’s upcoming Artemis campaign plans to place seismometers near the lunar South Pole.

Studies like Onodera’s are intriguing, Nunn says. However, longstanding questions remain about the causes of potentially dangerous moonquakes and their relationship to the moon’s internal structure. The Apollo missions landed only on a relatively small region of the moon, leaving open the possibility of detecting new moonquakes from unexplored areas that could upend current assumptions.

Scientists are hopeful the new seismometers will help them learn even more about the inner workings of the moon, says Nunn, “particularly because they will be in different places that we’ve never seen before.”

Rudy Molinek | READ MORE

Rudy Molinek is Smithsonian magazine's 2024 AAAS Mass Media Fellow.