The Moon Is Slowly Shrinking, Which May Be Causing ‘Moonquakes’ on Its Surface

Analysis of seismic data collected on the Apollo missions shows the moon is probably tectonically active

The moon doesn’t have volcanoes, tectonic plates that scrape against each other, or other complex phenomena that make planets like Earth so geologically complex. But a new study published in the journal Nature Geoscience that analyzed Apollo-era data suggests that the moon does have faults and some seismic activity, suggesting the moon is more dynamic than previously thought.

Astronauts placed seismometers, the instruments that detect earthquakes, on the Moon’s surface during all of the missions to the moon’s surface except for the last one, Apollo 17, according to NASA. In total, those instruments recorded many quakes on the surface of the moon between 1969 and 1977, ranging from two to five on the Richter scale.

The New York TimesRobin George Andrews reports that there were explanations for many of the shakes. Meteorite collisions caused some, others were attributed to Earth’s gravitational pull and others were caused by huge 500-degree temperature changes on the moon when night turns to day. Twenty-eight deeper quakes, however, were unaccounted for. Now, researchers have found evidence that active faults on the crust of the moon caused the shakes.

After its formation, the moon’s interior cooled over time, causing the surface of the moon to shrink the same way a grape shrivels up into a raisin, according to NASA. But because the moon’s surface is brittle, not flexible like grape skin, it cracked and formed slip faults as it shrank. Over the last few hundred million years, the surface has contracted about 150 feet. But most researchers thought the process of cooling was over, and the process that created faults ceased long ago.

But researchers began to suspect that the mystery quakes were caused by active faults in 2009 when examining images from the Lunar Reconnaissance Orbiter, reports Adam Mann for National Geographic. When study co-author Thomas Watters, a planetary scientist at the Smithsonian’s National Air and Space Museum, mapped scarps—or steep slopes usually caused by faults—on the moon, he noticed many of them looked relatively fresh, only 50 million years old or less with signs that boulders had rolled down the slopes.

To find out if those faults were still active, the team used an algorithm called LOCSMITH used to calculate where some of the Apollo moonquakes originated. They found that the epicenters of eight quakes were near young faults and occurred when tidal stress on the moon was the greatest, suggesting that the moon is still cooling and causing these thrust faults to form.

The team then simulated 10,000 seismic events, finding that the chance that these quakes are unrelated to the nearby faults is less than four percent. “That means, for all intents and purposes, the moon is tectonically active,” Watters tells National Geographic’s Mann. “To me, that’s an amazing result.”

The finding could mean astronomers do not know the full story of how small, rocky bodies cool off. “We’ve got these possibly active faults on the Moon, which means it isn’t this dead body,” Watters tells Ryan Mandelbaum at Gizmodo. “It flies in the face of conventional wisdom, that the smaller a rocky body, the quicker it loses interior heat and becomes geologically inactive.”

If the moon is more geologically active than we thought, it could have implications for where space agencies decide to build lunar bases or land missions. But not everyone is convinced we should start designing lunar earthquake shelters just yet.

“They use a lot of statistical arguments, and I think they do good science, but I wouldn’t say it’s definitely there,” Ceri Nunn of NASA’s Jet Propulsion Laboratory, not involved in the study, tells Mann. Although, without better data, she notes that the team has done as good of job as possible at pinpointing the location of the quakes detected on Apollo missions.

Watters and his team agree that more data is needed to confirm their finding, and hope that future missions to the moon will set up a high-quality network of seismometers and other equipment to determine if the moon is still restless.