The Moon Is Shrinking, Causing Moonquakes at a Potential NASA Landing Site, Study Finds

Though it might appear still and quiet, the moon is not an entirely peaceful world. Our natural satellite is slowly shrinking, and as it does, “moonquakes” rock its surface. These regular rumbles in the regolith could become a problem for future crewed flights to the moon, such as NASA’s planned Artemis 3 mission, according to a study published late last month in the Planetary Science Journal.

“There’s a lot of activity that’s going on in the moon,” study co-author Tom Watters, a lunar geologist and Smithsonian Institution scientist emeritus, tells USA Today’s Doyle Rice and Cybele Mayes-Osterman. “It’s just something that we have to keep in mind when we’re planning, especially, long-term outposts on the moon.”

The NASA-funded study suggests future astronauts and mission equipment might be endangered by moonquakes, though scientists say the risk is low.

“This is not to alarm anyone and certainly not to discourage exploration,” Watters tells CNN’s Jacopo Prisco, “but to raise the caution that the moon is not this benign place where nothing is happening.”

Born from a violent planetary collision more than four billion years ago, the moon still harbors remnant internal heat that’s slowly seeping into space. As its core cools, the moon gradually contracts—estimates suggest it has shrunk by roughly 160 feet in diameter over tens of millions of years. In a way, the moon is literally waning.

This process makes the lunar surface prone to puckering—just as a smooth grape dries into a wrinkly raisin. Unlike the Earth, our celestial companion lacks plate tectonics, so its brittle crust crumples into unstable ridges called thrust faults to accommodate its dwindling volume. The resulting creases can stretch tens of meters high, and the fact that they haven’t eroded away implies they might be relatively new, Watters tells the Washington Post’s Kasha Patel.

At the same time, Earth’s incessant gravitational tug on the moon adds stress to an already dynamic environment, contributing further to the formation of thrust faults.

The shrinking moon doesn’t affect us back on Earth. For one, the size reduction is too slight to do away with magnificent solar eclipses within our lifetimes. And although it’s contracting, the moon’s mass stays constant—as a result, so too do the lunar tidal effects on our oceans.

However, the effects of the moon’s shrinking can be felt on its surface. In the recent study, the researchers examined data on moonquakes detected by lunar seismometers, which have been on the moon since Apollo program astronauts left them there more than 50 years ago. They also used mapping data from NASA’s Lunar Reconnaissance Orbiter to identify the telltale creases on the moon. Through modeling, they linked these faults to seismic activity.

Scientists examined a cluster of faults around the lunar south pole and tied them to a large moonquake that notched a Richter scale magnitude of 5. On Earth, such a quake is unmistakably perceptible, though it might cause only minor infrastructure damage. On the moon, however, a magnitude 5 quake would feel much stronger and bring more devastating consequences.

The moon’s gravity—a sixth that of Earth’s—is too weak to secure surface objects in place, “so even a little bit of ground acceleration is going to potentially pop you off your feet, if you’re walking along,” Watters tells CNN. Moreover, the loose regolith is prone to landslides, especially along sloped surfaces. Moonquakes can also persist for hours—much longer than the typical seconds to minutes of terrestrial tremors.

Focusing on the lunar south pole, which has drawn scientific interest for potentially harboring water ice, Watters and his team modeled the surface slopes to determine the effects of quakes. They found that some permanently shadowed areas, such as the Shackleton Crater, are susceptible to landslides with even a light amount of shaking. Notably, this spot is a potential landing site for the NASA Artemis 3 mission slated for no sooner than 2026.

While the quakes do raise safety concerns for lunar visitors, astronauts may be unlikely to encounter shaking. Previous research estimated that shallow moonquakes occur only once every 100 days on average, per the Washington Post. But any human-made equipment intended for long-term use, such as a lunar outpost, might need to be quake-proof.

Still, moonquakes have a bright side: They are incredible tools for science. “They are like flashlights in the lunar interior that illuminate its structure for us to see,” says Jeffrey Andrews-Hanna, a University of Arizona planetary scientist who wasn’t involved in the research, to CNN.

Some researchers are skeptical of the findings. Per the Washington Post, a study in 2022 estimated that moonquakes are weaker than what the new study describes and projected they will stay clear of most of the Artemis candidate landing sites.

Additionally, not everyone agrees that the thrust faults on the lunar surface are the reason for moonquakes. Rather than shallow tremors, Yosio Nakamura of the University of Texas at Austin tells CNN the quakes rumble tens of kilometers deep inside the moon’s interior. “We still don’t know what causes shallow moonquakes, but it is not the sliding fault near the surface,” the geophysicist, who wasn’t involved in the study, says to the publication. “We need more data about them.”

Editor’s Note, February 9, 2024: This article has been updated to correct the estimate for the moon’s shrinkage.

Shi En Kim | | READ MORE

Shi En Kim is a Washington, D.C.-based freelance science journalist. Her work has appeared in National Geographic, Scientific American, the Atlantic, Popular Science and others. In 2021, she interned at Smithsonian magazine as an AAAS Mass Media Fellow.