For a long time, scientists gazing up at the night sky thought that Earth’s solitary satellite—the Moon—was about as geologically dead as a space rock could get.
But 50 years ago, seismometers deposited by Apollo astronauts began to pick up tremors beneath the lunar surface. Some were easy to explain: A few rumbles were probably aftershocks in the wake of meteorite impacts, or regular quivers that ebbed and flowed with the rise of the Sun; others, hundreds of miles beneath the surface, likely resulted from Earth’s gravitational pull.
And then there were the shallow moonquakes. Between 1969 and 1977, Apollo seismometers recorded a series of 28 oddball trembles, occurring in only the top few miles of the lunar crust. These tremors weren’t deep—but they were strong, sometimes striking with forces equivalent to magnitude 5 earthquakes. And they seemed to defy explanation.
But now, a team of researchers might finally have the answer to this long-standing puzzle. According to their study, published today in the journal Nature Geoscience, these mysterious rumbles appear to be linked to young fault lines triggered by heat escaping from the Moon’s interior half a century ago. If confirmed, the findings suggest that the moon is still in the process of cooling—and could even remain tectonically active today.
“The whole idea that a 4.6-billion-year-old rocky body like the moon has managed to stay hot enough in the interior and produce this network of faults just flies in the face of conventional wisdom,” study author Thomas Watters of the Smithsonian Institution told Adam Mann at National Geographic.
On Earth, quakes are produced by the grinding and shifting of mobile tectonic plates. Other rocky bodies like Mars and the Moon, however, lack tectonic plates (to be clear: “tectonics” broadly refers to the evolution of geologic structures in a planetary body’s crust, so something can still be tectonically active even if it doesn’t have tectonic plates). But even on Mars, the surface can shake: As the Red Planet continues to cool from its cataclysmic birth, its hard outer shell contracts and cracks like a prune, resulting in occasional quivers that can still be detected today.
Because the Moon is smaller than Mars, many figured it had released the last of its inner cache of heat long ago. But now, data from Watters’ team suggests that its surface, too, is still shifting.
Using a computer algorithm, the researchers mapped the epicenters of the shallow Apollo-era moonquakes. Eight of them fell within about 18 miles of a series of cliff-like lunar landmarks, called scarps, recently identified by NASA’s Lunar Reconnaissance Orbiter. These step-like geologic features are thought to represent thrust faults—places where the lunar surface has compressed as the Moon releases its heat—and they’re probably no more than 50 million years old.
Additionally, 18 of the 28 documented moonquakes, including six that were also scarp-adjacent, seem to have happened when the Moon was at its farthest point from Earth, when our planet exerts its strongest gravitational pull on the lunar surface. Thrust faulting and Earth’s tidal tug likely both play a role in the Moon’s shallow rumbling, but it’s the results from the scarps that hint strongly at residual lunar activity.
The data is 40 years old, but on geologic timescales, that’s the blink of an eye. “If we detected these...events 40 years ago, then these faults are still active,” Watters told Carolyn Gramming at Science News.
A few uncertainties remain, but other researchers are cautiously optimistic about the results. “It would be awesome to get more seismometers on the Moon...to test this out properly,” Anna Horleston, a planetary seismology at NASA who was not involved in the study, told Robin George Andrews at The New York Times.
Regardless of these quakes’ true origin, though, the lunar surface is still far more volatile than researchers once believed. And this information could soon prove critical—not just for understanding our Solar System’s past, but also for charting our own future. Pinpointing the sources of these quakes, for instance, could be important intel for researchers hoping to land spacecraft carrying scientific instruments, or even people, on the lunar surface.
“The more we learn about these small bodies, the more we realize that they are so much more interesting and dynamic than previously thought,” planetary geologist Amanda Nahm of the Arctic Planetary Science Institute, who was not involved in the study, told Gramming. “The Moon is no longer considered to be ‘dead.’”