Half a century ago, the Apollo astronauts left short-lived seismometers on the lunar surface. They found that the moon was alive and kicking. Some tremors deep below the surface likely were caused by Earth’s gravitational pull. Others were vibrations from meteorite impacts. Still others resulted from expansion of the moon’s chilly surface every two weeks when the sun rose.
There were also shallow moonquakes, just a few miles beneath the surface. Unlike the other categories of quake, these convulsions couldn’t be satisfactorily explained. But a study published Monday in Nature Geoscience suggests that they were triggered across myriad young faults by a combination of escaping internal heat and Earth’s gravitational pull.
The discovery suggests that the moon is still tectonically active, and raises the possibility that future lunar bases may be vulnerable to shallow moonquakes. It also prompts questions about the moon’s evolution.
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The moon, likely born from a violent impact four billion years ago, is just over one-quarter the radius of Earth. Its diminutive size led to the belief that any internal heat should have escaped into space long ago. As a result, the driving engine of most major geological activity should have shut down. But this new evidence suggests it hasn’t given up the ghost just yet.
Thomas Watters, a planetary geoscientist at the Smithsonian Institution’s National Air and Space Museum and lead author of the study, said that the finding “flies in the face of the conventional wisdom of how rocky bodies cool down.”
The spark for the study came in 2010, when NASA’s Lunar Reconnaissance Orbiter found evidence of young faults, no more than 50 million years old, on the moon. Dr. Watters and his colleagues wondered if those faults still might be active.
Between 1969 and 1977, seismometers at four Apollo landing sites picked up 28 shallow moonquakes. But partly because the instruments were less than ideally placed, the method used to determine the moonquakes’ locations was fraught with uncertainties.
With a bespoke algorithm named LOCSMITH, Dr. Watters’ team used those uncertainties to more accurately determine where the quakes originated. They discovered that several shallow moonquakes took place near some of the young fault features spotted by the NASA orbiter.
The team also simulated the surface shaking caused by those moonquakes, and found that strong shaking extended as far as 19 miles from the faults. Eight of the moonquake epicenters were within this distance, which strongly suggested that the quakes were created by the young faults.
These faults belong to a category known as thrust faults, in which a geological block slides upward against gravity when the surrounding region is being squashed. The moon’s thrust faults are a sign that the whole orb is contracting as it loses internal heat, cools and shrinks; Mercury is undergoing a similar process.
But if contraction alone were at work, the pattern of thrust-faulting and shallow moonquakes would be randomly distributed. The team’s mathematical models revealed that Earth’s gravitational pull is helping to focus this global contraction and make it less random: At different times during the moon’s orbit, fault slippage takes place on specific parts of the lunar surface.
The team reviewed the positions of the moon and Earth from 1969 to 1977 and found that the locations of the faults that were likely responsible for the shallow moonquakes made sense if the cause was a combination of cooling-related contraction and Earth’s tidal tugging. The model is bolstered by what appears to be recent surface shifting of boulders near these faults.
This evidence strongly suggests the moon is “tectonically active,” the study concluded.
On Earth, the phrase tectonics has come to be associated with multiple tectonic plates. But tectonics refers to large-scale processes — including those that facilitate heat loss — that influence the evolution of surface structures, said Ken Hudnut, a geophysicist at the U.S. Geological Survey who wasn’t involved with the study. Even though the moon lacks tectonic plates, he said, the team’s use of the phrase “tectonically active” is sound, and it is supported by compelling evidence.
Earth still leaks plenty of internal heat into space, which shows up at the surface as volcanic eruptions and massive earthquakes, but the moon doesn’t. “Maybe we can learn something about the future state of Earth from the present state of the moon,” said Dr. Hudnut.
Anna Horleston, a planetary seismologist and member of the InSight mission on Mars — which recently detected its first marsquake — said the study’s data looks solid, but “it would be awesome to get more seismometers on the moon and to test this out properly.”