A 2010 analysis of imagery from NASA’s Lunar Reconnaissance Orbiter (LRO) found that as the inside of the Moon cooled, it shriveled up like a raisin.
A recent study, using NASA’s data, suggests that the Moon could still be shrinking today. As it does, it is experiencing moonquakes.
“Just as a grape wrinkles as it shrinks down to a raisin, the Moon gets wrinkles as it shrinks,” NASA wrote.
“Unlike the flexible skin on a grape, the Moon’s surface crust is brittle, so it breaks as the Moon shrinks, forming “thrust faults” where one section of crust is pushed up over a neighboring part.”
“Our analysis gives the first evidence that these faults are still active and likely producing moonquakes today as the Moon continues to gradually cool and shrink,” said Thomas Watters, a senior scientist in the Center for Earth and Planetary Studies at the Smithsonian’s National Air and Space Museum in Washington. “Some of these quakes can be fairly strong, around five on the Richter scale.”
A team of researchers made the discovery possible by creating an algorithm to re-analyze seismic data from instruments placed by NASA’s Apollo missions in the 1960s and ’70s.
It also helped scientists understand where the moonquakes are actually coming from.
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They found that at least eight quakes were coming from true tectonic activity, rather than from asteroid impacts or rumblings deep within the moon’s interior.
“We found that a number of the quakes recorded in the Apollo data happened very close to the faults seen in the LRO imagery,” said Nicholas Schmerr, an assistant professor of geology at the University of Maryland, in a statement.
“It’s quite likely that the faults are still active today. You don’t often get to see active tectonics anywhere but Earth, so it’s very exciting to think these faults may still be producing moonquakes.”
Astronauts placed five seismometers on the moon’s surface during the Apollo 11, 12, 14, 15 and 16 missions. The seismic data was taken from those instruments.
Apollo 11 instrument died after a few weeks. But the rest kept measuring, eventually picking up 28 different, shallow moonquakes between 1969 and 1977.
Using the new algorithm they also found that of the 28 shallow quakes, eight of them were within 19 miles of faults visible in the LRO images.
The researchers also found that six of the eight quakes happened when the moon was at or near its apogee. Apogee is the point in the moon’s orbit when it is farthest from Earth.