To humans, the moon appears solid and unchanging, but new evidence reveals that it is actually shrinking over time, causing lunar wrinkles and moonquakes.

The phenomenon is similar to a grape shrinking into a raisin, a report from NASA describes. However, the surface of the moon is brittle, so as it shrinks, it creates cliff-like “thrust faults” or fault scarps with sections pushed up against others.

It turns out these faults keep the moon tectonically active, generating relatively strong quakes up to about a magnitude 5 on the Richter scale.

Why Is The Moon Shaking?

With the compact size and cooled off interiors of the moon, few expected to find tectonic activity on its surface. It came as a surprise to scientists when NASA’s Lunar Reconnaissance Orbiter captured more than 3,500 images of faults on the surface that are younger than 50 million years old.

A new study published in the journal Nature Geoscience analyzed data from the Apollo missions decades ago to shed more light on the geologic activities of Earth’s closest neighbor.

During five Apollo missions, astronauts set up seismometers on the moon. While one of the instruments only functioned for three weeks, the four others were able to record 28 shallow moonquakes from 1969 to 1977.

Each recorded quake was about a magnitude 2 to 5 on the Richter scale. It’s exactly the type of quake produced by the moon’s fault scarps.

The researchers behind the new study used an algorithm to hone in on these quakes, discovering that eight of the 28 were within 18.6 miles (30 kilometers) of the faults pictured by the orbiter. Furthermore, six of these eight quakes occurred when the moon was at the farthest point from Earth in its orbit, which causes additional stress and makes events along the faults more likely.

“We think it’s very likely that these eight quakes were produced by faults slipping as stress built up when the lunar crust was compressed by global contraction and tidal forces, indicating that the Apollo seismometers recorded the shrinking Moon and the Moon is still tectonically active,” said Thomas Watters, study lead from the Center for Earth and Planetary Studies at the Smithsonian’s National Air and Space Museum.

Apollo: The Gift That Keeps Giving

With the Apollo 11 celebrating 50 years in 2019, it’s almost poetic how the Apollo missions remain key in astronomers’ studies today.

“It’s really remarkable to see how data from nearly 50 years ago and from the LRO mission has been combined to advance our understanding of the Moon,” LRO Project Scientist John Keller said in a statement, adding that it also guides future missions on the interiors of the moon.

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