Goodnight moon?

Scientists Monday gave us the unsettling news that our favorite natural satellite is shrinking, according to a new study. The shrinking is also causing "moonquakes," which have only recently been detected.

But it's no cause for concern, said study co-author Nicholas Schmerr, a University of Maryland geologist, in an email to USA TODAY.

"As the moon cools, its overall size has contracted or decreased by about 100 meters over the past 4.5 billion years, which is why we say it is shrinking," he said. "This puts the crust under compression.

"If there's enough compressive stress, the crust can fail, producing earthquakes – or in this case moonquakes," he said.

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Fortunately, the moon's mass isn’t changing, "and the radius change is small, so the effect on Earth is minuscule, and it won’t affect tides or make the moon disappear," Schmerr said.

To detect the quakes, researchers reviewed data gathered by Apollo astronauts back in the late 1960s and early 1970s, along with new information from NASA's Lunar Reconnaissance Orbiter (LRO). The LRO is an unmanned probe in orbit around the moon.

Here's what's going on up there, according to the study: Just as a grape wrinkles as it shrinks down to a raisin, the moon gets wrinkles as it shrinks. Unlike the flexible skin on a grape, however, the moon’s surface crust is brittle. Therefore, the crust 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 study lead author Thomas Watters of the Smithsonian Institution's National Air and Space Museum. “Some of these quakes can be fairly strong, around five on the Richter scale."

What's impressive, scientists say, is that the data from the Apollo program decades ago is still paying dividends for researchers today: “It’s a great testament to the continued benefits of the Apollo program that seismic data collected over 40 years ago is helping to confirm that the moon is likely tectonically active today,” Watters said.

Schmerr, the University of Maryland geologist, said the findings also "emphasize that we need to go back to the moon. We learned a lot from the Apollo missions, but they really only scratched the surface.

"With a larger network of modern seismometers, we could make huge strides in our understanding of the moon’s geology. This provides some very promising low-hanging fruit for science on a future mission to the moon.”

Monday's study was published in the peer-reviewed British journal Nature Geoscience.