The moon is shrinking, and the process has become, literally, unsettling.
According to a new study, the moon has gradually been getting smaller. And as it shrinks, cracks form on the lunar surface that then form fault lines and generate moonquakes.
"Some of these quakes can be fairly strong, around 5 on the Richter scale," said Thomas Watters, a planetary scientist at the Smithsonian's National Air and Space Museum and the lead author of the study published Monday in the journal Nature Geoscience.
But there's no cause for alarm. The shrinking moon will have little impact on Earth.
The study found that the moon has gotten about 150 feet skinnier — over the course of the last few hundred million years.
The moon's mass isn't changing, University of Maryland geologist Nicholas Schmerr, another author on the study, said.
"So the effect on Earth is minuscule, and it won't affect tides or make the moon disappear," Schmerr told USA Today.
So just what is going on with the moon? According to an explanation from NASA, the moon contracts as its inside cools down. That causes its hard surface to crack and create fault lines.
"Just as a grape wrinkles as it shrinks down to a raisin, the moon gets wrinkles as it shrinks," the federal space agency said in a news release. "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."
Research on the quakes has been captured by seismometers left on the lunar surface about a half century ago by Apollo astronauts. Newer data also was obtained from NASA's Lunar Reconnaissance Orbiter, an unmanned probe that circles the moon.
"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," Watters said in the NASA statement.