At a Glance

Seismometers left on the moon by Apollo missions recorded 28 moonquakes between 1969 and 1977.At least eight are linked to thrust faults that are formed as the moon shrinks.

The moon is ever so slowly shrinking over time, which is creating wrinkles in the crust that may be triggering moonquakes, a new study says.

A team of researchers led by NASA used a new algorithm to re-analyze data from seismometers placed on the moon during several Apollo missions from 1969 to 1977. They compared the data with more than 12,000 photos taken by NASA's Lunar Reconnaissance Orbiter (LRO) since 2009 and determined the moon is 164 feet "skinnier" than it was in the early 1970s.

According to the study published this week in Nature Geosciences, the interior of the moon is shriveling up like a raisin as it cools, creating . Unlike a grape's flexible skin that conforms as it shrivels into a raisin, the moon's surface is brittle and it breaks and cracks as it shrinks. The thrust faults that appear on the moon's surface resemble stair-shaped cliffs or scarps and are typically tens of yards high and a few miles long.

The LRO has imaged more than 3,500 of these fault scarps on the moon since 2009.

The seismometers recorded 28 moonquakes ranging from 2 to 5 on the Richter scale between 1969 to 1977. The algorithm then allowed scientists to better pinpoint the epicenters of the quakes. They found that eight of those shallow quakes were located within 19 miles of thrust faults visible in LRO lunar images.

Two instruments placed on the Moon in July 1969 by Apollo 11 astronauts: a seismometer (foreground) and a laser ranging retroreflector designed by a University of Maryland-led team.

(University of Maryland)

The researchers also determined that six of the eight quakes linked to the thrust faults occurred when the moon was at its apogee, or at its farthest distance from the earth, where additional "tidal stress from Earth’s gravity causes a peak in the total stress on the moon’s crust."

“We think it’s very likely that these eight quakes were produced by 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,” Thomas Watters, senior scientist in the Center for Earth and Planetary Studies at the Smithsonian’s National Air and Space Museum in Washington, said in a press release.

The research has led to more questions about what is happening as the moon ages. While the seismometers were retired in 1977, imagery capturing landslides and fallen boulders suggests moonquakes continue to occur. Future research can also help scientists understand what is occurring on other celestial bodies like the planet Mercury, which is also shrinking and has thousands of thrust faults.

“For me, these findings emphasize that we need to ” said co-author Nicholas Schmerr, an assistant professor of geology at the University of Maryland. “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.”

"Establishing a new network of seismometers on the lunar surface should be a priority for human exploration of the moon, both to learn more about the moon's interior and to determine how much of a hazard moonquakes present," Renee Weber, study co-author and planetary seismologist at NASA's Marshall Space Flight Center, said in a statement.