Earth's closest natural satellite could be producing powerful moonquakes as it cools and shrinks, according to a newly-published study. The scientists behind the research analyzed data from instruments placed by astronauts who visited the Moon's surface in the Apollo-era, as well as more recent data collected by NASA's Lunar Reconnaissance Orbiter (LRO).

A total of five seismometers were placed on the Moon's surface by crewmembers of Apollo 11, 12, 14, 15 and 16. The instruments successfully detected 28 moonquakes that were likely caused by friction at tectonic faults before they were decommissioned in 1977.

The moonquakes detected in the 1960s and 70s shook the lunar surface with power the equivalent to magnitude two to five quakes back on Earth.

The team of scientists behind the new paper developed an algorithm that allowed them to accurately pinpoint the epicenters of these moonquakes. The refined Apollo-era location data was then overlaid on LRO imagery.

Eight moonquakes were discovered to have epicenters within 19 miles (31 km) of geological formations called thrust faults. These dramatic features, which are similar in appearance to cliffs found on Earth, can be miles long and tens of feet high.

Thrust faults are created when the Moon's interior cools, forcing the barren world to shrink. As it contracts, the fragile crust breaks, and parts of it are forced up and over neighboring sections. Some 3,500 such faults have been discovered by the LRO to date.

The relative proximity of the quakes to the faults suggest that they were triggered by geological activity rather than asteroid impacts or tremors from much deeper within the rocky body.

Of the eight moonquakes that occurred close to visible thrust faults in the LRO data, six took place when the Moon was at its most distant orbital point relative to the Earth. This point, which is called apogee, is also the period during which Earth's gravity inflicts the most stress, or tidal pressure, on the Moon's structure.

"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," comments Thomas Watters, lead author of the new paper, and senior scientist in the Center for Earth and Planetary Studies at the Smithsonian Institution in Washington.

The LRO images also revealed surface features that support the theory that the Moon continues to be active. For example, bright patches of ground have been observed near faults, which appear to be patches of lunar regolith that have yet to be darkened by weathering and radiation.

Similarly, tracks are apparent on slopes where boulders have been dislodged from perches on higher ground. This again suggests recent activity, as these trails would be quickly erased by micrometeoroid impacts.

"For me, these findings emphasize that we need to go back to the Moon," said Nicholas Schmerr, an assistant professor of geology at the University of Maryland and co-author of the paper. "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."

The study has been published in the journal Nature Geoscience.

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