Earth's Pull Is 'Massaging' Our Moon

Five years ago, in 2010, researchers took images from the Lunar Reconnaissance Orbiter's Narrow Angle Camera (NAC), and discovered 14 new cliff-like faults on the surface of the moon, known as "lobate scarps."

The discovery, published in the journal Geology, of these lobate scarps, coupled with the knowledge of about 70 previously known ones, has caused scientists to conclude that the moon is, in fact, shrinking.

These lobate scarps are generally less than 6.2 miles (10 km) long and usually only tens of yards (or meters) high. They're formed as a result of the moon's interior cooling down, causing global contractions. As the interior cools down, parts of the liquid outer core solidify, and the total volume of the moon decreases, and the crust cracks, according to the release. 

These faults are the most common tectonic landform on the moon. Scientists' analysis of the orientations of these small scarps brought forth an interesting discovery: the faults created are influenced by an unforeseen source - Earth's gravitational tidal forces.

"There is a pattern in the orientations of the thousands of faults and it suggests something else is influencing their formation, something that's also acting on a global scale - 'massaging' and realigning them," Smithsonian senior scientist and lead author Thomas Watters, of the National Air and Space Museum in Washington, said.

The fault scarps are still young, to the point that they're most likely forming actively today. The modeling done by the team shows that when the moon is at apogee, furthest from Earth in its orbit, the peak stresses occur.

When the tidal forces are superimposed on the global contraction, the combined stresses should cause predictable orientations of the fault scarps from region to region. Watters said that the faults predicted by tidal and contractional models has been strikingly accurate, according to NASA. 

"The discovery of so many previously undetected tectonic features as our LROC high-resolution image coverage continues to grow is truly remarkable," Mark Robinson of Arizona State University, coauthor and LROC principal investigator, said. "Early on in the mission we suspected that tidal forces played a role in the formation of tectonic features, but we did not have enough coverage to make any conclusive statements. Now that we have NAC images with appropriate lighting for more than half of the moon, structural patterns are starting to come into focus."

If these scarps are still active, scientists believe that shallow moonquakes may occur most frequently when the moon is at apogee. A long-term lunar seismic network could test this hypothesis.

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