As Earth and its moon dance their orbit around the sun, they're held in place by a careful balance: the centrifugal force that tries to pull them apart, and the mutual gravitational force that tries to push them together. The moon's gravity has an observable effect on the Earth, with the tides ebbing and flowing according to the moon's presence.
As it turns out, the Earth's gravitational force has an effect on the moon, too, even in spite of the latter's dearth of liquid water. Our planet is actually shaping formations of cliffs that form on the moon as the satellite slowly shrinks.
This shrinkage is nothing to be alarmed about. Here's what's happening: the core of the moon is cooling. As the moon's core cools, the partially molten regions in its mantle solidify.
This causes the moon to shrink a little, its crust "wrinkling" as it settles into its new size. These wrinkles form a type of cliff called "lobate scarps", like those seen on Mercury, usually less than 10 kilometres (6.2 miles) long and just tens of metres or yards high.
In 2010, NASA's Lunar Reconnaissance Orbiter found 14 of these scarps, adding them to the 70 or so that had already been identified, randomly scattered over the moon's surface. This led NASA scientists to conclude that the moon was shrinking.
Five years later, the LRO has imaged nearly 75 percent of the moon's surface in high resolution with its Narrow Angle Camera -- identifying in total over 3,200 scarps so far.
And a curious pattern has emerged. The contracting forces coming from inside the moon would have absolutely no effect on the orientation of the scarp lines; but they fall along specific lines.
"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," explained study lead author and Smithsonian senior scientist Thomas Watters of the National Air and Space Museum in Washington.
This force comes from the gravitational pull of Earth, the same tidal force that effects the Earth's oceans. The lines of the Earth's tidal force upon the moon align strongly with the orientation lines of the scarps.
"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," said study co-author and LRO Camera principal investigator Mark Robinson of Arizona State University. "Now that we have NAC images with appropriate lighting for more than half of the moon, structural patterns are starting to come into focus."
Because the moon's core is still cooling, the scarps are very likely still forming, with the strongest stresses on the scarps occurring when the Earth and the moon are the farthest apart. It is at this time when seismic activity is likely to be at a peak. NASA hopes to one day be able to monitor these hypothesised moonquakes with a lunar seismic network.
Correction, 6 a.m. PT: The reference to water's reaction to freezing has been removed. Unlike most materials, water expands when it freezes.