Cracks on the surface of the moon are being pulled open, and it's our fault — or at least our planet's fault, as the Earth's gravity pulls and tugs on our cosmic companion, astronomers say.
The Earth's gravitation field exerts a tidal force on the moon, just as the moon's gravity is responsible for the rise and fall of waters in the Earth's ocean and lakes, they explain.
While the opposing effects have been known for a long time, it's only recently been confirmed that the pull from Earth can actually open faults on the moon's surface.
"We know the close relationship between the Earth and the moon goes back to their origins, but what a surprise [it was] to find the Earth is still helping to shape the moon," says study lead author Thomas Watters, a planetary scientist at the Smithsonian Institution in Washington, D.C.
The researchers looked at observations made by NASA's Lunar Reconnaissance Orbiter, which provided data on around 3,000 cracks, or fault scarps, on the lunar surface that formed during the moon's lifetime as its liquid core cooled and solidified, causing the surface to shrink and crack.
However, if shrinkage was all that was behind their formation, the resulting cracks should be randomly distributed, the scientists note, oriented in all directions.
That's not what the LRO found, they say.
"It was a big surprise to find that the fault scarps don't have random orientations," says Watters.
Instead, he explains, the orientation suggests something in addition to the shrinking was at play, something acting on a global scale.
"That something is the Earth's gravitational pull," he says.
That pull creates stresses in the rocks of the moon's surface, and as the Earth alternately swings closer to and then farther away from the moon, the fault scarps arise in a recognizable pattern, the researchers say.
Only in the last five years have detailed observations by probes such as the LRO provided enough data from the patterns to become apparent, they add.
"Early on in the mission we suspected that tidal forces played a role in the formation of tectonic features, but we just did not have enough coverage to make any conclusive statements," says LRO team scientist Mark Robinson from Arizona State University's School of Earth and Space Exploration.
Many of the fault cracks are quite young and are probably still actively forming, the researchers say.
"The discovery of thousands of young fault scarps, influenced by tidal forces from Earth, is an exciting new dimension to our understanding of the close relationship between our planet and the moon," says Watters.
