More than 40 years since humans last walked on the Moon we are still learning about our nearest neighbor. The latest discovery is that the Earth's gravity is shaping the moonquakes that form cliffs on the lunar surface, causing a pattern in their orientation.
The Apollo missions detected around 70 cliffs called “lobate scarps”, all close to the equator. Similar features have been observed on Mercury and Mars. While these scarps can be as much as 10 kilometers (6 miles) long they are less than 100 meters (330 feet) high.
In 2010 the Lunar Reconnaissance Orbiter (LRO) detected 14 more of these scarps, triggering renewed interest in their formation. Since then the number discovered has risen dramatically. “Lobate scarps are among the youngest landforms on the Moon, based on their generally crisp appearance, lack of superposed large-diameter impact craters, and the existence of crosscut small-diameter impact craters,” the paper announcing the LRO's discoveries, published in Science, reported.
The Moon is largely geologically dead, but the scarps have been explained as being the result of global shrinkage as the last heat escaped the Moon's once blistering interior. If that was all there was to it, however, the scarps would be expected to run in random directions.
After more than six years in orbit, the LRO camera has imaged nearly 75% of the Moon's surface at high resolution, allowing the discovery of thousands more scarps. In a new paper, published in Geology, a team led by the Smithsonian Institution's Thomas Watters reports on an analysis of 3,200 of these features, concluding that they are less than 50 million years old and “may be actively forming today”.
More surprisingly, the authors find that their distribution is not random. “We propose that tidal stresses contribute significantly to the current stress state of the lunar crust,” the authors write.
Average directions of lobate scarps in lunar patches 40° by 20° showing a non-random orientation. Credit: NASA/LRO/Arizona State University/Smithsonian Institution
"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." Watters said in a statement.
That "something else" is our planet. Since the Earth's mass is 80 times that of the Moon it exerts a gravitational force far greater than the one that raises our tides. However, with one lunar face always turned towards the parental planet, this operates rather differently to the Moon's effects on Earth. Instead the forces change as the Moon's orbit brings it closer and farther from the planet, a softer version of the tidal stretching turning Io inside out.
When Watters and his colleagues modeled the effect of the Earth's gravity on the Moon they found a “pretty striking” match with the observations. The models indicate the stresses are greatest when the Moon is farthest from the Earth, not closest as might be expected. The authors suspect the moonquakes that cause the scarps happen at this farthest point, and say the next step is to establish a seismic network on the Moon to test this.