Quakes show that moon, gradually shrinking, is tectonically active

Federico Mansilla
May 14, 2019

This is despite a general scientific rule that smaller rocky bodies cool down more rapidly, Watters says.

"Our analysis gives the first evidence that these faults are still active and likely producing moonquakes today as the Moon continues to gradually cool and shrink", Thomas Watters, a senior scientist from the Center for Earth and Planetary Studies said.

Another Nasa project, the Lunar Reconnaissance Orbitor (LRO) spacecraft, has snapped thousands of images of the fault scarps on the moon since 2009, showing various landslides on the Moon's surface. The scarps form when one section of the moon's crust (left-pointing arrows) is pushed up over an adjacent section (right-pointing arrows) as the moon's interior cools and shrinks. Unlike the flexible skin on a grape, however, the moon's crust is brittle, causing it to break as the interior shrinks.

In a study published in Nature Geoscience on Monday, a team of scientists examined data from the LRO and compared it to the location of moonquakes recorded during the Apollo program in the 1960s and 1970s. These freakish moonquakes were detected by five seismometers placed on the lunar surface during the Apollo 11, 12, 14, 15, and 16 missions. They developed an algorithm that enabled them to get a more accurate location for the epicenters of each quake.

The researchers also found that six of the eight quakes happened when the moon was at or near its apogee, the point in the moon's orbit when it is farthest from Earth.

The quakes on the moon were recorded after astronauts placed seismometers - the instruments that measure quakes - during the Apollo 11, 12, 14, 15, and 16 missions, NASA said. This is where additional tidal stress from Earth's gravity causes a peak in the total stress, making slip-events along these faults more likely.

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Other LRO fault images show fresh tracks from boulder falls, suggesting that quakes sent these boulders rolling down their cliff slopes.

Of the 8 moonquakes most recently recorded, 6 of them occurred at these times.

"It´s quite likely that the faults are still active today", said Nicholas Schmerr, an assistant professor of geology at the University of Maryland who co-authored the study. Scientists trying to reconstruct the Moon's origin wonder whether the same happened to the Moon, or if instead it was only partially molten, perhaps with a magma ocean over a more slowly heating deep interior.

"This is exciting as it wasn't clear if the moon had already gone through this period billions of years ago and was tectonically dead, or if it was still active in the present", Schmerr said. With a larger network of modern seismometers, we could make huge strides in our understanding of the Moon's geology.

The discovery of young faults less than 50 million years old by the LSO's camera in 2010 has been interpreted as evidence of lunar tectonic activity. "It is also a testament to how much can be gained by human spaceflight to the surface of other worlds and underlines the wonderful potential for future missions back to the moon and, hopefully someday, Mars".

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