Moonquakes? New study suggests Earth's neighbour is still active

A new study has reached a remarkable conclusion: the moon is still tectonically active.

The findings come as a surprise because, throughout our solar system, there is only one body known to be tectonically active, and that's Earth.

The clue to the recent activity on the moon are called scarps, long, stair-like steps that can be tens of metres high and kilometres long that are littered across the moon's surface.

They were first seen in detail during the Apollo missions to the moon when the orbiters of Apollos 15, 16 and 17, fitted with high-resolution cameras, provided scientists with the first closeup images of the moon.

Initially, the scarps were believed to have been created when the moon cooled and shrunk billions of years ago.

But more tools would be deployed to unravel these mysterious formations. Not only did the Apollo missions have cameras, but missions 12, 14, 15 and 16 landing sites were equipped with seismometers measuring ground shaking, or moonquakes. It was thought that these could be behind the formation of the scarps.

"We're fortunate to have four seismic stations that operated on the moon from 1969 to 1977 … and that seismic network literally recorded thousands of moonquakes," said Thomas Watters, lead author of the paper, published in the journal Nature Geoscience and a senior scientist at the Smithsonian Institute's Center for Earth and Planetary Studies.

And the earthquakes weren't tiny. They measured anywhere from 1.5 to 5 magnitude.

In 2009, the Lunar Reconnaissance Orbiter (LRO), fitted with a camera system, provided even higher-resolution images of these scarps covering much more of the moon than the Apollo orbiters had found. 

This helped map them out, but the question of their age and what was causing them was still in the air. Watters wanted an answer. 

In a 2010 study, he suggested the scarps could be less than a billion years old, or even a hundred million years old — quite young, considering the age of our solar system is thought to be roughly 4.6 billion years old.

For this study, Watters and his colleagues studied 28 of the moonquakes recorded by the Apollo seismic network. Combined with the LRO data, Watters tried to zero in on the quakes' epicentres, developing an algorithm that would provide more precise locations. What he and his colleagues found was that eight epicentres occurred within 30 kilometres of a fault scarp, an area that would experience strong ground shaking. Of those, six occurred when the moon was less than 15,000 kilometres from its farthest point from Earth, called the apogee.

"I'm really confident that … we have a 40-year-old data set that is helping us make this connection that verifies that this shrinking and contracting of the moon was actually getting recorded by the Apollo seismic network — and, more important, that means that the moon is tectonically active today."

Unravelling the mystery

Earth is the only known body in the solar system with tectonic activity, where the large sheets of rock we sit on, called plates, move. We feel this motion as earthquakes.

This occurs because, after 4.6 billion years, Earth is still hot. Large planets in the solar system — Venus, Saturn and Jupiter, for example — are also still hot. But it was believed that smaller bodies, such as Mercury and our moon, would have cooled after such a long time.

Or would they?

In 2011, co-author of the new paper, Renee Weber, published a study that suggested the moon had a fluid outer core and a partially molten boundary layer.

"This fit right into the idea: Here I am, seeing these young thrust-fault scarps, and scratching my head. It's like, 'OK, where are these coming from? They've got to be coming from contraction of the body, but isn't the moon supposed to be cold?'" said Watters. "It turns out, no. The interior of the moon is still hot and is still cooling off."

See a visualization of images provided by the Lunar Reconnaissance Orbiter of the Lee Lincoln scarp at the Apollo 17 landing site.  

Gordon Osinski, a professor in the department of Earth Sciences at Western University in London, Ont., who was not involved in the study, is intrigued by the findings. 

"I think they've made a pretty convincing case," Osinski said. 

He said the findings illustrate another reason why we should go back to the moon, either with humans, or by deploying multiple seismometers more widely around the moon to better understand what's going on.

And, he said, it shows there's so much we don't know about our nearest neighbour.

"I think it provides more scientific impetus for going back. This is one of a few things," Osinski said. "They've found evidence for water on the moon. A decade ago, you would have been laughed at."

The new study suggests there are three things at work causing moonquakes, which in turn create the scarps: tidal forces, or the squeezing of the moon as it rotates around Earth; the fact that the moon is slowing receding from Earth; and, finally, the fact that the moon is still hot.

"It kind of flies in the face of the conventional wisdom that silicate bodies the size of the moon should have been cold a long time ago," Watters said. "I mean, we're talking 4.6 billion years that the moon is still tectonically active. The only other body we know of like that is Earth."

The squeezing can cause internal heat, but it wouldn't manifest in the manner of causing these moonquakes.

The next step, Watters said, is to find "the smoking gun" in order to better understand what exactly is happening on — and below — the surface of the moon.

"We are going to go back to the moon," Watters said. "This is more than just a scientific study about understanding the moon. We really need to know more about how seismically active the moon is."

Catherine Johnson, a professor at the University of British Columbia, was also involved in the research, but attempts to reach her were unsuccessful. Johnson is also a co-investigator for NASA's InSight mission to Mars.