The moon is quaking as it shrinks

The moon is quaking as it shrinks
This prominent thrust fault is one of thousands discovered on the moon by NASA's Lunar Reconnaissance Orbiter (LRO). These faults resemble small stair-shaped cliffs, or scarps, when seen from the lunar 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. New research suggests that these faults may still be active today. Credit: LROC NAC frame M190844037LR; NASA/GSFC/Arizona State University/Smithsonian

A 2010 analysis of imagery from NASA's Lunar Reconnaissance Orbiter (LRO) found that the moon shriveled like a raisin as its interior cooled, leaving behind thousands of cliffs called thrust faults on the moon's surface.

A new analysis suggests that the moon may still be shrinking today and actively producing moonquakes along these thrust faults. A team of researchers including Nicholas Schmerr, an assistant professor of geology at the University of Maryland, designed a new algorithm to re-analyze from instruments placed by NASA's Apollo missions in the 1960s and '70s. Their analysis provided more accurate epicenter location data for 28 moonquakes recorded from 1969 to 1977.

The team then superimposed this location data onto the LRO imagery of the thrust faults. Based on the quakes' proximity to the thrust faults, the researchers found that at least eight of the quakes likely resulted from true tectonic activity—the movement of crustal plates—along the thrust faults, rather than from asteroid impacts or rumblings deep within the moon's interior.

Although the Apollo instruments recorded their last shortly before the instruments were retired in 1977, the researchers suggest that the moon is likely still experiencing quakes to this day. A paper describing the work, co-authored by Schmerr, was published in the journal Nature Geoscience on May 13, 2019.

The moon is quaking as it shrinks
New surface features of the Moon have been discovered in a region called Mare Frigoris, outlined here in teal. This image is a mosaic composed of many images taken by NASA's Lunar Reconnaissance Orbiter (LRO).Credit: NASA

"We found that a number of the quakes recorded in the Apollo data happened very close to the faults seen in the LRO imagery," Schmerr said, noting that the LRO imagery also shows of geologically recent movement, such as landslides and tumbled boulders. "It's quite likely that the faults are still active today. You don't often get to see active tectonics anywhere but Earth, so it's very exciting to think these faults may still be producing moonquakes."

Astronauts placed five seismometers on the moon's surface during the Apollo 11, 12, 14, 15 and 16 missions. The Apollo 11 seismometer operated only for three weeks, but the four remaining instruments recorded 28 shallow moonquakes—the type produced by tectonic faults—from 1969 to 1977. On Earth, the quakes would have ranged in magnitude from about 2 to 5.

Using the revised location estimates from their new algorithm, the researchers found that the epicenters of eight of the 28 shallow quakes were within 19 miles of faults visible in the LRO images. This was close enough for the team to conclude that the faults likely caused the quakes. Schmerr led the effort to produce "shake maps" derived from models that predict where the strongest shaking should occur, given the size of the thrust faults.

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. This is where additional tidal stress from Earth's gravity causes a peak in the total stress on the moon's crust, making slippage along the thrust faults more likely.

The moon is quaking as it shrinks
Scientists have discovered these wrinkle ridges in a region of the Moon called Mare Frigoris. These ridges add to evidence that the Moon has an actively changing surface. This image was taken by NASA's Lunar Reconnaissance Orbiter (LRO).Credit: NASA

"We think it's very likely that these eight quakes were produced by faults slipping as stress built up when the lunar crust was compressed by global contraction and tidal forces, indicating that the Apollo seismometers recorded the shrinking moon and the moon is still tectonically active," said Thomas Watters, lead author of the research paper and senior scientist in the Center for Earth and Planetary Studies at the Smithsonian Institution in Washington.

Much as a grape wrinkles as it dries to become a raisin, the moon also wrinkles as its 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. This breakage results in thrust faults, where one section of crust is pushed up over an adjacent section. These faults resemble small stair-shaped cliffs, or scarps, when seen from the lunar surface; each is roughly tens of yards high and a few miles long.

The LRO has imaged more than 3,500 fault scarps on the moon since it began operation in 2009. Some of these images show landslides or boulders at the bottom of relatively bright patches on the slopes of fault scarps or nearby terrain. Because weathering gradually darkens material on the lunar surface, brighter areas indicate regions that are freshly exposed by an event such as a moonquake.

Other LRO fault images show fresh tracks from boulder falls, suggesting that quakes sent these boulders rolling down their cliff slopes. Such tracks would be erased relatively quickly, in terms of geologic time, by the constant rain of micrometeoroid impacts on the moon. With nearly a decade of LRO imagery already available and more on the way in the coming years, the team would like to compare pictures of specific fault regions from different times to look for fresh evidence of recent moonquakes.

"For me, these findings emphasize that we need to go back to the moon," Schmerr said. "We learned a lot from the Apollo missions, but they really only scratched the surface. With a larger network of modern seismometers, we could make huge strides in our understanding of the moon's geology. This provides some very promising low-hanging fruit for science on a future mission to the ."


Explore further

Moon's tidal stress likely responsible for causing deep moonquakes, new study confirms

More information: Shallow seismic activity and young thrust faults on the Moon, Nature Geoscience (2019). DOI: 10.1038/s41561-019-0362-2 , https://www.nature.com/articles/s41561-019-0362-2
Journal information: Nature Geoscience

Citation: The moon is quaking as it shrinks (2019, May 13) retrieved 16 September 2019 from https://phys.org/news/2019-05-moon-quaking.html
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May 14, 2019
All celestial mass bodies have the ability to generate quakes. The larger the mass body, the greater the chance for quakes.
The Earth alone averages tens of thousands of earthquakes, of varying intensities, each year.

These quakes are the result of its consistent force of gravitational acceleration on the mass, whose intensity is based on its mass density. The greatest gravitational force is expressed from within the center of mass, where the density is the greatest.

Increase the density of a mass and you increase the probability for quakes. A spinning mass provides for an even greater probability of quakes, as the varying stratified layers of the mass are massaged into place as the mass body attempts to adapt to the gravitational forces being applied to it.

As science fiction imitates science fact, you can gain a greater appreciation of what it would be like to live on the moon through reading the epic novel, 'Shadow-Forge Revelations'.

May 14, 2019
SJBauer: You use terms weirdly. "gravitational acceleration"? Perhaps you mean force, which is a function of its size and density. The gravitational force at the center of the Moon is zero. (Lot of pressure though!) Spinning does not increase stress, unless the rate of spin is changing "rapidly". Irrelevant as the Moon rotates only once per month. What was the point of your post, except perhaps to plug a book??

I'm a bit surprised to see the article note quakes occurring at apogee (farthest from Earth) due to tidal stress. I should think perigee (closest to Earth) would be more likely... Pity I don't see the amount of shrinkage quantified. I'm sure most readers of general press articles are thinking in terms of much greater shrinkage than is actually occurring.

May 14, 2019
SJBauer: You use terms weirdly. "gravitational acceleration"? Perhaps you mean force, which is a function of its size and density
Perhaps when you feel compelled to comment oonthings you dont know anything about, you ought to expect the people you are addressing know more than you do.

"A seismogram is a recording of ground motion. The recording can be in displacement (m), velocity (m/s), or acceleration (m/s2). Since acceleration is related to force (F=ma), the measured peak ground acceleration is the parameter which is frequently used in seismic hazard assessment. The acceleration is given in cm/s2 (or m/s2) or as a fraction of the gravitational acceleration (g=9.8 m/s2)."

May 14, 2019
Interesting, but I can't see how he was talking about seismic activity in
These quakes are the result of its consistent force of gravitational acceleration on the mass, whose intensity is based on its mass density. The greatest gravitational force is expressed from within the center of mass, where the density is the greatest.

The first sentence seems to be about how quakes are caused. (Nothing to do with measurement.) The second seems to be just wrong. (Or again, strangely worded.) Care to clarify your response??

May 14, 2019
Clarification: I can't see how he was talking about seismic measurement ...

May 14, 2019
As science fiction imitates science fact, you can gain a greater appreciation of what it would be like to live on the moon through reading the epic novel, 'Shadow-Forge Revelations'.
He's the author! So this borderline spam hiding under a bunch of sciency mumbo-jumbo.

May 15, 2019
Interesting, but I can't see how he was talking about seismic activity
-Of course not! You dont have the education. Neither do I. But I have enough common sense to do a google search and avoid speaking from hubris, which would only make me look doubly ignorant.

But I did know this

"An object moving in a circular motion—such as a satellite orbiting the Earth—is accelerating due to the change of direction of motion, although its speed may be constant. In this case it is said to be undergoing centripetal (directed towards the center) acceleration."

-which is pretty rudimentary. Didnt even know that did you?

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