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The Moon Is Cooling, Shrinking and Quaking

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The Moon is shrinking and quaking.  It is still tectonically active. 
 
The Moon is shrinking as its interior cools, getting more than about 150 feet (50 meters) skinnier over the last several hundred million years. Just as a grape wrinkles as it shrinks down to a raisin, the Moon gets wrinkles as it shrinks. Unlike the flexible skin on a grape, the Moon’s surface crust is brittle, so it breaks as the Moon shrinks, forming “thrust faults” where one section of crust is pushed up over a neighboring part.
 

This visualization of Lee Lincoln scarp is created from Lunar Reconnaissance Orbiter photographs and elevation mapping. The scarp is a low ridge or step about 80 meters high and running north-south through the western end of the Taurus-Littrow valley, the site of the Apollo 17 Moon landing. The scarp marks the location of a relatively young, low-angle thrust fault. The land west of the fault was forced up and over the eastern side as the lunar crust contracted. In a May 2019 paper published in Nature Geoscience, Thomas Watters and his coauthors provide evidence that this fault and others like it are still active and producing moonquakes today.

  Credits: NASA/Goddard/SVS/Ernie Wright
 

In 2010, an 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. Now a new analysis suggests that the moon may still be shrinking and actively producing moonquakes along these thrust faults.

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 team of researchers including Nicholas Schmerr, an assistant professor of geology at the University of Maryland, designed a new algorithm to re-analyze seismic data from instruments [seismometers] 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.

 
“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,” said Thomas Watters, senior scientist in the Center for Earth and Planetary Studies at the Smithsonian’s National Air and Space Museum in Washington. “Some of these quakes can be fairly strong, around five on the Richter scale.”

The team then superimposed this location data onto 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.

 

This is a view of the Taurus-Littrow valley taken by NASA’s Lunar Reconnaissance Orbiter spacecraft. The valley was explored in 1972 by the Apollo 17 mission astronauts Eugene Cernan and Harrison Schmitt. They had to zig-zag their lunar rover up and over the cliff face of the Lee-Lincoln fault scarp that cuts across this valley.

View of the Taurus-Littrow valley from the LRO spacecraft

Credits: [NASA/GSFC/Arizona State University

Although the Apollo instruments recorded their last quake 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.

“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 physical evidence of geologically recent fault 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.”

During the Apollo missions astronauts placed a number of different instruments on the moon, including 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 those 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.

“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.

 

The Taurus-Littrow valley is the location of the Apollo 17 landing site (asterisk). Cutting across the valley, just above the landing site, is the Lee-Lincoln fault scarp. Movement on the fault was the likely source of numerous moonquakes that triggered events in the valley. 1) Large landslides on of slopes of South Massif draped relatively bright rocks and dust (regolith) on and over the Lee-Lincoln scarp. 2) Boulders rolled down the slopes of North Massif leaving tracks or narrow troughs in the regolith on the slopes of North Massif. 3) Landslides on southeastern slopes of the Sculptured Hills.

Evidence for moonquakes on Lee-Lincoln fault scarp

Credits: NASA/GSFC/Arizona State University/Smithsonian

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 moon.”

Additionally, one of the revised moonquake epicenters is just 13 kilometers (8 miles) from the Lee-Lincoln scarp traversed by the Apollo 17 astronauts. The astronauts also examined boulders and boulder tracks on the slope of North Massif near the landing site. A large landslide on South Massif that covered the southern segment of the Lee-Lincoln scarp is further evidence of possible moonquakes generated by fault slip events.

“It’s really remarkable to see how data from nearly 50 years ago and from the LRO mission has been combined to advance our understanding of the Moon while suggesting where future missions intent on studying the Moon’s interior processes should go,” said LRO Project Scientist John Keller of NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

Since LRO has been photographing the lunar surface since 2009, the team would like to compare pictures of specific fault regions from different times to see if there is any evidence of recent moonquake activity. Additionally, “Establishing a new network of seismometers on the lunar surface should be a priority for human exploration of the Moon, both to learn more about the Moon’s interior and to determine how much of a hazard moonquakes present,” said co-author Renee Weber, a planetary seismologist at NASA’s Marshall Space Flight Center in Huntsville, Alabama.

The Moon isn’t the only world in our solar system experiencing some shrinkage with age. Mercury has enormous thrust faults — up to about 600 miles (1,000 kilometers) long and over a mile (3 kilometers) high — that are significantly larger relative to its size than those on the Moon, indicating it shrank much more than the Moon. Since rocky worlds expand when they heat up and contract as they cool, Mercury’s large faults reveal that is was likely hot enough to be completely molten after its formation. 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. The relatively small size of the Moon’s fault scarps is in line with the more subtle contraction expected from a partially molten scenario.

 

NASA will send the first woman, and next man, to the Moon by 2024. These American astronauts will take a human landing system from the Gateway in lunar orbit, and land on the lunar South Pole. The agency will establish sustainable missions by 2028, then we’ll take what we learn on the Moon, and go to Mars.

This research was funded by NASA’s LRO project, with additional support from the Natural Sciences and Engineering Research Council of Canada. LRO is managed by NASA Goddard for the Science Mission Directorate at NASA Headquarters in Washington. The LROC is managed at Arizona State University in Tempe.

The research paper, “Shallow seismic activity and young thrust faults on the Moon,” Thomas Watters, Renee Weber, Geoffrey Collins, Ian Howley, Nicholas Schmerr and Catherine Johnson, was published in the journal Nature Geoscience on May 13, 2019.

This work was supported by NASA’s Lunar Reconnaissance Orbiter Project and the Natural Sciences and Engineering Research Council of Canada. The content of this article does not necessarily reflect the views of these organizations.

Contacts and sources:

Matthew Wright
University of Maryland 
 

Citation: “Shallow seismic activity and young thrust faults on the Moon,” Thomas Watters, Renee Weber, Geoffrey Collins, Ian Howley, Nicholas Schmerr and Catherine Johnson, Nature Geoscience https://www.nature.com/articles/s41561-019-0362-2



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