Data from NASA’s Apollo and Lunar Reconnaissance Orbiter missions suggest that the moon is shrinking and quaking and therefore, is still tectonically active.
Back in 2010, an analysis of imagery from NASA’s Lunar Reconnaissance Orbiter (LRO) found that the moon both shrank and shrivelled as its interior cooled–thus leaving behind thousands of cliffs called thrust faults on its surface.
A New analysis — performed by a team of researchers including Nicholas Schmerr — an assistant professor of geology at the University of Maryland —adds to this finding by suggesting that the moon may still be shrinking today and still actively producing moonquakes along these thrust faults.
The team have even spotted geological activity like landslides and tumbled boulders. Schmerr says: “We found that a number of the quakes recorded in the Apollo data happened very close to the faults seen in the LRO imagery.
“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.”
Schmerr and the team designed a new algorithm to re-analyze seismic data from instruments placed during NASA’s Apollo missions in the 1960s and ’70s. This new examination allowed the team to pinpoint a more accurate epicentre location data for 28 moonquakes recorded between 1969 and 1977.
They then superimposed this revised location data onto the LRO imagery of the thrust faults. They found, that based on the quakes’ proximity to the thrust faults, at least eight of the quakes likely resulted from true tectonic activity — the movement of crustal plates — along the thrust faults. This is opposed to asteroid impacts or rumblings deep within the moon’s interior previously suggested as a mechanism for this activity.
Although the Apollo instruments recorded their last quake shortly before the instruments were retired in 1977, the moon is likely still experiencing quakes to this day — the paper describing the work— states. The research — co-authored by Schmerr — was published in the journal Nature Geoscience on May 13, 2019.
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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 epicentres of eight of the 28 shallow quakes were within 19 miles of faults visible in the LRO images — close enough for the team to conclude that the faults likely caused the quakes.
Schmerr led an 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. At this point, additional tidal stress from Earth’s gravity causes a peak in the total stress on the moon’s crust, thus making slippage along the thrust faults more likely.
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, says: “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.”
The researchers draw an analogy with the process that turns a grape to a raisin. Just as a grape may wrinkle as it dries to become a raisin, the moon also wrinkles as its interior cools and shrinks. But unlike the flexible skin on a grape — the moon’s crust is brittle, causing it to break as the interior shrinks. It is this breakage results in thrust faults, where one section of the crust is pushed up over an adjacent section. These faults — each roughly tens of yards high and a few miles long — resemble small stair-shaped cliffs when viewed from the lunar surface.
The LRO has imaged more than 3,500 fault scarps on the moon since it began operation in 2009 — with some of these images showing landslides or boulders at the bottom of relatively bright patches on the slopes of fault scarps or nearby terrain. As 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, which suggests that quakes sent these boulders rolling down their cliff slopes. These 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 intends to compare pictures of specific fault regions from different times to look for fresh evidence of recent moonquakes.
Schmerr adds: “For me, these findings emphasize that we need to go back to the moon. We learned a lot from the Apollo missions, but they really only scratched the surface.”
He concludes: “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.”
Original research: “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. http://dx.doi.org/10.1038/s41561-019-0362-2