In 2010, an analysis of images 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 lunar surface. A new analysis of archival data from seismometers deployed during Apollo missions gives the first evidence that these thrust faults are still active and likely producing moonquakes today as the Moon continues to gradually cool and shrink.
This thrust fault is one of thousands discovered on the Moon by NASA’s Lunar Reconnaissance Orbiter. 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. Image credit: NASA / GSFC / Arizona State University / Smithsonian Institution.
“We found that a number of the quakes recorded in the Apollo data happened very close to the faults seen in the LRO imagery,” said co-author Dr. Nicholas Schmerr, a geologist at the University of Maryland.
“The LRO images also show physical evidence of geologically recent fault movement, such as landslides and tumbled boulders.”
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 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, Dr. Schmerr and colleagues 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.
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 lead author Dr. Thomas Watters, from the Smithsonian Institution.
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 lunar 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 imaged more than 3,500 fault scarps on the Moon. 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.
“For me, these findings emphasize that we need to go back to the Moon,” Dr. 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.”
“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 Dr. Renee Weber, a planetary seismologist at NASA’s Marshall Space Flight Center.
The study appears in the journal Nature Geoscience.
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Thomas R. Watters et al. Shallow seismic activity and young thrust faults on the Moon. Nature Geoscience, published online May 13, 2019; doi: 10.1038/s41561-019-0362-2
