A fresh look at decades old data and imagery shows that the Moon may be shrinking as its interior cools.
Researchers from the University of Maryland have developed an algorithm that shows that the Moon has continued to shrink, while actively producing moonquakes that go along with thrust faults or cliffs on the surface that form as the moon shrivels.
The researchers first discovered that the Moon was shrinking after they analyzed imagery taken from NASA’s Lunar Reconnaissance Orbiter (LRO) in 2010. The new algorithm re-analyzes seismic data from instruments placed by NASA’s Apollo missions in the 1960s and 1970s and provides more accurate epicenter location data for the 28 moonquakes recorded from 1969 and 1977.
The researchers then superimposed the location data onto the 2010 imagery of the thrust faults and found at least eight occurrences where the moonquakes likely resulted from true tectonic activity along the thrust faults, as opposed to the impact of an asteroid or rumblings deep within the interior.
While the last recorded quake from the Apollo instruments occurred in 1977 when they were retired, the researchers believe that moonquakes have continued to occur over the last four decades.
“We found that a number of the quakes recorded in the Apollo data happened very close to the faults seen in the LRO imagery,” Nicholas Schmerr, an assistant professor of geology at the University of Maryland, said in a statement. “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.”
The Moon’s crust is very brittle, so as the interior shrinks, the crust breaks apart, resulting in thrust faults, where one section of crust is pushed up over an adjacent section.
Schmerr also said that the LRO imagery shows physical evidence of geologically recent fault movement like landslides and tumbled boulders.
The Apollo program was a NASA-run initiative with the goal of landing the first humans on the moon. The program ran between 1960 and 1972 with the first flight, AS-201, launched in 1966 and the final flight, Apollo 17, launched in 1972.
Astronauts placed five seismometers on the moon's surface during the Apollo 11, 12, 14, 15 and 16 missions, four of which recorded the 28 shallow Moonquakes, all of which are the equivalent of an earthquake ranging from a two to five on the Richter scale.
With the revised locational estimates, the researchers identified the epicenter of the eight quakes were within 19 miles of the faults visible in the 2010 images. This allowed them to conclude within a reasonable doubt that the quakes were likely caused by the faults.
They then produced “shake maps” derived from the models to predict where the strongest shaking should occur given the size of the faults.
Another discovery was that six of the eight quakes occurred when the moon was at or near its apogee—the point where it’s orbit is farthest from Earth, 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,” 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, said in a statement.
The LRO has imaged more than 3,500 fault scarps on the moon since it began operation in 2009 and the researchers believe their analysis prove a need to continue research to compare pictures of specific fault regions from different times.
“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.”
The study was published in Nature Geoscience.