Moonquakes Reveal the Moon is Still Shrinking & Shaking
Before humans go there again, it’d be smart to better understand the risks of lunar temblors.
Apollo astronauts left five seismometers scattered around the surface of the moon between 1969 and 1972, leading to the discovery of moonquakes and the realization that the moon is more geologically active than had been known. In 2006, a study revealed quakes that would equal up to a 5.5 temblor on the Richter Scale, the sort of event strong enough to crack plaster on a lunar wall or even topple a chimney, were there any.
“The moon is seismically active,” Clive Neal, an associate professor of civil engineering and geological sciences at the University of Notre Dame, said at the time.
Back in 2010, scientists showed how the moon had shrunk as it cooled over time, wrinkling like a raisin and leaving behind cliffs, made by thrust faults essentially the same as those seen in earthquakes. The evidence suggested the lunar diameter had become about hundreds of feet less sometime in the past billion years, possibly more recently.
Two new analyses indicates the moon is still shrinking, morphing the moonscape and actively producing moonquakes along ancient fault lines. The findings have meaning for space agencies and private companies hoping to return to the moon and set up outposts for exploration there and beyond.
The lunar-seismometer program ended in 1977. So in the new study, researchers created new algorithms to analyze that data, along with photos taken in 2010 by the Lunar Reconnaissance Orbiter (LRO).
“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 study team member Nicholas Schmerr, an assistant professor of geology at the University of Maryland.
The LRO images show landslides and tumbled boulders that indicate the faults had moved recently as tectonic shifts break the moon’s brittle crust.
“It’s quite likely that the faults are still active today,” Schmerr said. “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 findings are detailed May 13 in the journal Nature Geoscience.
Quakes Near Faults
The moon doesn’t have tectonic plates, like Earth, NASA says. But its tectonic activity, caused by the cooling and shrinking, results in faults and quakes that behave similarly to those on Earth.
The new analysis showed the epicenters of eight of 28 shallow lunar earthquakes studied were within 19 miles of known faults—close enough to conclude the faults likely caused the quakes, the researchers said.
Further, six of the eight quakes coincided with the moon being at a point in its orbit when the tidal pull of Earth’s gravity most stresses the moon’s crust, making it more likely a fault will slip.
That means these measured moonquakes were not likely caused by meteorite impacts or thermal expansion of the surface after the periodic two-weeks of deep freeze when half the moon faces away from the sun as it circles Earth.
“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, senior scientist in the Center for Earth and Planetary Studies at the Smithsonian Institution in Washington and lead author of the paper.
Another recent study found what scientists called “wrinkle ridges,” along with curved hills and shallow trenches, in one of the moon’s large basins, called mare.
These, too, show the moon is shrinking, researchers concluded in the March 7 issue of the journal Icarus They figure the ridges emerged in the past billion years, and some may be just 40 million years old, a relative eye-blink in the moon’s 4.5 billion-year lifetime.
“The Moon is still quaking and shaking from its own internal processes,” said study leader Nathan Williams, a post-doctoral researcher at NASA’s Jet Propulsion Laboratory. “It’s been losing heat over billions of years, shrinking and becoming denser.”
Whether NASA or anyone succeeds at returning humans to the moon remains up in the air. But Renee Weber, a co-author of the study in Nature Geoscience and a planetary seismologist at NASA’s Marshall Space Flight Center, would like to see more exploration, for safety’s sake at least.
“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,” Weber said.