Scientists have known for the last decade, or so, that the moon has shrunk by at least 150 feet (50 meters) over the last several hundred million years as its interior kept losing heat.
Giving the analogy of a shrinking and wrinkling grape as it transforms into a raisin, NASA says that the moon also shrank and wrinkled up as it cooled down.
However, owing to the fact that the lunar crust is brittle, unlike the supple exterior of a grape, it broke up, creating “thrust faults” where sections of the crust got pushed up over adjacent parts.
A team of researchers analyzing new images from NASA’s Lunar Reconnaissance Orbiter (LRO) spacecraft has found evidence that suggests the moon is continuing to shrink even today, causing thrust faults which, in turn, produce moonquakes as they slip.
“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, a senior scientist at the Center for Earth and Planetary Studies, Smithsonian Institution, Washington, DC, and the lead author of the research, published Monday (May 14) in Nature Geoscience.
“Some of these quakes can be fairly strong, around five on the Richter scale,” Watters added.
The new research was based on seismic data from the 1960s and 70s, recorded by four out of five seismometers left on the lunar surface by astronauts during Apollo missions 11, 12, 14, 15, and 16.
Barring the Apollo 11 seismometer, which lasted a mere three weeks, the remaining four registered a total of 28 shallow moonquakes, ranging from two to five on the Richter scale, between 1969 and 1977.
Using an algorithm, Watters and his team were able to get a better estimate of the location and epicenter of the quakes.
The new location-estimates revealed that 8 of the 28 quakes were not more than 30 kilometers (18.6 miles) from the thrust faults seen in lunar images, which led them to “tentatively” conclude that the quakes were caused by fault slips.
The researchers also noticed that six of the eight quakes occurred when the moon was at or approaching its apogee, the farthest point in its orbit around Earth, where tidal stress from Earth’s gravity is at peak levels, making the thrust faults more prone to “slip-events.”
To give more veracity to their conclusion, the researchers ran 10,000 simulations to determine whether so many quakes near the faults at the time of maximum stress could be a coincidence, only to discover that it was less than a four percent probability.
The possibility of meteoroid impacts causing the quakes was also ruled out because their seismic signature is different from that of quakes caused by slipping faults.
“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 Watters.
Further evidence of the faults being active comes from high-definition images from the camera onboard the LRO, which has photographed more than 3,500 fault scarps – step-like cliffs on the lunar surface that are generally tens of meters high and can extend for several kilometers.
A number of these images show boulders and landslides at the bottom of the fault scarp slopes or nearby areas, which are relatively brighter than the rest of the surroundings, indicating freshly exposed patches that have not been darkened by solar and space radiation.
Now, that could most likely be the result of moonquakes sending debris down the slopes of the fault scarps.
Further confirmation that these are recent lunar events comes from some of the other LROC images that show tracks made by boulders rolling down a scarp slope during a moonquake caused by slipping faults.
Had the tracks not been recent enough, they would have been obliterated pretty quickly, geologically speaking, by constant micrometeoroid bombardment that the lunar surface is exposed to.
Faults in the Schrödinger basin of the moon show boulder tracks that scientists say are the result of recent boulder falls caused by seismic activity.
Here’s what LRO project scientist John Keller of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, had to say about the latest findings.
“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.”
With a decade’s worth of LRO images at their disposal, Watters and his team are of the opinion that comparing images of specific fault areas from different times may provide more proof of recent moonquakes.
Study co-author Renee Weber, a planetary seismologist at NASA’s Marshall Space Flight Center in Huntsville, Alabama, says that more seismometers should be put on the moon for a better insight into lunar events.
“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,” he said.
Thomas R. Watters (lead author) – Center for Earth and Planetary Studies, Smithsonian Institution, Washington, DC, USA
Renee C. Weber (co-author) – NASA Marshall Space Flight Center, Huntsville, AL, USA
Geoffrey C. Collins (co-author) – Physics and Astronomy Department, Wheaton College, Norton, MA, USA
Ian J. Howley (co-author) – NASA Marshall Space Flight Center, Huntsville, AL, USA
Nicholas C. Schmerr (co-author) – the University of Maryland, Department of Geology, College Park, MD, USA
Catherine L. Johnson (co-author) – Dept. of Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, BC, Canada