Over the past few hundred million years, Earth’s moon has shrunk by about 150 feet in circumference. This may seem insignificant given the moon’s vast size, but the shrinking has led to notable changes on the lunar surface.
What’s Causing the Shrinking?
The moon’s shrinking is caused by the gradual cooling of its still-hot interior. Like Earth, the moon formed from debris left over after a giant impact billions of years ago. This violent collision generated intense heat, which has been slowly radiating out into space over the moon’s lifetime.
As the molten inner core cools and solidifies, the entire moon contracts. It’s similar to how a grape shrivels up into a raisin. But unlike a grape’s flexible skin, the moon’s brittle crust cracks and forms thrust faults where one section is pushed up over another. These faults are still active today, generating moonquakes and posing hazards for future astronauts.
Impacts on the Lunar Surface
The global contraction has caused the moon’s surface to warp notably. One region that has experienced significant deformation is the south polar region, specifically near the Shackleton Crater. This area is already of great interest for human exploration due to the suspected presence of water ice in permanently shadowed craters.
However, the same faults that could provide access to resources also generate hazardous moonquakes. In 1973, NASA’s Apollo missions recorded one of the most violent moonquakes to date, with an epicenter near the Shackleton Crater. The quake likely resulted from a relatively recent thrust fault event in the region.
The presence of active faults so close to a priority destination for human exploration raises concerns. Powerful moonquakes and shifting terrain could endanger settlements and equipment. Slope failures in nearby crater walls could also bury larger areas in debris. Scientists emphasize that while the risks do not preclude plans for exploration, they should be considered thoroughly.
Ongoing Monitoring Efforts
To better prepare for future missions, scientists are working to understand the scale and frequency of seismic activity associated with the moon’s contraction. Detailed gravity field maps from orbiters help locate surface disturbances linked to recent faulting events. Laser altimeters also precisely measure surface profiles to model slope stability and compare regions that may be more vulnerable to landslides.
In addition, seismic data allows scientists to pinpoint the epicenters of moonquakes and estimate their magnitudes. Evidence for recent thrust faulting can explain where quakes originate from and how destructive they might be. Gathering this information for priority destinations helps mission planners identify hazards to avoid. It also guides engineering decisions to ensure structures are designed to withstand both sporadic quakes and gradual shifting of the terrain.
The Next Giant Leap
As NASA prepares to return humans to the moon under the Artemis program, the lunar south pole is a leading contender for early landings. Its permanently shadowed craters likely harbor troves of frozen water that future explorers could utilize. However, unraveling the subsurface structure and activity in the region is key for keeping astronauts safe.
The moon’s seismic threat is a reminder that even airless, inactive-looking worlds can surprise us. But the knowledge and experience gained from facing these challenges will also equip us for more ambitious ventures to Mars and beyond.
