New research by NASA suggests the moon is shrinking and causing damaging moonquakes. The interior of the moon is cooling, causing it to grow smaller.
Over the last several hundred million years this cooling has caused the moon to shrink about 50m in size. As the interior shrinks the hard crusty, brittle exterior breaks causing “thrust faults” where one section of the crust is pushed up over a neighboring part.
“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 in the Center for Earth and Planetary Studies at the Smithsonian’s National Air and Space Museum in Washington.
“Some of these quakes can be fairly strong, around five on the Richter scale.”
Astronauts zig-zagged across moon cliffs
Astronauts have described these fault marks as a step-shaped cliff. They are usually tens of meters high and extend for several kilometers. When Astronauts Eugene Cernan and Harrison Schmitt landed in the moon’s Taurus-Littrow valley in 1972 they had to zig-zag their lunar rover up and over these cliff faces.
Seismometers - instruments that measure the shaking produced by quakes - were placed on the moon’s surface by the Apollo 11, 12, 14, 15, and 16 missions. Four of these seismometers recorded 28 shallow moonquakes from 1969 to 1977. The quakes ranged from 2 to 5 on the Richter scale.
Moon under tidal pressure
A recent study has analyzed this quake data using an algorithm that can better estimate the quake locations. They found that eight of the 28 recorded shallow quakes originated very close to faults visible in lunar images. Close enough that scientists can tentatively say the faults are the cause of the quakes.
They also discovered that the six of these eight quakes occur when the moon was at its farthest point away from earth. This position, known as the moon’s apogee put the moon under extra tidal stress from Earth’s gravity making movement events on the moon's fault lines 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 Watters.
Images show fresh moon surface
More evidence that the moon's fault lines are active comes from images taken by NASA’s Lunar Reconnaissance Orbiter (LRO) spacecraft. The Lunar Reconnaissance Orbiter Camera (LROC) took more than 3,500 images of fault scarps and in some of these landslides or boulders can be seen at the bottom of bright patches on the slopes of fault scarps or nearby terrain.
These bright patches indicate they have been freshly exposed to space as would occur following a moonquake.
“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,” said LRO Project Scientist John Keller of NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
NASA plans to send humans back to the moon in 2024.