The moon is shrinking because of clashing tectonic plates triggering moonquakes, according to new research.
Earth-style quakes are wrinkling it “like a raisin” – creating step like cliffs called ‘scarps’.
Each is roughly ten yards high and a few miles long – making them resemble giant stairway on the lunar surface.
They were first discovered more than 50 years ago by the Apollo astronauts.
Now an analysis has shown some are more recent.
Scientists superimposed epicentre location data onto images of the scarps from NASA’s Lunar Reconnaissance Orbiter (LRO).
Based on their proximity at least eight of the 28 recorded from 1969 to 1977 likely resulted from true tectonic activity.
This is the movement of crustal plates along the scarps or ‘thrust faults’ – rather than from asteroid impacts or rumblings deep within the interior.
NASA hopes to return to the moon in the next few years – and the discovery could speed up their plans.
Co-author Professor Nicholas Schmerr, a geologist at Maryland University, said: “We found a number of the quakes recorded in the Apollo data happened very close to the faults seen in the LRO imagery.”
The photographs also show physical evidence of geologically recent fault movement – such as landslides and tumbled boulders.
He said: “It’s quite likely 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 discovery of young faults less than 50 million years old by the LSO’s camera in 2010 has been interpreted as evidence of lunar tectonic activity.
But it was unclear how fresh this was.
So the US and Canadian team examined quakes recorded by seismometers at the landing sites of Apollo 12, 14, 15 and 16.
Using an algorithm they found the epicentres of eight of the quakes – more than a quarter – were within 19 miles of scarps visible in the LRO images.
This was close enough to conclude the Moon is currently tectonically active.
The LRO’s camera has identified thousands of the strangely shaped cliffs – stretching from the south to north poles.
An analysis in 2010 revealed they formed no more than one billion years ago. The latest findings suggest the moon is still getting smaller.
The Apollo instruments recorded their last quake shortly before they were retired in 1977.
But the moon is likely experiencing quakes to this day, say the researchers.
On Earth the shallow quakes – the type produced by tectonic faults – would have ranged in magnitude from about 2 to 5 on the Richter scale.
This would be enough to damage buildings and other structures.
The researchers also found six of the eight quakes happened when the moon was at or near its apogee – the point in the moon’s orbit when it is farthest from Earth.
This is where additional tidal stress from Earth’s gravity causes a peak in the total pressure on the moon’s crust – making slippage along the thrust faults more likely.
Lead author Senior Scientist Dr Thomas Watters, of the Centre for Earth and Planetary Studies at the Smithsonian Institution in Washington, said: “We think it’s very likely 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 the Apollo seismometers recorded the shrinking moon and the moon is still tectonically active.”
The moon wrinkles as its interior cools and shrinks – much as a grape does as it dries to become a raisin.
But unlike the flexible skin on a grape the moon’s crust is brittle – causing it to break as the interior shrinks.
This results in thrust faults where one section of crust is pushed up over an adjacent section.
The LRO has imaged more than 3,500 fault scarps on the moon since it began operation in 2009.
Some of these images show landslides or boulders at the bottom of relatively bright patches on the slopes of fault scarps or nearby terrain.
Because weathering gradually darkens material on the lunar surface, brighter areas indicate regions that are freshly exposed by an event such as a moonquake.
Other LRO fault images show fresh tracks from boulder falls, suggesting that quakes sent these boulders rolling down their cliff slopes.
Such tracks would be erased relatively quickly, in terms of geologic time, by the constant rain of micrometeoroid impacts on the moon.
With nearly a decade of LRO imagery already available and more on the way in the coming years, the team would like to compare pictures of specific fault regions from different times to look for fresh evidence of recent moonquakes.
Prof Schmerr said: “For me, these findings emphasise we need to go back to the moon.
“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 the journal Nature Geoscience.