The fault scarp or cliff forms when the crust breaks and is thrust upward along a fault as the Moon contracts. (Supplied: NASA/ GSFC/ Arizona State University/ Smithsonian)
Rumblings under the lunar surface captured by Apollo-era equipment may be a sign that the Moon is still tectonically active, researchers say.
The Moon was previously thought to be fairly geologically quiet, but a study published today in Nature Geoscience found moonquakes in the 1960s and 70s occurred near fault scarps, or small cliffs created by movement between fractured parts of the Moon’s surface.
This is more than a coincidence — the quakes were likely caused by slipping at these fracture points, said the study’s lead author Tom Watters of the Smithsonian Institution.
“It means the Moon has somehow managed to remain tectonically active over its 4.51 billion years,” Dr Watters said.
The fault scarps are “young” in that they are less than 50 million years old.
“The primary force creating what we now know is thousands of young fault scarps is cooling of a still-hot lunar interior,” he said.
“This cooling results in global contraction of the Moon.
“We have to figure out how small rocky bodies like the Moon can retain their interior heat over billions of years.”
The shallow moonquakes were detected by four seismometers from the Apollo program between 1969 and 1977 near the landing sites of Apollo 12, 14, 15 and 16. But researchers had not been able to accurately place the sources of the tremors.
Applying a relatively new algorithm designed for problems like this allowed Dr Watters and his team to locate the epicentres more accurately.
The research builds on findings made in 2010 by NASA’s Lunar Reconnaissance Orbiter (LRO), which discovered landforms that indicated the Moon was shrinking as its interior cooled — a little like the wrinkles that form as a grape shrinks to become a raisin.
The cliffs that form are called thrust faults. Slip events on these fault trigger shallow moonquakes that can cause strong shaking many tens of kilometres away.
The timing of the moonquakes was also important: many of the quakes occurred when the Moon was at or near its apogee — the furthest point in its orbit from Earth — when the tidal stresses on the rocky orb are at their highest.
By layering the seismic data over the LRO’s map of thrust faults, the researchers found the quakes were likely the result of activity in the Moon’s crust, rather than tremors caused by external forces such as asteroid impacts, or deep interior rumblings.
Of the 28 moonquakes measured, the team found at least eight were caused by true tectonic activity, with an equivalent earthquake magnitude of about 2 to 5.
The moon is not so “dull” after all
The Moon, “in its own little quiet way”, is much more interesting, geologically, than we previously thought said Craig O’Neill, a planet tectonics expert from Macquarie University.
“We’ve always assumed that a lot of the little bodies were very dull, very small, maybe a volcano back in the distant past but that’s about it,” said Dr O’Neill, who was not involved in the study.
“[In what] should be the death throes of this cold little world, it’s still able to generate a little bit of geological activity, something that’s interesting.”
The study also demonstrates the value of holding onto and re-analysing old data, said Jonti Horner, an astrophysicist from University of Southern Queensland.
“The work that they’ve done simply wasn’t possible when the Apollo landings happened because we didn’t have such high-resolution images of the moon, Professor Horner said.
“But equally, the work we’ve got here with the high-resolution maps wouldn’t be possible without that seismic data from the moon landings.”