Richard A Lovett
Scientists pouring over decades-old data from seismometers left on the moon by Apollo astronauts have concluded that its surface is tectonically active – so active, in fact, that moonquakes could pose a hazard to future lunar bases.
Four Apollo missions, beginning with Apollo 12 in 1969, placed seismometers on the moon. During the eight years in which they remained active, these instruments recorded thousands of quakes, says Thomas Watters, a planetary scientist at the Smithsonian Institution in Washington, DC, US.
Most were deep in the moon’s interior, but 28 were traced to the crust, within 200 kilometres of the surface.
That sounds deep, but the Apollo landing sites were too close to each other for an ideal seismic network.
“They were all on the near side of the moon, and all in the equatorial zone,” Watters says.
“So the Apollo data, great as it was, was not really good enough to pinpoint the locations of the epicentres or determine how deep those quakes were.”
Then, in 2010, pictures from NASA’s Lunar Reconnaissance Orbiter, which is capable of imaging the surface to resolutions as fine as 50 centimetres per pixel, revealed that the moon is littered with young-looking fault scarps.
“There are over 3000 of them,” Watters reveals.
Not that they are huge cliffs – they’re only a few tens of metres high. But they’re big enough that when Apollo 17 came down near one of them, Astronauts Gene Cernan and Harrison Schmidt drove over to it on their buggy for a closer look.
The rocks the pair brought back from its base were the first indicators that such scarps were extremely young, possibly younger than 10 to 12 million years.
“In geologic time, that’s almost like current,” Watters notes.
That and other data suggesting that the scarps truly were recent, he says, raised the question of whether they might be so young that they’re actually still being created.
Geologically they have the appearance of thrust-fault scarps, meaning that they are formed when a planet’s crust contacts.
“They occur when you’re pushing the crust together and literally push, or thrust, one part of the crust up over another and create a cliff-like landform,” Watters explains.
If contraction is the only force that forms them, however, such faults should be randomly aligned, with no discernable pattern. But these aren’t.
Rather, their alignment suggests that they were created by gravitational stresses from the Earth’s tides, he says.
Most people know that the moon’s gravity creates the tides in the Earth’s oceans, but most don’t realise that the Earth’s gravity has a similar effect on the moon.
“The moon is actually being flexed with solid-body tides,” says Watters.
These tides, which occur as the moon’s orbit causes its distance from the Earth to vary by about 40,000 kilometres, don’t stretch and contract it by a lot – only a few tens of centimetres or so – but they enough to generate stress.
Furthermore, he adds, the 28 shallow moonquakes recorded by the Apollo seismometers tended to occur at the points in the moon’s orbit when it was subjected to the highest tidal stresses.
To see if the quakes were indeed related to the scarps, Watters’ team reanalysed the Apollo seismic data using a new computer algorithm designed to tease additional data out of “sparse” seismic network – in this case, the four geographically clustered seismometers deployed by the Apollo missions.
When they did that, they found that a number of the quakes occurred close enough to the scarps to probably be involved in their growth.
In other words, these faults aren’t just “geologically” young.
“Now we can say that young is really young,” Watters says.
And that implies that the faults that created the scarps are still active – or, at least, were so only a half-century ago.
Furthermore, the quakes they produce aren’t just minor tremors. The largest came in at magnitude five on the Richter scale, large enough to be destructive on Earth, and potentially even more damaging in the moon’s low gravity.
That’s an important warning to space agencies looking to set up future moon bases.
“You might not want to be within 30 kilometers of one of these faults,” Watters says.
David Kring, a lunar geologist and senior staff scientist at the Lunar and Planetary Science Institute in Houston, Texas, agrees.
“Moonquakes are a hazard that needs to be mitigated when exploring the lunar surface,” he says.
He also notes that the new research is a good example of the extraordinary new insights that scientists are still able to extract from 60-year-old Apollo data.
But it is also an indication that we need to supplement the Apollo data with return visits to the lunar surface.
“We need to explore new regions,” Kring says.
A new paper written by Watters and colleagues is published in the journal Nature Geoscience.