Bengaluru: Ever since Apollo astronauts placed seismometers on the Moon five decades ago, it has been known that the Earth’s natural satellite produces quakes. But the exact nature of several shallow quakes originating near the surface has flummoxed scientists for years.
Now, a study based on data from these instruments has shown why this shallow tectonic activity on the Moon continues millions of years after conventional wisdom suggested it should have died down: While most quakes emanate from deep within the Moon, some occur because the Moon shrinks as its interior cools, and its brittle, crusty exterior quakes to absorb the change.
Apart from the plain and simple fact that this new finding helps us understand the Moon better, it is significant because humans are looking to establish a base there by the turn of the century, and information about tectonic activity would be necessary to mitigate risk and help decide optimal locations for explorers to reach.
Plate tectonics and lunar structure
Like all rocky bodies, the Moon has a core that is surrounded by a mantle, and then an outer crust.
But unlike other rocky bodies, the Moon’s core is only 20 per cent of its structure, not 50 per cent.
Inside Earth, the solid iron core is still very hot. It is surrounded by a liquid outer core, a viscous mantle, and the outer crust.
The crust is broken up into several tectonic plates, which float on the mantle and move around. They collide with and scrape against each other, and move away.
All these slow and bulky movements produce geological activity like volcanoes and earthquakes.
On the Moon, the reduced core volume and relatively small size meant that it cooled very rapidly.
Its solid iron core is surrounded by a viscous outer core, followed by a mostly-solid mantle, and a hard crust. There are no tectonic plates. The entire lunar surface is just one hard solid shell.
The lunar core isn’t as hot as it used to be the first three billion years since the Moon’s formation, and has been cooling for millions of years.
As it cools, the Moon also shrinks. Much like a grape turning into a raisin, this shrinking causes folds, disturbances, and breakages on the surface. Sometimes, the surface rises to form a mini cliff called a ‘scarp’.
A long-unsolved mystery
Between 1969 and 1972, the seismometers placed on the Moon by Apollo astronauts recorded thousands of quakes. The bulk of them came from deep within, indicating that Earth’s pull was stretching and tugging at the interior. But there were also shallow quakes that came from within the crust, much like the quakes on Earth.
According to the new study, 28 quakes in that period emanated from the crust, within 200 km of the surface.
Quakes coming from the surface would immediately imply tectonic activity, but the Moon was believed to have become tectonically inactive several million years ago.
The quakes were quite strong too, some measuring over 5.5 on the Richter scale, an intensity associated with damaged buildings on Earth.
However, while valuable, the Apollo data could not pinpoint the epicentres of the quakes to understand how they’re caused.
NASA’s Lunar Reconnaissance Orbiter, which was launched in 2009 and has been sending images of the surface since 2010, came to the rescue. The research team examined LRO image data from 2010 and identified several scarps and fault lines.
Studying the images along with the seismic data finally helped scientists piece together a picture of what is currently causing frequent and regular quakes on the Moon.
Of the 28 quakes, eight were within 30 kilometres of scarps. The researchers surmised that if the scarps were only forming because of the Moon shrinking, there would be a global presence of lunar scarps.
But they ran in a fixed pattern: Near the poles, they ran east to west, while near the equator, they ran north to south.
The only explanation for the quakes was again the nearest, most powerful body to the Moon — the Earth. The Moon is constantly under the influence of Earth’s gravitation, and gravitational attraction causes tidal forces on bodies.
The team examined the quakes in relation to the lunar orbit and found that 18 of the 28 occurred when the Moon was at its ‘apogee’, the farthest point in its orbit around Earth.
“From an analysis of the timing of the eight events, we found that six occurred when the Moon was less than 15,000 km from the apogee distance,” the researchers note in the study.
Of the eight located near scarps, five were during the apogee.
As the Moon goes around the Earth, at its apogee, it feels the maximum stresses created by the planet’s gravity.
This is compounded by the fact that the Moon also slows down at its apogee before making a return towards the Earth, thus giving time for changes caused by the planet’s pull to accumulate and intensify.
The shrinking of the Moon also offers up parts of the core that have loosened from the mantle. And once in a while, there’s a crack and a thundering quake slips out into the low-gravity environment, deadlier than it would be on Earth.
“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 Thomas Watters, lead author of the research paper and senior scientist at the Center for Earth and Planetary Studies at the Smithsonian Institution, US, in a press statement issued with the study.
The quakes also loosen the lunar soil or regolith, causing it to flow and expose material underneath, which can also be observed in images. They are also known to loosen boulders.
“It’s quite likely that the faults are still active today,” said Nicholas Schmerr, an assistant professor of geology at the University of Maryland and a co-author of the paper, in the press statement.
“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.”
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