Researchers believe they may have solved the long running mystery of how quakes happen hundreds of miles under the lunar surface.
They say the same gravitational force responsible for creating tides on Earth could be behind the ‘moonquakes’ about 800 to 1,200 kilometers (497 to 746 miles) below its surface roughly every 27 days.
Seismometers placed on the moon during the Apollo 12, 14, 15 and 16 missions revealed the strange events.
Moonquakes: This image shows the Apollo seismic stations and deep moonquake source regions on the moon used in the research. The lunar globe was taken from Lunar Reconnaissance Orbiter Camera (LROC)courtesy of NASA, and Apollo stations and deep moonquake nests were marked by the study’s authors.
The timing of the events – roughly the same as it takes the moon to make a complete circuit around Earth, caused scientists to suspect the moonquakes were a result of tidal stress, but their exact cause remained a source of debate until now.
The study in the Journal of Geophysical Research: Planets, a journal of the American Geophysical Union, combined readings from two different seismometers placed on the moon during the Apollo missions in a new way.
‘Apollo seismic observation discovered that the Moon is seismically active and the observation detected more than 13000 seismic events,’ they wrote.
‘Among the detected events, most frequently observed seismic events are deep moonquakes that occur at 800–1200 km depth in the Moon.
‘Although intensive studies have been carried out for decades, why and how deep moonquakes occur are remaining mysteries.’
In the study, the team reinvestigated the Apollo deep moonquake data.
‘This enabled us to study more than 100 deep moonquakes events while only one event was studied in the previous study, ‘ the team say.
‘With 131 deep moonquakes, we carried out comparative and statistic study of the fault characteristics.
‘Our study revealed that the deep moonquake faults are much smoother compared to the terrestrial counterparts and the stress release of the event is as low as the tidal stress between the Earth and the Moon.
This view of the north polar region of the Moon was obtained by NASA’s Galileo camera during the spacecraft flyby of the Earth-Moon system on December 7 and 8, 1992
‘The correlation between the deep moonquake occurrence and tidal stress has been pointed out and our study supports the idea that the tidal stress is not only triggering the deep moonquakes but also responsible of the whole stress release of the seismic activity.’
The tidal force causes Earth and its oceans to bulge out on the side closest to the moon and the side farthest from the moon.
On the moon, these same forces occurr, and cause a small-scale distortion of the entire planet, which scientists have named a solid-body tide.
These solid-body tides create faults or cracks on the moon, which rub against each other when tidal stress builds up and result in moonquakes.
The Apollo Lunar Surface Experiments Package (ALSEP) comprised a set of scientific instruments placed by the astronauts at the landing site of each of the five Apollo missions to land on the Moon following Apollo 11. This image shows the Apollo 16 package. ALSEP was a collection of geophysical instruments designed to continue to monitor the environment of each Apollo landing site for a period of at least a year after the astronauts had departed. Designed for a life of one year (Apollo 17 was for two), they ended up working for up to 8 years, the experiments permanently shut down by Mission Control on 30 September 1977.
Previous studies of deep moonquakes using the Apollo data were limited, since neither of the two seismic instruments on the moon entirely captured the signals from the moon’s activity—one detected high-frequency seismic activity and the other detected low-frequency seismic activity.
Researchers have uncovered over 200 previously unknown tremors on the moon by analysing data from the Apollo missions.
The find gives new insight into the moon’s geology, and reveals four distinct types of quake hit the orbiting body.
The algorithm-based program uncovered 210 previously unknown tremors and scientists at Max Planck Institute for Solar System Research say additional new discoveries will follow.
‘The system is trained to recognize deep moonquakes, impacts, and shallow moonquakes, and performs reliably,’ wrote the authors.
Apollo 11, the first spacecraft to land on the moon, planted not only an American flag on the mysterious lunar surface but also seismographic equipment.
Since that historic first landing in 1969, Apollo astronauts from four additional missions have placed seismometer stations on the lunar surface through 1972, Earth Magazine explains.
Until the hour they were decommissioned in 1977, these stations radioed seismic data from the moon back to Earth.
Analysing the data in the years since, scientists have identified about 13,000 separate tremors, some registering a 5.5 on the Richter scale – a magnitude strong enough to cause slight damage to buildings.
‘The algorithm’s demonstrated ability to detect rare events and flag previously undefined signal classes as new event types is of particular interest,’ wrote the authors.
THE FOUR TYPES OF MOONQUAKE
There are four separate kinds of moonquakes, registering as shadowy echoes on the Apollo mission seismometers.
The first type is deep, occurring about 700 km below the surface, and believed to be caused by tides and linked to its orbit around the earth.
A second type, the result of a meteorite crashing into the surface, takes the form of vibrations.
The third type is thermal in nature; after two weeks of lunar night (and deep-freeze temperatures), the morning sun causes an expansion, and ultimately cracking, of the moon’s frigid crust.
The software uncovered 210 previously unknown tremors, and gives new insight into the moon’s geology.
Finally, the fourth type is a shallow quake occurring 20 or 30 kilometers (about 12 to 19 miles) below the surface.
While deep moonquakes are generally only magnitude 2 or smaller, they occur on a monthly basis, whereas quakes occurring along the same fault line on Earth may be decades or centuries apart.
Moonquakes also last longer than earthquakes, which typically cease within a few minutes.
Since the moon is much drier and cooler than Earth, the vibrations carry for longer, whereas Earth’s more compressible structure acts like a sponge to absorb vibrations.
Moonquakes have been the only confirmed events recorded on any extra-terrestrial body so far, the researchers noted in their new study.
Importantly, lunar tremors look different from the seismic activity seen on Earth, according to NASA Science, and they arise from a different source.
At least one research team wondered, Could there be additional lunar activity not yet discovered in the Apollo seismic data?
To answer this question, Dr. Brigitte Knapmeyer-Endrun and her team developed a unique algorithm similar to those used in speech recognition programs.
After information from one moon tremor is fed into the new algorithm-based program, it is able to search new data and recognize similar patterns.
Running the program on a small subset of data from 1972, the researchers classified more than 50 percent of previously uncategorized events and discovered more than 200 new events not listed in the current lunar event catalog.
The researchers said their new program could identify additional undiscovered events in the existing data and may be useful in ‘future seismometer missions to other planets,’ including NASA’s forthcoming InSight mission to Mars.