According to the latest study, the gravitational force which is responsible for creating the tidal stress on Earth could be causing deep quakes on the moon. This study got published in the Journal of Geophysical Research: Planets. The scientists analyzed the latest data collected by the Apollo missions. They confirmed that the tidal stress or the gravitational pull between the moon and the Earth is responsible for causing deep moonquakes. These moonquakes are the lunar equivalent of the earthquakes.
Apollo Missions to study the Moon Activities:
During the Apollo 12, 14, 15 and 16 missions, the scientists placed seismometers on the moon to collect information regarding the activities occurring on the moon. During the analysis, they realized that the moon experiences deep moonquakes. These moonquakes occur at about 800 to 1,200 kilometers (497 to 746 miles) below the surface of the moon roughly every 27 days.
Since the moon completes a circle around the Earth in this time duration, so the scientists used to think that the moonquakes occurred due to tidal stress. However, they were not able to confirm the exact cause of the moonquakes.
During the latest study, the scientists decided to combine the readings obtained from two different seismometers placed on the moon during the Apollo missions. They selected three most active moonquake sites where moonquakes occurred during 1969-1977. They analyzed 131 moonquakes which occurred on the selected sites according to the new technique. And confirmed that those moonquakes are likely caused by the tidal stress building up on the moon.
As we all know that, the tidal force causes the water in the oceans of Earth bulging out on the side closest to the moon and the side farthest from the moon. These water bulges are termed as high tides. According to the scientists, these same forces are occurring on the moon as well. This causes a small-scale distortion of the entire planet. The scientists call it a solid-body tide. These solid-body tides are responsible for the cracks or faults on the moon. When the tidal stress builds up on the moon, the solid-body tides rub against each other resulting in moonquakes.
Reanalyzing the previous data obtained from the Moon:
If we look into the structure of the moon as proposed by the scientists, it has a core, crust, and mantle just like Earth. However, it doesn’t possess tectonic plates, unlike Earth. The Earthquakes occur when the tectonic plates move into or past each other. While this phenomenon does not occur on the moon as there are no tectonic plates on the moon.
The magnitude of the deep moonquakes is generally 2 or smaller. Moreover, they occur on a monthly basis. Whereas the Earthquakes occurring along the same fault line may be decades or centuries apart. The Earthquakes typically cease within a few minutes compared to the moonquakes. The vibrations which occur due to the moonquakes carry for a longer time. This happens because the composition of the moon is much drier and cooler than that of the Earth. Unlike Moon, the Earth has a more compressible structure which acts like a sponge to absorb the vibrations.
Previously, the scientists had limited data to study the deep moonquakes which were obtained during the Apollo missions. Moreover, the scientists couldn’t capture the entire signals from the moon’s activity by using two seismic instruments. Because they were detecting high-frequency seismic activity using one instrument and the low-frequency seismic activity using the other instrument.
In the previous technique, the scientists were using the readings from one of the seismometers only. And they were not considering the stress released by the moonquakes in their analysis. Due to which they were not able to see the full relationship between the tidal stress buildup and the stress released by the quakes.
In the latest study, the researchers used a new technique of combining the signals from the two different instruments. They combined the data from both the instruments and analyzed it. In this way, they were able to enhance their understanding of the faults where the moonquakes occurred.
By using this new method, the researchers were able to relate the buildup of tidal stress with the stress released during the moonquakes over a course of the month. They also found out that the two values agreed. They confirmed that the moonquakes occur as a result of the tidal stress.
Insight into the Interior of the Moon:
Previously, the scientists used to think that the interior areas of the moon were warmer where the deep moonquakes would occur. This would make the faults on the moon more supple and malleable.
Now after the recent study, the researchers got a new insight into the interior of the moon. They suggest that the mantle of the moon may be colder than it was previously thought. Moreover, the faults of the moon might be brittle to endure the observed tidal stress and create the recorded seismic readings.
After this study, the scientists are reconsidering the instrumentation that should be used for future space missions including the Insight mission to Mars. Similarly, they are planning to send both a short-period and a broadband instrument in the space missions. So that they would be using the similar technique of signal combination in the frequency domain as well while using the Insight instruments. Furthermore, they will be exploring the implications for the composition of the moon. Also its internal temperature in future as well.