The Size Of The Moon Is Decreasing And It Is Causing Moonquakes

  • Recent observations reveal that Earth’s Moon may be shrinking as its interior cool, and it’s causing fairly strong moonquakes. 
  • These observations are obtained from Lunar Reconnaissance Orbiter Imagery and 5 decade-old data. 

Launched in 2009, NASA’s Lunar Reconnaissance Orbiter (LRO) spacecraft revealed a lot of valuable information about the tectonic evolution of the Moon.

High-resolution images captured by LRO camera and decades of old data show that the Moon may be shrinking as its interior cools. Over the last few hundred million years, the moon has shrunk more than 50 meters.

Since the Moon’s crust is very brittle, it breaks apart as the Moon contracts, creating ‘thrust faults’ where one part of the crust is stacked up over an adjacent section.

Researchers at the University of Maryland analyzed the data obtained by LRO in 2010. They developed algorithms to reexamine seismic measurements taken by NASA’s Apollo mission in the ’60s and ’70s, which provide precise epicenter locations for the 28 moonquakes occurred between 1969 and 1977.

They then combined this data with thrust faults images captured by LRO, and discovered more than 8 events where moonquakes occurred due to true tectonic activity and thrust faults (instead of asteroid impacts).

The data shows that thrust faults are still active and causing fairly strong moonquakes as the Moon further shrinks.

Apollo Seismometers Captured The Shrinking Moon

Astronauts placed the seismometers (a device that responds to ground motions) on the surface of the Moon during five Apollo missions. These devices recorded 28 moonquakes, ranging from 2 to 5 on the Richter scale.

The new algorithm calculated the location of these shallow quakes and found that all of them occurred within 30 kilometers of thrust faults. In lunar images, these faults appear several meters high, extending up to a few kilometers.

Reference: Nature Geoscience | DOI:10.1038/s41561-019-0362-2 | NASA

In this study, researchers discovered that 6 (out of 8) moonquakes occurred when the Moon was near the farthest point from Earth in its orbit. At this point, Earth’s gravity adds tidal stress on the Moon’s crust, increasing the probability of slip-events along the thrust faults.

According to researchers, these 8 moonquakes were formed by faults slipping as stress built up when the Moon’s crust was squeezed by tidal forces and global contraction.

size of moon is decreasing LRO images showing fault scarps | Credit: NASA 

It’s quite impressive to see how scientists combined LRO imagery with 5 decade-old data to advance our knowledge of the Moon while giving directions to future missions that aim to study the Moon’s interior.

Mercury Shrank Much More Than The Moon

Earth’s moon is not the only celestial body that is getting shrunk with time. Mercury has faults (up to 3,000 meters high, extending up to 1,000 kilometers) much bigger than those on the lunar surface, which indicates that Mercury shrank much more compared to the Moon.

So far, LRO has captured more than 3,500 fault scarps of the lunar surface. In coming years, scientists will compare images of particular fault areas from different times to find any evidence of recent moonquakes.

Read: What’s The Mystery Behind Moon’s Lost Magnetism?

We have learned a lot from past missions but that’s just the beginning. Upcoming Moon projects could make huge strides in our understanding of lunar science.

Written by
Varun Kumar

Varun Kumar is an experienced science and technology journalist interested in machines, AI, and space exploration. He received a Master's degree in computer science from Indraprastha University. To find out what his latest project is, feel free to directly email him at [email protected] 

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