The Moon, our nearest celestial neighbor, is not as stable as it seems. A new study published in The Planetary Science Journal on January 25, 2024, has revealed that the Moon has been shrinking in size over the past hundred million years due to cooling of its interior. This has also resulted in unprecedented moonquakes, which are seismic tremors that can last for hours on the lunar surface.
How the Moon Shrinks and Quakes
The study, conducted by a group of scientists from NASA, the Smithsonian, Arizona State University, and The University of Maryland, used data from the Lunar Reconnaissance Orbiter Camera (LROC) onboard NASA’s Lunar Reconnaissance Orbiter (LRO) to detect thousands of relatively small, young thrust faults widely distributed in the lunar crust. These faults are cliff-like landforms that form where contractional forces break the crust and push or thrust it on one side of the fault up and over the other side. The contraction is caused by cooling of the Moon’s still-hot interior and tidal forces exerted by Earth, resulting in global shrinking.
The lobate scarps are formed when the lunar crust is pushed together as the Moon contracts. This causes the near-surface materials to break forming a thrust fault. NASA/LRO/LROC/ASU/Smithsonian Institution
The researchers estimated that the Moon has shrunk over 150 feet in its circumference while its core gradually cooled during the same period. The continuation of the shrinking process of the Moon is leading to the forming of moonquakes, which are slip events on existing faults or the formation of new thrust faults from ongoing global contraction. The study also presented evidence that some of these moonquakes and faults are located near and within some of the areas that NASA identified as candidate landing regions for Artemis III, the first Artemis mission planned to have a crewed lunar landing.
Implications for Future Lunar Exploration
The study has implications for future lunar exploration, especially for the Artemis campaign, which aims to send astronauts to the lunar south pole region by 2028. The south pole region is of strategic importance for scientific and commercial reasons, as it hosts potential resources such as water ice and minerals, as well as offers a unique vantage point for observing the solar system.
However, the study suggests that shallow moonquakes capable of producing strong ground shaking in the south polar region are possible from slip events on existing faults or the formation of new thrust faults. This could pose a risk for landing stability and outpost construction on the Moon. Therefore, the researchers recommend that the global distribution of young thrust faults, their potential to be active, and the potential to form new thrust faults from ongoing global contraction should be considered when planning the location and stability of permanent outposts on the Moon.
The epicenter of one of the strongest moonquakes recorded by the Apollo Passive Seismic Experiment was located in the lunar south polar region. However, the exact location of the epicenter could not be accurately determined. A cloud of possible locations (magenta dots and light blue polygon) of the strong shallow moonquake using a relocation algorithm specifically adapted for very sparse seismic networks are distributed near the pole. Blue boxes show locations of proposed Artemis III landing regions. Lobate thrust fault scarps are shown by small red lines. The cloud of epicenter locations encompasses a number of lobate scarps and many of the Artemis III landing regions. NASA/LROC/ASU/Smithsonian Institution
The study also highlights the need for more seismic data from the Moon to better understand its interior structure and evolution. The Apollo missions deployed four seismometers on the lunar surface between 1969 and 1972, which recorded 28 shallow moonquakes up to magnitude 5.5 until they were turned off in 1977. However, these seismometers were limited in their coverage and accuracy, leaving many uncertainties about the location and origin of moonquakes. The researchers suggest that a modern network of seismometers on the Moon could provide more precise information about moonquake activity and fault locations, as well as reveal new insights into lunar geology.
Conclusion
The Moon is shrinking and quaking as it cools down over time. This process is creating new features on its surface and posing challenges for future exploration. The study by NASA and other institutions provides new evidence for this phenomenon and calls for more seismic monitoring on the Moon to better understand its dynamics and history.