The moon is shrinking all the time. Will it shrink into the size of a raisin?

Produced by: Science Popularization China

Author: Denovo

Producer: China Science Expo

Every Mid-Autumn Festival, we look up and see the big, round full moon, which seems to have never changed and is silently protecting us.

However, you may not know that the moon has actually been slowly shrinking over hundreds of millions of years.

As early as 2010, NASA's analysis of Lunar Reconnaissance Orbiter (LRO) images found that as the moon's interior gradually cooled, many thrust faults appeared on the lunar surface, which was evidence of the moon's shrinkage.

Still, don't worry about the moon shrinking to a wrinkled raisin shape - research shows that over the past few hundred million years, the moon's overall diameter of about 3,474 kilometers has only shrunk by about 50 meters.

The moon shrank into a raisin

(Image source: generated by the author’s AI)

The structure of the moon

Why is the moon shrinking? We can start with the structure of the moon.

The structure of the moon can be divided into three layers: the lunar crust, the lunar mantle and the lunar core. The lunar crust is about tens of kilometers thick and is mainly composed of oxide-rich rocks. The surface is covered with a layer of lunar soil of varying thickness, which is composed of fragments and dust formed by meteorite impacts.

The structure of the moon

(Image source: Wikipedia)

Below the lunar crust is the mantle, which is about 1,000 kilometers thick and mainly composed of silicate minerals, with low density and high viscosity.

The core is located at the innermost part of the Moon and is divided into an outer core and an inner core. The outer core is thought to be a partially molten liquid metal composed mainly of iron and sulfur, while the inner core is solid iron. The Moon's core is relatively small, accounting for less than 3% of the total volume, unlike the Earth, which, by comparison, accounts for about 17% of the total volume.

Moon

(Photo credit: Photo taken by the author)

Moonquakes also occur on the moon

Because the moon's core is small and its thermal activity is limited, there is no plate movement and volcanic activity on the moon's surface like on Earth, but earthquake-like phenomena can also occur on the moon!

We often hear news about earthquakes at home and abroad. Earthquakes are natural phenomena caused by the accumulation and sudden release of stress in the earth's crust. When the rock layers in the earth's crust gradually accumulate energy under the movement of tectonic plates and exceed the bearing limit of the rock layers, the rock layers will suddenly break or shift, releasing a large amount of energy. This energy is transmitted in the form of seismic waves, causing the surface to shake.

Earthquakes typically occur at plate boundaries, such as subduction zones, transform faults or spreading ridges, where the movement of rock layers is most intense. The magnitude and impact of an earthquake depends on the size of the energy released and the depth of the earthquake's focal point.

There are earthquakes on the moon too.

(Photo credit: Photo taken by the author)

Do earthquakes also occur on the moon? Yes, moonquakes are earthquakes that occur on the moon. They are similar to earthquakes on Earth, but have different causes and characteristics.

Hundreds of deep moonquakes have been recorded, and 28 shallow moonquakes were recorded from 1969 to 1977. Deep moonquakes are caused by the tidal forces of Earth's gravity and occur at depths of 500 to 700 kilometers on the Moon, often occurring in groups.

Shallow moonquakes, which usually occur 20 to 30 kilometers below the lunar surface, are caused by the release of stress in the lunar internal structure. Although less common than deep moonquakes, shallow moonquakes are larger in scale, with body wave magnitudes > 5.5 and stress drops exceeding 100 MPa, which means that the energy released by moonquakes is very huge. Other sources of moonquake activity include meteorite impacts or landing factors of the lunar module. There is also a type of thermal moonquake, which is related to the thermal expansion and contraction of lunar surface materials caused by the temperature change between day and night.

The cooling and contraction of the moon is one of the main causes of moonquakes, especially as the moon's core gradually cools, the lunar crust contracts and forms faults. This process can cause shallow moonquakes, which can reach a magnitude of more than 5 on the Richter scale and last for a long time. Unlike earthquakes, the vibrations of moonquakes last longer. Shallow moonquakes can last for hours or even the entire afternoon because the moon's surface is relatively dry and rigid, similar to a vibrating tuning fork.

Would the moon have wrinkles like a raisin if it shrank?

Regarding the gradual shrinking of the moon, scientists have discovered through seismic data from NASA's Lunar Reconnaissance Orbiter and Apollo missions that the cooling of the moon has led to the formation of thousands of thrust faults on its surface.

A thrust fault is a type of fault in geology. It occurs when rock layers are squeezed, causing older rock layers to be pushed onto younger rock layers along the fault plane. This type of fault is usually formed in a horizontal or nearly horizontal stress environment, and the fault plane has a low dip angle (generally less than 30 degrees). The characteristic of a thrust fault is that the rock layers undergo significant displacement under horizontal compression, causing the shortening and thickening of the crust.

Digital model of the nappe fault in the lunar farside highlands

(Image source: Document 4)

It helps to compare thrust faults to the wrinkles on a raisin, but they differ significantly in their formation mechanisms and manifestations. The wrinkles on a raisin are caused by the shrinkage of the outer skin as it loses water, resulting in wrinkles on the surface.

A nappe fault is a large fault plane formed when geological or celestial rock layers are squeezed and pushed horizontally. These faults usually appear as small cliff-like terrain, similar to stairs. The formation of a nappe fault is due to the gradual loss of heat from the moon's interior, and the formation of faults in the lunar crust during the contraction process, which is also accompanied by the folding and cracking of the lunar crust.

Nappe fault near the lunar south pole

(Image source: Document 3)

These faults and the contraction of the moon are associated with shallow moonquakes, and studies have shown that these shallow moonquakes on the moon may last for a long time, even up to several hours. Because the surface of the moon is relatively dry and dense, the vibrations of moonquakes can trigger landslides through faults, especially in the south pole of the moon. This area is also one of the landing candidates for NASA's "Artemis" mission, so seismic activity may pose a potential risk to future lunar exploration and base construction.

In a new paper published this year in the Journal of Planetary Science, the research team linked a set of faults located in the lunar south pole to one of the strongest moonquakes recorded by the Apollo seismometer more than 50 years ago. The researchers also analyzed the surface stability of the south pole region through modeling and found that some areas are particularly susceptible to landslides and earthquake-induced impacts. Some of these areas are located in permanent shadow areas and may contain water ice resources, which are important targets for future exploration. Landslides caused by moonquakes may affect the stability of the lunar surface and increase the risk of future exploration missions.

Distribution of moonquakes in the lunar south pole

(Image source: Document 3)

Conclusion

The shrinking process of the moon is very slow, mainly manifested by the fractures and wrinkles on the lunar surface. Such changes are very small and will hardly affect the beauty of the full moon we see on Earth. Therefore, every Mid-Autumn Festival, we can still appreciate the round moon with peace of mind and experience the tranquility and peace it brings.

References:

1.Goins, Neal Rodney, AM Dainty, and MN Toksöz. "Lunar seismology: The internal structure of the Moon." Journal of Geophysical Research: Solid Earth 86.B6 (1981): 5061-5074.

2. Nunn, Ceri, et al. "Lunar seismology: A data and instrumentation review." Space Science Reviews 216.5 (2020): 89.

3. Watters, TR, et al. "Tectonics and Seismicity of the Lunar South Polar Region." The Planetary Science Journal 5.1 (2024): 22.

4. Watters, Thomas R., et al. "Shallow seismic activity and young thrust faults on the Moon." Nature Geoscience 12.6 (2019): 411-417.