Scientists have reported a decrease in the Moon's radius by 45 m.

The moon's radius has decreased by about 45 m over the past 400-800 million years. This assessment was obtained by scientists who studied a network of more than 10,000 young faults on its surface. Compression is associated with gradual cooling of the subsurface and may be accompanied by moonquakes, which is important to consider when choosing sites for long-term bases. The rate at which the size of the Moon is changing, why the slow process causes sudden movements, and which areas require special attention is described in the Izvestia article.

How much is the Moon losing in size

Thomas Watters, a planetary geologist at the Smithsonian Institution's National Air and Space Museum, led the new work. The space station has been studying the moon since 2009. During this time, an extensive archive of images has been accumulated, allowing us to examine small details of the relief and trace the distribution of tectonic structures over the surface.

According to the new estimate, over the past 400-800 million years, the radius of the Earth's satellite has decreased by about 45 m. If we divide this change by the duration of the period, we get about 0.06–0.11 micrometers per year, or about 6-11 cm per 1 million years.

This is a calculated recalculation of the published values. It shows the average rate over a huge geological period, rather than the result of directly measuring how much the radius has changed in one particular year.

The difference is important: the average velocity does not mean that the Moon has been contracting completely evenly and is now losing exactly the same amount annually. The study evaluates a long-term change based on its geological traces.

Compared to the size of the celestial body itself, the change is small. The moon's radius is about 1,740 km, so we are talking about a small fraction of a percent. But for the state of its solid shell, not only the total amount of decrease is important, but also the stresses that arise.

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Why does cooling change the lunar terrain

The process is based on a gradual loss of internal heat. As the moon cools, it shrinks in volume, and its outer solid shell is experiencing compression. At the same time, the bark cannot adjust smoothly and evenly to a decrease in size. Rocks deform and collapse, forming faults.

One of the characteristic results is thrusting. With this movement, one block of crust moves upward along the inclined surface of the fault and may move towards the neighboring one. A step-like ledge remains on the surface.

In NASA materials, such structures are described as relatively small cliffs tens of meters high, which can stretch for several kilometers. Their significance is determined not only by their prominence in the relief: they preserve evidence of deformation of the lunar crust.

Thus, the surface of the moon preserves the history of ongoing internal evolution. An outwardly calm landscape may include structures associated with the accumulation and release of stresses.

What do more than 10 thousand faults show?

The authors of the new work counted more than 10 thousand young faults distributed over the surface of the Moon. Their wide distribution helps to assess compression as a planetary-scale process.

The word "young" is used here in a geological sense. It does not mean that all these structures have been formed in recent years or during the operation of the orbiter. Millions of years in the history of a celestial body may belong to a relatively recent period.

The number of faults found should also not be equated with the number of registered moonquakes. The snapshot shows the structure of the surface, while the seismic recording captures a specific event and its fluctuations.

One fault may experience repeated movements. In another case, it is difficult to determine from the image when the last shift occurred. Therefore, a fault map is necessary to assess tectonics, but by itself it does not provide a timetable for future tremors.

The novelty of the work lies in expanding the understanding of the scale of the fault network and clarifying the estimate of the reduction in the size of the Moon. The very process of its compression and the possible connection with moonquakes have been studied before.

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Why slow compression can cause a sudden jolt

A small average annual speed does not mean the same smooth movement at every point on the surface. Stress in rocks can accumulate for a long time, and then be released with relatively rapid displacement along the fault.

It is this difference between slow accumulation of deformation and sudden movement that makes it possible to understand the connection with moonquakes. For a structure on the surface, the seismic event itself is important, not just the average decrease in radius over 1 million years.

On Earth, a significant part of tectonic earthquakes are associated with the boundaries of moving lithospheric plates. The moon is arranged differently: its outer rigid shell is not divided into a system of Earth-type movable plates. Stresses from global compression can be realized in faults that are widely distributed over the surface.

However, this does not mean that a strong moonquake is equally likely at any point and at any moment. To assess a specific site, information is needed about nearby faults, their activity, rock structure, and possible propagation of vibrations.

The Earth's gravity has an additional effect. It creates tidal stresses inside the moon, which can contribute to movements where rocks are already in a stressed state.

Why do they pay special attention to the South Pole?

The South Polar region is of interest for lunar exploration due to the combination of resources and lighting conditions. In permanently shaded areas, water ice may persist, and individual hills receive prolonged sunlight, which is useful for energy supply.

But the attractiveness of the area for the expedition does not guarantee geological stability. In a study presented to NASA in 2024, scientists examined young thrust faults and the possible effects of moonquakes in the South polar region.

They also modeled the stability of the slopes and found areas where even a relatively weak seismic impact can cause regolith, a loose surface material, to slide. This is a model assessment, not a report on the observed destruction of the base or landing site.

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For infrastructure design, the difference between the immediate impact and the secondary consequences is significant. The danger may be associated not only with the load on the structure, but also with the movement of soil on the slope next to it.

Therefore, the site selection should take into account the adjacent ledges, crater walls and areas where landslides are possible. The availability of ice and solar energy must be compared with geological conditions.

How the Moon's compression is changing the requirements for future bases

The new estimate does not determine where and when the next major event will occur. Its practical significance lies in the fact that tectonic activity must be taken into account in the long-term development of the surface.

For a short expedition and a base designed for many years of work, different time horizons are important. The longer the infrastructure has to function, the more significant the assessment of rare events and their possible consequences.

This leads to an engineering task: to compare the selected construction sites with faults, check the stability of the soil and determine which vibrations the structures and equipment should withstand. This is the direction of application of the results, and not the ready-made standards established by the new article.

Long-term seismic measurements are required for more accurate calculations. Orbital images provide a spatial picture, and instruments on the surface help determine the frequency of events and wave propagation patterns. These sources complement each other.

The main practical issue now is related to the transition from a global map to an assessment of specific sites. Reducing the radius by fractions of a micrometer per year is almost imperceptible in itself, but the movements accompanying this process are important for the sustainability of the future lunar infrastructure.