The Moon has long been viewed as a symbol of permanence, a static sphere altered only by external forces like meteorite impacts. This perception is now scientifically challenged by evidence showing that our satellite is an active and dynamic world. Data gathered from multiple space missions confirms that the Moon is gradually getting smaller due to internal planetary evolution.
Confirming the Shrinkage
The definitive answer from planetary scientists is that the Moon is indeed shrinking, a process that continues today. Over geological eras, this contraction has been significant enough to leave a permanent mark on the lunar surface. Precise measurements suggest that the Moon’s diameter has decreased by approximately 50 meters (164 feet) over the last several hundred million years.
This slow, steady change affects the entire global structure. While a 50-meter change in a body with a diameter of over 3,474 kilometers might seem minor, it represents a substantial volume loss. This measurable contraction confirms that the Moon is still tectonically active, a fact supported by the relatively pristine and young appearance of surface features.
The Mechanism of Contraction
The underlying cause of the Moon’s shrinking is thermal contraction, a fundamental aspect of planetary cooling. The Moon formed approximately 4.5 billion years ago in a massive collision, which generated immense heat and left it molten. Since then, the Moon has been radiating this internal heat into space, causing its interior to slowly cool and solidify. As the core and mantle cool, the material becomes denser and takes up less volume, forcing the rigid outer crust to adjust to a smaller circumference. Unlike Earth, the Moon lacks mobile tectonic plates. Instead, its brittle, single-piece crust must fracture and buckle to adapt to the shrinking volume underneath, driving current tectonic activity.
Visible Evidence on the Surface
Scientists observe this global contraction through features called lobate scarps, which are the Moon’s equivalent of wrinkles. These are small, cliff-like landforms resembling stair-steps, often running for many kilometers. They are formed by thrust faults, which occur when the crust is pushed together, causing one section to be thrust up and over the adjacent section. Images from NASA’s Lunar Reconnaissance Orbiter (LRO) have mapped thousands of these scarps across the lunar globe.
The sharp, well-preserved features of these scarps indicate they are geologically young, suggesting the shrinkage is ongoing. Many of these features cut across younger craters, providing further evidence that they formed relatively recently in the Moon’s history. Further confirmation comes from the Apollo Passive Seismic Network, which deployed seismometers on the Moon’s surface decades ago. Researchers have linked the epicenters of shallow moonquakes, which can reach up to magnitude 5, directly to the locations of the lobate scarps, showing the crust is actively fracturing today.
Shrinking vs. Receding
While the Moon is shrinking in size, this physical contraction is separate from its orbital movement away from Earth. The Moon’s global contraction is an internal geological process driven by thermal cooling, resulting in a loss of about 50 meters in diameter over hundreds of millions of years.
In contrast, the Moon is also slowly moving farther away from Earth, a process known as orbital recession. This movement is driven by tidal forces—the gravitational interactions that cause ocean tides—which slightly accelerate the Moon in its orbit. This tidal force causes the Moon to recede from Earth at a rate of about 3.8 to 4 centimeters (1.5 inches) per year, an external orbital dynamic entirely unrelated to its physical size reduction.