The idea of a hollow Moon has long captured the popular imagination, often arising from observations suggesting our nearest celestial neighbor is less dense than expected. Decades of scientific investigation, particularly geophysics, reveal a different picture. The Moon is not an empty sphere but a differentiated planetary body, possessing an internal structure composed of distinct, solid layers, much like Earth. Modern science provides a comprehensive model of the lunar interior, built on hard data and the physics of mass.
The Myth Versus Planetary Density
The primary evidence refuting the notion of a hollow Moon lies in the fundamental physics of mass and density. Scientists can precisely calculate the Moon’s total mass by observing its gravitational influence. When this mass is divided by the Moon’s known volume, the resulting bulk density is approximately 3.34 grams per cubic centimeter, comparable to that of terrestrial rock.
If the Moon were a hollow shell, its observed mass would require the outer layers to be composed of materials impossibly dense for a natural planetary body. Furthermore, the immense gravitational forces exerted by Earth would quickly cause a large, hollow structure to collapse inward under its own weight and tidal stress. The myth often gains traction because the Moon’s density is significantly lower than Earth’s (5.5 g/cm³). This difference is not due to a void but because the Moon has a much smaller, less dense metallic core relative to its overall size than Earth does.
Probing the Interior with Seismology
The most direct proof of the Moon’s solid structure comes from the Apollo Passive Seismic Experiment (PSE). During Apollo missions 11, 12, 14, 15, and 16, astronauts deployed a network of sensitive seismometers onto the lunar surface. These instruments recorded thousands of seismic events, including deep moonquakes, shallow moonquakes, and meteoroid impacts. Scientists also generated artificial seismic events by intentionally crashing spent Lunar Module ascent stages onto the surface.
By tracking the speed and refraction of the resulting seismic waves, scientists effectively X-rayed the Moon’s interior. When a seismic wave encounters a boundary between materials of different density, it bends, or refracts, and its speed changes. Both compressional waves (P-waves) and shear waves (S-waves) were observed traveling through the Moon, confirming a solid interior composed of distinct layers. This finding directly contradicts the behavior expected from a hollow body, where seismic waves would not transmit through a vacuum or show complex changes.
Defining the Moon’s Layered Structure
The seismic data confirmed that the Moon is a differentiated body with a crust, mantle, and core. The outermost layer is the crust, which is asymmetrical, measuring about 40 kilometers thick on the near side facing Earth and up to 60 to 100 kilometers thick on the far side. This crust is rich in anorthosite, a low-density, aluminum-rich rock that solidified from the ancient lunar magma ocean.
Beneath the crust lies the Moon’s thick mantle, which constitutes the majority of its volume. The mantle is composed of denser silicate rocks, primarily olivine and pyroxene, rich in iron and magnesium. This solid, rocky mantle extends hundreds of kilometers toward the center.
At the heart of the Moon is a small, iron-rich metallic core. Current models suggest this core has two parts: a solid inner core (radius about 240 km), surrounded by a liquid iron outer core (radius approximately 300 to 350 km). The Moon’s core is proportionally much smaller than the cores of other terrestrial bodies, occupying only about 20 to 25 percent of the Moon’s total radius. This small core size is the reason for the Moon’s relatively low overall bulk density, which has often been mistakenly interpreted as evidence of a hollow interior.