The Moon is a complex, differentiated world, much like Earth, with its own distinct internal architecture, despite often being perceived as a simple, dusty sphere of rock. Early understanding was limited to distant observations, suggesting a monolithic interior.
Scientific investigation confirms that the Moon possesses a layered structure, including a substantial mantle. This layering is a direct consequence of the Moon’s formation and thermal evolution. Understanding this structure provides insights into the Moon’s history.
The Layered Structure of the Moon
The Moon is a differentiated body, meaning its interior is separated into layers of differing density and chemical composition. This structure arose early in its history, likely following the cooling and crystallization of a global lunar magma ocean. That early process, known as planetary differentiation, caused denser materials to sink toward the center while lighter materials floated upward.
The Moon’s internal architecture is defined by three primary layers: a small central core, a thick surrounding mantle, and a thin outer crust. This arrangement is conceptually similar to Earth’s, though the relative sizes and physical states of the layers vary significantly. The core, for instance, occupies a much smaller proportion of the Moon’s total diameter compared to the cores of other rocky planets.
The crust represents the outermost layer, formed by the lowest-density rocks that crystallized and accumulated on the surface of the cooling magma ocean. Beneath this is the expansive mantle, which accounts for the vast majority of the Moon’s volume.
Defining the Lunar Mantle
The lunar mantle is the most substantial layer, situated between the crust and the central core. It extends downward for approximately 1,338 kilometers from the base of the crust. This thickness is significantly greater than the crust’s average 50 to 60-kilometer thickness.
The composition of the mantle is inferred to be ultramafic rock, meaning it is rich in iron and magnesium silicates. Specifically, it is thought to consist predominantly of the minerals olivine and pyroxene. These minerals are denser than the feldspar-rich rocks that make up the crust, which explains why they sank during the early differentiation process.
Some analyses of lunar basalt samples suggest the mantle is not entirely uniform, showing evidence of chemical heterogeneity. Certain regions appear to contain high abundances of titanium, likely trapped within the mineral ilmenite, indicating variations in the source regions of ancient lunar lavas. The mantle is generally considered to be mostly solid, a contrast to the churning, convecting mantle of Earth.
While largely solid, the deep lunar mantle, directly above the core, is thought to be partially molten. Estimates suggest this lower zone could be 10 to 30 percent melted, existing as a soft layer of rock. This state is influenced by temperature and pressure, with heat from the core contributing to the partial melting.
The Moon’s smaller size allowed it to cool more rapidly than Earth, resulting in a mantle that is far more rigid and static. This rigidity is why the Moon lacks the global plate tectonic activity that defines Earth’s dynamic geology. The cooler, more solid nature of the lunar mantle dictates the Moon’s internal heat flow and lack of current volcanism.
Seismic Evidence of the Moon’s Interior
The existence and characteristics of the lunar mantle were confirmed through the Apollo Passive Seismic Experiment (PSE). Beginning with Apollo 11, astronauts deployed a network of highly sensitive seismometers that operated on the Moon’s surface until 1977. This network recorded vibrations from both internal and external sources.
These seismometers measured thousands of seismic events, including deep-seated tremors known as “Moonquakes” and impacts from meteoroids. Deep Moonquakes, which originate between 700 and 1,100 kilometers below the surface, show a monthly periodicity, strongly linked to the tidal stresses exerted by Earth’s gravity on the Moon.
To precisely map the interior, scientists also used man-made impacts, specifically the controlled crashes of the discarded Saturn V S-IVB booster stages and the Lunar Module ascent stages. The energy from these known impacts generated seismic waves that traveled through the Moon’s interior.
By analyzing the travel times and paths of compressional P-waves and shear S-waves, researchers could detect density boundaries. When a seismic wave encounters a boundary between layers of different materials, its speed changes, or it may be reflected or refracted. The abrupt changes in wave velocity at specific depths allowed scientists to pinpoint the boundaries between the crust, the mantle, and the core.
The data revealed that seismic waves travel with unusual efficiency through the Moon’s interior, leading to long reverberations unlike those observed on Earth. This high efficiency suggests that the Moon’s mantle is much less fractured and more homogeneous than Earth’s, solidifying the model of a rigid, internally differentiated structure.