The Moon’s surface is extensively mapped, but its internal structure remains an active area of investigation. Understanding what lies beneath the lunar surface is a direct path to uncovering the Moon’s history and formation, particularly how it evolved from a molten body billions of years ago. Like Earth, the Moon is a differentiated world, meaning its materials separated by density, resulting in distinct layers. Scientists utilize indirect methods to peer into this deep interior. The composition and physical state of its innermost region offer clues about the early solar system.
Mapping the Moon’s Interior Layers
The Moon’s interior is organized into three principal layers: the crust, the mantle, and the core. The crust exhibits a notable asymmetry, being thinner on the side facing Earth and significantly thicker on the far side. Geophysical techniques estimate the crust’s average thickness to be approximately 60 kilometers on the near side, extending up to 100 kilometers on the far side.
Below the crust is the mantle, which accounts for the largest volume of the Moon’s interior, extending roughly 1,300 kilometers toward the center. This region is mostly solid, composed primarily of dense silicate minerals such as olivine and pyroxene. Evidence suggests a partially molten layer exists in the lower mantle, just above the core boundary. This layered structure resulted from a global “magma ocean” that cooled and crystallized early in the Moon’s history.
The Lunar Core: Composition and Physical State
The lunar core makes up only about 20% of the Moon’s total diameter. Its composition is predominantly metallic iron, likely alloyed with a small percentage of lighter elements such as sulfur and nickel. These elements lower the iron’s melting point and density.
The core is structured into two distinct parts. There is a solid, iron-rich inner core with an estimated radius of about 240 to 258 kilometers. This solid sphere is encased by a fluid, liquid outer core composed of molten iron and light elements. The liquid outer core extends the total core radius to approximately 360 to 380 kilometers.
The temperature within the core is estimated to be high, in the range of 1,330 to 1,430 degrees Celsius (1600–1700 K). This heat, combined with pressure, maintains the outer shell in its molten state while the innermost material remains solid. The estimated density of the inner core is about 7.8 grams per cubic centimeter, consistent with a composition dominated by iron.
Uncovering the Core: Scientific Methods
Scientists rely on geophysical measurements to infer the core’s structure and state. Data came primarily from the Passive Seismic Experiment deployed by Apollo astronauts between 1969 and 1972. These seismometers recorded “moonquakes,” which are seismic events caused by tidal stresses or meteorite impacts.
By analyzing how seismic waves travel through the Moon’s interior, scientists map the internal boundaries. P-waves and S-waves behave differently when encountering solid versus liquid material. The observation that S-waves are absorbed or blocked at a certain depth provided direct evidence for the existence of a fluid outer core.
Modern re-analysis of the Apollo data has refined the picture, confirming the presence of both a solid inner and a liquid outer core. These seismic findings are corroborated by data from the Gravity Recovery and Interior Laboratory (GRAIL) mission, which mapped the Moon’s gravitational field with high precision. Subtle variations in the gravitational field reveal the mass distribution beneath the surface, placing constraints on the size and density of the core. Lunar Laser Ranging experiments, which measure the Moon’s slight tidal flexing, also confirm the liquid nature of the outer core boundary.