What’s Inside the Moon

​Astronomers have recently confirmed that the Moon has a solid inner core with a density like iron, surrounded by a liquid outer core, making its internal structure somewhat analogous to Earth’s but on a smaller scale. This discovery was achieved using seismic data from lunar missions, lunar laser-ranging, and detailed models that combine various sources of observational data.
 
The Moon’s inner core is solid, with a density of around 7,822 kilograms per cubic meter, comparable to that of iron. The solid inner core is thought to have a radius of about 258 kilometers, while a liquid outer core extends outwards to about 362 kilometers. Above the core, the Moon’s interior shows signs of mantle overturn, a process where denser material sinks while lighter elements rise, influencing the Moon’s volcanic regions and surface composition. The near side (facing Earth) and far side of the Moon differ internally, with the near side’s mantle being warmer and less rigid, likely due to a higher abundance of radioactive elements, which also shaped the Moon’s magnetic field history.
 
A denser lunar mantle has important implications for the Moon’s geological evolution, surface features, and internal dynamics. Higher mantle density suggests the presence of heavy minerals such as olivine and pyroxene, which are iron- and magnesium-rich rocks like those found in Earth’s mantle. A dense mantle enhances the contrast at the crust-mantle boundary, which influences the way energy and materials move within the Moon, affecting heat flow, volcanic activity, and the creation of impact basins. The denser material in the Moon’s mantle means large impact events are more likely to excavate and expose deep mantle rocks when the crust is thin, especially in major lunar basins; evidence of olivine-rich mantle rocks has been seen in some basin regions. A mantle with higher density leads to a stiffer, less convective interior than Earth, explaining why lunar volcanic activity was less widespread and ceased earlier than on Earth.
 
The Moon’s mantle density and composition provide clues about its origin, differentiation, and the role of early magma ocean processes that separated lighter crustal rocks from heavier mantle materials. Geophysical data, such as from the GRAIL mission, help refine mantle density estimates and show that crustal thickness and mantle density together constrain the Moon’s internal structure and its geochemical evolution.
 
This robust confirmation of a solid, iron-like inner core, a fluid outer core, and complex mantle layering provides a new foundation for understanding the Moon’s geological past, its ceased magnetic field, and differences in surface features between its two hemispheres.
 
References
Spudis, P.D. (2014, April 4). The Moon’s Mantle Muddle. Smithsonian Magazine.
 
Starr, M. (2025, June 20). It’s Official: Scientists Confirmed What’s Inside Our Moon. Science Alert.
 
Wieczorek, M.A., Neumann, G.A., Nimmo, F., Kiefer, W.S., Taylor, G.J., Melosh, H.J., et al. (2012, December 5). 339(6120):671-675. The Crust of the Moon as Seen by GRAIL. Science.