The Moon’s heart revealed for the first time

Fifty years after Apollo 11 paved the way for the first surveys of the Moon, a team of scientists from CNRS, Université Côte d’Azur, the Côte d’Azur Observatory, and the IMCCE (Paris Observatory – PSL, Sorbonne Université) has shed light on part of its internal structure that had until now remained a mystery: the Moon has a solid core, like the Earth. In addition to this discovery, they also present evidence that explains the presence of iron-rich materials in the lunar crust. Their work was published in Nature on May 3, 2023.

Some twenty years after the identification of a fluid outer core, the team has revealed the existence of a solid inner core about 500 km in diameter, which is about 15% of the total size of the Moon. It is made up of a metal whose density is close to that of iron. Various methods, related in particular to the rotation of the Moon, had already enabled the fluid outer core to be clearly identified. However, the solid core remained undetectable, due to its small size. Its existence has now been demonstrated using INPOP ephemeris and data from various space missions and from lunar laser ranging.

Artist’s impression of the lunar interior. From the surface down to the centre: a thin crust, a very thick mantle, a low-viscosity zone at the core-mantle boundary, a fluid outer core, and a solid inner core. Crédits Géoazur/N. Sarter

In addition to this major discovery, various evidence appears to support the hypothesis of movement of material within the mantle, the intermediate layer between the core and the crust, during the Moon’s evolution. This is known as the “lunar mantle overturn”, and it helps explain the presence of iron-rich elements at the Moon’s surface. How did this process take place? Material could have risen to the surface, producing volcanic rocks that were deposited in the lunar crust. Subsequently, the materials that were too dense compared to the surrounding crustal material sank back down to the core-mantle boundary.

This work provides an important contribution to our understanding of the history of the Solar System and of certain events such as the disappearance of the lunar magnetic field, which was originally a hundred times stronger than that of the Earth today, and is now almost non-existent.


Arthur Briaud, Clément Ganino, Agnès Fienga, Anthony Mémin & Nicolas Rambaux, “The lunar solid inner core and the mantle overturn”, Nature, May 3, 2023.


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