New research (May 2023) combines space mission data and lunar laser‑ranging with modeling to conclude the Moon has a fluid outer core and a solid, iron‑like inner core. Best‑fit models give an outer core radius of ~362 km and an inner core radius of ~258 km, with an inner‑core density near 7,822 kg/m³. The results support mantle overturn and help explain the decline of the Moon's ancient magnetic field ~3.2 billion years ago. Future lunar seismology could directly confirm these findings.
Moon's Heart Revealed: New Study Confirms a Solid, Iron‑Like Inner Core
New research (May 2023) combines space mission data and lunar laser‑ranging with modeling to conclude the Moon has a fluid outer core and a solid, iron‑like inner core. Best‑fit models give an outer core radius of ~362 km and an inner core radius of ~258 km, with an inner‑core density near 7,822 kg/m³. The results support mantle overturn and help explain the decline of the Moon's ancient magnetic field ~3.2 billion years ago. Future lunar seismology could directly confirm these findings.
08:16 PM, 11/14/2025Science
Scientists confirm the Moon has a solid, iron‑like inner core
A comprehensive study published in May 2023 finds that the Moon contains a compact, solid inner core with a density close to that of iron. The research, led by Arthur Briaud of the French National Centre for Scientific Research (CNRS), combines space‑based measurements and lunar laser‑ranging data with geophysical modeling to resolve a long‑standing question: is the Moon's center molten or solid?
“Our results... support a global mantle overturn scenario and bring substantial insights on the timeline of lunar bombardment in the first billion years of the Solar System,” the team writes, highlighting implications for the Moon's magnetic history.
Seismic data from the Apollo missions provide a crucial baseline but lack the resolution to distinguish between a fully fluid core and a two‑layer core (fluid outer core plus solid inner core). To overcome that limitation, Briaud and colleagues incorporated orbital tracking, measurements of the Moon's tidal deformation, variations in lunar distance, and bulk density constraints into forward models of lunar interior structure. By testing many core configurations, they identified the models that best match the full set of observations.
Their best‑fit models indicate an Earth‑like internal structure: a fluid outer core and a compact solid inner core. Estimated dimensions are an outer core radius of about 362 kilometers (225 miles) and a solid inner core radius of about 258 kilometers (160 miles) — roughly 15% of the Moon's total radius. The inner core's density is estimated at approximately 7,822 kilograms per cubic meter, close to the density of iron.
These findings echo a 2011 reanalysis of Apollo seismic data by a team led by NASA Marshall scientist Renee Weber, which reported a solid inner core near 240 kilometers in radius with a density around 8,000 kg/m³. Briaud's study strengthens that earlier result by bringing independent orbital and ranging constraints to bear.
Why this matters: the Moon once hosted a much stronger magnetic field that began to decline around 3.2 billion years ago. Planetary magnetic fields are generated by motions in metallic cores (a dynamo), so the presence, size and composition of a solid inner core and a fluid outer core are central to understanding how — and when — the lunar dynamo operated and shut down. The models also favor deep mantle overturn, a process in which dense material sinks and lighter material rises, helping explain chemical anomalies in some lunar volcanic regions.
Looking ahead, renewed plans to deploy new instruments on the lunar surface mean that future seismic measurements could directly test these model‑based conclusions. For now, the study — published in the journal Nature in May 2023 — provides strong evidence that the Moon's interior is layered much like Earth's, with important consequences for its thermal and magnetic evolution.