Lunar CoreEdit
The Moon harbors a small but chemically and dynamically important core beneath its crust. Our understanding of the lunar core comes from a combination of indirect measurements—primarily seismology, gravity, and tidal response—and from models of planetary differentiation that describe how a body with the Moon’s size and composition would evolve as it cooled after formation. While the details are still debated, the consensus is that the Moon possesses an iron-rich region at its center that, at least early in its history, hosted enough heat and compositional buoyancy to drive molten behavior and, possibly, a transient magnetic dynamo. The core’s size and state have important implications for the Moon’s thermal evolution, its early magnetism, and how the Moon interacted with Earth over billions of years.
Structure and composition
The standard view is that the Moon has a relatively small iron-rich core, with a liquid outer portion and, in some models, a solid inner core. Estimates of the outer core’s radius generally fall in the range of a few hundred kilometers, with typical numbers around 300–400 km. If a solid inner core exists, its radius would likely be on the order of tens to a couple hundred kilometers. These figures are inherently uncertain because they are inferred from limited data and rely on models of how seismic waves travel through an object with a complex internal structure. The core is thought to be primarily iron-nickel alloy, with light elements such as sulfur or silicon helping to make the density profile consistent with the Moon’s overall mass distribution. For many geophysicists, the simplest working picture is a liquid outer core surrounded by a possible solid inner core, embedded in a partially molten or highly cooled mantle and crust.
The composition and state of the lunar core have been constrained by several lines of evidence. Seismology from the Apollo-era seismometers revealed Moonquakes and the way seismic waves propagate through the interior, providing constraints on core size and state. Gravitational data from orbital missions—especially high-resolution gravity models—refine estimates of density contrasts inside the Moon. Tidal and rotational (libration) measurements also inform the interior’s response to Earth’s gravity, helping pin down how rigid or fluid the deep interior behaves. See Seismology and GRAIL for related methods and findings. The overall picture remains that the Moon’s core is smaller than Earth’s and behaves differently under cooling conditions over geologic time.
Evidence for a liquid outer core comes from how the Moon responds to tidal forcing and from seismic reflections and refractions that indicate a distinct, low-velocity region at depth. The potential existence of a solid inner core is more controversial; some models require it to explain certain seismic signatures or the Moon’s historical magnetic record, while others show that the data could be explained without a solid inner core. The debate highlights how limited, uneven data coverage on the Moon constrains definitive conclusions. See Core and Planetary differentiation for broader context.
Evidence from geophysics
Seismology, gravity, and paleomagnetism together provide the strongest constraints on the core. Apollo-era seismometers detected different classes of Moonquakes, including deep moonquakes, that illuminate how seismic waves traverse the interior. Analyses of these waves, in combination with gravity-derived density profiles, yield a model in which a metallic core exists and is surrounded by a mantle whose properties influence how waves propagate. The seismic data are sparse compared with Earth, so alternative interpretations remain plausible, especially for inner-core size and the exact composition of the core material. See ALSEP and Apollo program for historical context on how the data were obtained, and Seismology for methodological background.
Gravity data from orbital missions complement the seismic results by mapping the Moon’s mass distribution. The Gravity Recovery and Interior Laboratory (GRAIL) mission provided high-resolution gravity fields that help constrain core size and the density contrast between the core and the surrounding mantle. These gravity models support the existence of a dense, metal-rich core but do not on their own determine whether an inner solid core is present. See GRAIL for mission details and Differentiation (geology) for how a body like the Moon differentiates into crust, mantle, and core.
The Moon’s magnetic record adds another dimension to core studies. Ancient lunar rocks show remanent magnetization that implies the Moon once hosted a magnetic field, likely generated by a dynamo in its interior. The current Moon is magnetically quiet, so any dynamo must have occurred in the past. Interpreting paleomagnetic signals depends on understanding the core’s convection and stability, which in turn depend on temperature, composition, and the presence or absence of a solid inner core. See Lunar magnetism and Lunar dynamo for related discussions and Magnetic field for a general treatment of planetary magnetism.
Dynamo history and magnetism
Several lines of evidence point to a dynamo that operated early in the Moon’s history. Paleomagnetic signatures in ancient lunar rocks indicate that a magnetic field likely existed several billion years ago, perhaps peaking in strength during the period when the Moon was still cooling rapidly after formation. If a liquid outer core existed and convection continued, it could have sustained a dynamo. As the Moon cooled and its interior grew more rigid, the dynamo likely shut down. The exact timing, duration, and mechanics of the lunar dynamo remain topics of active study, with debates focusing on whether heat-driven convection, compositional convection from core crystallization, or a combination of factors drove the ancient field. See Lunar magnetism for paleomagnetic evidence and Dynamo or Lunar dynamo for models of the magnetic field generation.
Controversies about the dynamo are common in planetary science, partly because the magnetic record in rocks depends on how they formed and later cooled. Some researchers argue for a relatively long-lived dynamo tied to core convection, while others favor early, short-lived dynamo activity or alternative explanations for the magnetic signatures observed in lunar samples. Proponents of different scenarios emphasize the limitations of the available data and the need for new measurements from future missions to resolve the timing and vigor of the Moon’s past dynamo. See Magnetic field as a general reference on planetary magnetism and Planetary differentiation for how internal structure influences magnetic history.
Formation and evolution
The Moon’s core formation is tied to its origin and early thermal evolution. Current leading theories hold that the Moon formed from debris produced by a giant impact on the proto-Earth, followed by rapid accretion, differentiation, and partial melting. In such a scenario, heat from accretion, short-lived radiogenic isotopes, and iron segregation would drive the development of a metallic core early in lunar history. The subsequent cooling of the Moon would promote the solidification of the inner layers, the possible crystallization of an inner core, and changes in the core's convection pattern that would influence magnetic behavior and tidal responses over time. See Moon for the object as a whole, Giant impact hypothesis for origin theories, and Differentiation (geology) for the process by which a body separates into distinct chemical layers.
Understanding how quickly the Moon cooled and how the core crystallized has implications for the Moon’s thermal evolution, volcanic history, and interaction with Earth. The size and state of the core influence the Moon’s tidal response and its gravitational and rotational dynamics, helping to explain how the Moon has come to occupy its current orbital and rotational state. Ongoing research integrates seismic, gravity, and magnetism data with laboratory and computational models of metals at high pressure and temperature to refine the core’s properties. See Planetary differentiation and Seismology for methodological contexts.
Implications for exploration and science policy
Advances in constraining the lunar core illustrate why investing in planetary interior science matters for understanding small bodies in the solar system. A better grasp of the Moon’s interior informs not only its geological history but also the design of future exploration missions, in-situ resource utilization, and the planning of instruments to capture deeper interior signals. The discussion around the core highlights how limited data can leave room for multiple viable models, encouraging continued investment in geophysical measurements, drilling concepts, and sample-return opportunities. See LRO and GRAIL for mission contexts, and ALSEP for historical methods that shaped current practice.