4.5 Billion-Year Mystery Of What's Inside The Moon Is Settled, It's Not Cheese

by Aaron Leong — Friday, November 14, 2025, 10:23 AM EDT

The Moon is not, in fact, a giant cosmic wheel of Gouda. In a landmark paper published in Nature, researchers from the French National Center of Scientific Research delivered a hard truth: the Moon is actually packing a solid inner core of material with a density suspiciously similar to iron. Rather than a massive cheese ball, we're dealing instead with a celestial cannonball.

This discovery basically settles a 4.5-billion-year-old structural mystery. As it turns out, our trusty satellite is built much like its Earthly parent, boasting Earth-like layering. That means a fluid outer core (coming in at 225 miles or 362 kilometers) wrapped around a distinct, solid inner heart—a tiny, metallic bullseye nestled 160 mi (258 km) deep. With an estimated density of 7,822 kg per cubic meter, the core could very well be made of pure iron.

moon interior1 — 2012 interpretation of the basic structure of the moon’s interior, with outer crust, mantle and outer and inner cores. The new study has the moon’s inner core as solid, with a density similar to iron, with a diameter roughly 300 miles, close to previous estimates. (Credit: kelvinsong / Wikimedia Commons)

To crack this lunar coconut, the team didn't just rely on low-res seismic wave records from decades-old Apollo captures. Instead, they compiled data from lunar laser-ranging and space missions to build various possible lunar profiles. They then ran models using various core types to determine which best matched the captured data. The solid iron inner core model won the fashion contest, perfectly matching the observed data.

It also turns out that interior structure has always been this static. What the research team found strongly supports a global mantle overturn scenario. Imagine the Moon’s mantle as a lava lamp in extreme slo-mo, where the denser, heavier material constantly decides it’s time for a nap and sinks toward the center, while the lighter, less dense bits float up to the surface. This geological activity may help explain certain volcanic activity observed on the Moon’s surface.

Perhaps the biggest payoff of this heavy-metal revelation is the solution it offers to the Moon’s long-lost core activity. Scientists have known for a while that early in its history, the Moon had a powerful magnetic field, which then promptly faded into nothingness some 3.2 billion years ago. Since magnetic fields are generated by the convection and motion of molten material in a planet's core, figuring out the precise architecture of the Moon’s heart is the key to understanding why its dynamo eventually gave up the ghost.

With humanity set to set foot on the Moon again, we might be able to obtain new data to truly listen to the Moon's proverbial heartbeat. Until then, we can at least sleep soundly knowing our faithful companion is structurally sound, highly metallic, and 100% dairy-free.