Scientists Confirm the True Composition of the Moon's Interior
In a comprehensive study released in May 2023, scientists confirmed that the Moon’s inner core is actually a solid sphere with a density comparable to iron. This discovery aims to resolve a longstanding debate over whether the core of the Moon is solid or molten, paving the way for a clearer understanding of the Moon's formation—and, by extension, the early history of the Solar System.
"Our findings," said a research team led by astronomer Arthur Briaud from the French National Centre for Scientific Research, "challenge previous ideas about the evolution of the Moon’s magnetic field. The confirmation of a solid inner core supports a global mantle overturn scenario, offering crucial insights into the timeline of heavy bombardment during the first billion years of the Solar System."
To investigate the interior composition of celestial bodies like the Moon, scientists often rely on seismic data. Seismic waves, produced by quakes, travel through and bounce off different materials within planets or moons, helping researchers construct detailed maps of their internal structures.
The Apollo missions did collect seismic data from the Moon, but its resolution wasn't high enough to definitively determine whether the inner core was solid or liquid. It was clear there was a fluid outer core, but whether it enveloped a solid inner core remained uncertain. Models of both a solid inner core and a completely fluid one were consistent with the Apollo data.
To settle the question, Briaud and his team gathered data from a range of space missions and lunar laser-ranging experiments. These measurements helped them build a detailed profile of the Moon, including its deformation due to Earth's gravity, changes in its distance from Earth, and its density.
Artist's impression of different instruments measuring the properties of the Moon to reveal its core. (Géoazur/Nicolas Sarter)
Next, the researchers ran simulations using different core configurations to determine which best aligned with their observational data.
Their findings were compelling. The models that matched the known characteristics of the Moon suggested that dynamic overturn is occurring deep within the lunar mantle. This process involves denser material sinking toward the Moon’s core while lighter material rises, a phenomenon that has been proposed as a possible explanation for the concentration of certain elements in the Moon's volcanic regions. The team's results provide additional support for this hypothesis.
Furthermore, the study revealed that the Moon's core has surprising similarities to Earth's. Like our planet, the Moon appears to have a fluid outer core surrounding a solid inner core. According to the simulations, the outer core has a radius of approximately 362 kilometers (225 miles), while the inner core measures around 258 kilometers (160 miles) in radius—about 15 percent of the Moon's total radius.
The research also determined that the density of the Moon’s inner core is roughly 7,822 kilograms per cubic meter, very close to the density of pure iron.
Interestingly, a similar discovery was made back in 2011 by a team led by NASA Marshall planetary scientist Renee Weber. Using what were then advanced seismological methods to analyze Apollo data, Weber's team also found evidence suggesting a solid inner core with a radius of around 240 kilometers and a density close to 8,000 kilograms per cubic meter.
Briaud and his colleagues see their own results as a strong validation of these earlier findings, reinforcing the idea that the Moon has an Earth-like core. This new data has significant implications for our understanding of the Moon's evolution.
We already know that shortly after its formation, the Moon had a strong magnetic field, which began to wane around 3.2 billion years ago. The source of such a magnetic field lies in the movement and convection within a planet’s core, making the composition of the lunar core crucial to understanding why that magnetic field eventually disappeared.
With humanity aiming to return to the Moon soon, the hope is that future missions may bring more precise seismic measurements that could confirm these findings once and for all.
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