Scientists have finally confirmed what lies at the heart of our Moon: a solid ball of iron similar in density to Earth’s inner core. This discovery, published in Nature in May 2023, settles a long-standing debate and sheds light on the Moon’s history, potentially revealing clues about the early Solar System.
For decades, researchers have debated whether the Moon’s core was entirely liquid or contained a solid inner portion. The Apollo missions provided some seismic data from moonquakes, but its resolution was too low to definitively determine the core’s state. Models with both completely fluid and partially solid cores aligned equally well with the available information.
To get a clearer picture, astronomer Arthur Briaud and his team at France’s National Centre for Scientific Research combined data from various sources:
- Lunar Laser Ranging Experiments: These experiments track the precise distance between Earth and the Moon, revealing subtle variations caused by gravitational interactions.
- Space Missions: Data from different spacecraft missions contributed to a comprehensive understanding of the Moon’s physical characteristics.
- Modeling Techniques: Using sophisticated computer models, the researchers compared various core compositions against the collected observational data.
The models that best matched the observed data pointed towards several key findings:
- Deep Mantle Overturn: The models suggest active circulation deep within the Moon’s mantle— denser material sinking toward the center, while less dense material rises. This process, known as overturn, is thought to explain the distribution of certain elements in volcanic regions on the Moon. Briaud’s team adds further evidence to this theory.
- Earth-like Structure: Most convincingly, the data supported a core structure remarkably similar to Earth’s: a fluid outer core surrounding a solid inner core.
The estimated radius of the outer core is about 362 kilometers (225 miles), while the inner core measures approximately 258 kilometers (160 miles)—representing about 15% of the Moon’s total radius. The density of this iron-rich inner core is around 7,822 kilograms per cubic meter, close to that of Earth’s inner core.
This discovery echoes earlier research in 2011 by NASA scientist Renee Weber, who also found evidence of a solid inner core using advanced seismological analysis of Apollo data. Briaud’s team further strengthens this conclusion.
The implications for understanding the Moon are profound: the presence of a solid inner core has significant implications for how and why the Moon lost its magnetic field billions of years ago. Magnetic fields arise from the movement and convection of molten material in the core, so the makeup of the lunar core is crucial to unlocking this mystery.
Future lunar missions promise more direct seismic investigations that will definitively confirm these findings.
