The mystery has finally been solved: the Moon is definitely not made
of green cheese.
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.