Well, the verdict is in. The Moon is not made of green cheese. A detailed study published in May 2023 revealed that the Moon’s inner core is actually a solid sphere with a density similar to iron. The researchers hope that this will help resolve the long-standing debate over whether the Moon’s inner core is solid or molten, and lead to a more accurate understanding of the Moon’s history and therefore the Solar System.

“Our results question the evolution of the Moon’s magnetic field by indicating the existence of an inner core and support the scenario of global mantle overturning,” writes the team, led by astronomer Arthur Briot of the National Center for Scientific Research in France. “They provide important insights into the chronology of the Moon’s bombardment during the first billion years of the solar system’s existence.”

Studying the internal composition of objects in the Solar System is most effectively accomplished with the help of seismic data. The way acoustic waves produced by earthquakes bounce off and pass through material inside a planet or moon can help scientists create a detailed map of the object’s interior.

We have lunar seismic data collected by the Apollo missions, but the resolution is too low to accurately determine the state of the inner core. We know there is a liquid outer core, but what it consists of is still questionable. The solid inner core and the all-liquid core models agree equally well with the Apollo data.

To find out for sure, Brio and his colleagues collected data from space missions and lunar laser-probing experiments to profile several properties of the moon, including the degree of deformation due to the moon’s gravitational interaction with Earth, its change in distance from Earth, and its density.

They came to some interesting conclusions. First, the models most similar to what we know about the Moon describe active overturning deep in the lunar mantle. This means that denser material at the center of the Moon sinks toward the center, while less dense material rises. This type of activity has long been suggested as a way to explain the presence of certain elements in the Moon’s volcanic regions. The team’s research adds another argument in its favor.

They found that the Moon’s core is very similar to Earth’s, with a liquid outer layer and a solid inner core. According to their simulations, the outer core has a radius of about 362 kilometers (225 miles), while the inner core has a radius of about 258 kilometers (160 miles). That’s about 15 percent of the Moon’s entire radius.

The team found that the inner core also has a density of about 7.822 kilograms per cubic metre, very close to the density of iron.

Interestingly, in 2011 a team led by NASA planetary scientist Marshall Renee Weber obtained a similar result using then-state-of-the-art seismological methods on Apollo data to study the lunar core. They found evidence of a solid inner core with a radius of about 240 kilometers and a density of about 8,000 kilograms per cubic meter.

Brio and his team say their results confirm previous findings and provide strong evidence that the moon’s core is similar to Earth’s. And this has interesting implications for the moon’s evolution.

We know that the Moon had a strong magnetic field shortly after its formation, and that this field began to wane about 3.2 billion years ago. Such a magnetic field is generated by movement and convection in the core, so what the Moon’s core is made of has a lot to do with how and why the magnetic field disappeared.

Given humanity’s hopes of returning to the Moon in a relatively short time frame, we may not have to wait long for seismic confirmation of these findings. The results of the research were published in the journal Nature.