Anyway, the decision has been announced. 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 similar in density to iron. Researchers hope 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 history of the Moon and, by extension, the Solar System.

The team, led by astronomer Arthur Briot of the National Center for Scientific Research in France, writes: “Our results question the evolution of the Moon’s magnetic field by demonstrating the existence of an inner core and support the scenario of global mantle overthrow.” “It provides important information about the chronology of the bombardment of the Moon during the first billion years of the solar system’s existence.”

The study of the internal composition of objects in the Solar System is most effectively carried out with the help of seismic data. The way acoustic waves produced by earthquakes pass and bounce 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 that there is a liquid outer core, but what this involves is still questionable. The solid inner core and completely 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 sounding experiments to profile various features of the moon. These include the degree of deformation of the Moon due to its gravitational interaction with the Earth, its change in distance from the Earth, and its density.

They then ran simulations with different types of nuclei to determine which ones best fit the observations. They came to some interesting conclusions. First, the models most similar to what we know about the Moon describe active overturning deep within the lunar mantle.

This means that the denser material in the middle of the Moon sinks towards the center, while the 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 that of the Earth, with a liquid outer layer and a solid inner core. According to their simulations, the radius of the outer core is about 362 kilometers (225 miles), while the radius of the inner core is about 258 kilometers (160 miles). This is about 15 percent of the Moon’s entire radius. The team found that the inner core also has a density of approximately 7,822 kilograms per cubic meter. This is 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 Moon’s 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 metre.

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 consequences for the evolution of the Moon.

We know that the Moon had a strong magnetic field shortly after its formation, and that this field began to decrease about 3.2 billion years ago. Such a magnetic field is produced 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 disappears.

Given humanity’s hope 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.