Well, the verdict is in. The moon is not made of green cheese after all.
A detailed study published in May 2023 found that the interior of the Moon is, in fact, a solid sphere with a density similar to that of iron. This, the researchers hope, will help resolve the long debate about whether the inner heart of the Moon is solid or molten, and will lead to a more accurate understanding of the history of the Moon – and, by extension, that of the Solar System.
“Our findings,” wrote the group led by astronomer Arthur Briaud of the French National Center for Scientific Research in France, “question the evolution of the Moon’s magnetic field due to its demonstration of the presence of an inner core and support the collapse of the Earth. The phenomenon brings great insight into the time of the moon’s bombardment in the first billions of Solar years System.
Investigating the composition of the interior of the Solar System is done primarily through seismic data. How acoustic waves generated by earthquakes travel and reflect off the interior of the Earth or moon can help scientists create a detailed map of the interior of an object.
We happen to have lunar seismic data collected by the Apollo mission, but its resolution is too low to accurately determine the conditions in the core. We know that there is an external water source, but the ingredients remain in dispute. Models of the solid interior core and the full fluid core work equally well with the Apollo data.
To see it completely, Briaud and his colleagues collected data from mission sites and lunar laser-ranging experiments to compile the shape of various moons. These include the degree of its evolution and its interaction with Earth, the variation in its distance from Earth, and its mass.
An expert’s view of the various instruments that measure the properties of the Moon to reveal its core. (Géoazur/Nicolas Sarter)
Next, they modeled with different intermediate values to find the best fit for the observational data.
They made many interesting discoveries. First, the models that most closely resemble what we know about the Moon describe strong fluctuations within the lunar mantle.
This means that the green matter inside the Moon falls in the middle, and the less dense matter rises up. This phenomenon has often been suggested as a way to explain the presence of other elements in the craters of the Moon. Team research adds another point to “the” tally of evidence.
And they found that the moon’s core is almost like Earth’s – with a liquid outer core and a solid inner core. According to their criteria, the outer core has a radius of 360 kilometers (225 miles), and the inner core has a radius of 250 kilometers (160 miles). That is about 15 percent of the total radius of the Moon.
The inner core, the team found, also has a density of 7,822 kilograms per cubic meter. It is very close to the abundance of iron.
Fortunately, in 2011 a team led by NASA Marshall scientist Renee Weber found similar results using the former seismological method on Apollo data to study the lunar core. They found evidence of a solid core with a radius of about 240 kilometers, and a density of about 8,000 kilograms per cubic meter.
Their results, Briaud and his team say, are a support of previous findings, and form a strong case for an Earth-like central moon. And this has some interesting implications for the evolution of the Moon.
We know that not long after formation, the Moon had a magnetic field, which began to decline 3.2 billion years ago. Such magnets are created by movement and convection within the core, so what the core of the moon is made of has to do with the effect and why the magnet disappeared.
Considering humanity’s hopes of returning to the Moon soon, we probably won’t have long to wait for seismic testing of these resources.
Research published in Nature.
A version of this article was originally published in May 2023.