It's Official: Scientists Have Confirmed What's Inside The Moon

Earth's Moon. (Thomas Campbell/NASA)

The verdict has been rendered. After all, the Moon isn't composed of green cheese.

According to a comprehensive study released in May 2023, the Moon's inner core is actually a solid ball with a density comparable to that of iron. Researchers believe that this will assist resolve a protracted dispute on the solidity or moltenness of the Moon's inner heart and result in a more accurate knowledge of the Moon's history, and consequently, that of the Solar System.

A team headed by astronomer Arthur Briaud of the French National Centre for Scientific Research in France wrote, "Our results support a global mantle overturn scenario that brings substantial insights on the timeline of the lunar bombardment in the first billion years of the Solar System and question the evolution of the Moon magnetic field thanks to its demonstration of the existence of the inner core."

Seismic data is the most efficient way to probe the composition of objects in the Solar System from the inside. Scientists can map the inside of a planet or moon in great detail by using the way acoustic waves produced by earthquakes travel through and reflect off of materials inside the object.

Although the Apollo program has provided us with lunar seismic data, the resolution is insufficient to precisely assess the condition of the inner core. Although we are aware that the outer core is fluid, it is still unclear what exactly it consists of. The Apollo results are equally compatible with models of a solid inner core and a completely fluid core.

Briaud and his associates gathered information from satellite flights and lunar laser-ranging studies to create a profile of several lunar features in an effort to definitively determine it. These include its density, the change in its distance from Earth, and the extent of deformation caused by its gravitational contact with Earth.

Artist's impression of different instruments measuring the properties of the Moon to reveal its core. (Géoazur/Nicolas Sarter)

To determine which core type best fit the observational data, they then performed modeling with a variety of core types.

They came to a number of intriguing conclusions. First, active overturn deep inside the lunar mantle is described by the models that most nearly matched our understanding of the Moon.
Accordingly, less dense stuff rises higher while denser material falls into the Moon's center. It has long been suggested that this activity explains why some elements are found in the Moon's volcanic areas. Another point is added to the "for" list of evidence by the team's research.

They also discovered that the lunar core, which has a solid inner core and an outer fluid layer, is quite similar to that of Earth. Their simulation indicates that the inner core has a radius of roughly 258 kilometers (160 miles), whereas the outer core has a radius of roughly 362 kilometers (225 miles). That represents roughly 15% of the Moon's total radius.

The Moon as captured by Andrew McCarthy and Connor Matherne.

The scientists also discovered that the inner core has a density of roughly 7,822 kg/cm³. That is extremely similar to iron's density.

Interestingly, a team led by NASA Marshall planetary scientist Renee Weber discovered a similar outcome in 2011 when studying the lunar core using Apollo data and then-state-of-the-art seismological tools. They discovered proof of a solid inner core with a density of roughly 8,000 kilograms per cubic meter and a radius of almost 240 kilometers.

According to Briaud and his team, these findings support those earlier conclusions and provide a compelling argument for a lunar core that resembles Earth. Additionally, this has some intriguing ramifications for the evolution of the Moon.

Perhaps we won't have to wait long for seismic confirmation of these results, considering humanity's aspirations to return to the Moon in a short period of time.

The research has been published in Nature.