Just like the Earth, the Moon has a solid, iron inner core in addition to its fluid outer core. This is the conclusion of a study by researchers from France, who created various models of the lunar interior. Understanding the structure of the Moon will shine more light on how Earth’s natural satellite was formed — and whether it once supported a magnetic field.

Back in 2011, planetary scientists at NASA analysed seismic data recorded on the lunar surface by astronauts as part of the Apollo programme.

The so-called “Apollo Passive Seismic Experiment” employed four seismometers, deployed on the Moon between 1969 and 1972, which remained operating until late in 1977.

In the same way that geologists use earthquakes to probe the interior of our planet, measuring “moonquakes” provides a window into the structure of the Moon.

This is because the path and speed of seismic waves can change based on the density of the materials they travel through.

At the same time, certain types of waves are not able to travel through liquids — meaning, for example, that the presence of the liquid core in the Earth and the Moon creates “shadow” regions where shear waves from a given source point cannot reach.

NASA’s analysis of the Apollo seismic data indicated that the Moon possessed a solid, iron-rich inner core — with a radius of nearly 150 miles — surrounded by a fluid outer core with a radius of around 205 miles.

Unlike in the Earth, the researchers said, the Moon appears to also possess a boundary layer between the core and the mantle — one with a radius of nearly 300 miles — in which the material is partially molten.

The team also concluded that the lunar core contains a small percentage of light elements like sulphur.

This mirrored the findings of similar studies at the time that argued sulphur and oxygen might be found in a layer around the Earth’s core.

In the new study, earth scientist Dr Arthur Briaud of the Université Côte d’Azur, France, and his colleagues performed a similar analysis to the NASA study, but in reverse

The team experimented with various models and simulations of different possible inner structures for the Moon, comparing these with observations of how the Moon deforms in response to tidal (that is, gravitational) forces from the Earth and the Sun.

The findings, the team said, yielded similar results to that of the NASA study.

Specifically, the best-fit model predicted an inner core of radius around 160 miles, with a density of around 7,822 kg per cubic metre — consistent with an iron composition.

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The team said: “Our results question the evolution of the Moon magnetic field, thanks to its demonstration of the existence of the inner core.”

Furthermore, they added, the findings support a “global mantle overturn scenario”, which explains how certain iron-rich elements have made their way from the lunar interior to the surface.

They explain: “Material could have risen to the surface, producing volcanic rocks that were deposited in the lunar crust. Subsequently, the materials that were too dense compared to the surrounding crustal material sank back down to the core-mantle boundary.”

The full findings of the study were published in the journal Nature.

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