For many years prior to spacecraft landing on the moon, astronomers have been perplexed by its structure. In the first half of the 20th century, there was a discussion regarding whether the moon was just a rocky object similar to Martian moons, or whether it had a more complex inner geology. Now, the debate has finally been settled.
Researchers developed models of the Moon and found it has a fluid outer core and a solid inner core, similar to Earth’s. The inner core is made up of a metal with a density similar to that of iron and measures about 500 kilometers in diameter, which is about 15% of the Moon’s total diameter. The research was published in the journal Nature.
“Our results question the evolution of the Moon magnetic field thanks to its demonstration of the existence of the inner core and 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,” the researchers wrote in their paper.
The Moon’s surprising structure
Looking at the inner composition of objects in the Solar System is best done through seismic data. This is because of the way seismic waves (which are essentially) acoustic waves propagate differently through different materials. So these waves generated by quakes move through the inside of a planet (or a moon) differently, depending on what they’re passing through. We have lunar seismic data from the Apollo mission, but the resolution is not sufficiently high to determine the core’s state.
Seeking a way forward, a group of French researchers gathered data from space missions and lunar laser ranging experiments and created a profile of the lunar features. These include its density and variation in its distance from Earth, among other things. They then did modeling with core types to find which matched best with observational data.
This allowed the team to make several findings. First, they found that denser material inside the Moon falls towards the center, and less dense material rises upwards. This has been previously suggested by researchers as a way of explaining the presence of certain elements in volcanic regions of the Moon. But that’s only one part of the picture.
The researchers also found that the lunar core is very similar to that of the Earth, with an outer fluid layer and a solid inner core. The core is about 500 kilometers in diameter, or 15% of the Moon’s width. Curiously, back in 2011, a team led by NASA experts found a similar result using seismological techniques on Apollo data to study the lunar core.
While the finding brings some light to the Moon’s structure, there are still many more mysteries that remain unsolved, such as what happened to its magnetic field. Not long after it formed, the Moon had a powerful magnetic field, which then began to decline about 3.2 billion years ago. The field was generated by motion and convection in the core.
More data about the Moon could soon become available for research, as government agencies and private space companies are working on their new lunar missions. NASA hopes to send four astronauts on the Artemis II mission in 2024 to fly near the moon, while companies such as ispace, Astrobotic and Intuitive Machines also have their own plans.