It’s official: the Moon isn’t made of green cheese. A comprehensive study published in May 2023 revealed that the inner core of the Moon is actually a solid sphere with a density akin to iron. This discovery is anticipated to end the long-standing debate on whether the Moon’s inner core is solid or molten and lead to a deeper understanding of the Moon’s origins, along with the broader history of our Solar System.
“Our results,” stated a team led by astronomer Arthur Briaud of the French National Centre for Scientific Research in France, “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 best way to examine the inner composition of celestial bodies is through seismic data. The movement of acoustic waves caused by quakes, as they travel through and bounce off materials inside a planet or moon, allows scientists to create detailed maps of these objects’ interiors.
Thanks to the Apollo mission, we have seismic data from the Moon. However, the resolution isn’t high enough to determine the exact state of the inner core. The data confirms a fluid outer core, but whether it surrounds a solid or entirely fluid inner core has been a subject of debate.
To resolve the matter, Briaud and his team gathered data from space missions and lunar laser ranging experiments to create a detailed profile of the Moon, including how much it deforms due to Earth’s gravity, its changing distance from Earth, and its overall density.
They then conducted modeling with various core types to see which best matched the observed data. Their findings suggest the Moon’s inner core behaves similarly to Earth’s: an outer fluid layer surrounding a solid inner core. The model they created indicated that the outer core has a radius of about 362 kilometers (225 miles), while the inner core’s radius is approximately 258 kilometers (160 miles), constituting about 15 percent of the Moon’s total radius.
The inner core’s density is around 7,822 kilograms per cubic meter, close to the density of iron. Intriguingly, this aligns with a 2011 study led by NASA Marshall planetary scientist Renee Weber, which used advanced seismological techniques on Apollo data and suggested a solid inner core with a similar radius and density.
Briaud’s team claims their findings support the earlier research, indicating that the Moon’s inner core might be more Earth-like than previously thought. This discovery has implications for the evolution of the Moon, especially considering its once powerful magnetic field, which began to wane around 3.2 billion years ago. A magnetic field is typically driven by motion and convection in the core, so understanding the core’s structure could shed light on why the Moon’s magnetic field vanished.
As humanity prepares for new missions to the Moon, we might not have to wait long for further seismic studies to confirm these results. The information gained could have a significant impact on our knowledge of both the Moon and the early Solar System.
