The Moon has captivated astronomers’ imaginations for ages, especially before we launched our first spacecraft. During the early 20th century, scientists engaged in heated discussions about whether the Moon was merely a rocky formation like its Martian counterparts or if it had a more intricate interior. Today, those debates are coming to a close.
Recent mathematical models reveal that the Moon boasts a fluid outer core and a solid inner core, much like what we see on Earth. Investigations suggest that its inner core, composed of metal akin to iron, spans approximately 500 kilometers across, constituting about 15% of the Moon’s total size.
What’s Beneath the Lunar Surface?
Knowing the Moon’s gravitational pull and its overall volume allows us to compute its average density, providing only a general idea of what it consists of. While unfortunate, it doesn’t paint the entire picture of its material composition. Previous missions have successfully sent landers to the Moon, capturing images of its surface and helping to reveal the composition of the lunar crust.
Yet still, that information alone is insufficient.
Since the famed Apollo missions of the 1960s, various expeditions have explored the Moon. Not only did the Apollo astronauts investigate its surface, but they also set up various scientific apparatus and brought back valuable lunar rocks for further examination. These contributions provided crucial data that helped us understand the Moon’s make-up. Looking ahead, the Chang’e mission in 2024 is set to return samples from the Moon’s far side,a first!
Moreover, the Apollo astronauts left seismometers on the Moon that delivered groundbreaking insight into its internal structure—much like what we do to learn about Earth’s geology, though we have a significantly greater number of seismometers on our home planet.
A newer seismometer has recently been placed on the Moon,providing crucial new data.
“Our findings challenge previous theories about the evolution of the Moon’s magnetic field by unveiling the existence of its inner core and pave the way for insights into the timeline of lunar disturbances during the Solar System’s early years,” researchers noted in their study.
Unpacking the Inner Lunar Secrets
Understanding the internal structures of celestial bodies like the Moon involves working with seismic data. Seismic waves—essentially sound waves—travel differently through various materials. As they move through the Moon, the behavior of these waves during lunar quakes reveals a lot about what lurks inside the Moon’s depths. Think of it as a geological X-ray.
India’s Chandrayaan-3, which successfully landed on the Moon in August 2023, made history by detecting the first lunar quake in over 50 years. Unfortunately, the researchers from Côte d’Azur University in France did not have access to this exciting new data beforehand. Therefore, they leaned on older data from the Apollo missions. The downside? That ancient data is rather low resolution—far from ideal for accurately gauging the Moon’s core conditions.
Hoping to move forward, the team combined recent data from launches and lunar laser ranging experiments, outlining the Moon’s characteristics—from its density to its shifting distance from Earth. They then tested various core configurations through modeling to figure out which ones aligned best with the observational data at hand.
The results were quite striking. They discovered that denser materials sink to the Moon’s center while less dense materials float towards the surface, potentially explaining the distinctive elements identified in the Moon’s volcanic regions. However, that’s only part of the puzzle.
The researchers further concluded that the Moon’s core has similarities to Earth’s—a dynamic outer fluid layer and a solid inner part. Their lunar core measures around 500 kilometers across, indicating that it’s roughly 15% of the overall size of the Moon. Interestingly, back in 2011, NASA scientists reached a comparable conclusion using seismological assessment of Apollo-era data. The consistency of two independent studies magnifies the reliability of these findings.
More Mysteries Await Discovery
Although recent findings illuminate some of the Moon’s inner structure, many mysteries persist, particularly regarding its magnetic field’s fate.
In contrast to Earth, The Moon lacks active plate tectonics, which means that its surface doesn’t undergo the type of recycling that gives rise to mountains and oceans on our planet. Still, the Moon underwent significant transformation not long after its formation. It initially sported a strong magnetic field, which began to wane around 3.2 billion years ago. This field was likely sustained by motion and convection of the innermost core.
On a more intriguing note, gravity data suggests that the Moon’s interior may be asymmetrical. Research conducted in 2025 indicates that volcanic activity on its near side could have significantly influenced its formation millions of years ago.
More recently, scientists have focused their efforts on examining areas closer to the Moon’s crust, not so much to unravel its geological story but to locate cavities that could be potential spots for upcoming lunar missions or even plans forpermanent bases on the Moon. One thing’s for sure: no matter how deep you dig, the Moon remains a captivating topic for further exploration.
The viewpoints in this piece were initially published in May 2023 and have since been refreshed.
Originally published by ZME Science. If you want to stay up-to-date on science, don’t forget to subscribe to our newsletter.
