For a long time, we just assumed the Moon was a dead, cold rock. It was easy to think that. It looks like a dusty graveyard from the outside, scarred by billions of years of cosmic abuse. But recently, things got weird. Scientists started looking at old Apollo-era data and mixing it with new gravity maps, and honestly, the picture of what's inside the moon has changed completely. It’s not just a boring lump of granite.
It's layered. It's got a heart. And that heart might still be warm.
The Crust is a Messy Shell
The outermost layer, the crust, is what we see, but it’s lopsided. That’s the first thing you have to realize about lunar anatomy. The "near side"—the part that always faces Earth—has a much thinner crust than the far side. Why? We’re still arguing about it. Some think a second, smaller moon smashed into the back of it billions of years ago. Others blame Earth’s gravity for pulling the molten insides toward us when the Moon was still a baby.
Basically, the crust is made of oxygen, silicon, magnesium, iron, calcium, and aluminum. It’s about 43 miles thick on the far side but maybe only 18 miles thick on the side we see. It’s a battered shield of anorthosite, a type of igneous rock.
The Mantle and the "Magma Ocean" Theory
Beneath that crust lies the mantle. This is the bulk of the Moon. If you want to understand what's inside the moon, you have to look at the Lunar Magma Ocean theory. Billions of years ago, the Moon was likely a giant ball of liquid fire. As it cooled, the heavy stuff sank and the light stuff floated.
The mantle is mostly made of minerals like olivine and pyroxene. It's solid, mostly. But there’s evidence of a "mushy" layer right at the bottom, near the core. This is where things get interesting for geologists like Arthur Nowack or researchers at NASA's Goddard Space Flight Center. They’ve found that the Moon actually deforms slightly because of Earth’s gravity—it "squishes" just a tiny bit. That wouldn't happen if the entire interior was a frozen, solid block of ice and stone.
The Heart of the Matter: A Solid Iron Core
We used to think the Moon didn't even have a core. We were wrong.
In 2023, researchers at the French National Centre for Scientific Research (CNRS) published a bombshell paper in Nature. They used seismic data and rotation measurements to prove that the Moon has a solid inner core, just like Earth. It’s about 310 miles in diameter. That’s roughly 15% of the Moon’s total size.
It’s made of iron. Specifically, a dense metallic solid with a density around 7,822 kilograms per cubic meter. Surrounding that solid ball is a fluid outer core. This is a big deal because it explains why the Moon used to have a massive magnetic field—stronger than Earth’s is today—about 4 billion years ago. Then, for some reason, the engine died. The core cooled enough that the "dynamo" stopped spinning, and the Moon lost its protection against solar wind.
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The Mystery of the KREEP Rocks
There’s this weird layer scientists call KREEP. It’s an acronym for Potassium (K), Rare Earth Elements (REE), and Phosphorus (P).
This stuff didn't fit into the crystals as the Moon cooled, so it got squeezed into a thin layer between the crust and the mantle. It’s radioactive. Because it's radioactive, it produces heat. This explains why the near side has all those "seas" or maria—the dark spots you see when you look up at night. The heat from the KREEP layer caused volcanic eruptions that flooded the lowlands with basaltic lava.
Without those radioactive elements hidden deep inside, the Moon would look like a uniform, featureless golf ball.
Water Where It Shouldn't Be
You’ve probably heard there's water on the Moon. But it’s not just in the shadows of craters at the poles. It’s inside the rocks. When the Apollo astronauts brought back "orange soil" (discovered by Harrison Schmitt during Apollo 17), they didn't realize they were looking at tiny glass beads formed by explosive volcanic eruptions.
Decades later, using better tech, we found water molecules trapped inside those beads. This suggests that the Moon's interior—the deep mantle—might have as much water as Earth's upper mantle. It's not liquid lakes, obviously. It's hydroxyl groups trapped inside mineral structures. But it changes everything about how we think the Moon formed.
If it came from a giant impact between Earth and a planet named Theia, how did the water survive the heat of that collision? It’s a massive hole in our current understanding.
Why Does This Matter to You?
We aren't just looking at what's inside the moon for fun. We’re going back. NASA’s Artemis program and various private ventures like SpaceX or Blue Origin are looking at the Moon as a gas station.
If the interior is rich in certain minerals or if we can tap into those deep-seated volatiles, the Moon becomes a stepping stone to Mars. Knowing the density of the core and the thickness of the mantle helps us stabilize satellites and landers. It also helps us understand our own planet's history. The Moon is basically a time capsule. Earth has plate tectonics that erase our history; the Moon just sits there, holding onto its secrets.
Reality Check: What We Don't Know
Let's be real: we've barely scratched the surface. Literally. We have only had seismometers in a few spots, all on the near side. We don't have a global seismic network on the Moon yet. Everything we think we know about the very center is based on mathematical models and "listening" to moonquakes caused by meteor impacts or Earth’s tidal pull.
There are still debates. Some scientists think the core is smaller. Others think the mantle is more heterogeneous—meaning it's a "plum pudding" of different rock types rather than nice, neat layers.
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Actionable Insights for Space Enthusiasts
- Track the Artemis Missions: Keep an eye on the Artemis III lunar landing (currently slated for the late 2020s). They plan to deploy the Lunar Effects on Agricultural Flora (LEAF) and other instruments that will measure internal heat flow.
- Use Online Visualizers: Check out the LRO (Lunar Reconnaissance Orbiter) QuickMap. You can toggle gravity layers to see where the crust is thickest and where the "mascons" (mass concentrations) are hidden inside.
- Support Citizen Science: Sites like Zooniverse often have projects where you can help categorize moon craters, which helps researchers map the age and thickness of the crust.
- Monitor Seismic Data: If you're a data nerd, NASA's PDS (Planetary Data System) hosts the original Apollo seismic tapes. People are still finding new "moonquakes" in that 50-year-old data today.
The Moon is a complex, differentiated world. It has a crust that varies in thickness, a mantle with a radioactive "hot zone," and a dual-layered iron core that used to power a magnetic shield. It’s far from a dead rock. It’s a frozen engine, waiting for us to figure out exactly how it used to run.