Does the Moon Have a Molten Core?

Is the Moon hot, or is it not? Scientists have wondered that question for ages. What looks like a completely dead world to us is still, in many ways, a mystery below the surface. Let’s take a look at what we’ve been able to decipher about the Moon’s center by first looking at what a core is.

Does The Moon Have A Molten Core

What Is the Core of a Rocky World?

Layers Of Mercury
Diagram of the planet Mercury, including all of its layers. (Image credit: A loose necktie on Wikimedia Commons CC BY-SA 4.0)

A core in the general sense represents the center of an object, be it the human body, an apple, or a giant rock floating through space. The core of a rocky celestial object is unique in being the hottest and most dense part of the world.

A Brief Look at the Earth’s Core

Layers Of Earth
Layers Of Earth. (Image credit: Kelvinsong on Wikimedia Commons CC BY-SA 3.0)

The Earth’s core has two distinct layers. The outermost layer of the core is entirely molten, while the inner core, the deepest part of our planet, is completely solid due to the amount of pressure at that depth.

Our core is so hot that it’s estimated to be the same temperature as the surface of the Sun. It makes up approximately 50% of Earth’s size and contributes to our seismic and volcanic activity.

How Do We Get Information About a Core We Can’t See?

Replik Seismometer Apollo 11 Science Museum
Replik Seismometer Apollo 11 Science Museum. (Image credit: Tohma on Wikimedia Commons CC BY-SA 4.0)

Even on Earth, we have no way of drilling down far enough to see what it looks like or exactly what it’s made of. Humans have only managed to travel 7.6 miles (12 kilometers) below the Earth’s surface, and that alone took 20 years to accomplish!

Therefore, scientists have to rely on data from other sources to hypothesize what the Earth’s core looks like. This data mainly comes from seismic activity and by studying our magnetic field.

On the Moon, Apollo astronauts placed seismometers on the surface to detect moonquakes in hopes of discovering more about its core. The seismometers collected data for six years before running out of juice, and scientists have been crunching the data ever since.

The Size of the Moon’s Core

Between that seismic data and calculations based on the Moon’s density, researchers have come to the conclusion that the Moon’s core must be quite small, relatively speaking. Compared to the Earth’s 50%, the Moon’s core only makes up about 20% of its total size.

Based on the Moon’s diameter, this places the core at a total thickness of roughly 400 miles (600 kilometers).

What Is the Moon’s Core Made Of?

As a rocky celestial object forms, the heavier materials naturally find themselves drawn toward the center. Based on analysis of the Moon’s density, scientists have been able to estimate that its core is almost entirely a metallic iron, with a small amount of nickel and sulfur mixed in.

Is the Moon’s Core Hot?

Based on recent calculations, researchers have been able to pin down that the temperature of the Moon right where the core meets the mantle ranges between 2,300 and 2,680 degrees Fahrenheit (1,300 and 1,470 degrees Celsius).

While indeed hot by most standards, the Moon’s core is quite cool when compared to objects like the Earth. The Moon has lost a lot of heat since its formation since it has minimal atmosphere to keep anything contained.

Is the Moon’s Core Molten?

Moon diagram
Moon diagram that shows its different layers. (Image credit:  Kelvinsong  on Wikimedia Commons  CC BY 3.0)

Temperatures at the very heart of the Moon are probably warm enough to melt the metallic iron it’s made of. However, the large amount of pressure at that depth keeps it all compacted into a tight, dense solid. This pressure is much higher than what’s required to keep the Moon’s inner core solid and will likely remain that way forever.

That being said, temperature calculations and seismic data have revealed that a small part of the Moon’s core, somewhere between 5 and 30 percent, is actually molten iron. This thin outer core meets up with the mantle and is liquid enough to flow.

What puzzles scientists is that the temperature at this depth shouldn’t quite be hot enough to melt metallic iron, but it’s clear that more than meets the eye is going on here.

A Piece of Earth?

Theories abound that the Moon’s small core is a result of its origin story. Scientists believe more and more that the Moon was once a part of our Earth and that a stray asteroid found our planet during its formative years.

That asteroid blew a large section of the Earth back into space, where it coalesced into the Moon we all know and love today. With most of the Earth’s heavier metals sinking toward its center, the debris cast into space was mostly lighter material.

In the Moon’s case, the little bit of heavy iron cast into space became its core, but the amount of iron was significantly smaller than that of the materials that now make up its mantle and crust. While unproven, this explanation explains well the Moon’s small core and low density.

Was the Moon’s Core More Molten in the Past?

Once upon a time, the Moon was a lot hotter than it is today. However, the entire Moon has cooled quite quickly over time due to its small size and even smaller core.

The Moon’s core certainly would have been more molten (or at least more fluid) than it is today, and this would have heated the mantle to a molten state as well. This is apparent in the large number of now dead volcanoes all over the Lunar surface, coupled with large, flat seas of basalt that were once covered in magma.

This would have also meant that the Moon once had a magnetic field. Magnetic fields form as a result of an abundance of molten iron in its core. Some believe that the Moon’s magnetic field, along with Earth’s, were primary factors in the formation of life on our planet.

This video talks about how the Moon’s magnetic field disappeared a billion years.

What Does This Mean for the Moon Today?

Today, the Moon’s entire composition is almost completely solid, leaving it a cold, dead rock. The little bit of heat the Moon does have trapped in its core is so far trapped under a solid mantle that it has no way of ever reaching the surface.

Final Thoughts

From what we know about rocky objects in our Solar System, the Moon is quite unique with such a small core. Once much hotter, we likely have a lot to thank the Moon for in the formation of our own Earth.

Scientists continue to study our closest neighbor, hoping to unlock more of its secrets. With no ability to retain heat, one can only speculate what the Moon’s future will hold. No matter how cold the Moon may be, it’s still beautiful to photograph on a clear night.

About Noah Zelvis

Noah is a content writer who has had a love of all things astronomy for as long as he can remember.
When not reaching for the stars, you’ll likely find Noah traveling or running.