Is the Moon's inner core solid or liquid? How does its exact state impact our understanding of lunar evolution?

The Moon's Core: A World with a "Hot Interior" but Gradually "Cooling Down"

Hello! This is an excellent question that touches the core of our understanding of the Moon and even the evolution of planets throughout the solar system. Regarding whether the Moon's core is solid or liquid, scientists have explored this for decades and now have a relatively clear answer.

Simply put, the Moon's core is layered, much like Earth's: it has a solid inner core surrounded by a liquid outer core.

Think of it like a well-made "molten chocolate cake": the very center is a solidified chunk of chocolate (solid inner core), enveloped by a layer of still-flowing hot chocolate sauce (liquid outer core). Of course, the Moon's outermost layer is a thick, solid "cake base" – the mantle and crust.

How Do We Know? – Like "Listening" to a Watermelon

You might wonder, since we haven't drilled to the Moon's center, how do we know this? The answer primarily comes from Lunar Seismology, a field akin to a doctor performing an ultrasound on a patient, or us tapping a watermelon to judge its ripeness.

The Legacy of Apollo: In the 1960s and 70s, NASA's Apollo missions placed several seismometers on the Moon. These instruments faithfully recorded internal vibrations (moonquakes) and transmitted the data back to Earth.
The Secrets of "Waves": Seismic waves mainly come in two types: P-waves (primary/compressional waves) and S-waves (secondary/shear waves). They have a crucial difference:
- P-waves can travel through both solids and liquids.
- S-waves can only travel through solids; they are "blocked" or disappear when encountering liquids.

By analyzing the propagation paths and speed changes of moonquake waves within the Moon, scientists discovered that S-waves could not pass through a certain region near the center. This clearly tells us: a liquid layer exists at the Moon's center.

Later, through more sophisticated models, re-analysis of Apollo data, and incorporating other observations like Lunar Laser Ranging (using lasers to precisely measure the Earth-Moon distance to infer tiny wobbles of the Moon), scientists further confirmed the existence of a denser solid inner core at the center of this liquid outer core.

Why Is This Important? – The "Three Revelations" of the Moon's Core State

Knowing the Moon's core is a "solid + liquid" combination isn't just about satisfying curiosity; it's crucial for understanding the Moon's past, present, and future. It's like finding a diary that records the secrets of the Moon's life.

1. Solving the Mystery of the Moon's "Lost Magnetic Field"

Revelation: The Moon once had a global magnetic field, but it has almost completely disappeared now. The state of the core perfectly explains this.
Simple Explanation: The generation of a planetary magnetic field works like a giant dynamo (professionally called the "dynamo theory"). It requires two conditions: a conductive fluid core (like liquid iron) and constant churning (convection) within it.
- In the Moon's early history, its liquid outer core was larger and churned more vigorously, strong enough to generate a powerful magnetic field. Ancient lunar rock samples brought back by Apollo missions bear traces of this past field.
- As the Moon slowly cooled over billions of years, its liquid outer core gradually "calmed down," with part of it solidifying into the solid inner core. As the churning weakened or stopped, the "dynamo" shut down, and the magnetic field vanished.

Therefore, the core's state is direct evidence of the Moon's evolution from a "magnetically active" body to its current "magnetically dead" state.

2. Recording the Moon's "Temperature" and Evolutionary History

Revelation: The state of the core is a "thermometer" for the Moon's internal heat.
Simple Explanation: A celestial body starts out hot at birth and slowly cools over time.
- If the Moon's core were completely solid, it would mean its internal heat had dissipated almost entirely – essentially "cooled off."
- But now we know it still has a liquid outer core, indicating the Moon's interior still retains significant heat today, not yet fully cooled. This heat comes partly from residual "warmth" from its formation and partly from the decay of radioactive elements inside.

This tells us the Moon's evolution is an extremely long process. Its core's transformation, from a fireball with a surface covered by a magma ocean, to generating an internal magnetic field, to the field disappearing and volcanic activity ceasing, down to its current state where the interior is "still warm," is a microcosm of the entire lunar life story.

3. Verifying the Moon's "Origin" – The Giant Impact Hypothesis

Revelation: The size and state of the core provide crucial support for the Moon's most widely accepted origin theory – the "Giant Impact Hypothesis."
Simple Explanation: This hypothesis proposes that early in the solar system, a Mars-sized body named "Theia" collided with the proto-Earth. The debris from this colossal impact coalesced in Earth's orbit, eventually forming the Moon.
- According to this model, the material forming the Moon came mainly from the impactor's and Earth's "mantles" (rocky parts), while most of the "cores" (iron-rich parts) of both Earth and the impactor merged into the new Earth.
- This leads to a predicted characteristic for the Moon: its iron core should be very small relative to its total volume.
- And our observations fit perfectly! The Moon's core makes up only 1-2% of its mass, while Earth's core accounts for about 30%. This "small core" feature is strong evidence for the Giant Impact Hypothesis.

To summarize:

The Moon's core is not a simple lump of iron, but a complex, layered structure (solid inner core + liquid outer core). This state is not only a direct reflection of the Moon's internal physical properties but also acts like a key, unlocking our understanding of the Moon's magnetic field history, thermal evolution, and origin mystery. Studying it is like tracing back the grand epic of a celestial body spanning 4.5 billion years.