What is the precise composition of the Moon? What have we learned from analyzing lunar rock samples brought back by the Apollo missions?

Okay, no problem. Imagine we're sitting together having a coffee, chatting about that big rock hanging in the sky. It's actually even more fascinating than you might think.

The Moon's "Ingredient List": Not Mysterious, But the Details Are Stunning

To talk about the Moon's precise composition, we first need to distinguish which part we're discussing. Just like an egg has a shell, egg white, and yolk, the Moon is divided into the lunar crust, mantle, and core. The part we know best is naturally what the Apollo astronauts collected – rocks and soil from the surface of the crust.

You can think of the lunar surface as a giant, ancient "construction site," primarily built from the following "materials":

1. Major Chemical Elements (by weight)

If you grind lunar rock into powder and analyze its chemical elements, you'll find its "ingredient list" is strikingly similar to Earth's crust, but with slightly different proportions:

Oxygen (O): ~43% – Absolutely the top ingredient! But don't get excited; it's not breathable oxygen. It exists as oxides within minerals, like silicon dioxide (SiO₂).
Silicon (Si): ~21% – The main component of sand, abundant everywhere on the Moon.
Aluminum (Al): ~10%
Calcium (Ca): ~9%
Iron (Fe): ~9%
Magnesium (Mg): ~5%
Titanium (Ti): ~2%

See? All elements very common on Earth. The Moon doesn't contain any "mysterious substance" not found on our planet.

2. Major Rock Types

Just looking at elements is a bit dry; let's see what interesting rocks they form:

Lunar Basalt: When you look up at the Moon, those large dark areas are the "lunar maria" (seas). They aren't actual seas but plains formed by cooled lava from massive volcanic eruptions billions of years ago. These rocks are basalt, rich in iron, magnesium, and titanium, hence their darker color. This is very similar to the black volcanic rock found in places like Hawaii on Earth.
Anorthosite: The bright areas you see on the Moon are the "lunar highlands," representing the Moon's oldest crust. The primary rock here is anorthosite, rich in calcium and aluminum, giving it a light color. Imagine that when the Moon first formed, it was a molten ball; these lighter anorthosite rocks "floated" to the surface, forming the initial crust.
Breccia: This is the coolest kind, in my opinion! The Moon has no atmosphere for protection and has been relentlessly bombarded by countless asteroids and meteorites for billions of years. These impacts mix surface rocks, debris, and glass beads (formed from instant melting and cooling during impact) together. Immense pressure then "welds" them into a new rock. So, a piece of breccia is like a "violent history book" of the Moon, recording multiple impact events.
Regolith: This isn't soil like we have on Earth for growing plants. Regolith is the layer of fine powder covering the lunar surface, formed over billions of years by the constant bombardment and grinding of lunar rock by micrometeorites. It's extremely fine but has sharp, angular particles like shards of glass. It's also electrostatically charged and easily sticks to spacesuits.

What Earth-Shattering Secrets Did the Apollo Moon Rocks Reveal?

Analyzing those 382 kilograms of lunar samples is undoubtedly one of the greatest achievements in planetary science history. It didn't just confirm the composition; it revolutionized our understanding of the Moon's history and that of the entire solar system.

1. Confirmed the Moon is a Bone-Dry Planet

This was one of the most immediate discoveries. The rocks brought back by Apollo contained almost no water whatsoever. Unlike rocks on Earth, which contain at least some water of crystallization, this showed the Moon has been extremely water-poor since its formation. This discovery also explains the absence of life on the Moon. (Of course, later missions detected possible water ice in permanently shadowed regions at the poles, but that's likely "foreign" – brought by comet impacts, for example – not "native" to the Moon itself.)

2. Revealed the Moon Once Had a "Fiery Heart"

Before Apollo, many thought the Moon had always been a cold, dead world since its birth. However, the existence of lunar basalts provided compelling evidence that the Moon experienced intense volcanic activity over 3 billion years ago. This means its interior was once extremely hot, even experiencing a global "magma ocean" phase. The "lunar maria" we see are remnants of this fiery period.

3. Gave Birth to the "Giant Impact" Hypothesis for Lunar Origin

This was the most significant discovery! By comparing isotopes in lunar and Earth rocks, scientists found a peculiar phenomenon:

The Moon's overall chemical composition is remarkably similar to Earth's mantle (the layer beneath the crust and outside the core).
But the Moon lacks heavy metals like iron (hence its small core) and is deficient in water, sodium, potassium, and other easily vaporized "volatile elements."

How to explain this? The "Giant Impact Hypothesis" emerged and is now the leading theory:

About 4.5 billion years ago, shortly after Earth formed, a Mars-sized planet named "Theia" collided obliquely with the proto-Earth. The impact blasted material from Earth's mantle and part of Theia itself into space. These fragments eventually coalesced under gravity to form the Moon.

This hypothesis perfectly explains the characteristics of lunar rocks:

Composition like the mantle? Because it mainly came from the mantle.
Lacks an iron core? Because the ejected material was primarily mantle; Earth's heavy iron core wasn't significantly involved.
Lacks water and volatiles? Because the immense heat of the collision "baked" them off, causing them to evaporate.

So, the Moon is likely Earth's "long-lost child."

4. Provided a "Time Capsule" for Solar System History

Earth is an active planet with wind, water, and plate tectonics. "Crime scenes" (impact craters) from billions of years ago have long been erased.

But the Moon is different; it's a perfect "museum." With no atmosphere or geological activity, impact craters from billions of years ago are perfectly preserved. By dating rocks brought back by Apollo astronauts from near different craters, scientists were able to precisely calibrate the early solar system's "impact chronology." They discovered that around 3.9 billion years ago, the solar system went through a frenzied period called the "Late Heavy Bombardment," where massive numbers of asteroids bombarded the inner planets, including Earth.

Studying the Moon, therefore, is like looking back at our own Earth's turbulent childhood.

In short, the rocks Apollo brought back are far more than just rocks. They are keys that unlocked the doors to the Moon's origin, evolutionary history, and the early story of the solar system. Every time we look up at the Moon, we should thank those brave astronauts who brought back these "messages from another world."