Astronomy and space

Composition and origins of the Moon

Since the dawn of civilisation, we have always wondered where the Moon came from. But until we finally managed to visit it in 1969, its origins remained a complete mystery to science.

The Apollo landings changed all that. By installing devices that could listen to natural tremors shuddering through the Moon, the Apollo missions provided precious data on what lies beneath. And today, these data are helping scientists gain a better understanding of what the Moon is made of, inching us closer to finally solving the mystery of how it formed.

Moon fact: The biggest moonquakes Apollo astronauts ever recorded registered 5.5 on the Richter scale and lasted for over 10 minutes. As this is enough to damage buildings here on Earth, any future Moon base will need to be super-sturdy.

How did the Moon get there?

Why looking deep inside the Moon can tell us how Earth gained its cosmic companion

4.6 billion years ago, in a dense cloud of gas and dust, a star sputtered into life. The gas left over formed a thin disk around the newborn star, and over time condensed to form dust grains. These microscopic particles collided and stuck together to slowly form bigger and bigger lumps. Attracting evermore particles under the force of gravity, over a few million years these lumps formed small planets, or planetesimals. And through smashing together and sweeping up the dregs of the gas and dust, they became the planets of the Solar System we know and love today. 

Hooked, grown or smashed?

But what about the Moon? Here the story gets murky. There are three main ways that moons are thought to form. Scientists have found that some planets sucked wandering planetesimals into orbit as they were passing close by. Phobos and Deimos, for instance, formed elsewhere in the Solar System before Mars captured them. This could explain why the Earth and Moon have different ingredients. But most moons captured by their parent planet in this way are oddly shaped and have wonky orbits. As our Moon is round and has an unremarkable orbit, it probably wasn’t a passing stranger snared by the Earth’s gravity.

Another idea is that the Earth and Moon formed together at roughly the same time. Many of Jupiter’s moons – like Io, Europa and Ganymede – are thought to have formed out of the crumbs left by Jupiter’s creation. The problem for our Moon though is that it’s metal-poor, while the Earth is metal-rich. If they formed from the same material, how could that be possible?

Most, but not all, scientists today think the Moon had a much more violent birth. They believe a Mars-sized object they have named Theia smashed into the young Earth. The impact spewed chunks of the Earth’s crust into space. Some of this debris bound together around what was left of Theia’s core to form the Moon as we know it today. But conclusive proof for this idea is lacking. 

Cartoon image of two celestial bodies, known by scientists as Theia and Proto-Earth, colliding to create the Earth and Moon

What lies beneath?

To settle the debate of how the Moon formed, our first clues are in what we see on a clear night and what our trips to the surface of the Moon have revealed about how it’s put together. For instance, we know that the atmosphere is extremely thin ­– there are only about 100 molecules in a bit of the atmosphere the size of a die, compared to about 100 billion billion molecules at sea level on Earth. We also know it contains various gases toxic to humans, and only trace amounts of oxygen. This meant that in 1969 Apollo 11 astronauts Neil Armstrong and Buzz Aldrin had to wear spacesuits throughout their time on the Moon. It also meant there was no wind or rain, allowing them to leave a permanent mark before rocketing back home – their footprints will remain there for millions of years.

These footprints were laid down in a layer of powdery rock and dust known as regolith. At 3–20 metres deep, regolith covers almost the entire lunar surface. Below this is a rocky geology ­– not Swiss cheese, unfortunately – consisting of impact craters, long dead volcanoes and ancient oceans of hardened lava.

So far, few clues. Perhaps we need to dig deeper to solve the lunar origin mystery. During the Apollo 11 mission and later Apollo landings, NASA astronauts installed a network of special instruments that picked up tremors from the Moon’s equivalent of earthquakes: moonquakes. From the shape of the seismic waves detected, scientists had compelling evidence that the Moon has a crust, mantle and core just like the Earth. And by combining this with measurements of the Moon’s gravity from orbiting spacecraft, they got an idea of how dense and thick the different layers of the Moon are, and even which are solid and which are molten.

From this, scientists generally believe that the Moon consisted of molten magma in its very early history. Heavy metals sank to form the core. Then, as it cooled, crystals began to form within the magma. Denser minerals sank down to later form the mantle, whereas lighter minerals floated to the surface to form the crust. What is left today is a small solid iron-rich core surrounded by a layer of liquid magma. Enveloping the core is a thick solid mantle 1350 kilometres deep that makes up most of the Moon’s volume. And wrapping the mantle is a roughly 50 kilometre deep crust, which is on average even thicker than the Earth’s.

Investigating the lunar internal structure in more detail has led to some interesting ideas about the Moon’s history. For instance, we now know that on the far side of the Moon the crust is much thicker and has a different geology than the familiar near side. Some scientists have suggested this could mean that the remnants of Theia’s impact with the Earth (if it happened) formed two separate proto-moons. Not long after, Earth’s twin moons gently collided and stuck together. 

Yet at the moment, this is all guesswork. Scientists need more data to draw any firm conclusions about the Moon’s origins. And those data are still hidden deep below the surface in the Moon’s internal makeup. Only by installing new moonquake-measuring instruments will we solve the mystery of how the Moon came to be once and for all.