Giant-impact hypothesis

The giant-impact hypothesis, sometimes called the Theia Impact, is an astrogeology hypothesis for how the Moon was formed. It was first suggested in 1946 by Canadian geologist Reginald Daly. This idea says that long ago, about 4.5 billion years ago, the early Earth crashed into a planet about the size of Mars. This planet is often called Theia, after a mythical Greek Titan who was the mother of Selene, the goddess of the Moon.

When these two large objects collided, pieces of both were thrown into space. Over time, these pieces came together and formed the Moon we see today. Scientists support this idea because several clues fit with what we see now. The Moon’s orbit and Earth’s rotation are angled similarly, and the rocks from the Moon and Earth have almost the same chemical makeup. Also, the Earth and Moon together spin and move in a way that needs a lot of energy — the kind that could come from a big crash.

Even though this hypothesis is widely accepted among astronomers, there are still some mysteries. For example, we do not yet have a perfect model that shows exactly how the pieces from the crash came together to make the Moon. But this idea helps us understand not just our Moon, but how other star systems might form planets and moons too.

History

In 1898, George Darwin suggested that Earth and the Moon were once one body. He thought a hot Moon had been spun from Earth because of the forces that push things away. Later, American and Soviet experiments proved that the Moon is slowly moving away from Earth.

However, Darwin's ideas couldn’t fully explain how the Moon could have come from Earth. In 1946, Reginald Aldworth Daly from Harvard University proposed that the Moon was created by a big crash, not just by forces pushing it away. His idea didn’t get much attention until 1974, when William K. Hartmann and Donald R. Davis brought it up again. They suggested that during the time planets were forming, a large object might have hit Earth, throwing material into space that later came together to form the Moon.

Around the same time, Alastair G. W. Cameron and William R. Ward proposed that a body about the size of Mars hit Earth at an angle. They believed this could explain why the Moon has less iron than Earth. In 1984, at a meeting in Kona, Hawaii, the idea that a big impact created the Moon became the most popular explanation.

Theia

Main article: Theia (hypothetical planet)

The name of the imagined planet comes from a story about a Greek titan named Theia, who was the mother of Selene, the goddess of the Moon. This name was suggested by a scientist in the year 2000 and is now commonly used. Scientists think Theia was one of many Mars-sized objects that existed in our Solar System about 4.5 billion years ago. It is believed to have moved in a special pattern with Earth and the Sun. The giant-impact hypothesis suggests that Earth had many big collisions while it was forming, and the one that created the Moon was just one of these huge impacts. Smaller space rocks might have hit Earth later, around 3.9 billion years ago.

Basic model

Astronomers believe that a big crash happened between Earth and a planet called Theia about 4.4 to 4.45 billion years ago, not long after our Solar System started. This crash happened at an angle while Earth was almost fully formed.

After the crash, pieces of both Earth and Theia were thrown into space. Some of these pieces stayed close to Earth and eventually came together to form the Moon. The crash also changed how fast Earth spins and added more material to our planet. There are ideas that maybe there was a smaller moon that later crashed into the Moon we see today, making one side thicker.

Composition

In 2001, scientists from the Carnegie Institution of Washington found that rocks brought back from the Moon by the Apollo program had the same chemical makeup as rocks from Earth. This was different from most other objects in space.

Later, in 2014, another group in Germany found a very small but important difference in the chemical makeup of Moon rocks compared to Earth rocks. One idea is that the object that hit Earth, named Theia, may have formed close to our planet.

These discoveries fit well with the giant-impact hypothesis. This idea says that Earth collided with a Mars-sized object about 4.5 billion years ago. The pieces thrown into space from this crash later came together to form the Moon.

Evidence

Scientists have found clues that support the idea that a big crash helped create the Moon. Rocks brought back from the Moon by astronauts show that their makeup is very similar to Earth’s rocks. This suggests that the Moon formed from material that once was part of Earth. The Moon also seems to have a smaller core than most planets, which fits with the idea that a large object crashed into Earth long ago.

Studies of special elements in Moon rocks compared to rocks from Earth and Mars also back up this theory. These elements act differently when heated, and the Moon’s rocks show signs of having been heated by a big impact. Scientists even found dust around distant stars that looks like what we’d expect from such a crash.

Difficulties

The idea that a big crash formed the Moon has some puzzles that scientists are still trying to solve. One puzzle is that this idea suggests Earth would have had a thick layer of melted rock after the crash, but we don’t see any signs that Earth ever had such a layer.

Composition

There are some things about what the Moon is made of that this idea doesn’t fully explain:

• The amounts of certain elements on the Moon don’t match what we’d expect if the Moon came from a big crash.
• Finding water and other materials on the Moon is harder to explain if the Moon formed from a very hot crash.
• The Moon has a certain amount of iron that doesn’t quite match what we’d expect if it came mostly from Earth or another planet.
• The Moon seems to lack some elements that we’d expect if it formed from another planet.
• The Moon’s oxygen looks almost exactly like Earth’s, which is surprising if it came from another planet.
• The Moon’s titanium also looks very similar to Earth’s, suggesting that not much of it came from another planet.

Lack of a Venusian moon

If the Moon formed from a big crash, we might expect other planets like Venus to have moons too. But Venus doesn’t have a big moon. One idea is that if Venus did have a moon, it might have been pulled in by the Sun or crashed back into Venus because of its closeness to the Sun. Big crashes like the one that might have formed our Moon were probably common, and they often result in a single moon. However, whether such a moon stays in orbit depends on many factors, like how far the planet is from the Sun and how the moon’s orbit is tilted.

Possible origin of Theia

In 2004, a mathematician from Princeton University and an astrophysicist suggested that Theia may have formed at special points in Earth’s orbit called L₄ or L₅ Lagrangian point, similar to some asteroids. As Theia grew larger, its movement changed, eventually leading to a collision with Earth.

Later studies suggested this collision may have happened a bit later than previously thought. Some researchers think other objects might have been created in this event and stayed in orbit for a very long time before moving away. There was also an idea that a later collision with a smaller object helped shape the Moon, but newer data has shown this might not be correct. In 2019, scientists found clues that Theia might have come from far away in the Solar System.

Alternative hypotheses

Main article: Origin of the Moon

Scientists have thought of several ways the Moon might have formed. One idea is that the Moon broke off from Earth when it was spinning very fast. Another idea is that the Moon was born somewhere else and Earth’s gravity pulled it in. Some believe Earth and the Moon formed together from the same cloud of dust and gas.

Another idea says a big space rock hit Earth later than usual, making the Moon mostly from Earth’s pieces. This would have covered Earth in hot liquid and created a early version of the Moon.

A final idea, from 2012, says the Moon and Earth formed together after two big objects crashed into each other. After this crash, Earth was covered in material that eventually came together to make the Moon.