Formation of the Moon
🎯 In this topic you will
- Describe evidence for the collision theory for the formation of the Moon.
🧠 Key Words
- collision theory
Show Definition
- collision theory: The idea that the Moon formed when a Mars-sized planet collided with the early Earth, causing debris to merge into the Moon.
🌕 Early Theories of the Moon's Origin
In the early twentieth century, scientists thought that the Moon was formed by splitting away from the Earth, soon after the Earth was formed. Calculations showed that, if this theory were correct, the Moon would still be slowly moving away from Earth. Accurate measurements made more recently have shown that the Moon is still moving away from Earth — at a rate of about 4 cm each year. These calculations could not completely confirm the theory that the Moon had formed by splitting away from the Earth.
🧪 Did you know?
The Moon is moving away from Earth at about 4 cm every year — that’s roughly the speed at which your fingernails grow!
💡 A New Idea: Collision Theory
In 1974, it was suggested that the Moon was formed in a different way, called the collision theory. The collision theory (also called the giant impact hypothesis) is another theory of how the Moon was formed. There is more evidence that supports the collision theory than any other current theory.
☄️ Theia and the Giant Impact
The collision theory refers to a collision that happened relatively soon after the formation of the Solar System. A newly formed planet, about the same size as Mars, collided with the newly formed Earth. Scientists have called the colliding planet Theia. The picture — drawn by an artist — shows what the collision between Earth and Theia may have looked like.
📌 Important Concept
Collision Theory (Giant Impact Hypothesis): This theory proposes that the Moon was formed from debris created when a Mars-sized object, named Theia, collided with the early Earth. It is currently the most widely supported explanation for the Moon’s origin.
🌌 How the Moon Formed from Debris
The collision would have caused rocks and dust to break away from both planets. The theory states that Earth was formed from the two planets joined together. The Moon was formed when the rocks and dust were pulled together by gravity. These stages are shown in the diagram:
⚠️ Common Mistake
Be careful not to assume the Moon was formed instantly — it took time for gravity to pull scattered debris into a single body.
| Evidence that supports the collision theory | Evidence that contradicts the collision theory |
|---|---|
| The Moon is less dense than the Earth. | The surface of the Earth does not appear to ever have been molten. |
| Samples of rock from the Moon show that its surface was once molten. | Venus has no moon. Collisions in the early years of the Solar System would have been common and scientists would have expected Venus to have a moon formed in the same way. |
| The Moon has a small iron core, similar to the Earth. | The composition of rocks on the Moon would be expected to be more similar to rocks on Theia than rocks on Earth. In fact, the composition of the Moon is more similar to Earth. |
| There is evidence outside the Solar System of similar collisions causing rings of rock and dust. | |
| The collision theory fits with the theory of how the Solar System was formed. | |
| The composition of rocks on the Earth and the Moon are the same. |
❓ QUESTIONS
1. The collision theory suggests that an object called Theia collided with Earth.
Which of these planets is closest in size to Theia? Write one letter.
A Mars B Earth C Neptune D Jupiter
👀 Show answer
2. Which of these statements could explain why the composition of rocks on the Earth and on the Moon is very similar?
Write one letter.
A The Moon is mostly formed from Theia’s rocks and the Earth only has a small quantity of Theia’s rocks.
B The Earth is mostly formed from Theia’s rocks and the Moon only has a small quantity of Theia’s rocks.
C Rocks from Theia and Earth were completely mixed during the collision.
👀 Show answer
3. The planet Mars has two moons called Phobos and Deimos. Scientists have evidence that Phobos and Deimos are asteroids that were captured by the gravity of Mars, so they remain in orbit around Mars.
Suggest how the composition of rocks on Phobos and Deimos would compare with the rocks on the planet Mars if Phobos and Deimos were:
a originally asteroids
b formed by a collision of another object with Mars.
👀 Show answer
b) If they formed from a collision involving Mars, their rock composition would be similar to Mars, as they would be made from Mars material ejected during the impact.
4. Models of the formation of the Solar System show that collisions between planet-sized objects would have been common soon after the Solar System formed. Suggest why these collisions are no longer common.