Formation of the Moon
Section: Earth and Space | Syllabus: Cambridge Lower Secondary Checkpoint Science (0893)
What is the Collision Theory?
The Moon formed approximately 4.5 billion years ago through a violent collision event. This is the leading scientific explanation for the Moon's origin.
- Collision theory (giant impact hypothesis): about 4.5 billion years ago, a Mars-sized planet called Theia collided with early Earth. This massive impact threw debris (rock, dust, and gas) into space. Gravity pulled this debris together to form the Moon
The Moon formed from debris thrown into orbit when the Mars-sized planet Theia struck the early Earth
Theia - The Mars-Sized Planet
Theia was a planet about the size of Mars (roughly half the diameter of Earth). Scientists named it after the Greek titan who was the mother of Selene, the Moon goddess.
- Size: similar to Mars - about 6,800 km in diameter
- Mass: about 10% of Earth's mass
- Orbit: was orbiting the Sun in a similar path to Earth
- Composition: made of rock and metal, similar to Earth
- Fate: destroyed in the collision - became part of Earth and the Moon
- Theia no longer exists as a separate planet. After the collision, most of Theia merged with Earth, and some material formed the Moon
- we can't see Theia today because it was destroyed 4.5 billion years ago
Other Theories Considered for the Moon's Formation
Before the collision theory became widely accepted, scientists proposed several other explanations for how the Moon formed. Exam questions sometimes list these alongside the collision theory and ask you to identify the correct one.
| Theory | Main Idea | Status |
|---|---|---|
| Collision Theory (Giant Impact Hypothesis) | A smaller planet (Theia) collided with early Earth. Debris from the impact formed the Moon | Currently accepted theory |
| Fission Theory (Spin-off Theory) | The early Earth spun so fast that a piece of it broke away and formed the Moon | Rejected - physics does not support Earth ever spinning fast enough |
| Co-formation Theory | The Earth and the Moon formed at the same time, from the same material | Rejected - does not explain why the Moon's core is so much smaller than Earth's |
| Capture Theory | The Moon formed elsewhere in the Solar System and was later captured by Earth's gravity as it travelled close by | Rejected - does not explain why Moon rock composition is so similar to Earth's mantle |
Worked Example: Identifying the Correct Theory
- Question: a question gives four descriptions of how the Moon might have formed and asks you to tick the correct one
- Answer: look for the option describing a collision/impact with a smaller planet, throwing out debris that formed the Moon - this is the only currently accepted theory
The Four Stages of Moon Formation
Stage 1: The Collision
- Theia (a planet about the size of Mars) collided with Earth
- The impact happened at an oblique angle (not head-on, but at an angle)
- Happened approximately 4.5 billion years ago
- Gravity pulled the two planets toward each other
- If Theia had hit Earth head-on, both planets would have been completely destroyed
- the oblique (angled) impact meant that some material could be thrown into orbit rather than everything being destroyed or merging completely
Stage 2: The Impact
The moment of impact released enormous energy and fundamentally changed both bodies:
- The collision released enormous energy
- Temperatures reached thousands of degrees
- Rock melted and vaporised from the heat
- Material was thrown into orbit around Earth
- Theia's iron core merged with Earth's core
- Outer material from both planets was thrown into space
Stage 3: Ring of Debris
After the impact, a ring of debris formed around Earth:
- Rock, dust, and vapour formed a ring of debris around Earth
- Similar to Saturn's rings, but made of molten rock and dust
- The material stayed in orbit due to its speed
- The debris ring glowed from the intense heat
- Material orbited at various distances from Earth
- The debris stayed in orbit because it was moving very fast (orbital velocity) - gravity pulled it toward Earth, but its speed kept it from falling, and this balance created a stable orbit
Stage 4: The Moon Forms
Gravity gradually assembled the debris into the Moon through a process called accretion:
- Accretion: the process where small particles stick together to form larger objects. Gravity pulls particles together, and as the clump grows, its gravity gets stronger, attracting even more material. This is how planets, moons, and many objects in space form
- Gravity pulled the debris together (accretion)
- Small clumps formed first, then collided to make larger clumps
- Larger clumps had stronger gravity, attracting more material
- Over time, most debris collected into one large body
- This body became the Moon
- Process took between months to a few years
- The young Moon was initially much closer to Earth (about 20,000-30,000 km away)
- the Moon was extremely hot - molten on the surface
- took millions of years to cool down
- eventually formed a solid crust
- has been gradually moving away from Earth ever since
Evidence Supporting the Collision Theory
Scientists have gathered many different types of evidence that support the collision theory.
Evidence 1: The Moon is Less Dense Than Earth
The Moon has a lower average density than Earth. This makes perfect sense if the Moon formed from the outer material of Earth and Theia.
| Object | Average Density | What This Means |
|---|---|---|
| Earth | 5.51 g/cm³ | Dense iron core + rocky mantle |
| Moon | 3.34 g/cm³ | Mostly rocky material, small core |
- During the collision, the outer, less dense material was thrown into orbit
- The heavy iron cores of both planets merged together and stayed with Earth
- The Moon formed from the lighter outer material
- This explains why the Moon is less dense - it's mostly made of rock, not heavy metals
Evidence 2: Moon Rocks Are Similar to Earth Rocks
Between 1969 and 1972, Apollo astronauts brought back 382 kg of Moon rocks to Earth. Analysis of these rocks provided crucial evidence.
- Composition: Moon rocks have similar minerals and chemical composition to Earth's mantle rocks
- Oxygen isotopes: Earth and Moon rocks have nearly identical oxygen isotope ratios (like a chemical fingerprint)
- Age: Moon rocks are about 4.5 billion years old - the same age as the collision
Interactive revision notes, videos and practice questions load below.