Collision Theory

Formation of the Moon – Collision Theory (Giant Impact Hypothesis)

• A Mars-sized body (Theia) collided with the early Earth about 4.5 billion years ago.
• The impact ejected debris from Earth's outer layers and Theia.
• Debris orbited the Earth and accreted to form the Moon.

Evidences Supporting Collision Theory

• The Moon is less dense than Earth.
• Moon rocks indicate it was once molten.
• Earth and Moon rocks have similar compositions.
• Fits current models of Solar System formation.

Evidence Contradicting Collision Theory

• Earth’s surface might not have been molten.
• Venus, with similar conditions, has no moon.
• The Moon’s composition is more like Earth than Theia.