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How did the Moon form?

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Quick answer

The Moon formed roughly 4.5 billion years ago when a Mars-sized body collided with the young Earth. The impact ejected a disk of vaporised rock and debris that orbited Earth, and within a short time—perhaps under a century—that material clumped together and became the Moon.

The Moon formed when a Mars-sized body struck the young Earth, blasting debris into orbit that gathered into the Moon. This giant-impact idea explains the Moon's lack of a large iron core and its similar oxygen chemistry to Earth.

Step by step

  1. A Mars-sized body, often named Theia, struck the young Earth at an oblique angle.
  2. The collision vaporised outer layers of both bodies and threw a disk of debris into orbit.
  3. In the disk, particles collided and stuck together, forming larger and larger clumps.
  4. These clumps merged into a single molten body, the proto-Moon, orbiting Earth.
  5. The Moon cooled and solidified, and tidal forces have since pushed it slowly outward.

Key things to understand

  1. About 4.5 billion years ago, a Mars-sized body often called Theia collided with the young Earth.
  2. The impact threw vaporised rock and debris into orbit around Earth, forming a hot disk.
  3. Particles in the disk collided and stuck together, growing into larger clumps called moonlets.
  4. These clumps merged into a single molten body that became the Moon.
  5. The Moon cooled, solidified, and settled into its current orbit, moving slowly away from Earth.

Labelled diagram

  1. Impact

    A Mars-sized body strikes the young Earth at an angle.

  2. Debris disk

    Vaporised rock and debris are thrown into orbit around Earth.

  3. Accretion

    Particles in the disk collide and stick, growing into larger clumps.

  4. Moon forms

    The clumps merge into a single molten body orbiting Earth.

  5. Cooling

    The Moon solidifies and settles into its present orbit.

How the Moon formed from a giant impact

Real-world examples

  • The Apollo missions brought back Moon rocks whose oxygen isotopes match Earth's, supporting the giant-impact idea.
  • The Moon's small iron core, inferred from its low density and seismic data, fits a debris-disk origin.
  • Computer simulations at institutions like NASA and the Southwest Research Institute reproduce the impact and debris disk.
  • The Moon's pale highlands formed from rock that floated on a global magma ocean, consistent with a hot, molten origin.

Common mistake

✗ Myth: The Moon was captured by Earth's gravity after forming elsewhere in the solar system.

✓ Fact: Capture is very unlikely because a passing body would need to lose enormous energy to enter orbit. The Moon's chemistry matches Earth's mantle, and its small core fits a giant impact far better than capture.

People also ask

What was the name of the planet that hit Earth to form the Moon?
Scientists often call the impactor Theia, after the Greek Titan mother of the Moon goddess Selene. The name is a convenient label, not an official designation, because no one knows exactly what the body was like or where it formed.
How long did it take for the Moon to form after the impact?
Most simulations suggest the Moon gathered from the debris disk quickly, probably within a few years to about a century, though estimates range from hours to thousands of years. The exact timescale depends on how much material was thrown into orbit and how sticky the collisions were.
Why is the Moon's core so small compared to Earth's?
The giant impact blasted away mostly outer, rocky material from both bodies, leaving iron-rich cores behind. The debris that formed the Moon therefore contained little iron, so the Moon ended up with a small core and a lower overall density than Earth.
Why does the Moon always show the same face to Earth?
The Moon rotates once on its axis in the same time it takes to orbit Earth, a state called tidal locking. Earth's gravity long ago slowed the Moon's spin until the two periods matched, so one hemisphere always faces us.
Could the Moon have formed without a giant impact?
Other ideas exist, such as the Moon and Earth forming together as a double system, or Earth capturing a passing body. Each struggles to explain the Moon's small core and its near-identical oxygen isotope mix with Earth, so the giant-impact model fits best.
What happened to the body that hit Earth?
Most of the impactor's material merged with Earth, while some was blasted into orbit and became part of the Moon. The collision mixed material so thoroughly that the Moon today carries a chemical signature of both bodies.
Is the Moon still moving away from Earth?
Yes. Tidal interactions transfer Earth's rotational energy to the Moon's orbit, pushing it about 3.8 centimetres farther away each year. This also lengthens Earth's day very gradually over millions of years.

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