Formation of the Moon
- According to the most widely accepted theory of the formation of the Moon,
the Moon formed when a Mars-sized object collided with the Earth about 4.5
billion years ago ('Giant Impact Theory').
- Consistent with the collision theory:
- The composition
of the Moon is very similar to the Earth's mantle. The Moon has very little
iron in its core.
- Computer simulations confirm the impact hypothesis.
- This
video goes over some of the history of this theory and has some nice
animations.
Failed theories on the origin of the Moon
- Fission theory--A molten Earth budded off the Moon. This theory failed
because it would imply:
- a ring of smaller moons rather than one big one
- the moons would fly off and then fall back to Earth (i.e., stable
orbit is unlikely)
- Co-accretion theory--The Moon formed at the same time as the Earth from
the same material. This theory failed because it would imply:
- Moon should have an iron-rich core like the Earth, bit it does not.
- The Moon is moving away from the Earth and, on "playback" appears to
have been born from the Earth.
- Capture theory--The Moon was captured by the Earth like many of the
moons of Jupiter. This theory failed because:
- A captured moon would follow an elliptical orbit. However, the Moon
follows an almost circular orbit.
- The Moon is too big to be captured. A collision with Earth would be
more likely.
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