Origin of Moon

1. Origin theory must explain

Moon has low average density - hence no iron core.
Moon's surface rocks have no water.
Moon formed at least 4.4 billion years ago.
Moon's orbital plane inclined to plane of Earth's equator.
Moons is a large satellite.
Venus has no satellite.
Mercury has no satellite.
Mars has two shall satellites.

Classical Origin Theories
Fission Theory - Moon thrown off young rapidly spinning Earth.

• explains low Moon density.
  
• cannot explain Moon's inclined orbit.
  

• problem incoming object speeds up and avoids capture. Earth's gravity accelerates incoming object.
  

• problem Moon & Earth different composition
  
• why don't Venus and Mercury have satellites?
  

Presently Accepted Lunar Origin Theory
- Giant Impact Theory

A Mars- sized body struck young proto-Earth a glancing blow, causing an explosion (no volatiles, no water) and hurling material in orbit to later form the Moon.

Explains:

• inclined orbit
  
• lack of water
  
• low density of Moon
  
• why only Earth has large satellite - compared with Venus, Mars, Mercury
  
• formed long ago, lots of colliding objects around.
  

Apparent size Moon = apparent size Sun.
Moon's rotation period = Sun's rotation period.

Lesser gravity on Moon permits building larger telescopes. Where is the giant impact crater left over from Moon's formation? Removed by 4 billion years of continent building and erosion.

Can Saturn's ring particles coalesce to form a larger satellite? No they are too close to the planet, so any material that made the Moon had to be far enough away from Earth.