How did this get here?

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The moon has a density of 3.3 grams per cubic centimeter. Is this a problem?

The Origin of the Moon:

Until recently, this was not understood. The moon has long been an anamoly because its mass compared to the earth is 1/80 and there is very large for a planetary satellite

Possibilities for Lunar Origin:

• Formed in place by accretion --> NO! its composition is wrong
• Formed someplace else where the average density is 3.5? --> Implies formation near Mars --> how did it get here?
• Fissioned from the earth's mantle (note the density of the earth's mantle is around 3.5) - but by what mechansism?

The above 3 scenarios for lunar formation are all physically implausible so why is the moon there?

Let's imaging the following sequence:

• Shortly after the formation of the earth a leftover planetisimal roughly the size of Mars had either struck the earth or had a close passage to it.
• Here is the Sequence if there was an impact
  • 1. Liberation of Debris
  • 2. Formation of Debris Ring
  • 3. Moon forms via acretion

• Here is the Sequence if it was a close passage
  • Path of Encounter
  • Deformation of Molten Earth
  • Fission
  • Tidal induced debris ring
  • Moon forms via acretion

Since the moon formed from the condensation of a debris ring in relatively near-earth orbit, the moon was initially quite close to the earth. At this time (4.6 billion years ago) the earth was rotating quite rapidly (about once every 5 hours). The nearby moon exerted large tidal forces on the spinning earth causing it to slow down. This process continues to day. To conserve total system angular momentum, the response of the moon is then to move farther away from the earth.

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