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:
The above 3 scenarios for lunar formation are all physically implausible so why is the moon there?
Let's imaging the following sequence:
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.