5. The Moon: stepping stone to the planets
Origin of the Moon
There are three classical hypothesis for the origin of the Moon that have been advocated for more than a century. They are the fission, capture and accretion models, nicknamed the daughter, pickup and sister theories. But as Sherlock Holmes said in The Adventure of Silver Blaze, "I am afraid that whatever theory we state has very grave objections to it". So, we will move on to the more successful giant impact hypothesis.
The giant impact mechanism permits the Moon to form initially in the same part of the solar system as the Earth and to undergo a process that explains both the dearth of metals and volatile elements, as well as an enrichment of refractory elements, before solidification. It might have shattered the colliding object to smithereens and vaporized parts of the iron-poor upper layers of the Earth, blasting off a mix of terrestrial and impactor material into orbit where it coalesced to form the Moon that we know. If the impacting object had a heavy iron core, it might have tumbled into the still-forming Earth, merging with the planet's core.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University