The Origin of the Moon

When we first explored the Moon, we weren't sure what we would find.  We expected to find similar material that we find on Earth.  What we found is even more unusual.

First, why did we expect similar material on the Moon?  Based on the location of the Earth in the Solar System (and by default, the location of the Moon), we expected to find refractory elements because they have a higher boiling point than volatile elements, i.e. they vaporize at high temperatures which in the early Solar System included the location of the Earth.  All planets close to the Sun are generally made up of this type of material because they are close to the Sun.  This is why we believed that terrestrial planets are close to their central star and Jovian planets are far from their star.  Because of this, we expected to find the Moon was made up of refractory elements.

However, when we actually went to the Moon, we discovered something really strange; the composition of the Moon is nearly identical to that of the interior of the Earth.  Not that we found the same material, but that the concentrations were the same.  So what does this tell us?

This tells us that when the Earth was very young, something collided with the infant Earth to create the Moon.  A Mars-sized body collided with the Earth to create the Moon and to leave some material on the Earth.  This helps explain three things:

1. The Moon's composition and why it is nearly identical to the Earth's composition
2. The Moon is receding from Earth. Recall that the Moon is moving about 2 cm away from the Earth every century (See blog post on the eclipses)
3. The 1:1 Moon-Earth resonance since the Moon formed from the Earth

Daily Galaxy