How Come We Have A Moon?

But I didn’t want one! Stop your whining, Earthlings. We have a serious question to answer, courtesy of Tamara:

What’s the moon like below its surface, moving into the interior? And what’s the current thought on its formation?

Well, we do know a lot about the Moon’s insides the same way we know about the Earth’s; just like the Earth has earthquakes, the Moon has moonquakes. These tell us about the Moon’s interior. From back when we landed on the Moon, we planted the Apollo seismic experiments and saw 28 moonquakes. From these, we learned that the Moon has a crust about 60 km deep, a deep mantle, and a tiny core that’s less than 25% of the Moon.

But now let’s get to the really interesting one: how come we have a Moon at all? Of all the other rocky planets out there, Mercury and Venus have no Moons, and Mars just has a couple of captured asteroids. But Earth is special because we have a huge Moon! So let’s pause to take a good look at it, and then let’s figure out how we got to have one:

Well, the Moon has a bunch of interesting things about it that let us figure out the answer. These are:

1. The Moon has the same density as the Earth’s Mantle.
2. Surface rocks on the Moon (brought back by Apollo) have the same composition as rocks on the Earth. But…
3. There is practically no iron on the Moon, whereas there’s a bunch of it on the Earth (mostly in the core).
4. Rocks on the Moon have been “baked” by the Sun, and have virtually no moisture in them.

There are a bunch of discredited theories about how the Moon formed, but the one that works is awesome, and is called the giant impact theory.

Basically, a huge chunk of rock smacked into the Earth when it was still very young, and caused a big chunk of the Earth’s mantle to fly off around it. The chunks that didn’t fall back to Earth coalesced because of gravity and formed the Moon; the giant impact possibly created the Pacific Ocean on Earth. So the reason we think we got a Moon is just that we got “lucky” enough to get hit by a giant rock!

I was going to show you a computer simulation video of the formation of the Moon, but oh, my God, is it boring. (If you have 4 minutes and really want to watch it, go ahead and click the above link anyway, but I warned you.) It’s much more entertaining to watch Cookie Monster sing about eating the Moon, and if you’ve made it this far, you deserve the entertainment.