What is the Earth made of?

Simplicio: It is made of earth, of course.
Salviati: Woa! We got over earth, air, fire and water a long time ago. Now we know about chemistry .

Sagredo: I have an idea. Probably the Earth is made of rocks. So let's see what rocks are made of.
Salviati: Well, silicon and oxygen are pretty common. For instance, the common mineral quartz is silicon dioxide SiO₂. But maybe the inside is different from the outside.
Simplicio: I object. Didn't you say before that a simple theory is better than a complicated one.
Sagredo: Yes, I agree with Simplicio.
Salviati: Well, then, let's look at the density of the Earth. The density of common rocks is in the range 2000 to 3000 kg/m³.
Sagredo: So that must be the density of the Earth if the earth is made of rocks.
Salviati: Yes, but we can measure the density of the earth:

• We know the radius of the earth, radius ~ 6000 km , so we can calculate its volume.
• We can measure the gravitational force on an apple, and we know Newton's constant G, so we can use the mass of the Earth.
• Or, we could measure the orbital period of a satellite and its distance from earth and get the mass of the earth.
• Either way, we get

  density of Earth ~ 5500 kg/m³

Sagredo: Wow, that means that Simplicio was wrong!
Simplicio: Well, I admit it, but if the Earth isn't all made of rock, what else could it be?
Salviati: Lots of meteorites that fall on us from space contain a lot of iron ...
Sagredo: so how about iron? What is the density of iron?
Salviati: About 7800kg/m³.
Sagredo: So iron in the middle and rock on the outside would do it.
Simplicio: I still object. How could the iron get to the middle?
Salviati: Given the evidence of Mercury's surface, it looks as though the early history of the planets might have been pretty violent. Perhaps the energy released by a continual rain of material onto the new earth was enough to heat it up until it melted.
Sagredo: If it melted while it was being stirred up, that would just make it into a uniform mixture.
Salviati: Yes, except that then the heavy iron would sink to the middle while the lighter rocks would float to the top.
Sagredo: I see.

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