Composition of the Earth's core

[crazylum]

I understand that most scientists believe that the Earth's core is composed of iron, nickel and perhaps some other elements such as sulfur and oxygen. I also understand that they believe that the Earth's core formed during a time when the Earth was molten and that the core was formed by heavier elements sinking and compacting and lighter elements rising on convection currents. My question is if it was formed by this process why is it not composed of elements heavier than iron such as uranium?

 

[billiards]

First you might consider the cosmic abundance of uranium in relation to iron. I think you'll find there's a hell of a lot more iron in the universe than there is uranium!

Second, uranium tends to get obund up in not especially dense minerals, like zircons, which makes it hard to sink.

 

[LURCH]

Now this is interesting; I had always assumed ther were non-negligable amounts of heavy elements in the core. I heard a long time ago that the original heat of compression from Earth's formation should have been radiated away eons ago, and one speculation for why the core is taking so long to cool is decay of radioactive elements. From there, I guess I just assumed that these heavier elements must be more abundant in the core, having sunk there during planetary formation.

 

[billiards]

I don't know where you heard that, but it's wrong. Some of the heat generated in the Earth is radiogenic, although the decay of U, Th, and K is mainly occurring in the mantle. Note that the outer core is liquid, a lot of heat is released as latent heat of crystallization, as the outer core gradually freezes to develop the ever expanding inner core.