Differentation in early earth history?

[j1j2j3]

I've just heard something from my brother and I just can't grasp it so I put it here and hope some learned person can help enlighten me.

Obviously the earth at the time of it's formation was homogeneous. From what I learned in uni it was radioactive decay which caused heat which in turn caused differentation in the earth's composition.

What he's saying is that a large asteroid hit the earth early in it's lifetime causing large amounts of thermal energy which in turn is supposed to be the primary cause of differentiation. Also that Luna is a by product of this impact in where the earth spewed out large amounts of matter.

Now I argued that since Mercury , Venus , Mars don't have large sattelites and yet have a similar interior composition to earth, that heat inducing impacts could not have been the primary cause. Yet he says that Earth had a large impact and the others could have had numerous small impacts.

I admit impacts could have caused some thermal heating, but is it the primary cause of interior differentation? Or could it be a combination of both?

[Surlethe]

The Earth was much hotter in the past simply because when it coalesced from the dust cloud into the planet we all know and love today, the force of gravity did work on the particles which had to bleed off. To my understanding, it was primarily this excess heat which, if I understand your question correctly, permitted the heavy elements to sink to the core and the lighter elements to float.

[j1j2j3]

What I learned is that this excess heat was caused by radioactive decay. What my brother is saying is that this excess heat was caused by outside impacts.

You're saying that it had excess heat from lumping up together?

[Surlethe]

It's quite possible I'm wrong. I was under the impression that early heating was caused by gravitational work, but if I'm wrong, I'd think it likely that it was at least a combination of radiation and impacts.

[GrandMasterTerwynn]

What I learned is that this excess heat was caused by radioactive decay. What my brother is saying is that this excess heat was caused by outside impacts.

You're saying that it had excess heat from lumping up together?

When the planet first formed, it formed via numerous collisions between smaller objects. Each object carries a significant amount of potential energy, which is converted to heat when the object was brought to an abrupt halt by impacting the early Earth. However, these planetesimals and other debris tended to be relatively cold. This sort of heating was fairly localized and shallow, and would've (for most impacts) only melted the top layers of a large planetesimal.

What generated the bulk of the heat responsible for planetary differentiation was the accumulation and decay of radioactive nuclei by the growing planets. Your brother's statement is only partially correct, a large Mars-sized object did collide with Earth early in it's history, and that collision did produce the Moon. However, both bodies were already well-differentiated at the time of the impact.

The reason we know this is that the Moon is very poor in heavy metals, and its composition closely resembles that of rock from Earth's mantle. The Moon was formed from materials lofted into orbit from the mantles of both Earth, and its impactor. The impact took place at such an angle that the iron core of the impactor plowed into Earth and eventually merged with Earth's own core, throwing up a lot of material from Earth's outer layers. If both bodies were homogeneous at the time of the collision, we would expect the Moon to be as dense as Earth, and have a large, dense iron core. But, it's rather less dense than the whole of Earth, and has this puny little core.