Quote:
Originally Posted by Jim Mash
My second question is how is it that the Earth has not cooled down by now after 4.5 billion years? You cannot seriously explain it with radioactivity because that would not explain why other planets give out much more heat than they receive from the sun.
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Actually, it's fairly simple. The earth does not emit much more heat than it receives from the sun - it's pretty close to equilibrium externally. As for the interior heat? The heat is residual from formation, and as the core is slowly solidifying, the latent heat of fusion released is keeping the area around it warm and convecting. It definitely does not require any radioactivity or other similar ideas to explain. As for the planets that are emitting significantly more heat than they receive (gas giants), they are still shrinking as they cool, with the slow compression keeping the interiors hot. Also, it takes a larger object longer to cool from the same initial temperature, as the radiating surface area goes up by diameter squared, while the volume (and therefore the amount of stuff to be cooled) goes up by the diameter cubed. All of the gas giants are very large. It makes sense that they would be farther from equilibrium than the smaller planets.
(I know it's only one of your points, but I don't have time to go over any more right now)