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From: fish@ihu1g.UUCP (Bob Fishell)
Newsgroups: net.misc,net.physics
Subject: Re: What heats up the center of the earth?
Message-ID: <277@ihu1g.UUCP>
Date: Tue, 3-Apr-84 17:53:35 EST
Article-I.D.: ihu1g.277
Posted: Tue Apr  3 17:53:35 1984
Date-Received: Thu, 5-Apr-84 03:21:55 EST
References: <255@heurikon.UUCP>
Organization: AT&T Bell Labs, Naperville, IL
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You should properly be asking what keeps it from cooling off.
The Earth was formed, along with the other planets of the 
Solar system, out of gravitational coalescence of floating
gas, debris, dust, etc, as was the sun.  It and many of the
other planets have never cooled off from that time; originally,
all the larger bodies in the Solar system were hot.  Only the
sun, and possibly Jupiter, were hot enough and heavy enough 
for sustained nuclear reactions to occur.  This is why the
sun shines and Jupiter appears to have an energy source at
its core, while the other planets have cooled, or at least
crusted over. This is an oversimplification; composition of
the bodies is also a factor, but you get the idea.

As to what keeps the Earth's core warm, all the silicate material
in the planet's crust is a pretty good insulator.  The planet
doesn't radiate that heat very well, and in fact collects a good
deal of what it does radiate from the sun.  This does not necessarily
imply a state of dynamic equilibrium, but the Earth  is cooling only
at a very slow rate.  The USGS could probably give reasonable figures.
-- 

                               Bob Fishell
                               ihnp4!ihu1g!fish