[ExI] Heat of the Earth [WAS Re: Efficiency of wind power]

[Richard Loosemore]

Kelly Anderson wrote:
> 2011/4/9 John Clark <Jonkc at bellsouth.net>:
>> On Apr 9, 2011, at 2:03 AM, Kelly Anderson wrote:
>>
>> Given that the core is heated by nuclear fission,
>>
>> Geothermal heat does not come from nuclear fission, 80% of it comes from the
>> decay of radioactive isotopes, primarily Potassium Uranium and Thorium; the
>> remaining 20% comes from the formation of the Earth when a large cloud of
>> matter was compressed by gravity into a small ball 8000 miles in diameter,
>> all that gravitational potential energy was converted into heat.
> 
> Thank you very much for this clarification. I somehow missed the
> difference between fission and nuclear decay. Lord Kelvin estimated
> the age of the earth at something around 8000 years based on the
> cooling without the nuclear component... what is science's best guess
> as to when the decay of isotopes will stop? Is it before or after we
> are swallowed by the sun? I ask because it seems bad things happened
> to Mars after it's core became more solid and it lost it's
> magnetosphere.
> 
> I would add that a lot of the initial heat in the formation of the
> earth came from the impacts of comets and asteroids into the earth.
> This may be what you meant by gravitational potential energy converted
> into heat, but the picture is so much more vivid when you look at
> comets slamming into a molten orb at 20,000 MPH! :-) That's a LOT of
> heat, although assuming Kelvin was right about that part, it seems not
> to account for the majority of the heat over the eons... but I don't
> know if Kelvin was correct even on that point. He may have just been
> trying to match the Biblical numbers with pseudo science.

Gravitational PE converting to heat is really a continuous process, 
which applies to everything from individual atoms to entire 
planetesimals falling down Earth's potential well.  Drop a ham sandwich 
onto the ground right now and it will warm the planet up just a little.

So, the Earth was already bleeping hot before anything recognizable as 
comets started raining down on the surface.

I am guessing that the half life of the Earth's heat would be determined 
by the isotope with the longest half life, which would be Thorium at 
about 14 billion years.  Comparable to the Sun's lifetime, and well 
before I plan to move to another solar system, so there is no need to 
worry yet.


Richard Loosemore