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Name: Bernardo F.
Status: Other
Age: 20s
Location: N/A
Country: N/A
Date: Summer 2001


Question:
I´ve seen everywhere that Earth´s internal heat comes from both residual heat from the time it was formed, some 5 billion years ago! and from radioactive decay. But something doesn´t seem to fit, due to the long time period involved: how long can it take the most radioactive isotope to become a stable atom, thus not emitting any more heat? How many tons of radioactive isotopes do we need to emit so much heat as to keep the mantle in a fluid, if viscous state?

What I find most strange is that nobody seems to have realized that Earth´s iron core, which everybody accepts as being solid, and so has a density much higher than any other part of Earth, and which indeed rotates at a different speed than the mantle and crust, must generate in this process, combined with tidal forces from Sun and Moon on a rotating Earth, quite a lot of heat just due to friction. Or doesn´t it? If you put a boiled egg, which has a fairly similar density in all of it, it does rotate at a much faster speed than a raw one, in which components are not uniform, so Earth´s rotation would surely be faster, and its magnetic field would be lower, than with the accepted model of Earth´s composition.

I´d be most grateful if you took some time to explain on what I´ve exposed.



Replies:
I do not know how much heat you could expect from different rotational velocities of the earth's core and mantle. Planetary dynamics is not my field.

However, I can comment on radioactive decay. The earth is still plenty young to be heated by the decay of radioactive elements. Estimates are that the earth formed about 4.6 billion years ago. Plenty of natural radioactive isotopes have persisted from that time to the present. The most important are Thorium-232, Uranium-235, and Uranium-238, but there are many more. Here is a list of some long-lived naturally occurring radioactive nuclei and their half-lives:

Nucleus Half-life (billions of years)
Potassium-40 1.28
Rubidium-87 48
Indium-115 510,000
Lanthanum-138 110
Neodymium-144 2,100,000
Samarium-147 106
Gadolinium-152 110,000
Lutetium-176 36
Hafnium-174 2,000,000
Rhenium-187 40
Platinum-190 600
Platinum-192 >60,000,000
Thorium-232 14.1
Uranium-235 0.7038
Uranium-238 4.468


So, you can see that many of these nuclei, though radioactive, have not had time to completely decay away since the formation of the earth.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois



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