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Earth's Heat Source
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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Update: June 2012
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