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Earth Cool Down Time
Name: John
Status: Student
Age: K-3
Location: New York
Country: United States
Date: December 30, 2004
Question:
The earth has been slowly cooling since it was formed
with some heat gain from solar energy.
After 4-5 billion years why has the earth not cooled off? Radiational
cooling to outer space likely exceeds solar gain, so there must have been
a huge net loss of heat over time.
Replies:
In addition to the heat retained from the formation of Earth, heat is
generated by the decay of radioactive elements. While the concentration of
radioactive material is not particularly high, the total amount of heat
generated is significant because the mass of the earth is very great. So,
you can say that the planet is nuclear-powered as well as solar-powered..
Andy Johnson
Thanks for your question, John… You are absolutely correct in
stating that Earth has been cooling since its formation approximately
4.6 billion years ago… and with that statement, you have hit on the
important issue; what is the source of that heat in the first place?
The primary energy source of Earth is radioactive decay. The sun,
gravity, and meteorite impacts all contribute some energy, as well,
but not nearly as much as that provided by radioactive decay
(estimated for the bulk Earth at around 6.18x10-12 watts/kilogram).
As a radioactive isotope decays, particles are ejected from its
nucleus for the purpose of stabilizing the atom. Radioactive decay
processes produce electromagnetic radiation (gamma rays, for example)
which transmit energy from the nucleus to the environment.
Additionally, the ejected particles have kinetic energy that
ultimately converts to thermal energy as the particles are
mechanically resisted by their environment. The crust, mantle, and
core of Earth contain varying amounts of radioactive elements, the
most important for heat production being Uranium-238, Uranium-235,
Thorium-232, and Potassium-40, with half-lives of roughly 4.47 billion
years, 704 million years, 14.1 billion years, and 1.28 billion years,
respectively. From the half-lives of these isotopes and a comparison
with the age of Earth, you can see that internal heat production via
radioactive decay will likely persist at near current levels for quite
some time to come.
As an aside, the discovery of radioactivity by Henri Becquerel in the
1890’s later aided Sir Arthur Holmes (during the early twentieth
century) in providing a mechanism by which Alfred Wegener’s theory
of continental drift (now called “sea-floor spreading”) could
work. Prior to that time, continental drift theory was harshly
criticized as lacking any plausible method by which such large pieces
of Earth’s crust could migrate. Holme’s suggestion was that
radioactive heat could cause convective upwellings and downwellings in
the partially liquid portion of the mantle which would, consequently,
enact a traction-like force at the base of the crust (similar to a
conveyor belt). This idea is still considered a somewhat plausible
mechanism to this day, though there are certain physical limits to its
successful operation. I hope this explanation helps…
Scott J. Badham
Department of Geology and Geophysics
University of Wyoming
The core of the earth is molten, but very viscous, so one mechanism for
heat generation is the movement of this molten viscous core. However, the
main reason is believed to be that the core contains radioactive elements,
primarily uranium, that produces alpha particles. This fission reaction is
believed to be the primary heat source. Since 238U has a half life of about
5 billion years most the primordial uranium is still around. If you do a
Google search on the term "Oklo reactors" you will find well documented
sites describing "natural" nuclear reactors that have been discovered. When
these were first found the reaction of much of the geological and physics
community was "NO WAY!!" But after many investigations, it seems that
Mother Nature has indeed made such nuclear devices.
Vince Calder
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Update: June 2012
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