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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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