47  Helium from inside the Earth
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Radioactive decay processes which occur inside of the Earth produce helium and heat. However, the quantity of helium escaping amounts to only four percent of that expected in relation to the escaping heat. One possible explanation is that the major portion of the helium is retained on the inside of the Earth. Another possibility would be that the Earth still has a large store of heat from its origin, meaning that not all heat results from radioactive decay. Neither of these possibilities is compatible with the model of an old Earth.


Helium escapes from the inside of the Earth because of radioactive decay. Simultaneously, heat is produced which escapes to the Earth’s surface. The decay of uranium, thorium and potassium to produce the inert gases helium and argon is undisputed. The quantity of heat produced during this decay process should correspond to the quantity of helium escaping from the Earth’ crust. To generate one Joule of heat it would be necessary to produce 1012 He atoms and 2 x 1011 Ar atoms inside the Earth.

 
Now, the heat and helium escaping from the inside of the Earth has been measured:
 
The heat escaping through the Earth’s crust in the area of the oceans is 0.1W/m2. The quantity of helium escaping from the inside of the Earth in the area of the oceans is 4 x 109 helium atoms per square metre per second. The resulting calculated helium/heat ratio is 4 x 10¹º atoms per Joule, amounting to only four percent of what is expected on the basis of the quantity of helium escaping (1).
  

Two alternatives and two problems with the model of a 4.5 billion year old Earth:

a) Imagine that no helium at all is present on the inside of the Earth. If we now start a decay process producing the quantity of heat we measure today, it could, in fact, be expected that initially only a small portion of the helium would find its way to the Earth’s surface. The major portion would be retained by the Earth’s crust. However, a balance would have to build up in the long run. It is not imaginable that an incongruity between the quantity of helium produced and the quantity escaping still exists after 4.5 billion years.

b) Imagine that the quantity of heat escaping is the result of radioactive decay processes to only a small part. This is also hardly imaginable using a model of a 4.5 billion-year- old Earth. It is difficult to imagine that the Earth has still not cooled off after such a long period.


Helium in the Earth’s atmosphere:

Interesting in this context is also the fact that the Earth’s atmosphere contains less helium than predicted by the model of an old Earth.  However, it is not yet clarified how much helium can escape from the atmosphere into space and how many helium nuclei are added to the Earth’s atmosphere by solar wind (2).



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References:

(1) E. Ronald Oxburgh und R. Keith O´Nions, Helium Loss, Tectonics, and the Terrestrial Heat Budget, Science 237, 25 September, 1987, pages 1583–1588.
(2) Melvin A. Cook, Where is the Earth´s Radiogenic Helium?, Nature 179, 26 January, 1957, page 213.
 
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