Atmospheric Helium

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Helium constitutes 0.0005% of the earth's atmosphere. Helium is produced underneath the surface of the earth by the alpha decay of different radioactive isotopes, including Uranium. This very small, mobile gas migrates through pores in the rock and dirt and escapes into the atmosphere. This rate of introduction into the atmosphere is 13 million helium atoms/square inch/second [or 67 grams/second]. Helium can escape into outer space at a maximum of 0.3 million helium atoms/square inch/second.

Dividing the known amount of helium in the atmosphere by this rate of accumulation gives a maximum age of the earth at no more than 2 million years! This assumes that there was no helium in the atmosphere to begin with, and that nothing has disturbed the earth’s surface to cause the Helium to bubble out quicker. The only way to get an old earth out of this measurement of atmospheric helium is to invent unknown ways of Helium escape into outer space, ways that do not exist now.

Helium escape the atmosphere? To escape our atmosphere, Helium molecules must exceed the "escape velocity", which is 24,200 miles/hour. The average Helium molecule velocity is 5,600 miles/hour. This is why very few Helium molecules escape the atmosphere.

Summary: Considering the amount of radioisotopes in the earth’s surface, and the amount of Helium being produced, then an old earth should have an atmosphere laden with Helium, but it does not. Only a young earth model explains our low helium levels satisfactorily.

It is claimed that the atmosphere's level of Helium-4 (which is created by radioactive decay) should be much higher if radioactive decay has been going on in the Earth's crust for 4.6 billion years. The calculation behind this claim assumes that helium escapes by a simple thermal-gravitational process, as it does from planets without large atmospheres. In reality, the Earth's magnetic field causes it to give off a stream of particles from the poles, in a process identical to the formation of the solar wind. This "polar wind" carries atmospheric particles away from the Earth, and is primarily responsible for removing Helium-4 from the atmosphere at a faster rate than gravitational escape alone would do (see page 8 of D.M. Hunten, Icarus, v.85, pp.1-20 (1990)).(1)

But if we're going to use this to explain the lack of helium, we need to know the wind's rate of removal.

Looking back at the original text, we read: "Dividing the known amount of helium in the atmosphere by this rate of accumulation gives a maximum age of the earth at no more than 2 million years."

Does it really work fast enough for 2 million years worth of helium to accumulate instead in over 4 billion? This is a far stretch, in my opinion, and seems unlikely. (If we divide the 4.2 by .002, we find that for an old earth to have existed, polar wind would have had to remove helium a couple thousand times faster than regular molecular escape.)

To try to find the removal rate, I looked up the reference he gave: The Icarus Journal, v.85, Issue 1, May 1990, Pages 1-20, ISBN 0019-1035

Unfortunately, I wasn't able to, since the article must be purchased. I did manage to find an overview, though: The escape of gases from atmospheres is discussed from a historical viewpoint, with emphasis on concepts rather than details, and on the interaction of ideas and observations. Although rooted in theoretical ideas from the kinetic theory of gases, the subject remained almost sterile until space-age data could be brought to bear. Major advances followed from the first sounding-rocket data on the Earth's upper atmosphere, from the Mariner 5 results on hydrogen near Venus, and the nitrogen isotopic composition found by Viking at Mars. These results stimulated concepts of diffusion-limited flow, non-thermal escape mechanisms, formation of bound non-thermal coronae, energy-limited escape, and mass fractionation during early blow-off. As a consequence, we not only have acceptable explanations of a wide range of phenomena, but also prospects of explaining the xenon isotopic patterns in meteorites and various atmospheres, and the still-mysterious behavior of Io's atmosphere and plasma torus. <further research needed>

100,000 tons of rock are added to the earth each year and will contain He. This also has an effect on the amount of helium in our atmosphere over a long period of time, and the response above gives no indication of having taking this into account. <further research needed>

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