You may have read that helium is obtained from natural gas, extracted by gas wells from underground. How did that helium get there? As a gas lighter than air, would it not be more logical to find it high in the atmosphere, not deep underground?

Believe it or not, helium exists underground because it was produced there. It is a product of the radioactivity of uranium, thorium and similar heavy elements, found in the Earth's crust.

A nucleus of uranium is held together by the nuclear force, which pulls together its 92 protons and 126 neutrons. But another force is trying to disrupt this neat arrangement. Each proton carries a positive electric charge, and since charges of the same sign repel each other, each proton is repelled by the 91 other protons sharing the same atomic nucleus.

The larger a nucleus, the harder it is for the nuclear force to hold all its protons together, which is why the uranium atom is the largest (and heaviest) atom found in nature: larger ones can be produced in the laboratory, but they tend to fall apart, in radioactive disintegration. Even uranium is just barely stable and ultimately disintegrates by expelling some of its positive charge, though it may take billions of years to do so.

You might think it would expel a proton, but from the point of view of energy, a more favored process is expelling two protons together with two neutrons, because these together form a very stable combination. That combination, in fact, is the same as the nucleus of helium, which is how helium comes to be underground.

Such helium nuclei are ejected with great velocity--a few percent of the velocity of light!-- and are then known as "alpha particles," a term dating back to the early years of research on radioactivity. Uranium and thorium atoms are widely scattered throughout the Earth's crust, and the energy released by their "alpha radioactivity," together with electrons from radioactive potassium, is the main reason why the deep interior of the Earth is so hot.

Those alpha particles are stopped within a very small distance, by collisions with atoms around them. Each decaying atom also lets go of two of its electrons, and these (or a pair like it) find their way to the alpha particle, producing an ordinary helium atom. When natural gas forms, from decayed plants (the way coal or oil form), some of these atoms are swept up by it--and from there, occasionally, they end up in our helium balloons and blimps.

Postscript: There is plenty of helium in the Sun, but a small fixed fraction of it is of a lighter variety, with only one neutron. We know helium in natural gas has a different origin, because all of it is of the ordinary kind.

The author, Dr. David P. Stern, retired from GSFC in 2001 but continues as emeritus. He has created several large educational web sites, including "From Starrgazers to Starships," home page http://www.phy6.org/stargaze/Sintro.htm. One of the many files there, http://www.phy6.org/stargaze/Sun7enrg.htm, tells about the energy of the Sun, about the unusual stability of the helium nucleus (see graph there) and about its role in creating solar energy.