Is the Earth Young?

A Response to the Helium Argument

3. The atmosphere has less than 40,000 years worth of helium, based on just the production of helium from the decay of uranium and thorium. There is no known means by which large amounts of helium can escape from the atmosphere. The atmosphere appears to be young. a) Melvin A. Cook, Prehistory and Earth Models (London: Max Parrish, 1966), pp. 10-14.

Cook, as it turns out, was wrong. There are two basic mechanisms for getting helium out of the earth's atmosphere, thermal escape (which Cook must have known about) and ionic escape (which Cook either did not know about or underestimated).

Thermal escape is the standard method. Helium being the lightest thing around save hydrogen, it will tend to predominate at the highest levels of the earths atmosphere, the thermosphere. Some of the helium atoms will gain enough energy, either by collision, of through absorption of solar energy, to escape from earth altogether. If Cook made use of only the thermal escape mechanism, he would have underestimated the rate of helium escape, and derived an inconsistency between the relatively high rate of production, low rate of escape, and amount present, if one assumes an old earth.

However, it has been shown that helium ionizes and can then escape preferentially along the lines of the earth's magnetic field, where the field is concentrated near the poles. By adding up all of the known escape mechanisms and production mechanisms, it now appears that helium is in equilibrium in the earth's atmosphere. The present abundance is not a problem for the old earth.

Larry Vardiman made the same argument in I.C.R. Impact #143, May 1985. He mentions ionic escape, but assumes it is insufficient. But the papers I reference below came out some time later, and show that the ionic outflow mechanism is quite sufficient to solve the problem.


Helium Escape from the Terrestrial Atmosphere - The Ion Outflow Mechanism
O. Liesvendsen & M.H. Rees
Journal of Geophysical Research (Space Physics) v101(A2): pp2435-2443 (1996 Feb 1)

We have computed global He+ escape fluxes for a range and a variety of diurnal, seasonal, universal time, and solar activity geophysical conditions. We average over the short-term variables and compute the globally averaged escape flux for a range of cutoff latitudes, which separate regions of open and closed field lines, during one solar cycle. The global escape flux averaged over a solar cycle was computed, and we find that a cutoff latitude of about 60 degrees or lower is sufficient to balance the outgassing from the Earth's crust.

Helium Escape from the Earth's Atmosphere - The Charge-Exchange Mechanism Revisited
O. Liesvendsen, M.H. Rees & K. Stamnes

Planetary and Space Science v40(12): pp1639-1662 (1992 Dec)

We have studied the escape of neutral helium from the terrestrial atmosphere through exothermic charge exchange reactions between He+ ions and the major atmospheric constituents N2, O2 and 0. Elastic collisions with the neutral background particles were treated quantitatively using a recently developed kinetic theory approach. An interhemispheric plasma transport model was employed to provide a global distribution of He+ ions as a function of altitude, latitude and local solar time and for different levels of solar ionization. Combining these ion densities with neutral densities from an MSIS model and best estimates for the reaction rate coefficients of the charge exchange reactions, we computed the global distribution of the neutral He escape flux. The escape rates show large diurnal and latitudinal variations, while the global average does not vary by more than a factor of three over a solar cycle. We find that this escape mechanism is potentially important for the overall balance of helium in the Earth's atmosphere. However, more accurate values for the reaction rate coefficients of the charge exchange reactions are required to make a definitive assessment of its importance.