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[personal profile] mellowtigger
I noticed the warning a few days ago that our primary supply of helium is nearly exhausted. The world's largest reservoir of it is located in the Texas panhandle (near Amarillo) and will likely run dry around 2016. The article explained how helium is produced on Earth, but it also stated, "... any helium ultimately released into the atmosphere by users, drifts up and is eventually lost to the Earth." This statement contradicts my own calculations on the matter, so naturally I was curious to learn the details.

Near as I can tell, my original calculations are appropriate but they should be applied in more specialized circumstances. It is not, apparently, the temperature at the surface that matters for giving molecules their kinetic energy for escape velocity. Our atmosphere has different layers, and it is the temperature in the highest layer, the exosphere, that determines the possibility of thermal escape for various molecules of gas.  It's 1800 degrees in the exosphere, a lot hotter than down here on the surface, and that's plenty of energy for sending atoms or even whole molecules zooming off into space.
Aside: I didn't know before, but this process is called the Jean mechanism, though I can't find why it has that name. There is also the possibility of electric escape, with ions snatched away by solar flux. On Venus, in contrast, it is electric repulsion of atoms from the atmosphere itself after just the electrons are lost via thermal escape.
But then which molecules at the surface manage to make it up to the exosphere?  I don't know.  Knowing that information is necessary in order to make my calculations again.  Apparently Earth does lose more atmosphere than I originally thought.  I wish I knew better how to calculate that loss.

Back to helium though.... apparently helium exists near equilibrium these days (being produced in the Earth and lost to space at equal rates).  But with the convenient ground sources nearly exhausted, and with important scientific and medical uses for helium taking precedence, it may be only a few years before party balloons disappear unless we find another safe alternative.  I sure can't imagine anyone selling hydrogen balloons.  (Mini-Kaboom!)  Helium is already more expensive (per liquid gallon) than gasoline.  It would get a lot more expensive if producers have to harvest it from the atmosphere directly.


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