points by thaumasiotes 3 days ago | 0 comments
In the air? That's the one place you can't find helium. The earth doesn't have enough gravity to hold it down; if it gets into the air, it escapes into space.
Any helium we have comes from underground, which is why you can see discussion of "mining" helium upthread.
[–] adrian_b 3 days ago
[–] BizarroLand 3 days ago
The earth has plenty enough gravity to hold down helium. It's the rest of the atmosphere, being denser and heavier than helium, that counteracts earths gravity and forces it up to the top where solar winds dislodge it.
[–] thaumasiotes 2 days ago
What are you trying to say? You just described the earth not having enough gravity to hold helium down. We've got other planets that can do the job, and the earth can't.
While it is true that helium escapes from the atmosphere, it is also produced continuously in the Earth's crust.
Thus there is an equilibrium concentration of helium in air, from generation and loss, and it is greater than for the heavier noble gases krypton and xenon, which do not escape from the atmosphere.
So there is more helium in air than krypton and xenon, which are nonetheless extracted from air, despite their low concentration. However, their extraction from air is easier than for helium, because when liquid air is evaporated the lighter and more abundant gases evaporate first and krypton and xenon remain in the small liquid residuum.