> One thing about helium: once its released to the atmosphere its gone forever. Because it is so light it not only heads upwards, the velocity of the atoms is high enough that some exceed the Earth's escape velocity and don't come down.
I'm all for preserving helium, but sensationalist BS doesn't help.
> On the surface of the Earth, the escape velocity is about 11.2 km/s
There is no way that Helium reaches escape velocity without some sort of mechanical assistance.
I've heard it theorized that Helium, because it is prone to float at the outer edges of the atmosphere, is particularly susceptible to being stripped away by solar winds. I don't have sufficient expertise to evaluate this claim, but it seems at least plausible. This can't be happening very much, though, or our atmosphere would much more closely resemble that of Mars, which lacks a protective magnetosphere.
I had this impression as well, but there's something else going on. If you run the numbers, at T=293, there's a negligible fraction above 5 km/s. The atmospheric loss models typically use an exosphere temperature of T=1000, but again, negligible fraction above 10 km/s.
However, there were some contradictions I couldn't immediately understand. This source [1] says "Only about one in a million helium atoms is lost from Earth via Jeans’ escape.". But it also cites a plot from [2] showing the rule-of-thumb for Jeans' escape: 1/6th the escape velocity vs the RMS thermal velocity. In this case, He has an RMS velocity of 2.5 km/s at T=1000, which is greater than 1.9 km/s.
Agreed the article is not very clear. From the context of the surrounding paragraph though, I think it means one in a million helium atoms is lost via Jeans’ escape every year. Which would obviously be quite a significant factor on any geological timescale.
I'm all for preserving helium, but sensationalist BS doesn't help.
> On the surface of the Earth, the escape velocity is about 11.2 km/s
https://en.wikipedia.org/wiki/Escape_velocity
The thermal velocity (at room temperature) of He is 1.245 km/s
https://en.wikipedia.org/wiki/Thermal_velocity
There is no way that Helium reaches escape velocity without some sort of mechanical assistance.
I've heard it theorized that Helium, because it is prone to float at the outer edges of the atmosphere, is particularly susceptible to being stripped away by solar winds. I don't have sufficient expertise to evaluate this claim, but it seems at least plausible. This can't be happening very much, though, or our atmosphere would much more closely resemble that of Mars, which lacks a protective magnetosphere.