3 ms·
> There is no way that Helium reaches escape velocity without some sort of mechanical assistance. Thermal velocity is an average - there is a distribution of v
by martinpw 7y ago
> There is no way that Helium reaches escape velocity without some sort of mechanical assistance.
Thermal velocity is an average - there is a distribution of velocities around this average. It says that right in the article you linked to.
At the high end of the distribution, some Helium atoms will indeed have velocities greater than the Earth's escape velocity.
- liuhenry 7y agoI 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. [1] https://geosci.uchicago.edu/~kite/doc/Catling2009.pdf https://geosci.uchicago.edu/~kite/doc/Catling2009.pdf [2] http://geosci.uchicago.edu/~kite/doc/Catling_and_Kasting_ch_5.pdf http://geosci.uchicago.edu/~kite/doc/Catling_and_Kasting_ch_...
- martinpw 7y agoAgreed 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.