gmalivuk wrote:Is high-atmosphere hydrogen monatomic? If it's H2, then we'd only have 9km/s or so. Which is "good" news for Venus, with its lower escape velocity but more than adequate atmosphere.
This general issue is something I've struggled with a great deal with space exploration topics. It just gets complicated. My primary thinking is that we might just have stray Hydrogen ions by themselves, and those
could get hit. That would, of course, decrease the rate of this type of event by many orders of magnitude. There's also the issue that even an H2
molecule that gets stuck in cislunar space might just be blown away by the solar wind (charge interactions), or sunlight. I don't actually know how easily that would happen. If molecules are not regularly disturbed (which I think may be the case), then a molecule rocketed off the top of our atmosphere at 9 km/s will be a Newton's cannonball - meaning that it will rejoin the atmosphere in due time.
But the concept of a stray Hydrogen or Helium atom getting blow away has quite a few orders of magnitude reduction in event frequency by itself. The Helium atom is harder to blow away, but the sun's spectrum still has plenty of photons which are 2x or 3x as energetic as the mean, so we have to recognize that has some event frequency. I feel like the real question is what the photo-absorption mechanism is in the first place, because there's plenty of sunlight.
Ultimately, this is the picture it fits into, these are mass flows in and out of Earth's atmosphere.
Now that I revisit this, the space loss is quite unimpressive. The ISS is about 500 tonnes, which is 5x105
kg. So space only gives us like two ISS's worth of Hydrogen every year. That sounds like the least important fact you might ever learn, but over billions of years, maybe it affected the elemental availability to life?
Oh right, but the Hydrogen gas dissociation energy. I looked that up, and put in terms of velocity, it is this:
sqrt((2 * (4.52 eV)) / (2 amu)) = 20,883.336 m / s
Chemical bonds have a lot of energy. Although, this is what it takes to break two H atoms apart, so it's not really useful for this discussion. Even if a photon from the sun broke apart a H2
molecule, they wouldn't have much kinetic left after that. But we know the space loss is rather low anyway, so I'm kind of beating a dead horse.
Also, Helium use rate is something like 15 million kg per year (by first google result). So if my numbers were right (and they might not be), the Helium isn't actually floating into space. It's really just accumulating in our upper atmosphere, and it'll take the sun a long time to blow it away (okay, maybe 1000s of years, which is like a sneeze to the sun).
EDIT: last post kind of beat me to some of these points, which I didn't see until I submitted.