Saw a guy asking about helium balloons, got me thinking, where does used helium go ? Straight up ?
It goes up, but doesn’t stay there. Helium tends to escape the atmosphere into space.
Ok. related question.
What kind of gravitational field would Earth need to hold on to Helium?
(For the sake of making a useful comparison, let’s say that Earth’s radius stays the same and only its mass changes. What would g be at the surface if it was massive enough to retain He?)
Somewhere less than that of Uranus (14.5 x earth mass,) which does retain it. But I don’t think you can get a definite answer, as other variable factorsalways come into play. And g isn’t relevant exactly - Uranus has a g of 0.89 of Earth’s, but it has an H-He atmosphere. Of course, its V[sub]e[/e] is nearly twice ours (21.2 km/s vs 11.2 km/s) and that’s the thing that matters.
If Earth were as far from the Sun as Uranus I’m pretty sure it would have helium in its atmosphere too. Even Titan has a trace of helium and hydrogen, despite having a very low escape velocity.
Forgive me. Uranus is huge compared to Earth. Are you saying it’s mass is less and its gravity field is weaker?
Please note that helium does not go “straight up.” Like the other gases, it’s concentration is highest at the surface of the earth, but the height at which the concentration is half as much is at a greater altitude than nitrogen.
The air isn’t stratified, it’s all mixed up because gravity is such a weak effect (compared to the force of collisions between molecules). There isn’t a layer of CO2 near the ground, and there is no layer of He at the top of the atmosphere. It’s the same reason you don’t find a layer of pure alcohol at the top of a wine bottle. (Oil & water is a different matter, it has to do with how non-polar and polar molecules behave against each other.)
And all components of air escape into space. But the lighter the molecule, the faster it’s moving, so helium escapes from the earth’s atmosphere at a faster rate than other gases.
Uranus has a significantly higher mass and a slightly lower surface gravitational field than the Earth. Neither of these is directly relevant to the question at hand, though: More relevantly, it has a significantly stronger gravitational potential.
By analogy, think of a mountain. A tall mountain is analogous to a strong gravitational potential. A steep mountain is analogous to a strong gravitational field. While a steeper mountain may well be taller, it doesn’t necessarily need to be. Uranus is like a mountain that’s tall but not very steep, compared to the Earth.
Um…
Why?
Edit: I’m guessing radius and density figures into it. (I’m not sure I understood the mountain analogy…)
Yes. For a planet of given mass, further away from the center of the planet, the weaker the gravity. So on a planet that’s massive but not very dense, the “surface” is pretty far from the center, so the gravity is weak.
About the “mountain analogy” - he was explaining the difference between gravitational field and potential. Imagine you have a 1kg weight on a massless string, suspended from very high above the planet. Gravitational field describes how strongly the weight is pulling on that string. Gravitational potential describes how much energy it takes to reel that weight all the way out of the planet’s gravity well. On a small dense planet, the gravitational field decreases quickly with altitude, but on a large, less dense planet, it decreases more slowly. So it takes more energy to pull the weight all the way out from a large, less dense planet.
I hear what I read in my head.
ROFLLLLLLLL
So how come when you hear astronauts talking in a space station, their voices aren’t comically high?
Oh, never mind, I figured it out. They have helium filters in the space station.
:: gears in head working, moving stiffly, but moving… ::
Thanks for the post.
Let me give this a shot.
The Sun is constantly ejecting millions of tons of helium.
The helium does not stick to the Earth but will stick to Uranus because
Uranus has a better “gravity well.”
My horse sense says helium itself is a lot colder and heavier when it reaches
“heavy” Uranus. But helium is hotter and lighter when it reaches our atmosphere.
I know this isn’t correct but it sounds correct.
For what it’s worth, I pronounce “Uranus” as something like “OOR-a-NOOS”, with the second syllable unaccented, and no “Y” sound at the beginning. This is both reasonably close to the Latin pronunciation, and avoids scatological puns.
^^^ that’s how my English teacher taught us to say it.
nm
FWIW, from Wikipedia article on Helium: