Hmmmm, really? I mean seriously, a butterfly... a bird...a house fly... a bumble bee...even a grass hopper... can create enough "escape velocity" when they are right here on the surface with full gravitational forces....
That's not ESCAPE velocity. ESCAPE velocity is where you are able to travel beyond the distance required for the force of GRAVITY not to pull you back down. Butterflies, birds, flies, bumble-bees, and/or grasshoppers don't ESCAPE from Earth.
How, then, does a He2 molecule.. that just floated up there, against the pull of gravity.... get held there at the far reaches of our atmosphere... by that weaker gravity... in competition to the very high pressure difference of space, a vacuum, and our atmosphere?
I believe that the point is that Helium molecules (which are lighter than any other molecules other than Hydrogen molecules) can and do escape from the Earth's atmosphere, because of the lessened effect of gravity upon these molecules (due to their small mass).
Heavier molecules (for instance oxygen, nitrogen, etc.) are too heavy to drift out of Earth's gravitational field. Just as you are too heavy to drift out of Earth's gravitational field.
Also note that the atmosphere thins as you (or a helium atom) gain altitude. Thus, a low-mass molecule can traverse Earth's atmosphere to reach the very thin upper reaches where the atmosphere finally gives way to the vacuum of space.
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