I’ve had a conversation with a guy that claimed “it’s a myth”. An obvious face-palm, I know, but I realized I’d like to know.
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If you do a home-scale experiment, you might get confused. Say that you have a box divided in two halves, and then you suck the air out of the top half to make a vacuum. So, when you pull the divider away, the air in the other half shoots up instantly, but that’s not how our atmosphere works, so what gives?
The answer is that while gravity might be comparatively weak, it has an infinitely long range. In the scope of a box, gravity has no time to really affect how the air moves, but when you’re looking at the planet as a whole, suddenly for an air molecule to “shoot up” into outer space means that it has to travel a hundred kilometers directly up, all the while fighting Earth’s gravity. And that’s only the beginning, because even after all that travel, Earth’s gravity is only about 4% weaker.
For anything to escape Earth’s gravity well permanently, it needs to reach the escape velocity of 11 km/s (at the surface). If you shoot a molecule up any slower than that, it moves so slowly that Earth’s gravity has enough time to slow it down until it stops and starts falling back down. Particles are constantly fighting the gravity and bouncing all over the place, but they all still eventually “fall down” which is why our atmosphere is so densely packed near the surface of the planet.
In comparison, air molecules move at ~500m/s. Which, while fast, is an order of magnitude lower than the required speed. Solar radiation can skim some gases off the top and give individual particles enough of a boost to kick them off permanently, but the amount is insignificant.
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