As far I understand gravity holds the air back down, but surely it isn’t like a seal where air can’t escape
In: Planetary Science
Yes, it’s gravity. It’s not a “seal” but it is strong enough to hold the bulk of gas in the immediate area pretty close to the surface (relatively speaking). Mind, there’s no hard line where on one side there’s air and on the other there isn’t. There’s no line that defines where the atmosphere ends and space begins. It’s a pretty smooth transition where the air gets thinner and thinner and thinner until there’s almost nothing, but almost nothing isn’t *nothing*. Even the Sun’s gravity increases the density of stuff in space by a little bit – interstellar space is a little less dense, and then intergalactic space is even less dense.
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