Is it a force field like the glass dome in the Simpsons movie or the one surrounding Wakanda in Black Panther? Or is it just a region of gas? How does it hold itself together? Isn’t space a vaccum, so wouldn’t the atmosphere get sucked into space? Or is gravity strong enough to counter the vaccum of space?
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To start, the thing holding it down is just gravity. Gasses “feel” the pull of gravity the same way you and I, they are just in a different physical state.
So there’s no force shield or anything holding them in, it’s the earths gravity that is pulling it all down and keeping it on earth. But some can and does get knocked away, during solar storms we can lose gas to space sometimes.
But overall, the gravity of earth is strong enough to hold those gasses near earths surface and keep them from floating off into space. The same way earths gravity keeps our feet on the ground. Or causes water to flow downhill.
It is exactly because of gravity. It’s also worth noting that the atmosphere never really “ends”; it just fades and fades and fades until at some point it’s so thin as to not be worth thinking about. But even up where geostationary satellites orbit, there are still gas particles getting pulled down towards the earth by gravity.
The atmosphere is a region of gas that fades smoothly into the vacuum of space the higher you go.
The reason for this shape is that there’s two competing forces involved:
* Gravity wants to pull everything in the atmosphere closer to the ground. Or, in more familiar terms, air “weighs something”. If you draw a tiny square on the ground, one inch by one inch, the air above that square, all the way to the top of the atmosphere, weighs about 15 pounds. (For our metric users, this is about 1 kg of air over a square 1 cm by 1 cm.)
* The air is a gas exposed to a vacuum, so it wants to spread out. In other words, air has an outward pressure the more you squish it down toward the Earth.
The actual shape of Earth’s atmosphere, and other planetary atmospheres like Venus’, Mars’, or even those of the gas giants, is the result of a balance between these two forces. To put it in your question’s terms, both gravity and the suction of exposure to vacuum are involved, and the shape of the atmosphere is the result of a balancing act between the two.
It turns out that these two opposing effects create an atmosphere that drops off roughly exponentially the higher you go. Typical atmospheres have a shape defined by a value called their [scale height](https://en.wikipedia.org/wiki/Scale_height): each time you go up 1 scale height, you divide the thickness of the atmosphere by *e* (about 2.718). The scale height depends on three main factors: the strength of the planet’s gravity, the planet’s temperature, and the specific gases that make up its atmosphere. Stronger gravity, lower temperature, and heavier gases result in a more compressed atmosphere (= a smaller scale height).
Earth’s scale height is about 8.5 km, meaning that the air at 8.5 km up is 1/e = ~37% as dense as it is at the ground, the air 17 km up (two scale heights) is 1/e * 1/e = ~14% as dense as the air at the ground, and so on. This works pretty well up to a high altitude. (It isn’t a perfect approximation because some other forces are involved, but it’s good enough as an answer to your question.) Mars’ scale height, on the other hand, is about 11 km: Mars is smaller than Earth, so its gravity can’t “squish” its atmosphere down as much even though Mars’ atmosphere is made of heavier gases than Earth’s.
After a bunch of these divisions, the atmosphere becomes so thin that it’s not really distinguishable from the occasional atoms you run into flying around in space. For the purposes of a satellite, for example, the drag from Earth’s very thin upper atmosphere is basically zero once you get up to a few hundred kilometers, although the exact distance varies due to space weather (no, really, that’s actually what it’s called: the behavior of the sun at the time affects the extent of the Earth’s upper atmosphere).
It’s a region of gas. There’s no glue holding it together – every individual molecule of air is pulled down by gravity and by piling up atop one another they manage to create a significant pressure downward.
Vacuum doesn’t apply force – it applies no force at all. Air applies a force outward. This outward force counteracts gravity. Otherwise our atmosphere would collapse into a flat sheet over the surface. If gravity got stronger, our atmosphere would squish down more, and if gravity decreased our atmosphere would expand.
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