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
Vacuum doesn’t “suck”.
The reason vacuum “sucks” in air on earth is because it’s surrounded by air under pressure. So to be more specific, the surrounding air forces itself into unoccupied space. It’s pushed, not pulled.
Once we establish principle, it’s easier to explain vacuum of space. Without anything else acting on it, the air on earth would like to push out and disperse too. But there IS something acting on it, pulling it down. The Earth, it’s gravity. It’s what’s causing the air to be under pressure in the first place, because that’s how gas resists being squished. It’s not pushing out, it’s just resisting being pushed down even more. That’s why air pressure drops the higher you go, until it transitions into the vacuum of space.
TL;DR There’s only one force acting on it, towards earth’s surface
Well, that’s ALMOST all. Light enough gasses, like hydrogen and helium, float to the very top of the atmosphere where they can be blown off by the solar wind. So earth loses a some atmosphere all the time.
Gravity holds almost all the air down, but yes, Earth loses a bit of air (tens of thousands of tons, which sounds like a lot until you compare it to Earth’s atmospheric mass) every year. The gradual loss is called *outgassing*.
However, our gravity also pulls in tons of little rocks every year, some of which vaporize as they hit the atmosphere (and some of them even contain ice), so that replaces some of the loss. But over millions of years, yes, we should very, very gradually run out of atmosphere.
Vacuums don’t suck things, it’s just that things that can move around (like air molecules) are more likely to spread out because some of the ones on the outside are going away from the center. It’s kind of like if you released a bunch of ants in the middle of a room, eventually they’ll spread out just by moving in all directions.
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.
> surely it isn’t like a seal where air can’t escape Why not? What’s going to make it leave the earth? This “vacuum” isn’t constantly “pulling” on the air. The force that air molecule experiences is from the **pressure** from the air **underneath** that molecule. Once that molecule is in a place where the air below it isn’t pushing on it anymore, it’ll stay there.
It helps to imagine if the atmosphere consisted only of a single air molecule.
It wouldn’t get “sucked” into outer space, and what would happen is that it would just randomly bounce up and down, but Earth’s gravity is always strong enough to eventually pull it back down no matter how high it got. For the particle to actually leave, it would have to surpass the escape velocity, and normal molecules just don’t go that fast unless they’re hit by something like a cosmic ray or solar wind.
Bits of air do get blown away by solar winds constantly. The Earth also replenishes its atmosphere through volcanic activity.
An air molecule needs to be moving around 11km/s to escape Earth’s gravity, otherwise it will just fall back down. The only reason all the air molecules don’t fall all the way down to the surface is because they’re constantly bouncing into each other, and the ones near the ground are bouncing off that. They’re moving fast, but a few hundred meter’s per second fast, not 10+km/s fast.
Air does escape, in a small amount as it’s heated/sped up by energy from the sun, but mass is also added as the stellar wind collides with Earth and some of it slows down and is captured (for a while) by gravity. Water ice and bits of carbon-rich rock also fall into Earth and are vaporized before hitting the ground, adding to Earth’s atmosphere.
It’s not a closed system, but an ongoing one that is constantly seeking equilibrium between several different factors.
Ok, so instead of explaining fluid dynamics at an ELI5 level, lets just say that for this example, air and water are similar enough.
Think about when you have a bucket full of water, lets say you fill it up right to the top, so it spills out, just a little.
The water is now right at the very top. Any more and it will over flow.
This is how the air on the Earth is. The Earth is the bucket. And the air is the water. You would not expect the water in the bucket to get sucked out of the bucket. Even though the pressure of the air is less than the pressure of the water.
Gravity is holding the water down, the air is floating on top of that, and it fills up the capacity of the planet.
Stray molecules of air are lost to space, but that is mostly due to how solar wind, and other charged particles act when the atmosphere is very thin.
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