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Old 08-03-2005, 07:28 PM
duffer duffer is offline
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Why doesn't Earth's atmosphere get sucked away into space?

Before going into work this morning I was watching coverage of the space walk where the guy was about to fix the belly of the shuttle. Somehow the conversation got around to welding in space. The main point being they just didn't know the effects of welding in a vacuum.

All the while there was a live shot from the shuttle looking down upon the planet. Then I started thinking, a vacuum, in essence, will suck in any material not in a vacuum. (Well, you know what I mean).

So what I wonder is, what holds an atmosphere in place around a planet? Why isn't it just whisked away into the void? I could understand if there were a physical shell made of titanium or something, but it's just gas. What keeps it all together?

Now, I realize an exact answer will probably read like the dissertation of an MIT grad, but I'm hoping to glean enough to have a basic understanding of this. I'm almost certain it has mostly to do with gravity, but again, we're talking about gasses.

What gives?
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Old 08-03-2005, 07:34 PM
Padeye Padeye is offline
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Gravity
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Old 08-03-2005, 07:35 PM
Captain Amazing Captain Amazing is offline
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The Earth's gravity keeps the atmosphere where it is, doesn't it?
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Old 08-03-2005, 07:35 PM
wevets wevets is offline
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Some of it is. One of the reasons Earth's atmosphere is mostly substances like Oxygen, Nitrogen, etc. rather than the substances far more common in the universe: Hydrogen and Helium is that Hydrogen and Helium are extremely light, and can more easily obtain high enough speeds to escape or evaporate out of the Earth's atmosphere.

Gravity keeps what we do have together - mostly.
Undoubtedly some molecules of the heavier substances escape too, but not enough to be concerned about.
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Old 08-03-2005, 07:36 PM
Larry Borgia Larry Borgia is offline
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IANA physicist, but I believe it's just gravity. You don't see a lot of hydrogen around the earth, because hydrogen is light enought that if it gets disturbed it can float away. The heavier gases don't reach escape velocity and are pulled back to earth.

The more massive the planet, the more atmosphere it will have. Compare the vast hydrogen sky of Jupiter to the airless waste of the moon.
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Old 08-03-2005, 07:41 PM
An Arky An Arky is offline
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Because North Dakota sucks!

Bwahahahahaha!

I'm sorry, such a clean set up and all.

I'll be here all week

What they said.
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Old 08-03-2005, 07:41 PM
Blake Blake is offline
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The Erath's gravity does indeed keep the amtosphere 'in.

The mistake you are making duffer is believing that a vacuum sucks. A vacuum doens't suck. Vacuum is a void, a nothing, it has no properties beyond non-existence. The reason a vaccum appears to suck on Earth is because there is atmosphere all around it trying to push into the void created. The apparent suction isn't caused by the vacuum itself sucking, it's caused by the atmosphere blowing into that vacuum to try to equalise the drop in pressure.

Of course in space there is no atmophere, which is precisely why the vacuum of space has no sucking power. Whether it's a vacuum cleaner or a perfectly evacuated jar if you operate them in space there woudl be absoluetly no effect on anthything around them because there si no presuure difference inside the vacuum and outside.

You are astually better off thinking of the atmophere pushing dust into a vaccum cleaner rather than imagining it being sucked in. It is far closer to reality.
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Old 08-03-2005, 08:46 PM
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But it is,
but it is!
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Old 08-03-2005, 08:58 PM
Squink Squink is offline
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Escape velocity from earth is about 11,000 meters per second.
The average (rms) velocity of N2 at 25°C is about 511 meters per second.
To get that up to escape velocity, the atmosphere would have to be very hot.
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Old 08-03-2005, 09:08 PM
Derleth Derleth is offline
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Quote:
Originally Posted by Squink
Escape velocity from earth is about 11,000 meters per second.
The average (rms) velocity of N2 at 25°C is about 511 meters per second.
To get that up to escape velocity, the atmosphere would have to be very hot.
Then all those sources I've heard saying terrestrial H2 and He are leaving the atmosphere entirely are wrong? Or am I misapplying your data?
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Old 08-03-2005, 09:12 PM
duffer duffer is offline
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Thanks Blake, I was hoping it was something like that. (Unless you're on the "in" and are sworn to keeping the rest of us trogs in the dark)

Like I said, I knew it was likely just plain ole gravity. What I started wondering about was the concept of a vacuum. Here's what I was likening it to. Imagine a Ziploc bag with an apple in it. Then you use one of those gadgets that sucks all the air out creating a vacuum. When you open the bag again (or jar, whatever) you can hear the air rush in. The vacuum, like a Hoover, created a void that the available air fills.

Now, I know there are differences that make this all happen that may not occur in space. For instance, I have a working understanding of ambient atmospheric pressure. I'm sure these laws of physics, while fundamentally the same, are affected differently based on whether it's taking place on my kitchen counter or in the troposphere. At some point, the particles are lost to the space, otherwise our atmosphere would be even larger. Seeing as a larger level of gravity affects potential size of an atmosphere, there must be some sort of leeching of a given body.

So maybe the question should be, how is the concept/pysical laws of an atmoshere different than what most of us are familiar with when we think of a vacuum?

Thanks for everyone helping me out on this.


An Arky, you live near one of the largest black holes known to man. Stones, glass, housing, etc. You know the rest
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Old 08-03-2005, 09:58 PM
Manduck Manduck is offline
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The air pressure that causes air to rush into that container when you open it is itself caused by gravity. The weight of all the air that is piled on top of that quantity of air pushes it into whatever void becomes available. It doesn't rush into outer space in the same way because the vacuum of space is above the air, and the air pressure is caused by the air pressing down under the influence of gravity. So, the force that is causing atmospheric pressure is acting away from space and toward the earth.

That's my layman's understanding, anyway. It will probably be eviscerated by the next scientist to post
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Old 08-03-2005, 10:03 PM
Squink Squink is offline
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Quote:
Originally Posted by Derleth
Then all those sources I've heard saying terrestrial H2 and He are leaving the atmosphere entirely are wrong?
Hydrogen goes quite a bit faster, about 1930 meters/sec at 25°C.
That's still well below escape velocity, but that's the rms speed, so some molecules travel much faster, and are lost to space.
Take a look at the Maxwell speed distribution here. (There's even a calculator, which I found after crunching the numbers by hand )
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Old 08-03-2005, 10:43 PM
Excalibre Excalibre is offline
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Quote:
Originally Posted by Derleth
Then all those sources I've heard saying terrestrial H2 and He are leaving the atmosphere entirely are wrong? Or am I misapplying your data?
The speed that gas particles move is inversely proportional to their mass (well, the littler ones move faster. I don't remember high school chemistry enough to know the actual equations anymore.) In a gas, the individual particles are moving at all different speeds, so with a light gas, the fastest ones will be moving fast enough to escape Earth's gravity - they're moving faster than escape velocity. Soon enough all of them will work their way out of the atmosphere. With heavier gases, only a tiny, tiny percentage would be moving quickly enough. H and He are light enough to escape, but oxygen, nitrogen, carbon dioxide, argon, and the other major components of the atmosphere are not.
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Old 08-03-2005, 11:15 PM
Derleth Derleth is offline
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Squink, Excalibre: That makes sense. Thank you.
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Old 08-03-2005, 11:31 PM
ultrafilter ultrafilter is offline
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If there were no gravity, a (non-rotating) planet's atmosphere would diffuse into the surrounding vacuum. I'm not sure how the rotation would affect it.

Quote:
Originally Posted by Manduck
The air pressure that causes air to rush into that container when you open it is itself caused by gravity. The weight of all the air that is piled on top of that quantity of air pushes it into whatever void becomes available. It doesn't rush into outer space in the same way because the vacuum of space is above the air, and the air pressure is caused by the air pressing down under the influence of gravity. So, the force that is causing atmospheric pressure is acting away from space and toward the earth.

That's my layman's understanding, anyway. It will probably be eviscerated by the next scientist to post
Next time, try holding the bag upside down, so that the vacuum is above the air.
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Old 08-04-2005, 12:47 AM
Manduck Manduck is offline
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Quote:
Originally Posted by ultrafilter
If there were no gravity, a (non-rotating) planet's atmosphere would diffuse into the surrounding vacuum. I'm not sure how the rotation would affect it.



Next time, try holding the bag upside down, so that the vacuum is above the air.
It's not above the huge column of air that's causing the pressure, though.
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Old 08-04-2005, 02:47 AM
Mangetout Mangetout is online now
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Quote:
Originally Posted by Derleth
Then all those sources I've heard saying terrestrial H2 and He are leaving the atmosphere entirely are wrong? Or am I misapplying your data?
I remember reading in some 'amazing facts for kids' book that hydrogen or helium released at ground level would accelerate upwards through the atmosphere and just keep zooming away from the Earth, however, I think the statement was based on the misconception that h and he have some kind of antigravity property.

They would tend to float to the top of the atmosphere (in the same way that oil tends to float on water); this probably means that they are statistically more likely to get skimmed off by the solar wind.
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Old 08-04-2005, 12:43 PM
chemistrydork chemistrydork is offline
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In case anyone finds it amazing that gravity holds the atmosphere in, keep in mind that 90% of the atmosphere is within 10 miles of the Earth's surface -- and that the Earth has a diameter of some 8,000 miles.

The thickness of the atmosphere is akin to the skin on an apple.
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Old 08-04-2005, 12:50 PM
Otto Otto is offline
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Quote:
Originally Posted by Blake
You are astually better off thinking of the atmophere pushing dust into a vaccum cleaner rather than imagining it being sucked in. It is far closer to reality.
Not to hijack too severely I hope, but do vacuum cleaners actually create any sort of vacuum?
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Old 08-04-2005, 01:52 PM
Captain Lance Murdoch Captain Lance Murdoch is offline
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There's sort of a part two to this as well.

The Earth's magnetic field prevents the atmosphere from being blown off into space by solar winds.
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Old 08-04-2005, 03:04 PM
Lemur866 Lemur866 is offline
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Yeah, the big misconception is imagining that vacuums suck. Actually, it is the atmosphere that blows. If you were in the space shuttle and opened the door, you wouldn't be sucked out by the vacuum...you'd be blown out by the air the space shuttle deciding to leave the shuttle.

Or...remember Alien Ressurection? In one scene someone gets sucked into space through a tiny hole in the hull of a spaceship and turned into spaghetti. Completely impossible. The force to push you out the hole would come only from the air inside the ship. There's a bit of energy potential there, but how much? How much pressure is the air inside the ship exerting on the hull of the ship? One atmosphere, likely. Can one atmoshere of pressure pull you into spaghetti? No, you'd need vastly greater pressure. That's why your eyes can't get sucked out of their sockets by a vacuum...there would have to be air behind your eyes blowing your eyes out that's only counterbalanced by 1 atmosphere of pressure pushing your eyes in. There might be small amounts of air behind your eyeballs, but very very little, and once that air escaped into the vacuum there'd be nothing left to push your eyeballs out.

So, there is a certain atmospheric pressure on earth caused by the earth's gravity pulling all that air in. The atmosphere wants to blow away into space, but gravity won't let it. The amount of atmospheric pressure is determined by the weight of the column of air above a certain spot. If there were more air on earth but the same gravity, we'd have higher atmospheric pressure, like on Venus, with less air we'd get the opposite. If gravity were lower, atmospheric pressure would be lower, and if gravity were higher pressure would be higher. The total amount of atmosphere is determined by how much gasses are generated over time balanced by how much atmosphere manages to reach escape velocity over time. Oxygen and Nitrogen molecules in the air on average are moving much slower than escape velocity. However, some are moving slower than average, some are moving faster. A few are moving much slower, a few much faster. A very very very few are moving at escape velocity. Since the rate of loss of atmosphere is constant, and the rate of production of atmosphere is roughly constant, the earth's air pressure has reached an equilibrium. Venus is in equilibrium too, but has much higher air pressure, Mars is in equilibrium but has much lower air pressure.
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Old 08-04-2005, 03:27 PM
Squink Squink is offline
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Quote:
Originally Posted by Otto
Not to hijack too severely I hope, but do vacuum cleaners actually create any sort of vacuum?
Yes, they do. A vacuum cleaner equipped with a centrifugal fan will easily maintain a pressure differential of 100cm H2O between the input and output ends. ShopVacs typically use this type of blade, while many uprights use a less efficient straight impeller design, but they all work by creating a vacuum.
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Old 07-20-2012, 03:37 PM
hontjenmab hontjenmab is offline
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Would the air be sucked out if you would install a theoretical vacuum resistant tube / hose with one end near the earths surface and the other end in deep space away from any atmosphere so full vacuum? This would create a differential pressure of 1 bar causing the air to flow through the tube into space? And since all the air from the top will be sucked out no air column would prevent the air from flowing towards space.

Friction losses should be possible to reduce by increasing tube diameter.

Also with regard to the vacuum cleaner. The goal of the vacuum cleaner is to maintain a vacuum for a certain loss rate. If the flow would stop the vacuum cleaner would reach a lower absolute pressure limited by how much resistance the fan will be able to overcome!
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Old 07-20-2012, 04:12 PM
beowulff beowulff is offline
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Quote:
Originally Posted by hontjenmab View Post
Would the air be sucked out if you would install a theoretical vacuum resistant tube / hose with one end near the earths surface and the other end in deep space away from any atmosphere so full vacuum? This would create a differential pressure of 1 bar causing the air to flow through the tube into space? And since all the air from the top will be sucked out no air column would prevent the air from flowing towards space.

Friction losses should be possible to reduce by increasing tube diameter.

Also with regard to the vacuum cleaner. The goal of the vacuum cleaner is to maintain a vacuum for a certain loss rate. If the flow would stop the vacuum cleaner would reach a lower absolute pressure limited by how much resistance the fan will be able to overcome!
This is a zombie thread, but no, that wouldn’t work.
Think about it - what’s magical about a tube? The upper part of the atmosphere is already touching a vacuum...
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Old 07-20-2012, 04:13 PM
Malacandra Malacandra is offline
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In answer to the first part (and please note this thread hasn't been posted to in 7 years, and some of those who did post to it have been banned, while others may not be posting any longer for reasons of their own), no. You can see this on a smaller scale using a vacuum tube filled with a denser fluid than air - a mercury barometer. There's a very good vacuum above the mercury, not an absolute one but close enough for present purposes such that the pressure differential is one bar to within a small rounding error, and it won't support a mercury column of infinite height but only one of about 30" depending on local atmospheric pressure. That models what you'd get with your vacuum-proof tube into deep space - when the weight of the air in the column matches the push of the atmospheric pressure from below, you have equilibrium, and no irresistible force sucking your atmosphere into space.
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Old 07-20-2012, 09:26 PM
Senegoid Senegoid is offline
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Vacuum sucks.
But gravity (on Earth at least) sucks even more.
YMMV (like, if you're on Mars or the moon).
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Old 07-21-2012, 10:12 AM
Der Trihs Der Trihs is offline
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Ha! I knew that the tube-to-space question had been asked relatively recently.
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Old 07-21-2012, 11:34 AM
BrotherCadfael BrotherCadfael is offline
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During my freshman orientation at UVM, a LONG time ago, I attended "Uncle Donny's Traveling Magic Show", an entertaining lecture hosted by Dr. Donald Gregg, retired professor (emeritus) of chemistry.

At one point, he had a long-necked flask filled with ammonia gas, sealed at one end. He put the end in a tank of water and removed the stopper. The gas instantly dissolved into the water, and the water fountained up into the flask. He had added phenolphthalein to the water, so also turned pink as it rushed up the neck of the bottle.

He then said, "Now all of you are thinking that the water was just sucked up into the flask. That is WRONG. I know some people say UVM sucks, but this is not an example of that! What actually just happened is that after the gas dissolved into the water, the water was PUSHED into the now-empty space by the weight of the air surrounding it."

Made the point very effectively, and I have never forgotten it.
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Old 07-21-2012, 12:36 PM
clairobscur clairobscur is offline
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Interesting zombie. I too had the misconception that vacuum somehow sucked.
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Old 07-21-2012, 12:44 PM
John Mace John Mace is offline
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Nature abhors a vacuum, but not Zombies.

BTW, Mars did lose its atmosphere long ago. But then, Maris has only 1/10 the mass of Earth, and so proportionately less gravitational pull.
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Old 07-21-2012, 07:04 PM
BrotherCadfael BrotherCadfael is offline
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Quote:
Originally Posted by John Mace View Post
Nature abhors a vacuum, but not Zombies.

BTW, Mars did lose its atmosphere long ago. But then, Maris has only 1/10 the mass of Earth, and so proportionately less gravitational pull.
"why is the hatrack moving?"

"That's not a hatrack, that's Maris!"
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Old 07-21-2012, 07:06 PM
Der Trihs Der Trihs is offline
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Quote:
Originally Posted by John Mace View Post
Nature abhors a vacuum, but not Zombies.

BTW, Mars did lose its atmosphere long ago. But then, Maris has only 1/10 the mass of Earth, and so proportionately less gravitational pull.
And as said above it also lacks a magnetic field, which makes things worse since the solar wind strips away much of what atmosphere its gravity lets it hold on to.
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Old 10-30-2016, 02:21 AM
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Hi, what I think is happening is that the cold and coagulated gases at the top of our atmosphere is the one being pulled so hard to keep the whole atmosphere in place. But the mystery is that the magnitude of low pressure or vacuum in space is so great, the gravity should not be able to hold it up. Let use an analogy (it's the best thing to explain),: so let look at a jar made of a thin glass material; the region in it is kept at absolute Zero pressure or vacuum. If this jar is placed in space, nothing happens, but if placed in within earth's atmosphere, the jar burst inward due to the difference in pressure OR if the same jar is filled tightly with a gas, in space it burst-outward. So if the jar is our coagulated-gas at the top of our atmosphere and the internal region of the jar is our much more lower-gaseous region of our atmosphere. The stronger the jar, the harder it is to burst. So it could be possible that it's our gravity which is keeping our atmosphere in place but the void out there is stronger and beyond the critical-threshold of this gravitational pull-strenght.
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Old 10-30-2016, 03:25 AM
naita naita is offline
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Quote:
Originally Posted by oyinzyinks View Post
Hi, what I think is happening is that the cold and coagulated gases at the top of our atmosphere is the one being pulled so hard to keep the whole atmosphere in place. But the mystery is that the magnitude of low pressure or vacuum in space is so great, the gravity should not be able to hold it up. Let use an analogy (it's the best thing to explain),: so let look at a jar made of a thin glass material; the region in it is kept at absolute Zero pressure or vacuum. If this jar is placed in space, nothing happens, but if placed in within earth's atmosphere, the jar burst inward due to the difference in pressure OR if the same jar is filled tightly with a gas, in space it burst-outward. So if the jar is our coagulated-gas at the top of our atmosphere and the internal region of the jar is our much more lower-gaseous region of our atmosphere. The stronger the jar, the harder it is to burst. So it could be possible that it's our gravity which is keeping our atmosphere in place but the void out there is stronger and beyond the critical-threshold of this gravitational pull-strenght.
Unlike this ramble, the previous, decade-old, answers in this thread explain the OP's question adequately.

As it slowly approaches a vacuum the atmosphere "closest to empty space" is hotter and less dense than that down here, making your "cold coagulated-gas" explanation terribly, terribly wrong.
  #36  
Old 10-30-2016, 07:26 AM
samclem samclem is offline
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Since the OP was answered 10 years ago, let's close this Zombie.

samclem, moderator
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