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?


The Earth’s gravity keeps the atmosphere where it is, doesn’t it?

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.

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.

Because North Dakota sucks!


I’m sorry, such a clean set up and all. :smiley:

I’ll be here all week

What they said.

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.

But it is,
but it is!
:eek: :eek: :eek:

Escape velocity from earth is about 11,000 meters per second.
The average (rms) velocity of N[sub]2[/sub] 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 H[sub]2[/sub] and He are leaving the atmosphere entirely are wrong? Or am I misapplying your data?

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) :stuck_out_tongue:

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 :smiley:

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 :slight_smile:

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 :smack: )

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.

Squink, Excalibre: That makes sense. Thank you.

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.

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.

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.

Not to hijack too severely I hope, but do vacuum cleaners actually create any sort of vacuum?