Why doesn’t the atmosphere (air we breathe) just float off into space?
Some objects in space have atmospheres like Earth and Jupiter. Others like asteroids and the moon don’t have an atmosphere.
Why doesn’t earth atmosphere just drift away into space?
What am I missing about the concept of atmosphere?
In: Planetary Science
because the air is kind of heavy.
the helium and hydrogen that was here already did float off into space because they are light.
One added point.
All the comments on gravity are correct and covered that well. I have no need to add to that.
I just want to point out that we often mistakenly think of the atmosphere as a blanket of gas that has a distinct upper edge. A stopping point. This may contribute to wondering why the atmosphere doesn’t just drift away. However, that’s not true. There isn’t really an edge to the atmosphere. The atmosphere gradually gets thinner and thinner out to about 10,000km. There it eventually becomes so thin it’s indistinguishable from ’empty’ space. It doesn’t do much stop as far away.
The international space station orbits at a mere 408km up. It is constantly experiencing drag and slowing down. It needs occasional boosts.
What you’ll often hear called the edge of space is something called the Karman line. It’s about 100km up. Really it’s just the point where the atmosphere is no longer thick enough to scatter light and make the sky blue. It’s where you get to see stars even in the day. But there’s still plenty of atmosphere there. Enough to measure.
Gravity. That’s all. Same reason you don’t float off the surface into space. All the heavier/more dense particles are closer to the earth’s surface and the lighter/less dense particles will be further up in the atmosphere.
Things that have atmospheres have enough gravity & magnetic field to prevent the relevant gasses from being dispersed into space.
Earth is big. Big thing heavy. Heavy thing pull hard. Asteroid small and not so heavy. Not so heavy thing not pull so hard.
Air is matter. Matter has mass. Mass has weight (in Earth’s gravity) so it is pulled towards the Earth.
WHen they say atmospheric pressure is “14.6 pounds per square inch,” that’s literally the weight a column of air above a square inch of surface. That’s the force gravity is exerting on that column of air keeping around the Earth.
In the same vein, the air pressure gets lower as you go higher up because there’s less gas above a given square inch. Less pressure, lower density. The amosphere gets thinner and thinner as you go up and just gradually transitions into “empty” space. There’s no real end to the atmophere. It jst gets to a point where it’s thin enough to call “space.”
Fun fact.
It is.
Gravity acts to hold it down. There is no force trying to pull it away. So left alone, it’ll settle down and stay stuck to the planet.
However, the air also moves as it is warmed by the sun. And if itoves fast enough it won’t come back (this speed is escape velocity)
Most of the air is cool and goes slowly. It doesn’t go fast enough to fling itself into space. But the speeds are randomly distributed. Even at low temperatures some small percentage is moving with enough energy to get away. So eventually the gas will leave.
And lighter gases go faster. So really light gases like hydrogen are more likely to go “to fast” and escape into space faster, on the order of tens of millions of years for Earth.
Helium is hundreds of millions of years before most is gone.
Oxygen (O2), nitrogen (N2) and co2 are much heavier and take even longer. Like a couple billion years. That should be plenty long enough…
..
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But Earth is 4.5 billion. Even the normal gas in our air should be gone!!!
But don’t panic more gas is being added through “outgassing” processes than we lose, for now. Gas is liberated from the earth through chemical, physical or thermodynamic processes. Like volcanoes.
When it comes to gases, the temperature of a gas determines the average kinetic energy that a gas molecule has, and so also dictates its speed. To escape the gravitational field of Earth, the gas molocules have to exceed the escape velocity of Earth. Depending on the weight of the molecule, the average kinetic energy may or may not exceed that. A light molecule, like hydrogen, may exceed that escape velocity, which is why there’s not much hydrogen or helium in the atmosphere.
For heavier molecules like oxygen or nitrogen, their average speed does not allow escape. It’s also important to note that the speed occurs in a distribution, not a single value, so some gas molecules of any given molecule will still have enough speed to escape, but the fraction in that range will be much smaller for heavier molecules than lighter ones.
It’s very not intuitive but every object, from the biggest boulder to the tiniest sand is falling towards earth at the same speed.
Air is made of molecules and molecules are tiny objects. They too fall towards earth. If there were just one molecule of air, it would free fall.
Now in fact there’s a lot of air and they block each other from free fall. It’s because they bounce back and forth so if a molecule would fall towards earth, it will eventually hit another molecule and bounce back. And because the hit is not necessarily on a straight line, they may bounce in an angle like pool balls.
So the atmosphere as a whole is a continuous fall-bounce state but gravity makes sure that the fall part is always there. That’s why there’s more air doing this fall-bounce closer to earth and so the air goes thinner as you go higher.
If you studied closely the edge of the atmosphere (define where you want as long as it’s up in the air) you will see that half of the molecules are leaving, and half of the molecules just outside your boundary are coming back. But there aren’t as many molecules just outside your boundary as there are just inside so on average there are slightly more leaving than there are returning. This is diffusion. It is true no matter where you define the boundary, and it means that the atmosphere is actually leaving. It is being replenished from solid and water parts of the Earth.
Also take note that the atmosphere is incredibly thin. I live closer to the edge of the atmosphere than to a supermarket. Astronauts traveling to outer space from Cape Canaveral travel the same distance (vertically) as they would if they were traveling horizontally to space mountain in Disney world.
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