Space as a vacuum and earth's atmosphere

[Leonidas]

Awesome web site, but its mostly over my head... with my physics expirence limited to high school.

I have always wondered this... if space is a vacuum... and gasses in a vacuum tend to ... uh... for a lack of a vocabulary... de-concentrate... disperse... what causes Earth's gases to stay where they are?

is it merely gravity overcoming the vacuum? if so... then how would it ever begin? with everything naturally attempting to be as-spread-out-as-possible... then how would masses large enough to have enough gravity to influence other particles form?

inform my ignorance, please.

 

[selfAdjoint]

Yes, it's gravity. The planets began as a gravitating concentration of dust in the wider dust cloud that became the solar system. Depending on their masses, they were able to trap some of the gasses that were mixed in with the dust; the heaviest clots, which were to become Jupiter and Saturn, were able to retain the lightest, and therefore fastest, gas molecules, Hyrogen and Helium. Somewhat heavy planets like the Earth and Venus were able to retain gasses like oxygen and nitrogen, and water (H2O, molecular weight 18). Mars apparently retained water for a long time, but due to lower mass, lost it at a greater rate than Earth, and now only retains carbon dioxide (CO2, molecular weight 44).

 

[Leonidas]

so... if its gravity... then... I'm not really sure how to phrase this... why aren't the gasses as compressed as possible?

also... doesn't that mean that the pressure within the Earth's atmosphere continually rises as more and more gasses slowly accumulate as the pull (Push? don't understand how that works either) of gravity brings more molecules of gasses into earth... or does the Earth's atmostphere slowly grow outwards?

 

[selfAdjoint]

The gravity of the planets is not strong enough to "suck in" new gasses, not is there any significant free gas left to suck in. What the gravity does is to slow down the escape of gasses. But even today a few molecules are always getting up to escape velocity due to collisions and, well, escaping.

As to the compression, gravity is a finite force, and gases have a definite weight. The weight of a column of air one square inch in cross section and as high as the atmosphere is about 14.7. pounds. That's what presses on you all the time, although it varies a bit with the weather (hence barometers).

 

[Leonidas]

i'm not sure i understand this...


the added mass of the gass would cause our gravitational pull to be stronger, not weaker, right? so... if we were able to suck in little bits of gas in the first place... and we have a stronger force of gravity now... why can't we do it now?

I get so confused...

 

[neutroncount]

Because there is no material to suck in at a significant amount. The solar system has formed. There aren't huge of dust and gases for our planet to collect anymore. So the atmosphere we have is either drifting off to space or being slowly added to as with volcanoes, which output gases that are locked within the earth.

 

[russ_watters]

the added mass of the gass would cause our gravitational pull to be stronger, not weaker, right?

While factually true, you may as well ignore it because the mass of our atmosphere is utterly insignificant when compared to the mass of the planet itself.

 

[Phobos]

I get so confused...

Welcome to Physics Forums!

Don't be shy about asking for clarification even if many discussions here are complicated. We're happy to discuss any aspect of astronomy.

 

[Phobos]

the added mass of the gass would cause our gravitational pull to be stronger, not weaker, right? so... if we were able to suck in little bits of gas in the first place... and we have a stronger force of gravity now... why can't we do it now?

Piecing together the above responses...
(1) The solar system formed from a disperse cloud of material.
(2) Gravity pulled the material together to form the sun & planets.
(3) That formation process is pretty much complete and there's not much free material left in this solar system to add anything significant to the Earth. We still get a daily dose of space dust & particles from the sun as well as the occasional asteroid/comet impact, but the additions are small compared to the whole Earth, so the gravity increase is very small.
(4) Over time, some light, fast moving atoms in the atmosphere (like hydrogen) do escape out into space.
(5) Our atmosphere is the result of the balancing act between the Earth's gravity pulling matter downwards and the outward "diffusion pressure" you're asking about. If Earth's gravity were much higher, then the atmosphere we have would be compressed more.

 

[lazy01001]

My theory here:

Gravity is the force causing the universe to expand. For some reason all the planets and debres and everything moving in space(and time)casue an indentation in space and time, sending somewhat of a wave through space, arcing, and hittig the ends of the univers, over millions and millions and billions of light years, pushing it, and forcing it to expand. Space is full of matter, and at the ends of the univers, there "is no matter" so law of equillibreum must come into play where the huge vaccume of space is succing in all the emptyness, thus making it bigger, EXPANDING, and whatever is outside space, decrease... or something, someone please inform me!