# If gasses escapes Earth, then how do they reach escape velocity?

1. Jan 27, 2014

### acesuv

2. Jan 28, 2014

### davenn

they dont have to, they just float upwards.
its only the lightest gasses that are lost to space, eg, Hydrogen and Helium

google is a wonderful thing ;)

Cheers
Dave

Last edited: Jan 28, 2014
3. Jan 28, 2014

### willem2

But the gasses can only escape if their atoms/molecules do get to escape velocity,
and if that happens far enough up for the molecules to escape without colliding with other air molecules again.
This can happen because of random thermal motion, or collisions with protons from the solar wind.
In both cases, the lightest molecules will get the highest speeds and will be far more likely to escape.

4. Jan 28, 2014

### acesuv

so theres gas in orbit around earth?

5. Jan 28, 2014

### Pythagorean

Escape velocity is dependent on mass. Gas molecules have very little mass so they require very little escape velocity.

Though, there's probably other factors for gas molecules because of their small inertia. A small inertia means even the smallest forces will push you around.

6. Jan 28, 2014

### nasu

It doesn't. See for example here:
http://en.wikipedia.org/wiki/Escape_velocity
You can see that there is one value for a given planet.

The velocity of the gas molecules, for a given temperature, depends on the mass of the molecule.
Lighter molecules like hydrogen have a higher average speed than heavier molecules, for a given temperature. So it's more likely for them to have speeds above the escape velocity.

7. Jan 29, 2014

### BruceW

how do you do the red highlighting? that would be really useful. also, I agree. the lighter molecules will be bouncing around a lot more, so are more likely to reach escape velocity, but the escape velocity for them is not less than it is for heavier molecules.

8. Jan 29, 2014

Just change the colour.

9. Jan 29, 2014

### BruceW

I was trying to find the tags, but I found them now. to do this write $color="red"$to do this$/color$

I've just put \ before every [ or ] so that it doesn't actually do the tags, and you can see what it is.

p.s. thanks for the hint "change the colour". I went on the advance reply, and used the "colors" button, and it showed me the tags.

Last edited: Jan 29, 2014
10. Jan 29, 2014

### D H

Staff Emeritus
No!

Escape velocity is $v^2 = G(M+m)/r$. When m is 50 orders of magnitude smaller than M (e.g., the mass of a molecule versus the mass of a planet), it is quite safe to approximate this as $v^2 = GM/r$ . In other words, the escape velocity of a molecule of gas is independent of the mass of the molecule.

The primary reason lighter molecules and atoms tend to escape is temperature. The kinetic energy of a gas is proportional to temperature, and this energy is more or less equally partitioned amongst the individual components of the gas. In a gas comprising multiple kinds of molecules, the less massive components will have a larger velocity than the more massive components.

The composition of the gases in the atmosphere is pretty much independent of altitude in the lower parts of the atmosphere (water vapor excluded, for obvious reasons). The very short mean free path (average distance a gas molecule travels between collisions with other molecules) in the lower atmosphere means that this part of the atmosphere is very well-mixed. This is no longer true in the upper atmosphere. Here the mean free path is very long, longer than one orbit in the uppermost reaches of the atmosphere. Coupling that long mean free path with the velocity distribution means the uppermost reaches of the atmosphere is predominantly made up of very light components.

11. Jan 29, 2014

### sophiecentaur

There is a constant stream of lost gas molecules from any atmosphere. The actual rate of loss depends upon the temperature, the planet's mass and how near the planet is to the Sun (which can pinch the highest flying molecules).
I think the reason that Mercury and the Moon have lost most of their atmospheres is due to their particular temperatures and masses of their nearest neighbour.

Otoh, there is a steady stream of stuff reaching every planet in the form the material it sweeps through in its orbit and from the Sun etc.

12. Jan 29, 2014

### nasu

I select the text and then click on the list associated with the "A" with a bar underneath to change the color. It's next to the "Sizes" list.

13. Jan 29, 2014

### A.T.

To find out how someone does something on a message boat, press quote on his post, and you see his tags.

14. Jan 29, 2014

### sophiecentaur

FGS, what's a "message boat"?
I am so out of touch. (I only learned how to use Spotify today).

15. Jan 30, 2014

### Devils

Its called the atmosphere.

16. Jan 30, 2014

### QuantumPion

No, the atmosphere is not in orbit around the earth any more so than the ocean is.

I guess it's possible for some molecules to be actually in orbit, e.g. a particle near escape velocity gets a kick from a cosmic ray in just the right direction to put it into orbit. Or rocket exhaust. But how many and for how long, I don't know.

17. Jan 30, 2014

### CWatters

Temperature is only an indication of the average velocity of gas molecules. Some will be a lot faster than average and some will be fast enough to escape...

http://en.wikipedia.org/wiki/Atmospheric_escape