# What is the barrier that keeps in molecules?

1. Oct 26, 2012

### Leoragon

There is the atmosphere that is composed of 4 parts: the troposphere, the stratosphere, the mesosphere, and the thermosphere.

I know that air is a gas made up of molecules. And a gas fills up the container it is contained in, which is the atmosphere. 1Most of the mass of molecules are in the troposphere, and there is ozone production in the stratosphere. Plus, there is oxygen and nitrogen in the thermosphere. But what keeps most of the molecules in the troposphere?

1There are other elements in there right?

2. Oct 26, 2012

### Staff: Mentor

Gravity!!! The gas is pulled towards the Earth but the molecules have energy and are bouncing around and off of each other and such, so they don't just fall into a big glop. This results in a slow reduction in the density of the air as you rise in altitude. And realize that the troposphere is defined as the part of the atmosphere where the temperature falls with altitude. At a certain point, known as the Tropopause, this trend suddenly stops, and then reverses in the Stratosphere where the temperature increases at altitude increases.

http://en.wikipedia.org/wiki/Troposphere
http://en.wikipedia.org/wiki/Tropopause
http://en.wikipedia.org/wiki/Stratosphere

Sure, there are trace amounts of many different elements.

3. Oct 27, 2012

### Leoragon

I don't get it. So gravity pulls in the molecules, but the molecules have energy that they bounce off of each other. I don't get what you mean by slow reduction.

Oh, and why doesn't the atmosphere just extend all the way if the molecules keep bouncing off of each other?

4. Oct 27, 2012

### Staff: Mentor

The density of the air, meaning the amount of air in a given volume, drops off slowly as you go up in altitude. Perhaps I shouldn't say "slowly", just realize that air gets thinner as you get higher in altitude because it is getting less dense.

Perhaps this can help.

I can't vouch for the validity of the answer, so I wouldn't base your Exam studying off of it.

5. Oct 27, 2012

### Leoragon

Okay, so because of the increased volume of space, the density is less. But what why is there the top of the atmosphere? Why doesn't it reach further into space?

6. Oct 27, 2012

### Staff: Mentor

It does! There is no sudden boundary. The atmosphere simply keeps thinning out as altitude increases until the density is so low that its pretty much equal to the average density of space in the solar system.

7. Oct 27, 2012

### Leoragon

So, as you said, the atmosphere keeps lowering density until it becomes equal to the density of space. Now why does the temperature drop as you go up in the troposphere, then increase when you reach the stratosphere or somewhere up there?

8. Oct 27, 2012

### Staff: Mentor

Because of the absorption of UV radiation I believe. See the stratosphere article I linked.