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Q & A: atmosphere goes way up

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Most recent answer: 10/22/2007
Q:
Why is the atmosphere up to 1000 km?
- Tan Andromeda (age 10)
CHIJ BT, Singapore
A:
The atmosphere actually doesn't just stop at some height. It gradually gets less and less dense the higher up you go. The number somebody gives for the 'height' of the atmosphere depends on what they'e trying to keep track of: how high can you go before there's not enough air to breathe, or how high must a satellite go before the air friction isn't important over its expected useful life, etc.

The pattern of how the atmosphere gets less and less dense is, approximately, simple. Roughly speaking, it gets half as dense for each 5 km higher that you go. The reason has to do with statistical mechanics. The higher that a nitrogen molecule is, the more gravitational potential energy it has. The more energy it has, the less energy other things around it have. If the nearby things (like the Earth's surface) have less energy, than there are fewer different microscopic states available to them. There are more states with that nitrogen molecule near the Earth than with it high up. So if every one of the possible states is equally likely, you're more likely to find the molecule near the Earth. Every 5 km farther away takes enough energy to reduce the number of states of the neighbors by roughly a factor of two, depending on the temperature.

Mike W.

(published on 10/22/2007)

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