Requirement For An Atmosphere

beerformyhorses

1st October 2008 - 07:14 AM

Hi,

I just the other day read a science magazine which said that in order to have an atmosphere of a given gas, a planet needs to have an escape velocity > 6*the average molecular speed of that gas.

I understand the concept, but why exactly the constant six?

Enthalpy

3rd October 2008 - 07:18 PM

To have a probability low enough for the atmosphere to stay for billions of years.

Anyway, the temperature in the outer atmosphere fluctuates a lot, so the factor-of-six can't be that accurate.

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