What factors determine the composition of a planet's atmosphere?

[Drakkith]

I'm curious as to how the mass of a body affects the kind of atmosphere it can hold. I know that Earth tends to lose its lightest gases over time, which is one reason why helium doesn't build up in the atmosphere, whereas planets like Saturn and Jupiter have held on to lots and lots of hydrogen and helium. However, I haven't read much on how other gases are affected. Does anyone have any good information on this?

 

[Charles Link]

I don't have a good source, but $\bar{v}=\sqrt{\frac{8 kT}{\pi m}}$. This means with sufficiently high temperatures, atoms of lighter masses are going to be able to thermally acquire the necessary escape velocity more readily. The escape velocity, with $\frac{1}{2}(m)v^2=\frac{GM}{r}(m)$, (the $m's$ cancel) , will be higher for a planet of heavier mass $M$ . Thereby a cold planet with a heavy mass favors being able to contain the lighter atoms in its atmosphere.

 

[Drakkith]

That matches with what I was thinking, but I hadn't seen any real information on the subject before. Makes me wonder if all those hot Jupiter's are slowly losing mass.

 

[Ophiolite]

This wikipedia article deals with the issue. It includes a useful overview of the importance of non-thermal losses (especially solar/stellar winds) in some circumstances. There is a useful chart plotting escape velocity versus temperature for some of the more important gases.

 

[Drakkith]

This wikipedia article deals with the issue. It includes a useful overview of the importance of non-thermal losses (especially solar/stellar winds) in some circumstances. There is a useful chart plotting escape velocity versus temperature for some of the more important gases.

Thanks! Just what I was looking for!