Gas Giant atmsophere composition and star types

[Gruffty]

OK:

a) Do all gas giants have Hydrogen in their atmospheres? Or is it possible to have a gas giant that doesn't?

b) Does the size/type/magnitude/class of star a gas giant orbits have an effect on the size a gas giant can be? And do these same factors affect the composition of the atmosphere of said gas giant(s)?

Answers on a speculum to the usual address.....

 

[Malenfant]

a) Yep. That's basically what makes them a gas giant. You don't even have to have that big a world to hold onto hydrogen in its atmosphere either. Planets that are only a few thousand km bigger than Earth could do it, and once it can hold onto H2, it's pretty much guaranteed to would 'snowball' into a fully blown gas giant by pulling in the surrounding H2.

It's conceivable that you could have a 'helium world' that is massive enough to retain He, but not massive enough to retain H2. But the difference between the two gases is so slight that it would be very difficult to arrange this (plus as soon as it accumulated enough He to increase its mass, it could start pulling in H2 as well...)


b) Not as far as I know. You do get a tendency for brown dwarfs to form around low mass stars though. (generally speaking, companion 'stars' should be at least 10% of the mass of the primary. So a brown dwarf could be found around an K V or M V star (and indeed often are), but it's unlikely that they'd be found around a star more massive than about 0.8 solar masses).

 

[Anthony]

Originally posted by Malenfant:
It's conceivable that you could have a 'helium world' that is massive enough to retain He, but not massive enough to retain H2.

Question about that. From looking at minimum retained molecular mass formulas, it seems like the earth is big enough to hold on to neon (it's heavier than methane, ammonia, or water, and it's only 40% lighter than N2), and neon's a really common element in the universe, but the earth has very little neon.

Why?

 

[Malenfant]

This has a little bit of info on Ne in the atmosphere:
http://www.windows.ucar.edu/tour/link=/earth/past/earths_primordial_atm.html

It seems to me that while one might expect there to be more Ne than N2 based on cosmic abundances in the primordial atmosphere that was pulled from the protoplanetary nebula, the fact that there isn't indicates that a secondary process - i.e. volcanic eruptions and outgassing - greatly increased the amount of nitrogen relative to neon .

So Earth has about as much Neon as you'd expect it to have - it's just swamped by all the other gases that were put in the atmosphere by other processes later on.

 

[Anthony]

Originally posted by Malenfant:
So Earth has about as much Neon as you'd expect it to have - it's just swamped by all the other gases that were put in the atmosphere by other processes later on.

Assuming that's true, why does the earth's atmosphere have 0.93% argon? Argon isn't going to be replenished by volcanic activity any more than neon will be, and it's significantly less common than neon. It certainly sounds like neon must have been leaking away (or was blown away by some historical event).

 

[Fritz_Brown]

Well, it's all been used up in Vegas, right? And cheap motels all over....

 

[Malenfant]

Well, Argon is a radioactive decay product: Potassium-40 decays to Argon-40, with a halflife of about 1.26 billion years. There's a lot of potassium in the earth's crust (it's a very common rock-forming mineral, found in many silicates. The radioactive form of K is about 1 in every 10,000 Potassium atoms) - the Argon is still seeping out from the earth's interior (and is also liberated when rocks melt).

Since it's an inert gas it's not going to go anywhere once it seeps into the atmosphere, and as it's continually being replenished the proportion should go up with time.

These URLs have a bit more info about K-Ar dating:
http://id-archserve.ucsb.edu/Anth3/Courseware/Chronology/09_Potassium_Argon_Dating.html
http://hypertextbook.com/physics/modern/half-life/