How Do Gas Giants Retain Massive Atmospheres?

[Nakisima]

Hi guys,

I've had a question nagging at me for a while - When I was at school, my chemistry teacher taught us that the outer electrons of atoms were bound to the attraction of the nucleus, and the more layers of electrons that were added, the weaker the attraction due to electron "shielding" until no more layers could form (ergo, giving us a limited number of elements etc). With the gas giant planets, it is thought that they have relatively small, possibly metallic cores with therefore relatively huge atmospheres. So my question is, how is this possible? I mean, I accept that the force of attraction is gravity rather than opposite charges, but surely the gravitational pull of gaseous substances are weaker, and therefore could not hold the volume of gas together that Jupiter does, for instance.

 

[Drakkith]

Gravity is an attractional force between everything with mass. Normal matter does not repel other normal matter under the force of gravity the same way that like charges repel. The reason Jupiter has a metallic core is that it has a very large mass, which pulls in all the matter that composes it. This makes the pressure get increasingly higher the further down you go into the atmosphere due to the weight of the gas above you. At a certain point, the pressure gets so high that the gas is compressed into a metallic liquid, and beyond that is the superhot compressed core.

Edit: Imagine that Jupiter was an atom. In this case the outer layers, or electrons, do not repel each other and neither do the protons that compose its nucleus. Instead, all of it attacts every other bit of it. In a normal atom the protons which provide the positive charge are located in a small area which is the nucleus. The electrons orbit around this nucleus, with an increasing distance the more electrons there are. As the electrons get further away from the nucleus, they experience less attractiveness due to the increasing distance. In our Jupiter scenario the attractive force is everywhere, not simply in the nucleus, or core of the planet. Even the gas on opposite sides of the planet attract each other towards themselves.

 

[Nakisima]

I may have worded my question a little obscurely; I meant to ask why Gas Giants, using Jupiter as an example, can have such a large volume of gas around a very small core when the core's mass should not be generating much gravitational pull. Is gravity not fully responsible for this? Are there other factors involved? Sorry for the confusion.

 

[Nakisima]

Thanks for the edit, it makes more sense to me now!

Cheers

 

[Drakkith]

I may have worded my question a little obscurely; I meant to ask why Gas Giants, using Jupiter as an example, can have such a large volume of gas around a very small core when the core's mass should not be generating much gravitational pull. Is gravity not fully responsible for this? Are there other factors involved? Sorry for the confusion.

Even though I answered your question already, I just wanted to clarify that the core doesn't generally have a set boundary where the density suddenly jumps. The density gradually increases the further down you go and at a certain spot it's so high that the matter is compressed into a liquid/solid form that we call the core.

 

[qraal]

Hi guys,

I've had a question nagging at me for a while - When I was at school, my chemistry teacher taught us that the outer electrons of atoms were bound to the attraction of the nucleus, and the more layers of electrons that were added, the weaker the attraction due to electron "shielding" until no more layers could form (ergo, giving us a limited number of elements etc). With the gas giant planets, it is thought that they have relatively small, possibly metallic cores with therefore relatively huge atmospheres. So my question is, how is this possible? I mean, I accept that the force of attraction is gravity rather than opposite charges, but surely the gravitational pull of gaseous substances are weaker, and therefore could not hold the volume of gas together that Jupiter does, for instance.

Good question. The problem is a matter of scale. Gases might seem diffuse to us, but gather a big enough volume and even diffuse gases can be bound by their own gravity. What drives gases apart is the energy inside them, because that produces a net outward pressure - a ball of gas surrounded by empty space will expand to fill it if there's insufficient gravity. That's why small moons can't have bound atmospheres, because of the ceaseless motions of gas particles.

Also gas giant cores are small relative to their whole, but probably began as solid bodies bigger than Earth. At a certain size the primordial core began rapidly attracting gas on to it from the nebula of gas the planets formed in.

 

[Nakisima]

I hadn't thought of that. I knew that a body of rock or metal had to be a certain size before its gravity shaped it into a sphere, but thinking of Jupiter's core as larger than Earth hadn't even crossed my mind. Thanks!

 

[qraal]

I hadn't thought of that. I knew that a body of rock or metal had to be a certain size before its gravity shaped it into a sphere, but thinking of Jupiter's core as larger than Earth hadn't even crossed my mind. Thanks!

Another thing to realize is that while the core began that big it may not exist as a distinct region anymore, due to the heat of the gravitational capture of all the gas. The core might have melted away and mixed with the rest of the planet. Hard to imagine a core bigger than Earth melting away, but it's possible.