[chris (OP)]

Colin contacted me to ask this:

How does hydrogen get concentrated at the core of gas giant planets, when it's the lightest of gases?

What does everyone think?

[PmbPhy]

Colin contacted me to ask this:

How does hydrogen get concentrated at the core of gas giant planets, when it's the lightest of gases?

What does everyone think?

My question is to whether its actually true that hydrogen is actually at the center of gas giants.  And why did Colin use the term "concentrated"? Just trying to understand his usage of that term, not whether its right or wrong to use it or not.

[Kryptid]

It probably doesn't. During formation, the gas giants would have accumulated large amounts of heavier elements as well (like iron and silicon). These would have preferentially sunk to the planet's center.

[chris (OP)]

@PmbPhy - that wording is lifted directly from his email to me this morning.

It may be a reference to the fact that massive gas giants like Jupiter have "metallic" hydrogen towards the centre, don't they, owing to the powerful gravitational compression of the gas.

[evan_au]

The atmosphere of Jupiter has a lot of Hydrogen (75%) plus Helium (24%) and merely traces of heavier elements - somewhat similar to the Sun, and probably similar to the nebula from which the solar system condensed..

Hydrogen is much more concentrated in Jupiter than on Earth because the mass of Earth is not enough to hold Hydrogen (while Jupiter is 318 times more massive than Earth) and the temperature of Earth gives Hydrogen molecules enough energy to escape (the cloud tops of Jupiter have a temperature around -145C).

It is thought that Jupiter probably has a rocky core - some estimates put it at 10-40 times the mass of the Earth.
This is expected to be surrounded by successive layers of less dense elements, including layers of helium, plus metallic and supercritical hydrogen. (This year a team claimed to have created metallic hydrogen on Earth, using a diamond anvil to produce the extreme pressures required.)

So rather than say hydrogen is "concentrated at the core of gas giant planets", I would say it is "concentrated in layers within the gas giant planets, according to its density". The layer with concentrated hydrogen extends all the way to the top of the atmosphere.

See: https://en.wikipedia.org/wiki/Jupiter#Internal_structure

[chris (OP)]

To complete the answer to this question, I think the misconception here was why Jupiter is made of hydrogen (mainly), which is exceptionally light, when there are lots of other heavy things around that could have been incorporated.

The answer is that we don't know for sure, because we've only got our own Solar system to study directly, but the most likely scenario, as alluded to by @evan_au, is that in the early solar system, which was a conglomeration of gas and dust falling together under the influence of gravity, the planets formed as this material aggregated; in some areas - such as where Jupiter is, there was a large amount of material. Jupiter became sufficiently big, in terms of its rocky core, to hang on also to hydrogen. It also remained in a cooler part of the solar system so that the gas was never given enough energy to be driven off.

Eventually a positive feedback look ensues whereby the addition of matter increases the mass of the planet, making it even more gravitationally active so that it attracts even more material and hangs onto it even harder. And because Jupiter has never migrated in any closer to the Sun, it's still got its hydrogen payload...

Reasonable summary everyone?

[evan_au]

We talk above of Jupiter starting with a rocky core (iron, nickel, and silica), but the gravitational collapse of gas onto the surface, and nuclear decay of heavier elements have heated the interior to a temperature estimated at over 20,000K.

This is hotter than the surface of the Sun, and so is a plasma rather than a solid. Probably still rich in iron, nickel, silicon and oxygen nuclei, with the denser ones more common towards the center; how well mixed they are depends a lot on the dynamics of convection at these temperatures and pressures.

[chris (OP)]

We talk above of Jupiter starting with a rocky core (iron, nickel, and silica), but the gravitational collapse of gas onto the surface, and nuclear decay of heavier elements have heated the interior to a temperature estimated at over 20,000K.

This is hotter than the surface of the Sun, and so is a plasma rather than a solid. Probably still rich in iron, nickel, silicon and oxygen nuclei, with the denser ones more common towards the center; how well mixed they are depends a lot on the dynamics of convection at these temperatures and pressures.

Good point!