Saturn: Helium Rain, or non-indigenous planet?

[JDoolin]

I'm using Chaisson'/McMillan's "Astronomy, a Beginner's Guide to the Universe"/7th Edition

In Chapter 7, it describes the gas giants, and says that Jupiter, Uranus, and Neptune all have "86% Hydrogen, 14% Helium" in their atmospheres, while Saturn has about 92% hydrogen, and 7% Helium, in its atmosphere.

As an explanation, it offers that on Saturn, Helium liquifies, and condenses toward the center of the planet. If that's accurate, it would have to mean that Helium must undergo an inelastic collision in the core... What mechanism could cause that? Helium is a noble gas, so it doesn't interact easily with other chemicals. It is also is incredibly stable as an isotope, so it isn't going to interact easily with other nucleons. How does Helium get trapped towards the surface or core of Saturn?

Also, Saturn is an outlier as far as densities go. 710 kg/cubic meter for saturn, as opposed to (1300, 1200, and 1700) for Jupiter, Uranus, and Neptune, respectively.

How strong is their modeling for Helium rain? Should an alternative hypothesis be explored that the bulk of Saturn might actually be captured from a more ancient solar system? Or has such an alternative hypothesis already been explored and rejected?

 

[rootone]

Should an alternative hypothesis be explored that the bulk of Saturn might actually be captured from a more ancient solar system?

Not unless there is found at least some shred of evidence which could justify it.
All of the planets in the solar system have aspects which make them unique.
Mercury is exceptionally dense, Venus spins 'backwards' in comparison to most planets while Uranus spins on it's side, the Earth-Moon system is almost a binary planet, no other planet has such a relatively large companion, some planets have strong magnetic fields others almost none, Jupiter emits huge amounts of EM in radio frequecies. I am sure there are many other features of planets which make them unusual in some way.
On that basis we could equally well argue that every one of the planets originated somewhere other than in the solar system.

 

[JDoolin]

All very good points.

I totally agree... We don't really need an explanation for why Saturn has less helium in the atmosphere... The planet's are all unique.

 

[Drakkith]

As an explanation, it offers that on Saturn, Helium liquifies, and condenses toward the center of the planet. If that's accurate, it would have to mean that Helium must undergo an inelastic collision in the core... What mechanism could cause that?

I'd think the effect is more like mist forming near the ground than like raindrops falling.

 

[JDoolin]

I'd think the effect is more like mist forming near the ground than like raindrops falling.

But is there something special about Saturn that would make this phenomenon happen there, but it wouldn't happen on Jupiter, Uranus, or Neptune?

 

[Drakkith]

But is there something special about Saturn that would make this phenomenon happen there, but it wouldn't happen on Jupiter, Uranus, or Neptune?

That I don't know.

 

[JDoolin]

Helium is a noble gas, so it doesn't interact easily with other chemicals.

I stand corrected by the phase diagrams here.

http://ltl.tkk.fi/research/theory/helium.html

It appears that at 10, 20, 30, 40 atmospheres, Helium does become a liquid at the sorts of temperatures you'd find on Saturn.

(Actually, I'm not a hundred percent sure how to read these phase diagrams... It might become a compressible liquid, or a supercritical fluid?)