Hydrogen Oceans

[ajtribick]

Is the mass at which a gas giant starts to develop hydrogen or metallic hydrogen oceans known? As far as I've been able to find out, Uranus and Neptune don't have metallic hydrogen, whereas Jupiter and Saturn do, so I'm guessing the value would be somewhere between 20 and 80 Earth-masses...

[ArneB]

Is metallic hydrogen the same as hypercritical hydrogen? (Compressed hydrogen with fluid like features) If so, it does not only depend on mass, but also density of the planet, the total mass of hydrogen and the temperature of the planet. Perhaps you could figure that one out yourself, browsing through thermodynamic tables and "integrating your way" down through the atmosphere. Just keep in mind that the pressure gradient increases as the atmosphere thickens.

[eburacum]

The temperature of the gas giant might be a wild card here; it depends on the distance from the star, but also on the albedo and greenhouse effects,

and any internal radiation souce of heat;
and the gravitational contraction factor, which changes with time as far as I can figure, but must affect the core and the pressure gradient.

[granthutchison]

Is metallic hydrogen the same as hypercritical hydrogen? (Compressed hydrogen with fluid like features)

No, metallic hydrogen is ionized so it conducts electricity. What you're describing would still consist of neutral molecules of hydrogen.
Because temperature increases with depth, none of the gas giants contain true liquid hydrogen - the temperature at depth is always higher than hydrogen's critical temperature, so there's no sudden transition to a liquid; just a gradual increase in density and viscosity until the gas is behaving like liquid hydrogen, as you describe.
The transition to metallic hydrogen kicks in at ~1.4Mbar, and that seems to be constant across a pretty wide range of temperatures (1000K-7000K) likely to exist at such depths - so small differences in the "surface" (ie cloud-deck) temperature of the gas giants aren't going to be very relevant.
You basically just need enough depth of atmosphere to hit the 1.4Mbar isobar ... ~10000km in diagrams of Jupiter, but ~30000km for Saturn.
The difficulty for ice-giants like Uranus and Neptune is therefore two-fold: while they may have enough mass to generate the necessary pressure, they certainly don't have the atmospheric depth above their large cores to generate the high pressures required. The pressure at the centre of Uranus and Neptune is around 8Mbar, so you can imagine that if their mass consisted entirely of hydrogen and helium they might well be able to form metallic hydrogen near their centres.

Grant

[ArneB]

Oh, thanks.

they certainly don't have the atmospheric depth above their large cores to generate the high pressures required

That's what I thought of.

You ought to see the BBC documentary "The Planets". They have had a computer artist trying to visulaize hydrogen oceans, and the result is really beautiful and convincing.