Beneath the clouds of our Jovian planets are oceans of liquid hydrogen, or in the case of Uranus & Neptune, an ocean that may be superheated water/ammonia/liquid hydrogen/whatever. Someone posted a link here that said Jupiter's huge ocean of liquid hydrogen has no surface-that the air pressure steadily increases until it inperceptably turns into liquid.

How sound is this hypothesis? What's it based on? Could there be a scenario where one would have a stormy ocean surface whipped about by the violent weather above and heat from below?

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Consider the state diagram of a fluid. The two axes of the diagram are temperature and pressure, so any point in the diagram represents the behaviour of the fluid at a certain temperature and pressure.

The liquid state and the gaseous state will coexist along the condensation curve. At higher temperatures or lower pressures only the gaseaous state exists; at lower temperatures or higher pressures only the liquid state exists.

However, the condensation curve has two endpoints. One is the triple point, where the solid state may exist as well. The other one is the critical point, where the difference between the liquid state and the solid state ceases to exist, as both have the same density.

If a probe were to descend into Jupiter's atmosphere it would see both pressure and temperature rise as functions of the depth it has reached. The descent could therefore be plotted as a curve in the state diagram of the ambient fluid (which I think is mostly hydrogen). If this curve intersects the condensation curve, the intersection marks the depth where the probe will encounter a liquid surface. If, however, the curve passes beyond the critical point, no liquid surface will be found.

There may however, be a solid surface if pressure rises high enough.

I have read that the latter possibility is realized on Venus. Here the ambient fluid is carbon dioxyde, and the high temperatures ensure that the fluid is "supercritical" at the solid surface. Venus is of course not a gas giant; the solid part of Venus is rock rather than solidified carbon dioxyde. Even so, the conditions at ground level might resemble those on the bottom of an ocean more than those on the bottom of a gaseous atmosphere. One might, for example, feel resistance if one tries to move.

The critical point of hydrogen is at 34 Kelvin and 7 bar, so there will be a liquid surface if the pressure rises above 7 bar before the temperature rises above 34 Kelvin.

I think this is enough to rule out a liquid surface on Jupiter, but perhaps not on Neptune. If there is a liquid surface on Neptune, the term "liquid giant" might be a more appropriate term than "gas giant".

A recipe to make a "liquid giant" would ask for:

(1) a large mass, to ensure strong gravity, and thereby a steep increase of pressure with increasing depth

(2) a large distance to the sun, to make the outer regions as cold as possible

(3) preferably no satelites or radioactive elements, to prevent heat from being generated inside the planet, because this will steepen the increase of temperature with increasing depth

(4) preferably some other fluid than hydrogen, to give a longer condensation curve. Oxygen would be a lot better than hydrogen; it has a critical point at 152 Kelvin and 60 bar.

An interesting idea would be to have conditions suitable for a liquid surface at the poles (colder, higher gravity) but not at the equator. Now one would have a surface which ends at a certain latitude. A ship sailing this sea would not sink when it encountered the end of the sea; it would float merrily on, as the fluid would still have enough density to support it. But the idea of a sea which just... stops... is mind-boggling.

Nice post Relmuis. However, I'm not sure that it is relevant to the question that was asked. Phase diagrams like you mentioned are for conditions that are, at least, somewhat near what we call "normal". Conditions deep in a gas giant planet are far outside of the area where those diagrams apply. The pressure deep inside Jupiter is in the gigabar range!

From the Nine Planets website:

"Recent experiments have shown that hydrogen does not change phase suddenly. Therefore the interiors of the jovian planets probably have indistinct boundaries between their various interior layers."

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If a liquid-gaseous interface were to exist on any of the giant planets, it should of course be located at points described by the usual state diagram. Which means, not all that deep into the planet. It would therefore not matter what kind of conditions were to be found at the core.

To elaborate, if the critical point of the ambient fluid (hydrogen, helium, oxygen or whatever) were to lie at 20 bars, then no surface of any "sea" could be found beneath the 20 bar level.

Except that the "ambient fluid" here is liquid metallic hydrogen, which cannot exist at less than megabar pressures!

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If something cannot exist at the pressures found on the upper levels, it wil not be found there. But who said that the "sea" must be metallic? You can have liquid hydrogen at 7 bar and 30 Kelvin, for example. If there is any place in Jupiter where the pressure is 7 bar, the temperature is 30 Kelvin, and the dominant stuff is hydrogen, a liquid surface will be found there.

Personally, I do not think that this combination of conditions can be found anywhere on Jupiter. But perhaps one can find it on Neptune.

By the way, a state diagram is just that: a diagram. There is nothing to prevent us from extending it to the gigabar range, the megaKelvin range, or both. We may not know exactly what to put inside these far part of the diagram, as it may not be possible to compute this from first principles, but there is nothing conceptually wrong with the idea that even the core of Jupiter is to be described by a state diagram.

In fact we might be able to explore the further regions of the hydrogen state diagram by sending probes inside Jupiter.

Actually, they're more like "gas-slush" giants down to certain levels.

No, the phase diagram is valid under all conditions. It's where you draw a curve on the diagram that determines what the conditions are (or, of course, vice-versa). One would expect that the conditions in the deep interior of a gas giant would be far removed from the "interesting" part of the diagram, where the critical point & triple point are found, and I think that's what you mean to say.

We expect that the core of Jupiter is made of "rock", perhaps surrounded by metallic hydrogen (i.e., Metallization of fluid hydrogen at 140 GPa (1.4 Mbar): implications for Jupiter). But if we go down from the top of the atmosphere, we will surely pass through the part of the phase diagram where the critical point & triple point are found. But what we see depends on the physical conditions, which will determine the curve on the phase diagram, which represents what we see on the way down. I think it is generally agreed that there will be no distinct transition from gas to liquid to solid, nor from gas to solid. But that depends on the model you use for the atmosphere, which in turn depends on what you think the atmosphere is made of, which is determined from observation. So there is some wiggle room. We do see clouds, and if there is any "surface" below them, I think it must be a liquid surface, since the light elements that are the primary constituent, will not solidify at such high temperatures unless the pressure is much higher.

See, for instance, The interior of Jupiter, a chapter in the book Jupiter: The Planet, Satellites and Magnetosphere, Cambridge University Press, 2004.

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I fully agree with this. However, I wonder why often (for instance in this link
http://www.astro.washington.edu/lars...aturanept.html) layer of liquid hudrogen is mentioned in between gaseoud hydrogen and mettallic hydrogen on Jupiter, when according to the logic developed in the quote, liquid hydrogen cannot exist on Juoiter.