Originally Posted by Relmuis

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.