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Older pages like this "Exploration of the Solar System" course page describe the transition as being a few hundred kilometers down.

More recent findings seem to put the boundary deeper. See

The result was a surprise for the Juno science team because it indicated that the weather layer of Jupiter was more massive, extending much deeper than previously expected. The Jovian weather layer, from its very top to a depth of 1,900 miles (3,000 kilometers), contains about one percent of Jupiter’s mass (about 3 Earth masses).

- nasa.gov, March 7, 2018: "NASA Juno Findings - Jupiter’s Jet-Streams Are Unearthly"

and

Below whirling jet streams, 3,000 kilometres deep, lies a dense, rotating core of liquid hydrogen and helium.

- abc.net.au, March 7, 2018: "Jupiter: Juno uncovers deep jet streams, dense liquid core — and cyclone mystery"

Given the updated understanding of the mass of gas above it, what would the pressure and temperature be at that level?

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    $\begingroup$ I don't see much reason yet to change from older structure models in light of those new results... Pressure-Temperature structure seems to be still pretty much describable by pre-Juno results... And that you can look up $\endgroup$
    – AtmosphericPrisonEscape
    Commented Mar 10, 2018 at 1:38
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    $\begingroup$ I don't understand how that could hold true. For the pressure to still be the same for a given area at the boundary under revised depth of 3,000km, wouldn't the column of gas above have to have same mass as envisioned before? Wouldn't that mean there's a higher temp. in the middle atmos. fluffing the the same mass of gas out? Then it'd have to converge back to the same temp. as before for bottom atmos.?One way or another,it seems like P/T values as one goes down must trace a different path through phase diagram than previously thought, or boundary depth would've been more accurately predicted. $\endgroup$
    – Jacob C.
    Commented Mar 10, 2018 at 7:15
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    $\begingroup$ I have yet to actually read those papers, but we know that at those depths (measured by the Galileo entry probe) Jupiter's atmosphere is already convective. There is only one possible P-T profile for a convective region (that's why I'm confused about their use of 'weather layer' which would indicate a tropopause, which shouldn't exist there), also dynamic effects can only be a first order perturbation on top of the general hydrostatic profile of the planet. So I'm pretty skeptical about this deep weather layer doing anything to the planetary structure. $\endgroup$
    – AtmosphericPrisonEscape
    Commented Mar 10, 2018 at 12:12
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    $\begingroup$ Technically there isn't really a gas-liquid boundary because temperatures are well above the critical point of hydrogen. It's a supercritical fluid. There are important changes at various depths though including the bottom of the circulating winds, the transition to metallic hydrogen and (it now appears) an increasing density of dissolved/suspended heavier elements making up a somewhat diffuse core $\endgroup$
    – Steve Linton
    Commented Mar 10, 2018 at 15:45
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    $\begingroup$ @SteveLinton - You should make that an answer. $\endgroup$
    – David Hammen
    Commented Mar 10, 2018 at 20:35

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Technically there isn't really a gas-liquid boundary because temperatures are well above the critical point of hydrogen (33K and about 18bar). It's a supercritical fluid. There are important changes at various depths though including the bottom of the circulating winds, the transition to metallic hydrogen and (it now appears) an increasing density of dissolved/suspended heavier elements making up a somewhat diffuse core. There is lots of "pre-Juno" information easily accessible, including temperature and pressure at the transition to metallic hydrogen (10000K and 20GPa). This source gives a pressure of 100kBar at the bottom of the circulating winds layer (about 10 GPa), but I can't find a temperature.

I just found this very interesting diagram:enter image description here from these lecture notes which shows how the expected temperatures and pressures inside all the giant planets relate the various phases of hydrogen. Notice how all of them except Jupiter do cross the short curved black line at the left, so that they do have a gas-liquid boundary at pressures of a few atmospheres and temperatures of a few tens of Kelvin

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    $\begingroup$ Just fyi, there's currently no answer to “Rivers of metallic hydrogen” in the atmosphere of Jupiter? and I've just added a bounty. $\endgroup$
    – uhoh
    Commented Mar 11, 2018 at 9:42
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    $\begingroup$ Hrm... regarding the transition to metalic, the diagram on p. 27 of the arXiv PDF of Guillot, T. 1999, "A comparison of the interiors of Jupiter and Saturn" (Planetary and Space Science, Volume 47, Issue 10-11, p. 1183-1200) "A transition region, assumed to lie between 1 and 3 Mbar, is represented, but experiments and theory are still unclear as to whether the separation between the molecular and metallic regions is sharp or not." So (converting from megabars), it was thought to be at 100 to 300 GPa according to that? $\endgroup$
    – Jacob C.
    Commented Mar 14, 2018 at 4:17
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    $\begingroup$ From which I think we simply conclude that no one really knows yet. Juno might give us some idea of the depth at which Jupiter becomes conductive and/ or of the density profile $\endgroup$
    – Steve Linton
    Commented Mar 14, 2018 at 9:47

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