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Ice giants are currently not considered suitable for life due to temperature, pressure and composition. This paper gives some parameters for life in regards to temperature, pressure and composition:

  • Temperature: $-15\ ^\mathrm{o}C$ to $122\ ^\mathrm{o}C$
  • Pressure Upper Limit: $1,100\ \mathrm{atm}$, and
  • Composition to Include: liquid water, $C$, $N$, $S$, and $P$.

I'm trying to find out what the conditions of the hot dense ammonia/water mantle border with the hydrogen/helium atmosphere are, in regards to temperature and pressure. We know the mantles are under very high pressures, have liquid water (though in a likely different state due to pressures), C, N and S. What are the pressures and temperatures at this mantle border? Specifically in reference to Uranus and Neptune.

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A basic approach would be to estimate the air-pressure at the top of the mantle.

Uranus' surface gravity is a bit less than Earths, but, somewhat counter-intuitively, it's atmosphere is light enough that it's gravity increases as you move inside the planet towards the top of the mantle. (the same is true for Earth). For ballpark estimate, lets say the gravity works out to be about the same as Earths, because I don't want to calculate, but it might be a bit more than that.

The Mass of Uranus' atmosphere, by Wikipedia is about 0.5 times the mass of Earth. It's surface area is about 16 times Earth (4 times the Radius), but the surface area of the Mantle would be less, about 80% of the radius which works out about 10.2 times the surface area.

If you spread 0.5 Earth masses over 10.2 times Earth's surface area, you get a pressure between 50,000 and 60,000 atm. Some 50 times the upper limit you provided in your question. That's probably a limiting factor for most "ice giants". There's simply too much hydrogen and helium atmosphere to allow liquid water at 1,100 ATM. Ice giants might create their own hydrogen by heat and pressure on Methane, creating diamond like structures and free hydrogen.

The heat below all that atmosphere (about 20% of Uranus' radius) would be outside your criteria too. I couldn't find any good estimates for Uranus' mantle's temperature. There's too many unknowns about why Uranus is as cold as it is on it's surface, so estimates on it's mantle temperature seem pretty tentative and unavailable, but what little I know about how temperature rises inside a planet, I don't see any way the top of Uranus' mantle would come close to the temperature criteria you gave. Not at a depth of 20% of Uranus' radius.

Consider this chart from Uranus' wikipedia at 200,000 mbar Uranus temperature is approaching 340 K. 200,000 mbar is about 200 atm. The mantle begins, by my ballpark estimate at about 50,000 atm. The temperature wouldn't be close unless the planet had a very long time to cool down, perhaps tens of billions of years.

I could run ballpark estimates on Neptune too, but I suspect the results would be similar.

For an ice giant to meet your criteria, it would need to have comparatively very little hydrogen and helium compared to Uranus. Perhaps if an ice giant was close enough to it's star that it had a hot enough surface temperature to lose much of it's hydrogen and helium through Jeans escape, and then, it had a long time to cool down, it might get the right criteria of liquid water at the proper range of temperature and pressure. I don't see any other scenario where it would work.

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  • $\begingroup$ So the mantle border is out due to pressure and temperature. But above you mention 340k (88C) at 200 bar on Uranus, and the Neptune Wikipedia page mentions 0C at 50 bar, within T & P parameters, and that the composition increases in ices (water, methane, ammonia, H2S, hydrocarbons, etc.) as you get further in depth. The atmosphere above 1 bar at 78% H, 19% He, 2% methane, the rest in trace amounts. So the question remains, closer to the temperature limit, 110C? and 250 bar?, is the concentration of C, N, O, S (maybe 30% ices/volatiles vs. 70% H, He) high enough for life to form/survive? $\endgroup$
    – Brooks Nelson
    Commented Aug 9, 2017 at 9:06
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    $\begingroup$ @BrooksNelson Life exists on Earth everywhere, high in the atmosphere for example and frozen in the ice in Antarctica. I see no reason why life couldn't exist in the rain and clouds of water vapor in the regions you suggest. The the standard argument is that life probably formed in liquid water. I can't access the paper you posted, but to my understanding, bacterial "cloud" life could in theory be carried to Uranus and survive there but it's not a good candidate for life forming there or advancing very far. $\endgroup$
    – userLTK
    Commented Aug 9, 2017 at 12:25
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    $\begingroup$ The overriding problem is chemistry. Water (I've heard methane and ammonia as possible alternatives, and that's pretty much the entire list for solvents that support the seemingly necessary chemistry) is, as far we know, necessary as a solvent, but it is not sufficient. We need a lot more chemicals than that to form life. We expect that the complex chemistry of a suitable water-rock interface will be necessary to produce all of the chemicals, in sufficient concentrations, necessary for life. But things like temp and pressure also impact the chemistry. It gets complex really fast. $\endgroup$
    – zibadawa timmy
    Commented Aug 9, 2017 at 19:17

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