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My book is set far, far into the future, in a widely settled solar system. An expedition is sent to Neptune, and the expeditioners find out that there are massive cave systems reaching deep into Neptune's mantle, which is not so much bigger than Earth's diameter. There is also water trapped down here, and the heat is okay (my science might be wrong on this). A project to colonize Neptune is devised, and it begins with sealing off the cave system, before using displacements pumps to remove a lot of the air down there. This way, they lower the pressure to habitable levels. They are enduring the pressures due to their space suits, but that isn't an option for long-term settlement. After depressurizing the caves, they fill them up with breathable air, extracting gases like oxygen and nitrogen from Neptune's atmosphere.

After that, they set up nutrition sources, creating a nutrition-wise diverse ecosystem within the waters of the cave system. Maybe the have some soil with them as well and start cultivating inside the caves. They have wind turbines outside drawing power from the immensely intense winds of Neptune.

Obviously, this is not a very fleshed out idea, but it doesn't need to be. My book is soft sci-fi, so my ideas only need to make sense on a surface level and be theoretically possible. Thing is, I'm wondering if this idea is completely nonsensical. I'm a bit confused as to the nature of Neptune; is everything that surrounds the mantle just a huge ass ocean? If so, how does this plan work then? I mean, the wind turbines could just be replaced by a water turbine. Instead of extracting water from within the caves, they could just open up a hatch in the roof and let the water in from above. And is the temperature down there livable?

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    $\begingroup$ You would have an easier time setting up your colonies high above the solid planet surface, floating buoyantly at the level where the remaining atmosphere above your colonist was only as heavy as earth's normal. $\endgroup$
    – Henry Taylor
    Commented Aug 2, 2020 at 1:46
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    $\begingroup$ Neptune is a gas giant. Its atmosphere extends down to its mantle. At that point the pressure should be about 100,000 times that of Earth's atmosphere. If you handwave that issue away, the rest can easily work for you. $\endgroup$
    – Alexander
    Commented Aug 2, 2020 at 2:28
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    $\begingroup$ I think you have a good foundation for perhaps half a dozen interesting questions in here. As written, your query doesn't actually focus on a single worldbuilding problem. It seems like you're doing a little market research here. $\endgroup$
    – elemtilas
    Commented Aug 2, 2020 at 4:01
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    $\begingroup$ If you're looking for cave-dwellers, then if you try and write hard sci-fi you're going to get in trouble with finding metabolic pathways to give them life, not to mention that tall the water in their bodies will be frozen very, very solid at those temperatures and pressures, see phase diagram water. Might work for science-fantasy though. $\endgroup$
    – Escaped dental patient.
    Commented Aug 2, 2020 at 16:15
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    $\begingroup$ You might prefer Titan. It has a rocky surface and liquid oceans beneath a dense atmosphere. Neptune's rocky core is far beneath the meta-liquid mantle. $\endgroup$
    – James McLellan
    Commented Aug 2, 2020 at 17:09

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I don't believe it would work

enter image description here

Earth's diameter is 7,917 miles. Neptune's diameter is 30,599 miles. The temperature and pressure at the Earth's inner core are 5,200° Celsius (9,392° Fahrenheit). The pressure is nearly 3.6 million atmosphere (atm). I can't even imagine what the temperature and pressure would be near Neptune's core. But we need to know.

And that's the problem.

Neptune's internal structure resembles that of Uranus. Its atmosphere forms about 5% to 10% of its mass and extends perhaps 10% to 20% of the way towards the core, where it reaches pressures of about 10 GPa, or about 100,000 times that of Earth's atmosphere. (Source)

And that was just to get past the atmosphere, which is only 10%–20% of the distance to the core. The rest of the distance is ice... about ten thousand miles of ice. Think "tunnel 1.5X the diameter of the Earth."

And what's amazing is that despite the enormous pressure caused by the atmosphere, and (we think) despite all the pressure of the upper ice mantle, we still think the frozen layer reaches all the way to the core. That's a whole lot of cold to survive that much pressure. Here's the relevant chart:

enter image description here

This scenario puts you in the Ice Type X range.

So, I apologize, but no. I don't think this scenario is plausible.

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  • $\begingroup$ The internal structure resembles Uranus? TMI. $\endgroup$
    – DKNguyen
    Commented Aug 3, 2020 at 2:49
  • $\begingroup$ On the one hand, I want to say that, since this is a soft setting, there could be some sort of inefficient exotic tech that requires absurd amounts of mass, and being farthest from most of the action, Neptune got mined just that heavily. But that would have so many other implications I wouldn't know where to begin. $\endgroup$
    – CAE Jones
    Commented Aug 3, 2020 at 4:59
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Nah, I don't think so.

Neptune, although it does have a solid, rocky core, is so massive, that the hydrogen and helium in the lower atmosphere are crushed into a liquid, which further crushes the rocky core. And you would need something like a Dyson Sphere around the Sun to remove a planet's worth of liquids and gases away to expose the rocky core.

Due to the increased amount of heavier gases in the atmosphere, you could use hydrogen as a lifting gas in the Neptunian atmosphere, to create a rudimentary zeppelin-like "cloud city" on Neptune. You might be able to use the ferocious winds on Neptune to harvest wind energy via wind mills, but again, 2,000 kmph isn't something that can be harvested by windmills or turbines.

Forget Neptune for now, we need to focus on Mars and Venus first, and move onto gas giants.

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