Atmospheric pressure in an almost empty world with a specific habitable zone far above the ground

Question

I'm currently working on the following concept of a world for a high fantasy setting:

• The world's core has a radius of around 100 km. Inside is a magical object that creates a large-scale (planet-sized) gravitational field around it.
• The core is enveloped in a mantle of around 10-15 km of water with no additional notable land masses, such as continents.
• The "habitable" zone begins at around 8.000-9.000 km away from the core. This is where humans and humanlike species live under conditions similar to earth's sea level (same air composition and pressure, gravitational constant of approximately 10, etc.) on magical floating islands.
• No other celestial bodies exist in the universe, as sun, moon, and so on are divine constructs (comparable to e.g. Helios' chariot from Greek mythology).

With the basics out of the way I'm wondering about certain properties (especially relating to atmosphere and pressure as these are some of the most difficult topics for me personally) this world would have and how they could be explained with something other than "it's magic"? I don't require any hard numbers, I'm just very curious about what parts of this concept could be explained sensibly with the help of science.

• How high is the approximate pressure inside the water mantle and below the inhabited zone. Could any realistic creature survive inside it? Could the water physically even be liquid?
• How large would the "habitable" zone be? Just as earth's atmosphere gets too thin for humans to breathe above a certain height, how dense would this much larger atmosphere be at bigger heights? Would the pressure fall-off be linearly compared to earth's atmospheric pressure (e.g. a constant factor of x times earth's pressure for any given height or would this factor be changing depending on height)?

This is my first question here, so I'm very sorry if I made a mistake while asking this question! Please tell me if I did so and I'll try to correct it!

Thanks a lot in advance!

Edit: I forgot to mention that the magical object at the core generating the gravity creates it in a manner as if it was an extremely high amount of mass condensed into a small point.

I also went a bit overboard with the block of questions, sorry. I would summarize them as "Given the setting above, how would pressure affect habitability and atmospheric composition at different altitudes, assuming otherwise realistic physics?"

Answer 1

TL;DR: you won't have 8000km of air with a thin ocean below it, but thousands of kilometres of weird high pressure dense materials like the inside of a gas giant.

It doesn't sound like you want your work to work like that, and that's fine, so turn up the magic and turn down the "realistic" physics to compensate. No-one will mind.

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Terminology note: I'll call the 8000km radius where gravity and pressure is intended to be Earthlike the datum.

Easy bit first... the simple equation for scale height depends on the constitution of the atmosphere, its temperature and the strength of the local gravitational field. Given that conditions at the datum altitude are the same as on Earth, you'll get basically the same change in atmospheric conditions as you go up... this means that 8000m above the datum would still be the death zone on your world.

Now, to get an Earthlike gravitational field strength at the datum, the gravitational field generated by the core is equivalent to that of a planet with ~1.57x Earth's mass. This means that if the magical core has all that mass in it, the gravity at the surface of the core is ~6400 gees. It being magic, it doesn't have to have all that mass in it... if the magical "virtual mass" was spread out through the whole of the volume of the gas bubble below the datum line, the gravity at the core would be very small indeed... about an eightieth of a gravity. Given that the core is made of pure handwavium, it is entirely up to you which of these is true.

It is almost irrelevant to the habitability of the "ocean" though, because bearing down upon it is 8000km of air... or at least, 8000km of material with the same average elemental composition as Earth's atmosphere. What it definitely won't be is a gas. Regardless of how your local gravity works, there's a lot of material pushing down from above, and all that pressure is going to do interesting things to materials beneath it.

The simple barometric formula can help for short distance below the datum, but it is hard to get a good idea of what is really going on because the pressure of all that atmosphere will build and build and build. I'll ignore the effects of temperature because building a temperature profile of your weird world is far too hard, but this simple approach will do to illustrate the problems you're facing. By the time you get past 17km below the datum (still thousands of kilometres above the core!) the simple barometric formula starts giving pressures of megapascals, and at that point air starts getting weird.

Here's a nitrogen phase diagram from Wolfram alpha:

Once you reach a high enough pressure to enter the "supercritical region" (about 3.4 megapascals) nitrogen isn't really a gas anymore, but has become a supercritical fluid. You'll get a big increase in density below that point... not a liquid as such, but much denser than a merely compressed gas would be. Pressure will increase more quickly below this point, because density has also increased as so there's a greater mass of stuff. At some point you hit the "melting curve" and get exotic high pressure nitrogen "ice"... this is effectively the "crust" of your world.

Go deep enough and eventually you'll probably find a layer of metallic nitrogen around the core. You might, in fact, get a layer of exotic high pressure ice underneath this around the core, as water ice will probably be denser than metallic nitrogen, but it won't quite be a an "ocean".

I think it is fair to say that nothing could live down there.

Answer 2

It is all ok.

Suppose instead of a high fantasy fiction you are making a porno movie. It has all the porno movie things going on.

But now halfway through you have a new character show up and you worry "Are those pants too tight? Too revealing? Is that in bad taste? Maybe people will be offended?".

No. They will not be offended. The people watching your porno movie will be fine with those tight pants. They are watching the movie expecting that sort of thing because it is that sort of movie.

So too your fiction. You have started with a magic gravity generator in a magical universe. People who read past that are OK with that. The fact that now your habitable area is comparably magical is OK too. Your readers are taking in the story expecting magic.

That does not mean there are no rules. In the porno movie I might decide to fill time with some scenes I made for my gory horror movie project. My porno audience might rightly object to that; not in keeping with the themes of the rest of the film. If you have magic with no rules - people and things arrive out of nowhere, actions with no apparent limits, so much magic that nothing makes sense then you might lose even your magic digging audience. Unless you are filming a music video in which case if the music is good and the idols are cute it is ok.

Lay down the rules and then tell your story within them. You have the water planet. You have your floating islands. They are the way they are, and your characters live on them. Some of your characters wear tight pants. All OK!