Could this work? How to? Terraformed gas-giant

I have an idea for a scify/lost technology world, that I would like to check the plausibility of.

What I want A terran world of large size. The crust of the world is thin, and hostile gas-organisms live underneath. During rare earthquakes poisonous gasses can rise, and once in a bicentennial blue moon sometimes landmasses can even shift to new global positions, blown of when huge amounts of gas erupts somewhere else. This catastrophe-prone world is habitable only because of its size - even a shift in global position will entail a smaller shift in climate zones if the planet is large enough.

How I thought it could be done A terran world of gas-giant proportion, where the tectonic plates are really thin and rests on the compressed clouds of the (jupiter-style) gas giant.

Proposed method For human life, it had to be shifted into a goldilocks-zone, which meant to pass it through an asteroid-belt. The tectonic plates are essentially hollowed out egg-shell shaped asteroids and cannibalized moons which simultaneously where sunk on the gas-giant, trapping light gasses underneath. Later fused, with gasses and water harvested from the world itself.

Question Is this idea completely bonkers? What would need to be done differently to make it work? Are there better ways of achieving my initial desire?

• Welcome to Worldbuilding! By "life" could you edit the question to be a bit more specific? Does "bacteria that reproduces with some molecule" count, or do you want to lean towards bipedal, DNA-based, intelligent beings? And does life have to first start on this planet, or can it be introduced (panspermia)? – Zxyrra Dec 21 '16 at 8:10
• So are we talking Gas giant, Jupiter style, or more solid surface and toxic athmosphere, Venus style or something completely different. I am confused. – Alexander von Wernherr Dec 21 '16 at 8:11
• @AlexandervonWernherr He means a combination of the two - sort of like an egg. The "yolk" is a gas giant, but the "shell" is solid; some external atmosphere may be present if the planet "leaks" – Zxyrra Dec 21 '16 at 8:21
• Would it work at all? The gravity from the solid core would break the outer shell, I think. – Alexander von Wernherr Dec 21 '16 at 8:24
• what would affect the probability of this? – Tell Dec 21 '16 at 8:37

What you are proposing sounds much like the Supramundane Worlds concept of Paul Birch. A gas giant planet like Jupiter is not usable in its current form by humans, but if you could build a shell over the atmosphere you would create a "world" with many times the surface area of the Earth.

While this isn't possible with today's technology, in the far future, sheets of woven carbon nanotubes or Graphine could be used to build the "deck" over the atmosphere of Jupiter, and then an ecosystem overlaid on top of the "deck". Obviously, such a structure isn't going to be in one piece, and the seams where the various panels are attached would serve much as the edges of the continental plates on Earth. It may even be possible that the edges are deliberately unattached, to allow for flexing and thermal expansion in order to relieve stress on the overall structure.

Obviously these engineering zones would have been marked and put off limits during the initial construction and early habitation of the Supramundane world, but after thousands of years of habitation, the memories and instructions of the initial colonists are long forgotten, so wandering into the area of one of these expansion joints would have much the same effect as you are looking for.

• +1 for 'the dangerous bits have had their hazard stripes worn off' – Joe Bloggs Dec 21 '16 at 13:40

1: Is it bonkers?

Yes. This can't happen the way you described. A solid won't come to rest midway down a liquid (or gas) that it otherwise sinks in (though Terry Pratchett has a beautiful section describing dead ships sailing underwater in the Diskworld series and there are liquids dense enough to float on), so your 'plates' won't float unsupported. If the whole crust is one shell surrounding the core of the planet then any kind of crack, fracture or instability would shatter it like an eggshell under a stiletto.

The second issue is the method of capture for these rocks: You describe the planet passing through an asteroid belt. Firstly it would have to be one hell of a dense asteroid belt to coat a gas giant in rock. Asteroid belts aren't the common trope of dense fields of rocky shards bouncing off each other. Even planetary ice rings don't achieve the kinds of density I think you'd need for this. Not only that: but these rocks are going to be hitting your gas giant at one heck of a speed: more than enough to punch down past whatever fragile equilibrium point you established.

Even if you manage to find a neutral buoyancy rock and have it suspended by thermal updrafts or stratified liquids or similar you hit another problem: Your human beings can't possibly survive. If the gas the plates are floating on is dense enough to support rock then either the gas above is dense enough to kill the humans, the humans are dense enough to sink through whatever makes up the plates or the plates are under a lot of pressure from below (which would tear the plate apart). In any of these cases humans die horribly.

2: What would need to be done differently to make it work?

Strangely enough this is a concept that I fleshed out for one of my first fully featured worlds, and it's a common trope throughout steampunk fiction or any kind of 'floating island' world where magic isn't allowed. Some form of element that is a room temperature superconductor. In your case specifically it would be held up by magnetic locking in the magnetosphere of the planet's metallicised hydrogen core. If this element is common and ubiquitous enough in the crust of the planet then you can fine tune the height of the plates in the atmosphere. I'm fairly sure there is another question on WB about whether there is an idea height for this, so have a look around and see what you can find.

3: Are there better ways of achieving my initial desire?

Yes, if your 'large' world is only a little large (so not a gas giant) and you're willing to give up on a totally thin crust in favour of a very 'pocketed' one.

If instead of a thin crust hanging over nothing you go with a cavern riddled planet with rare but violent bouts of vulcanism you can achieve poisonous gases, creatures living in (fairly small but still hostile) subterranean gas environments and violent rearrangements of the world that can literally sink whole continents and cause new ones to be pushed up from under the ocean, forming new pumice-stone-like continents for both the human populations and the subterranean to migrate to. This isn't exactly what you asked for, but hopefully it sparks a couple of ideas (in fact it's got me thinking of a question I'd like to ask...)

Happy world building!

I doubt such a planet would be stable.

As soon as a piece of crust would separate from the rest (imagine a quake) the buoyancy would make it sink (high pressure gas cannot sustain a lump of rock). It won't take much before the crust would be entirely gone to the core of the planet and all living beings would be faced with the challenge of learning to fly life long in a poisonous gas.

There are many reasons this would not work

A crust surface would be denser than what is beneath it.

The density of metallic hydrogen, a large part of Jupiter's core, is about 600 kg/m$^3$. Water is 1000, rocks in the 2000-3000 range, solid steel around 8500, etc. The crust surface would want to sink through the core into the center of the planet...where most of a gas giant's rocky material probably is already.

Its too hot

Jupiter's metallic hydrogen starts at around 10,000 K. That will melt any sort of crust material sitting on it. The molten parts will then drip and fall through the less dense metallic hydrogen into the core.

Too much convection

The roof of the former Superdome in New Orleans was kept afloat by air pressure. This is what you are asking to do with Jupiter, to build a shell where crust-like plates can be supported by Jupiter's immense pressures. Unfortunately, Juptier's clouds, storms, and bands are kept that way by massive convective forces. The magnitude of those forces (of which there are no good measurements) would certainly overwhelm any known material. Your crust would be torn apart, the broken pieces melted, and all of it pulled by gravity and its higher density into the core.