What could life be like on a Water Giant?

[speaker-to-trolls]

Inspired by this article which describes the possibility of a planet like Uranus or Neptune in the habitable zone of a star, where it could potentially hold an ocean of water thousands of kilometres deep. Also reminded me of a planet described in Iain M. Banks The Algebraist which had such a setup, although Banks said that the liquid ocean would only extend about 100 kilometres before even the water was compressed into a layer of ice.

What would such a planet be like? Would it be reasonable to expect life to evolve on such a planet where organic molecules would be so few and far between? Would it be reasonable to expect an oxygenation event as happened on Earth (allowing complex multicellular life) when there's such a huge volume of water to oxygenate?

[Broomstick]

I'm not sure you can assume that organic molecules would be "so far and few between" - there may be mechanisms that concentrate them in certain areas, and even if they were a small percentage overall of such an ocean the actual objective amount could still be enormous.

If life did evolve I see no reason an "oxygenation event" couldn't happen, even without any land. Earth's O2 event happened long before anything crawled out onto land.

I see no reason you couldn't have multicelluar life under such circumstances, either.

[Solauren]

It's really hard to say how life would evolve in such an environment.

All of our aquatic life 'points of reference' all had a solid surface to work with in some way. Either near one (Coastal), or on the ocean floor, and no more then a few kilometers down.

Without the stability of an offered surface, near a viable light or heat source, we, as far as is known, severly limit the life forms that can evolve.

For example, I can't see something like coral evolving under thousands of miles of ocean, resting on a 'mantle' of ice.

As far as I'm aware, the most likely forms of life to evolve on such a planet would probably be simple plants that exist on or near the surface of the water, with the kind of food chain that could support. For example; floating plankton, some kind of plankton eater, then something that eat's them.

THat's not to say you couldn't get an amazingly complicate ecosystem and genetic diversity. I just wouldn't expect anything massive or physically complex.

That being said, I wouldn't be surprised if somewhere, out in the cosmos, there's a world like that, where the dominate lifeforms could inspire HP Lovecraft stories, and compete with Dinosaurs as a dominate life form.

[Broomstick]

How about something like the sargasso sea? That's an ecosystem floating in open water without being anchored to anything with complex as well as simple life forms.

[OsirisLord]

The best I can see would be the sort of thing we have on the mid ocean ridge, where geothermic vents spew hot nutrients that bacteria use for energy.

[Dark Flame]

Small nit-picky question. Would water really condense into ice? I'm thinking that since liquid water is denser than ice, than the immense pressures of a 100 km deep ocean would make it so that there is no ice below the surface.

[Rahvin]

Small nit-picky question. Would water really condense into ice? I'm thinking that since liquid water is denser than ice, than the immense pressures of a 100 km deep ocean would make it so that there is no ice below the surface.

No. Ice is actually less dense than liquid water (which is why it floats, and why the bottom of the ocean is liquid despite the temperature being well below freezing).

[Starglider]

No. Ice is actually less dense than liquid water (which is why it floats, and why the bottom of the ocean is liquid despite the temperature being well below freezing).

Wrong. Or rather, that is only correct for the relatively narrow range of conditions we have here on earth. This is the phase diagram for water;



As you can see, beyond 10,000 atmospheres or so it starts crystalising even at high temperatures. The exotic, high-pressure crystalline phases have higher density than the low-pressure one we're used to; higher than the liquid phase in fact.

[Ryan Thunder]

So it's not actually ice, then, its just water compressed to the point that it acts like a solid. Is that an accurate description?

[Surlethe]

It's water compressed to the point that it is a solid. At such high pressures, the atoms simply don't have space to move around and fall into a lattice. If you want to call "ice" the solid phase at low pressures, then it's not ice; if "ice" is just solid H2O, then it is definitely ice.

[Ryan Thunder]

It's water compressed to the point that it is a solid.

Sorry, yes. I guess that would be more accurate.

[Starglider]

So it's not actually ice, then, its just water compressed to the point that it acts like a solid. Is that an accurate description?

'Ice' means 'solid water'. Everyone in the scientific community calls these phases 'Ice XI', 'Ice VII' etc. Kurt Vonnegut's famous sci-fi novel 'Cat's Cradle' postulated a (fortunately fictional) form of ice, 'Ice IX', which was solid at room temperature and which converted water in contact with it into more Ice IX (with disasterous effects on planet earth). The actual Ice IX phase wasn't discovered until later.

So no, you're wrong.

[speaker-to-trolls]

Besides the accumulation of nutrients without any contact with the planets crust, my other thought about the problems in developing life was that there wouldn't be any volcanic vents to provide energy for early life forms. Is there anywhere else energy could conceivably come from?

I see no reason you couldn't have multicelluar life under such circumstances, either.

Sorry, I really meant that as a followup to the thing about no oxygenation event, since as I understand it only oxgen based processes can provide enough energy for multicellular life on Earth.

[Akhlut]

Besides the accumulation of nutrients without any contact with the planets crust, my other thought about the problems in developing life was that there wouldn't be any volcanic vents to provide energy for early life forms. Is there anywhere else energy could conceivably come from?

Solar energy; a lot of gas giants are actually fairly close to their parent stars so it's not inconceivable for a gas giant to be within the Goldilocks Zone. Further, a lot of these planets have enormous magnetospheres, so I imagine those could provide some sort of energy to the oceans to help life along.

Nutrients can be accumulated through comet and asteroid strikes, as well as significant stores of methane and other carbon compounds commonly found in gas giants.

Sorry, I really meant that as a followup to the thing about no oxygenation event, since as I understand it only oxgen based processes can provide enough energy for multicellular life on Earth.

On earth, yeah, but there could be alternative electron acceptors in alien biologies. The primary purpose of oxygen is to serve as the final step of a process to gradually move an electron around to release energy to fuel metabolic processes. Alternate electron acceptors at the end of such a chain could be found, doubtlessly, and could therefore serve to allow the evolution of multicellular organisms.

[Erik von Nein]

Oxygen in our electron transport chains is the most effective way of producing ATP for heterotrophs, yes, but it only uses oxygen as a final electron receptor. Otherwise oxygen is only involved in functional groups like aldehyde. Electron transport chains in autotrophs generally doesn't involve oxygen, at least not as an electron receptor. Thus why they tend to produce oxygen instead of use it. NADP+ is the final receptor in the linear electron flow of C3 photosynthesis.

The way oxygen's involved is by the plant splitting water for H+ molecules and electrons. Oxygen's a byproduct. Worse, in C3 photosynthesis it's a competitive inhibitor for CO2 in C3 photosynthesis.

So, no, you don't need oxygen to produce high potential energy molecules, but you most certainly need some form of energy to start these processes.

EDIT: As far as getting nutrients in the first place if they crust is completely cut off from the surface by ice then you're kind of boned. Comet or asteroid strikes would take a while to give you enough to support any really complex lifeforms.

Certainly, though, you can have life that spends its entire life-cycle suspended in the ocean. The notion that you need some sort of solid ground for all life is silly, especially if the life you're talking about evolved in this environment in the first place.