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I have created for my novel a planet with extremely deep oceans. The ocean water itself is fairly similar to the ocean water of earth (phytoplankton, zooplankton, saline content, aquatic microbes, etc.) The average depth is twenty miles downward, but there do exist fissure and trenches that lead even deeper, leading through the crust and mantle, almost to the outer core of the planet (this would be about 2000 miles).

For the sake of this question, please ignore the implausibility of an ocean trench extending into the mantle.

On this planet, there exist huge cephalopodic creatures that are fairly similar to Architeuthis dux, or giant squid, with some differences.

How can I design this squid so that it can dive all the way to the extremity of the 2000-mile trench without it suffering injuries from the pressure? The squid can be as big as you need to answer this question, but it may not be smaller than 57 feet in total length (the largest known Architeuthis Dux).

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  • $\begingroup$ Well, simply make the creatures diet contain some handwavium and unobtanium $\endgroup$
    – Topcode
    May 13 at 19:55
  • $\begingroup$ If you want to ignore the implausibility of an ocean trench extending into the mantle, why are you Asking any such Question? Either way, why can your squid not have circulatory, respiratory and whatever other necessary systems adaptable to external pressure/temperature/whatever? $\endgroup$ May 15 at 21:55
  • $\begingroup$ My question is concerning how its anatomy can be designed to survive the pressure. In regards to the ocean trench extending into the mantle, I am not asking about it because I already have an explanation formed. I merely mentioned it to give a gauge of the depth and since I didn't want answers to how it could exist. $\endgroup$
    – Wyvern123
    2 days ago

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You would need to not only make it able to withstand the pressure, but at the depth that you require, they'd need to burrow through the ice too.

On Earth in the occeans, the water increases in pressure at a rate of 1 bar per 10 metres. At a depth of 2,000 miles (3.2 million metres), the pressure will have increased to 320,000 bar. The phase-diagram of water will give you solid water (at 0 Celsius) at about 6 kbar:

Phase diagram of water.

First you'll get ice VI (1.31 g/cm3). If you want to make it liquid, you'll need to raise the temperature there to 50 Celsius at 10 kbar. The deeper, the hotter you'll need it to be to be liquid, 'till at 20 kbar, it'll need to be at 100 Celsius to still function as water.

Ice VII ( 1.65 g cm−3), the same ice thought to comprise the sea-floor of Europa, is a disordered lattice. If you want it liquid, you'll need it at a temperature of 350 Celsius, and that'll only take you a third of the way to the bottom - the rest of the way is solid or you need 100s of Celsius to keep it liquid.

A solution might be to have underwater hydrothermal-vents that are constantly pumping heat into the ice from below, particularly at the spot where our hero dives.

Any detritus from the ecosystem above would be inclined to produce an anaerobic environment, strongly inimical to Earth-squids.

But it still doesn't strike me as practical with a normal Earth-like squid

  • It would need to withstand very high temperatures, maybe up to 600 Celsius.

  • Or have quickly self-replacing tungsten teeth on it's tentacles, and be incredibly strong and have endurance like Superman.

  • It's legendary ability to survive by metabolising it's body-fat as an oxygen substitute (like the Crucian carp) makes it way, way more resilient and remarkable than Earth squids.

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  • $\begingroup$ This is a good criticism, but the question doesn't actually specify a temperature. The diagram stays liquid almost to the pressure you specified at 650 K, and the question also didn't specify a gravity. (Given it's an ocean planet, it would be slightly less if Earth-sized due to the lower density of water than rock, but that's not enough...) I had a related question here $\endgroup$ May 13 at 20:40
  • $\begingroup$ I've added some stuff. Something a bit more helpful to the OP, I hope. @MikeSerfas $\endgroup$ May 13 at 21:11
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Nope

For context a waterjet can cut through a block of steel. Waterjets are about 100,000 PSI. You can compare that to ocean pressure at 2000 mile depth. Every ten metres you get another atmosphere of pressure. An atmosphere is about 14 PSI. Add them up and you get about 4,500,000 PSI on the squid.

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    $\begingroup$ What would be the lowest they could believably go, and how then? It's not a reality-check question, so your answer is more frame-challenging than answering 🐙. $\endgroup$
    – Tortliena
    May 14 at 0:49
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This is really tough, but you have some things on your side.

enter image description here I'm going to start with the same water phase diagram as the other guy, but I added a line to it for a question about Uranus I posted a while back. That line is based on an uncertain source about the pressure vs. temperature on Uranus, but it should illustrate that a real planet might contain liquid-water conditions that extend to substantial pressures and temperatures. Your octopi don't have to burrow into ice, but they do have to contend with extremely hot temperatures as they go down (which shouldn't really surprise us, considering where those cracks are!)

Now there's a lot to fudge with that line - water is heavier than hydrogen, so the pressure should build up faster than it does on Uranus; water has a large heat capacity; it could move heat by convection faster. Maybe your planet has super-thick crust in large part because the water fissures are cooling it to a much deeper depth than Earth. You can blame the tendency to fracture deeply on some aspect of the geological composition of the planet (which is even further beyond my competence than the rest of this). We'll just say somehow the line stays on the green as we go down your fissure.

Now we're cooking squid in a pressure cooker, and our expectation is for cooked meat. But this planet has had a while for evolution to do its magic, and we can at least find some contrary tendencies - the same tendencies that keep the water liquid. On one hand, every atom and molecule in the squid is getting shaken up with an amount of energy proportional to the kelvin temperature. On the other hand, the pressure favors molecules sticking together. Under the ideal gas law, every molecule takes up the same amount of space, but tying two together reduces that by half. So you can get the same amount of energy out of tying two molecules together as if you had bled that volume of gas out through a hole into a vacuum. Put together, you can argue that an ammonia or CO2 molecule that might have just flown off a heated protein, will instead be forced to remain in place.

That's helpful, but not nearly helpful enough. You still have plenty of situations like trans fats. The place we're going has temperatures and pressures comparable to those used in the chemical manufacture of "partially hydrogenated vegetable oils", a bright idea that has killed about 500,000 people a year for a very long time now, because a fat with a cis double bond isn't the same as one with a trans double bond, and the heat can twiddle one to the other. You can imagine there are many, many ways to shake up a biomolecule that doesn't disperse it into small bits of gas, and those are still likely to occur to your squid.

So? We evolve it. Molecule by molecule, all the weak links have to go. Everything has to be buttressed up and made sturdy. This may seem quite impossible, but bear in mind that there are bacteria known that can survive autoclave conditions (that's only 394 K, but it's a start). Hydrothermal vents in Earth trenches are not lacking for life. So we're going to have to handwave - a lot - and say that there's been a massive change in the overall biochemistry of this squid, to allow survival under much higher pressures and temperatures. I don't think it's impossible but I don't know how to do that either! I doubt anybody knows how to draw an ecosystem for a very different planet than ours.

At 32GPa I should also note that you're getting into the boundary of ice and supercritical fluid. I don't think that's a tremendously big deal - there are kinds and kinds of supercritical fluid, and while it is in some sense it's the same as a supercritical gas, it's not entirely gaslike in properties. But at the end your octopus might be doing something closer to falling, perhaps with a need to pay attention to updrafts, in a way that was not as true at lesser depths. Try to stay on the right side of the Fisher-Widom line at least.

The hardest part is probably the journey. You need the same squid to be able to survive the sunlit shallow sea, the freezing depths, and a super-pressure cooker in an oven set to self clean. This isn't something that Earth life is terribly good at - many of our deep sea organisms die immediately upon retrieval to the surface. You may need something very remarkable, such as a symbiosis between two different kinds of life that regard themselves as "the same organism" and share a genetic code and mechanism of memory, in order to pull this off.

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