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Let's say there's a habitable planet somewhere, covered with very deep cool water ocean. By very deep I mean thousands of km, but the exact numbers don't actually matter.

According to water's phase diagram, at 0.6 GPa and 0oC, or 1GPa and room temperature denser than water ice VI becomes stable. The water is significantly salty, and I have no idea how it would influence ice formation. Say it would just drive the boundary a bit deeper. Also I see no reason for temperature to be far from 4oC where the water is densest, at least that's what we see in the Mariana trench. So let's say there's icy ocean floor somewhere between 60 and 100 km deep, until Titan or Neptune is actually explored.

Now there's also a layer of buoyant organic matter covering most of ocean surface. It is actually thick enough for a small human colony to survive on top of it. And that colony is desperate for metals. From the ocean composition they think there must be at least iron, copper, and gold somewhere in the sediments on the ocean floor. Also there must be a lot of sticky organic ooze there.

So the question is: how to explore the floor and bring back samples without using too much metal?

The colony has a few portable, reliable, efficient fusion reactors capable of using any hydrogen-rich substance. However, they don't want to risk any of those unless absolutely needed. Also they can mine nearby asteroids/moons for metals and silicates, but that's expensive and again they don't want to risk the only ship too much. They can 3D-print any material at hand into any imaginable form, let's say big crystals are out of reach though (a diamond monocrystal submarine, huh). A robot capable of any formalized, non-creative task can be programmed. Other than that, let's say they are on todays mankind level. Calling for interstellar help is prohibitively expensive, they're already in debt and they also sunk a leased spaceship.

What I could think of:

  • brute force - make a very thick steel bathyscaphe. I have little idea if steel can withstand that kind of pressure.
  • use graphite bricks (hello RBMK-1000). How do they patch the seams between bricks?
  • Power - accumulators? wires? just H2+O2?
  • handling the ooze - how? Freeze it with liquid nitrogen, then drill through?
  • How long would the journey to the bottom take (w/o humans on board)?

Feel free to correct/criticize my assumptions here if you feel they are wrong. Links to existing similar worlds are appreciated as well.

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  • $\begingroup$ Thousands or hundreds of kilometers? Or tens? $\endgroup$ – Trevortni Nov 22 '16 at 23:33
  • $\begingroup$ @Trevortni let's just say very-very deep. I don't want to postulate exact numbers so that I can fine-tune planet composition, radius etc. Anyway I believe the bottom of the ocean to be at around 1GPa/100km, see my 2nd paragraph. Oh, yes, the gravity is like here on Earth (probably the planet is bigger and less dense). $\endgroup$ – Dallaylaen Nov 22 '16 at 23:40
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    $\begingroup$ Could they perchance find the sunken spaceship and use it? It may be useful even for spare parts/metal. $\endgroup$ – Landon Boyd Nov 22 '16 at 23:40
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    $\begingroup$ If you don't need a low pressure cabin to keep humans alive in, just build your robot like normal and fill it with something incompressible like gasoline or mineral oil. Pressure isn't really an issue if you are not trying to fight it. $\endgroup$ – John Meacham Nov 23 '16 at 0:50
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    $\begingroup$ How powerful is energy the source they have? The chemical composition of seawater - potentially they can extract a lot of elements just from sea water itself. Numbers are not particles per million, but effectively they are mg per kg of sea water - so Iron is about 3.4 tonne per cubic km of water. They can use selective membranes to extract the stuff. And how deep is actual sediment floor - it may be important for some approaches. $\endgroup$ – MolbOrg Nov 23 '16 at 3:33
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The Challenger Deep of the Mariana Trench is 11 km deep, and that is just a small fraction of the depth you want to explore (the tallest mountains on Earth are all less than 9 km above sea level), so you'd be three to five times the total elevation range on Earth. This is very, very deep.

A Dredge System Won't Work

Even a 60km to 100km cable or chain reaching from the surface to the bottom would require a lot of material and weigh a lot, and would have to be of a material that would be function at every combination of pressures and temperature from the surface to the bottom. This would be on the same order of magnitude as a cable for a space elevator (low Earth orbit is about 100 km up), and is basically impossible (Pugno 2006) to do so, even with state of the art materials, technology and resources (one estimate cited in the linked paper was $10 billion for an approach that it determined wouldn't come close to working), so there is no way your folks could build that.

What About A Robotic Mining Submarine?

As noted in the comments, there is also no sensible reason to maintain a low pressure part of submarine to retrieve materials from this depth, which makes the engineering problem much greater than one in which an incompressible fluid inside the cavities in the submarine can counterbalance the exterior pressure. But, you would still need to have a control system (either wirelessly remote controlled - possibly with intermediate depth relay buoys, or AI), a power system, a propulsion system, and something to sift through the sediment to distinguish useless material from the materials needed (a non-trival task for materials you can't grab with a magnet).

Mining Sediment Would Be Very Challenging

Any sediment to sift through would tightly packed and might very quicky become sandstone at such high pressures, with a minimal sandy layer, in which case you'd need to break up the sandstone. And, before you got to the sandstone which might have minerals in it, you'd probably have to clear away slushy near ice and organic mud on the bottom. So, you'd need a slush shovel, then a sand mover/sifter, then a sandstone breaker that would make bits fine enough to analyze, and then something to analyze the content.

And, it is entirely possible and indeed, likely that the metals would be tens to thousands of meters under the sea floor, so you might need some underground mining of the sea floor as well to get at the good stuff.

Then, once you segregated out the good stuff, you'd need to bundle it up somehow and have enough propulsion to get it back to the surface (perhaps some sort of compressed air could adjust boyancy and float it up). Also, the good stuff in all likelihood wouldn't be pure elements. It would be, at best, rich ores or oxidates or something like that which would require further processing once you obtained it.

The materials you used would have to not corrode in cold salt water, would have to withstand sometimes strong currents, would have to be big enough to carry an appreciable amount of mined material plus everything you brought to mine it, so even if you unmanned mining sub is barely bigger than a backhoe attached to a small oceanic submarine, you'd probably need something on the order of 10,000-100,000 kg at a minimum. Air independent propulsion rather than nuclear propulsion could probably work, as the total distance travelled wouldn't have to be very great between refueling.

A big investment mining submarine concentrates limited resources in a few high risk projects

But, this is still a major engineering undertaking, and you only get one or two major mistakes and your efforts end up at the bottom on the unthinkably deep ocean. This is a big problem in an environment where there are necessarily lots of unknown unknowns.

What About Filtering Water For Trace Useful Elements?

On the whole, sieving low concentrations of materials dissolved in water (perhaps using a pump to get water from deeper parts of the ocean where mineral concentrations might be greater) seems like a better plan.

For example, on Earth, typical seawater contains the following:

Seawater composition (by mass) (salinity = 3.5%)

Oxygen 85.84

Hydrogen 10.82

Chloride 1.94

Sodium 1.08

Magnesium 0.1292

Sulfur 0.091

Calcium 0.04

Potassium 0.04

Bromide 0.0067

Carbon 0.0028

Vanadium 1.5 × 10−11 – 3.3 × 10−11

All sorts of goodies are present in sea water in more trace concentrations.

Filtering Water Is Much Simpler And Disperses Risk Across Many Filters

A system that filters impurities from cold high pressure water would have a lot less moving parts and would be easier to set up in multiple small units that could be planted in a line, and could be floated at different depths for different materials.

With a water filtering strategy, it wouldn't be important to land your filters right where rich deposits were found in the sediment the way that it would if you were digging for it. A filter might have five or ten bins each, fitted with balloons that would inflate with something a bit less dense than the surrounding water and float up to the surface (or at least to a manageable retrieval depth like 1-5 km from the surface), that would return collected material as it was gathered. Then, the entire filter system would float itself back up when it ran out of fuel and retrieval bins. A distributed approach would also make occasional failure of a particular filter system or retrieval bin (even say 20%) tolerable in a way that it wouldn't if you put all your eggs in one basket building a massive and complex sea floor mining system where one fail could exhaust all available resources.

The Sea Floor Might Be A Good Place For Simple Filter Systems, Or Not

Indeed, maybe rather than digging in the sediment, you set up an osmosis filter on the sea floor (on the same concept as an industrial strength reverse osmosis seawater filter except that you'd be after the contaminants rather than the filtered water itself) on the theory that mineral concentrations are greater at the interface of sediment and water, rather than actually digging at all.

If you wanted, you could even deliberately fill an area of the seabottom with lots of contaminants by dropping bombs on the sea floor to turn sediment into fine grained debris and then filter out that debris.

But, actually this intuition isn't all that sound. Empirically, all sorts of different trace elements are found at different depths and in different conditions. Lots of elements are most common near the surface, iron seems to be common at mid-depths, and other elements (e.g. lead) tend to favor the bottom of the ocean.

So actually, you'd want filter systems at all sorts of depths after first running probes at different depths and locations to figure out where the elements you want are at the highest concetrations.

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  • $\begingroup$ Can a small pressure-resistant casing be built for electronics? And I just realized that sodium may be used as underwater fuel (probably packed into organic-coated pellets/wires to make it controllable). If these assumptions hold, a small robotic explorer that only brings back water/mud samples looks doable. Of course, that doesn't eliminate the need to cut through the sediments to actually mine anything. $\endgroup$ – Dallaylaen Nov 24 '16 at 1:21
  • $\begingroup$ As long as you don't use any air/gas you should not need a pressure-resistant case. It's the difference in pressure that leads to items breaking/imploding underwater. This is why a steel rod won't have a problem going down hundreds of kilometers under water but a steel can that is full of air would implode before coming even 1/100th of the way. Also, a mini-submarine controlled by radio won't work because of massive signal distortion underwater. $\endgroup$ – Selene Blok Nov 24 '16 at 11:20
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    $\begingroup$ It's not just signal distortion, it's absorption. The signal just can't travel far through sea water. en.wikipedia.org/wiki/Communication_with_submarines $\endgroup$ – Tim B Nov 24 '16 at 12:01
  • $\begingroup$ @JoostBlok Radio is unneeded. As stated in the post, they can assemble a robot and then instruct it to "search the floor, collect samples and return, run away in case of danger" (literally!). $\endgroup$ – Dallaylaen Nov 24 '16 at 15:54
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    $\begingroup$ should note you don't need to bomb the ocean floor, most of the ocean floor is already loose sediment. $\endgroup$ – John Nov 27 '16 at 5:52
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You don't need a submarine, remote dredging will work. Assuming that a chain to the ocean floor will work (and fail) like a space elevator is wrong. If the individual segments are buoyant, you can have the cake and eat it too. The tensile strength of the segments then only needs to cope with the dynamic stresses, and you can minimize those by moving real slow.

Whatever heavy elements you have, or gain, can be used in streamlined impactors (add explosives and propulsion as needed) that race to the bottom and mix things up to better dredge them.

If you can spare some tech, the chain segments can be made to control their own buoyancy, if not, you will need to print them from organics to fit into some specific slot on the pressure\ density\depth scheme of things. You then place them there by means of the one variable buoyancy- device that you have, or if you're extra cheap, by attaching salt crystals designed to dissolve in the needed time.

Depending on you budget and the specifics of your topside dwelling, the dredging mechanism may vary.

If there are sizeable currents, this idea goes the space- elevator way to the farm upstate...

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  • $\begingroup$ If the (deep) currents are constant, that's just one more coordinate to consider in the buoyancy + weight + pull == 0 equation. And I don't think there should be alternating currents beyond certain depth (are there on Earth?) $\endgroup$ – Dallaylaen Apr 11 '18 at 15:41
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    $\begingroup$ Not so sure about that, with current, you get the problem of every meter more adding to the preceding meter, and so on, the very thing that makes the ultra-long space elevator cable so problematic. Buoyancy will only cancel the effects of gravity. $\endgroup$ – bukwyrm Apr 11 '18 at 17:00
  • $\begingroup$ Yes, you are right. $\endgroup$ – Dallaylaen Apr 12 '18 at 0:28
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Posting an answer to my own question here just to share some further thoughts on the setting.

Magnesium

Magnesium is abundant in seawater. It is also light, stiff, and hard metal, although subject to corrosion and flammable. So in the given setting it is going to become the colony's structural material of choice.

Magnesium is also quite conductive (not as good as Al or Cu though).

Robots

As pointed out in comments, robots needn't fight the pressure and can be filled with liquid (like gasoline) instead. I still believe that semiconductors may need casing, but that's going to be a small and easy to do one (think raspberry pi mounted inside a bowling ball).

Acсumulators

I "invented" a galvanic cell with sodium anode slowly dissolving in organic compound, graphite cathode in the outside salty water, and polymer membrane separating the two. However, both Trieste and Deepsea Challenger seem to just use normal batteries.

Deep water vessels

A graphite or steel sphere with thick enough walls looks capable of withstanding the pressure, however, communication with the outside is going to be tricky. In particular, efficient heat exchange and thus submerged fusion reactor is out of question.

Communication

As pointed out in comments, radio would be inefficient in water. Not sure if ultrasound could be used at least for location, heartbeat, and short orders.

Wires

As pointed out in another answer, hanging wires would not bear their own weight. However, metallic wires in a thick buoyant insulator could be self suspending. But the compression profile of water and insulator is likely to differ, as water is not quite incompressible at required depths. So additional ballast/float units will have to be attached in order to balance the whole thing.

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    $\begingroup$ I like the idea of innovating on a few concepts, such as batteries - The Trieste could simply buy those, you will need to build the whole thing from scratch,-> potential for optimization. ---On the subject of semiconductors: just because they are solid state does not mean they are not impressed (pun intended) by pressure - The bandgaps they work on are intimately related to atomic distances, which are the very definition of compression. ---- On the subject of comms: Whales use lower frequencies for long reach. Ultrasound will give higher (still terrible) bandwidth, short reach. $\endgroup$ – bukwyrm Apr 13 '18 at 22:11

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