Establishing an under-ice base on Europa - ice drilling/melting?

Question

If life were discovered in the supposed ocean on Europa, it would be a popular destination for scientists and researchers. I am curious as to what life on Europa would be like, in terms of engineering and building a sustainable habitat.

First, to reach the ocean, is it more practical to melt down through the ice, or drill it? How hard would the ice be at 10km thick (in 1/9th of Earths gravity)

If melting/drilling a tunnel down to the ocean, would it be possible to passively leave this borehole open, or would the pressures cause the ice to flow together again? What kind of engineering would be required to keep the borehole open, assuming my colonists want an elevator to the surface... how would tunnels and rooms at the bottom of the ice behave, structurally?

If the ocean was reached, at about 10km depth, the water pressure would be close to 100bar with some rough calculations. Would it be practical to build habitats mounted on the underside of the ice, in the water, or would drilling rooms within the ice be easier?

Answer 1

It is much more practical to melt through ice than to drill through it. The most likely way of doing this is to place either radiothermalgenerators or a reactor on the front of your craft.

It would not be possible to keep the borehole open after passing through without installing a thick steel tube the full width and length of the tunnel. This would still snap as soon as the ice moved though (which might happen every Europan day due to tidal forces). [correction: Europa is tidally locked and thus has no days.]

My rough calculations suggest 133 bar, which is more or less what you got. This is roughly 3x greater pressure than modern large submarines are able to survive, so your pipe would have to be roughly 3 times stronger than a Seawolf pressure hull. That is a heck of a lot of material to cart across the solar system, and any flaw in its entire 10km length would result in collapse.

If it was absolutely necessary for your story, I would probably build the tunnel from some modern version of Pykrete. https://en.wikipedia.org/wiki/Pykrete Perhaps using a carbon nanotube fiber rather than cellulose (and ideally manufacturing the CNT on site). You would probably do best to situate the tunnel at a point which experiences the least ice movement. (Presumably far from any fault zone.)

Ice at the bottom of the hole would presumably not be very strong as it is under enough pressure that it is very close to becoming liquid again. You should transition away from Pykrete before reaching the water layer, and use some other material.

This whole endeavor is pretty unrealistic with any technology we are likely to have in the next 100 years though. (IMHO) By the time humans have the technology to do something like physically surviving a trip to the Europan ocean, they will have long since developed many technologies which would undermine the plot of this story, like very advanced genetic engineering which make humanity unrecognizable to readers, super tough nanobots who would have already scouted the Europan oceans quite thoroughly, and super advanced sensors which would have scanned the Europan oceans in detail right through the 10km of ice.

Also, it's likely that if life were discovered on Europa (especially macroscopic life) the entire moon would be utterly and completely off limits to human explorers for fear of contamination.

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I just read JDługosz's answer and he makes a lot of good points that have made me rethink my answer.

Ok, the mass of the ice column that needs to be removed is M=density×height×π×r². Let's say the radius of the hole is 5 meters. Based on that I'm getting a mass of the column of 7.2E9 kg (neglecting the compressibility of ice). For the potential energy of raising those tailings to the surface I'm getting 11 MW years. (11 megawatts for 1 year). If your removal system is 50% efficient that would require a 22 MW (electrical) reactor, which is pretty serious hardware. At 35% efficiency, that would put out 64MW (thermal). Your boring machine is going to need more power to run the cutters, but I have no idea how to calculate this.

The energy needed to raise the temperature of the ice column to −50° C (which, from the phase diagram of water, appears to be the temperature needed to sublimate ice rapidly in a vacuum) is E = mass × specific heat × change in temperature. The specific heat of ice is roughly 2000 joules/kg°C. I'm getting a heat energy of 1.7E15 joules or 52 MW years (thermal).

The enthalpy of fusion of ice is 333KJ/KG. The enthalpy of vaporization of steam is 2257KJ/KG. I assume you just add these two together to get the enthalpy of sublimation? (I never did that in school.) That gives me a 1.86E16 joules to phase change the ice to steam or 589MW years, which is a really shockingly large number.

It also occurred to me after reading JDługosz's post that the sublimated ice would possibly recondense on the walls of the tunnel (although maybe not if the tunnel is in vacuum?) Regardless, the tunnel should probably be insulated in a melting scenario.

Regardless the vast majority of the energy need for the melting and vaporization plan goes into the boiling part of the phase change (rather than the melting part). This makes intuitive sense now that I think about it. Water quenching of hot things is crazily effective, and I once saw a TV show demonstrate that melting a hole through a glacier was way more efficient than drilling.

Based on these results, and taking into account the (probably) infeasibility of shoring up the tunnel, I would like to propose a new solution.

Covering the tunnel mouth with a lid to keep it pressurized to roughly 1 atmosphere. You still melt your way through the ice, but now you coat the interior of the ice tunnel with insulation rather than shoring. (Pykrete makes a great outer layer of insulation, but you would want something else on the inner layer.) Melted water stays liquid, meaning you don't have to spend energy boiling it off. This reduces the phase change energy requirements from 589MWY to 87MWY. Adding that to the temperature rise energy of 77MWY (you now need 0 degrees C, so this is a little higher than before) leaves you with 183MWY (thermal), which is still a heck of a lot of energy. I didn't even include the heat loss through the walls of the tunnel, but I think it's the least challenging part of the entire mission. Especially since the reactor needs no coolant radiators which, on a space ship in vacuum, would be enormous.

The weight of the water maintains more than enough pressure on the walls to keep them from collapsing. The reactor heat drills the hole without the need for thousands of tons of drilling equipment (and repair parts and machinery to let you install the repair parts) and ... billions (?) of tons of shoring.

Your explorers would need to live inside a super strong submarine which is 3 times stronger than a modern military submarine, but much less strong than the Alvin deep sea submersible. (The submarine would serve as an excellent space habitat by the way.) The reactor would need to be kept running permanently in order to prevent the column of water from refreezing, so you wouldn't just let it drop into the ocean once you broke through the ice. Probably the biggest challenge is now the mass of the insulation.

It would be really cool if there was a good way to manufacture cellulose (or other similarly performing polymer) from materials found on Europa. The ice on Europa is not 100% water. http://people.virginia.edu/~rej/papers09/Carlson4019.pdf Perhaps the ammonia and carbon dioxide ices can be used as feed stocks in a small factory to produce a hydrophilic polymer? The inner surface of the insulation might be a gel or a hydrate of some sort? You want something where most of the mass of the insulator is water, which is captured and held immobile.

Answer 2

It is much more practical to drill (or tunnel or quarry) than it would be to melt the ice.

Consider: we have tunnel boring machines and other excavating techniques here. Do we melt the rock into lava and pump that? No, we just cary it away in chunks.

On a cold world, ice is a mineral. So think of it like rock.

To melt it, you have to raise its temperature from a mean of −170°C up to 0 while the surrounding material is conducting the heat away, and then further add the amount of energy that would have raised the temperature of ice 80° before it budges again—the heat of fusion.

Then you still have not supplied the energy needed to lift the material out of the hole. And it has to be well insulated to be piped because it’s surrounded by cold.

(People in colder climates have to shift ice off roads and driveways. Do you ever melt it and drain the water? Or is it easier to just shift it while in solid form?)

Finally, the ice is ductile and under high pressure: the hole has to be kept open by adding a liner as you go (in the manner of tunnel boring machines) or filling it with a dense fluid (in the manner of oil/gas wells). Simply adding fiber to the ice to make a composite will not be anywhere near strong enough. A back of the envelope calculation indicates a pressure of 2 MPa (13% of the Earth weight of a column of ice 15 km tall). Note that on Earth the glaciers are fluid at the bottom under pressure, and rock is fluid in the pressure of the mantle. The top (ultracold) will have the consistency of granite and the bottom (ultra pressure and warm) like roofing tar.

The best way to tunnel will the same as how we treat hard rock here: apply pressure to shatter it, then sweep out the pieces. Think about oil drilling bits as your model. You have the further advantage in that ice is rather light weight, so you can arrange for it to float in the drilling fluid. That makes it easy to keep the chips away from the working face when digging straight down.

Maybe you won’t use teeth or pressure to crack off chips, but can use radiation (microwaves or lasers) to melt small spots and crack the working face into chips without using mechanical wearing tools. But, you don’t have to melt the whole thing!

For much more on ice mantles in general, see this talk in the SETI weekly colloquium series. Europa in particular appears to have a “cold, static lid”.

summary

The advantage of melting is the lack of complex moving machinery. But you’re not talking about getting a small probe through the ice without leaving a hole; you want to leave a (rather large) hole as a perminant access point. So you do need complex moving machines to lay the liner, and material must be evaculated.

Melting requires significant amounts of power, and must be intense enough to not be carried off by surrounding material.

I suspect that different techniques will be applicable at different depths, and you may include cooling the deeper part to stabilize the material!

Answer 3

First, to reach the ocean, is it more practical to melt down through the ice, or drill it?

The Russians drilled a hole 4 km down to Lake Vostok. They used a drill. The technology they are using is prefectly capable of drilling a similar borehole on Europa.

Would it be practical to build habitats mounted on the underside of the ice, in the water, or would drilling rooms within the ice be easier?

In this question I calculated the water pressure at the base of a 20 km thick layer of ice at 237 atm. Since hydrostatic pressure scales linearly with depth, at 10 km thick, pressure would still be 118 atm, equivalent to 1250 m in our ocean. Modern submarine are rated to a pressure of about 500 m. Assuming that transportation cost of materials is a significant factor (i.e. moving all that structural alloy from another moon/asteroid, then down a 10–20 km shaft), it is probably not worth making a large permanant living structure that deep. Humans need a lot of air space at 1 atm to live comfortably, and that is very expensive to make available.

Also in that other post, I calculated radiation exposure. 10 m of ice is really all you need, so there isn't a lot of value in going too deep.

If melting/drilling a tunnel down to the ocean, would it be possible to passively leave this borehole open, or would the pressures cause the ice to flow together again?

Ice has a viscocity. From this textbook, the viscocity is about $2\times10^{13} \text{Pa}\cdot\text{s}$. This means that an internal pressure of $2\times10^{13} \text{Pa}$ will impart an expansion of 1m/s to an ice mass. 118 atm is about $1.2\times10^{7} \text{Pa}$, so the imparted speed of expansion will be $\frac{1.2\times10^{7}}{2\times10^{13}}= 6\times10^{-7}\text{m/s}$ which is about 5 cm per day.

The pressure of the ice on any structure made to hold the tunnel would be about 12 MPa. That pressure isn't excessive, but since ice is viscous you can't just put some support struts in there. The ice will ooze around it at 5cm a day. To put a cylinder in to maintain the size of the hole you need it to be.. well at least 10 km long. Too expensive.

The Russians drilling in Lake Vostok have similar problems here. Their hole is only 4 km, but since gravity on earth is higher, pressure is higher too, up to 350 atm at the bottom of the ice. They don't use a structure to maintain the hole, they simply melt all the ice that seeps in with a mixture of kerosene, freon, and antifreeze, and then pump it out.

This solution is a bit harder on Europa. Lake Vostok itself is about −3C, while the surface temps can be as low as −89C (coldest place on earth, incidentally). However, they don't drill in the winter so −20 to −50C is more like what the drill team sees at the surface. The surface temperature of Europa's surface is −160C, but the liquid ocean would be warmer than Vostok, based on the phase diagram of water and anticipated pressure of 12 MPa.

What kind of engineering would be required to keep the borehole open, assuming my colonists want an elevator to the surface.

After drilling is complete, the hole is kept open by pumping an anti-freeze solution around the edges of it to melt it. The anti-freeze interacts with the ice, lowering its lowering its freezing point below whatever temperature the ice is at. The rate of ice encroachment is relatively small, but since the shaft is big, the amount of ice to be removed is large. Assuming 2.5cm on average of ice encroach along the entire 10km length of the shaft, and with a 4m radius borehole, 12600 m$^3$ of ice must be removed every day, or 12 million tons of it. Fortunately, by melting the ice with anti-freeze and letting it flow to the bottom of the hole by force of gravity, the flow rate of 0.14 $\frac{\text{m}^3}{\text{s}}$ is not unrealistic. That would take 4 standard, 3" firehoses, and is about 50% more than you can get out of a single fire hydrant.

The biggest engineering challenge is removing the ice that is collapsing in the top half of the ice sheet, where the temperatures are closer to −160C than zero. No anti-freeze is going to work at those temperatures; car anti-freeze freezes at -40C, and alcohol at -110C. The anti-freeze itself will freeze. Some sort of heating system will be needed. It would be much more effective, once the borehole is dug, to maintain it from the bottom, since that is the warmer side of the ice, and since gravity will pull melted ice down into the warmer regions without need for pumping; you only have to pump antifreeze and not melted ice too.

So you basically have to pump your heated anti-freeze solution from the base of the ice sheet to the top, and recover it at the bottom, separating the antifreeze out for reuse, and presumably dumping the water/ice into the ocean.

Pumping up is a big issue, due to shutoff head limits for centrifugal pumps. I deleted the math as extraneous since this post is already forever long, but, suffice to say, a centrifugal pump, which is good at high volume pumping, will not get the pressure you need. However, any good pressure washer can get the pressure you need (3000 psi = 204 atm) and they do this with positive displacement pumps. So you will need some enormous positive displacement pumps; flow rate has to be relatively high or your heated antifreeze will cool and freeze before it reaches the service. Not an impossible engineering challenge, since I have seen them. If you want 200 gpm of 3000 psi reciprocating positive displacement pumps, you will need about 400 kW of electrical power, based on the pumps I've seen.

So that brings us to generating both a.) enough heat to unfreeze a 10km hole and b.) enough power to run a 400 kW electrical load forever; for reference this is what a 100 kW diesel generator looks like and c.) doesn't take a ton of fuel. The solution with today's technology is a nuclear reactor. Fortunately, they already have them in submarines, so it's not too much to ask for to bulk up the pressure hull to handle higher pressure, and install one at the bottom of the ice to keep the hole open. Though, keep in mind, you can't assemble it 10 km under the ice, so the hole has to be big enough to get the thing down there in the first place. Also, you have to replace it every 10–15 years once it runs out of fuel.

In conclusion

Most people would permanently live in habitats dug a few meters into the ice. This would give plenty of room for expansion by digging more warrens into the ice, without having to go into high pressure areas, and also keeping the colonists close to the outside world.

The hole would have to be significant. A submarine style pressure hull with reactor would have to be inserted into the hole. However, since the smallest nuclear submarine had a pressure hull about 4 m in diameter, the size doesn't have to be unreasonably large. Maybe an 7 m hole and a 6 m pressure hull with nuclear reactor and ice melting equipment. This could be operated remotely, its not a threat to human life if the hole closes if there are no people below the hole. The worst you have to do is re-drill the hole.

In fact, I don't anticipate humans going down the hole at all, too dangerous. Just some construction-bots to install your ice-melter and some submarine-bots to explore. Maybe an Alvin for exploration, but you'd never want to try to dock and transfer people to the ice-melting hull at 12 Mpa.

Answer 4

when drilling/melting a hole in such a cold environment one important factor is time taken for the new ice layers to form. So the process should be as quickly as possible or we could figure out ways to prevent ice sheets from forming in a localized area.

Ice forms When the water molecules move slowly because of low energy and it is easier for them to hook on to each other by sharing electrons. when salt is added salt molecules arrange themselves around the water molecules like little fences and keep the water molecules from hooking together. But if it gets cold enough, about 28.5 degrees Fahrenheit or -2 Celsius, ocean water will freeze too.

source: http://quatr.us/chemistry/atoms/ice.htm

So If we are able to build nano structures that can prevent water molecules from hooking easily while staying localized to that particular area and be degradable easily after sometime. Then we just have to put the nano material and wait without worrying of contaminating alien environment. There have been some research going in this area

Answer 5

There have been reports of cryo-volcanoes and water ice geysers coming from Europa, most likely caused by cracks in the ice due to tidal forces.

It seems that rather than trying to drill through the ice the simplest thing would be to wait for natural crevices or other openings to form and then move in through them. Even if not naturally wide enough they would provide a starting point to help the drilling and the flow of water would carry away drilled ice.

Of course this would be risky as if the crevice started to close the forces involved would be immense, but that is a hazard faced by any attempt to penetrate the crust. Most likely we would be sending through unmanned vehicles, in which case they could be made small and just inserted through the crevices and cryo volcanoes directly with no need to drill at all. Some sort of swarm system with members of the swarm stopping at intervals to act as communications relays would make sense.

Answer 6

One thing which the other answers overlooked is the need for getting below the ice as quickly as possible. The immense radiation fields around Jupiter make this imperative, since humans and unprotected electronic devices will receive a lethal dose of radiation in a relatively short period of time. High speed is essential.

Because the ice is going to be as hard as rock on the surface, due to direct exposure to space and heat energy rapidly radiating away, there are a few possibilities. This technique uses high speed projectiles fired down a tube to strike the working face at speeds measured in kilometres per second. This sort of energy would shatter rock. In the case of ice, it would shatter the ice and possibly melt the walls of the tube, providing an impromptu smooth surface to lay down the actual tunnel walls (for insulation and to economize on materials, a foamed material made from silicate rocks imported from another Jovian moon should suffice).

Ram Accelerator schematic

Hammering the ice like this has a disadvantage in that there will be large areas of cracked ice radiating away from the tunnel exterior. This broken ice may eventually "flow" back together from static pressure and the action of the Jovian tides, but that is both long term and does not have the sort of quality control that engineered solutions havre.

As an alternative, if the spaceships and landers have fusion or nuclear drives, the power of the exhaust could be used to rapidly melts through the ice. The jet of plasma will rapidly melt through the ice, and one issue would be the venting of clouds of steam released by the process, or protecting the dismounted engine assembly while it cuts through the ice. Conceptually, the engine could be held in an articulating frame which grips the sides of the tunnel and can point the exhaust plume in the desired direction. After the ice has melted and the steam cleared, the frame can be "walked" by moving the supporting legs, while a "finishing machine" follows and lines the tunnel. This method also allows the device to carve larger chambers in the ice, once sufficient depth was reached for radiation protection. The frame can be swivelled so the exhaust plume moves in a cone or spiral shape to excavate larger areas under the ice.

If a small plasma cutter can rapidly carve through steel, ice should not be an issue

While there will also be a problem of cracks radiating away from the tunnel or opening, the heat energy should be able to create a relatively thick wall of fresh ice, providing support until the engineers can stabilize the area with "Rock bolts", injecting hot water like grout to fill and seal cracks (something like a giant Zamboni machine used to prepare ice surfaces at arenas), and liners installed.

To protect the tunnel from damage caused by heat leakage from the base, insulating the base from the intense cold and protecting the base from the inevitable movement of the ice, I believe the best solution there would be to have an inner liner with separation between the ice walls and the manned part of the base, like an insulated flask.

Several of these ideas could be used in conjunction with each other. The Ram accelerator could be used to drill pilot holes for the dismounted fusion engine to direct the exhaust plume. Extra holes could be drilled in parallel so when the main jet is used, steam can flow into the parallel tunnels through cracks in the ice and escape, protecting the driving platform from working in a steam bath. Even the meltwater could be pumped into forms and refrozen to create bricks of pure ice, free of trapped gasses and having no internal flaws or cracks. These ice bricks can then be used to create the initial liner of the tunnel, much like a barrel vault except completely circular.

Drilling is relatively well known technology. The difference here is you need to drill deeply and rapidly, in order to make livable structures in a reasonable time.