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What would colonists building a base on Europa use for building materials? I am interested in habitats in three locations:
Can metals or other building material be extracted from dust on the surface or in the ice? Is it plausible to harvest or mine minerals and metals from the ocean floor?
Given the radiation environment of Europa, stacking ice to create igloos could become an issue. You would need several metres of ice to block the radiation from Jupiter, which would make building igloos somewhat challenging, to say the least.
The most likely location for Europan colonies would be i bubbles melted into the ice sheets. The problem here is that the ice itself will be very cold, and on a large scale somewhat flexible (the ice will move with the Europan tides, for example), so the colonists will want insulation.
Based on the idea that Jupiter has 67 moons, it would be a small issue to gather silicates and metals from the other moons and sue them as construction materials. Europan colonies would look a bit like a thermos bottle in cross section: the ice bubble would have a smaller "bubble" of foamed rock and metal supporting structure suspended within, and a vacuum would be maintained between the two bubbles for insulation. Inside the rock and metal bubble would be the colony itself.
The size and scope of such a colony wold depend on the amount of resources and time the colonists would be willing to invest in the project, but a spacefaring civilization with access to interplanetary ships and fusion energy could conceivably "blow bubbles" of very large size, even kilometres in diameter if desired.
If it was decided to go under the ice in the oceans. then the bubble structures of foamed rock and metal would suffice so long as there was no desire to go very deep into the oceans where the static pressure would endanger the bubble.
At the surface colonists will be surrounded by an abundance of ice that can easily be carved into blocks and stacked igloo-style.
Snow is used because the air pockets trapped in it make it an insulator. On the outside, temperatures may be as low as −45 °C (−49 °F), but on the inside the temperature may range from −7 °C (19 °F) to 16 °C (61 °F) when warmed by body heat alone. (Wikipedia)
Within the ice sheet itself ice can be hollowed out as you mentioned. In both cases, colonists will need insulating material to help retain heat without melting the ice walls, and to contain air pressure approximating Earth's. Unfortunately, without knowing what minerals and such might be suspended in the ice or subsurface water, or until we know the actual composition and structure of the subsurface layers and the pressures and hazards involved, it's impossible to anticipate how or what colonists might manufacture locally. They would need to bring material with them.
With an abundance of ice-water, colonists will have no shortage of growth medium for microbial or bacterial mats. Such mats may in fact be the first signs of life native to Europa, as (if they exist) they would be growing on the underside of the ice crust at the ice-ocean interface.
Genetically engineered microbes can manufacture petroleum-free bioplastics, given enough oxygen or nitrogen and carbon to work with. Sheets of this material could act as insulation, and layers pressed together could form rigid interlocking plates.
It's conceivable that native microbes might produce even more useful materials if farmed and harvested correctly.
The obvious solution is to repurpose the ship that brought them to Europa in the first place. In addition to whatever raw materials are brought for the purpose, the insulation and modules themselves could be designed for the task. Habitats on or under the ice crust could be built from the hulls of colonist ships landed on the surface and lowered into position.
I'll aim my answer primarily at this:
What would colonists building a base on Europa use for building materials?
Europa is likely a captured moon that did not form in the outer regions of our solar system. Presence of a magnetic field and its density suggests that Europa is closer in make-up to the terrestrial planets than the outer ones, suggesting that somewhere within it is a rocky planet along with a metallic iron core.
This would suggest that there is the capability to mine almost any material that you would be capable of mining on Earth (well, at least non-organic compounds). It would also appear that Europa is geologically active (mostly due to interactions with other moons and Jupiter itself) which could suggest that underneath this ocean will have volcanic peaks (or less likely, tectonic mountains)...which would mean although a good chunk of the 'land' of Europa will be far under water, there should be peaks under a much shallower ocean that could allow for mining.
The red of Europa is currently speculated as magneisum sulphate, or perhaps even sulphuric acid. Possible the intrepid colonists could make use of that.
Outside of that, much of Europa's composition is speculative...you can take quite a few writers liberties in its composition.
As a side note...I should point out that Europa is radiated. A lethal dose to a human would be less than one day of exposure. This highly limits the 'igloo' solution as it would need to be a radiation shielded igloo if humans were to survive.
Europa resides inside Jupiter's equivalent of the Earth Van Allen radiation belt (upwards of 10× stronger than Earth’s). I think it would be planet wide regardless of facing. There is a time period though where Europa is in Jupiter's Magnetotail where it would be protected. https://en.wikipedia.org/wiki/Magnetosphere_of_Jupiter will kinda describe that.
This could be a really interesting plot point. The ice would work as radiation protection (option number two on your list), so they would be hiding from radiation under an ice sheet for a period of time. When Europa enters Jupiter's Magentotail, the radiation levels will drastically reduce, allowing for a time frame where humans could wander the surface of Europa with limited radiation protection. This time frame would be short...Europa orbits jupiter every 3.5 Earth days and the magnetotail would protect about 1/3 of that time. 1 day in 3.5 that they could make surface expeditions and hope they get back down in time.
Also of note...Juptier has some qualities of a pulsar and emits some pretty powerful radio waves in the kHz and mHz ranges (pretty much what we call AM and FM in radio terms). This could make for some heavy communication challenges if we are still relying on radio for communication during Europa colonization
Radiation will kill you on the surface
Several other posts hit the same point, but to summarize briefly, Europa's surface receives 540 rem per day, which is probably a fatal dose. However, the 1080 rem you get in two days is definitely a fatal dose. You need shielding, water (and ice) are great shielding, there you go.
Water pressure is too high in the ocean
Let me make a few super-general assumptions to simplify the math. First lets assume atmospheric pressure of zero, lets assume that the ice crust is pure water ice (density: 0.9167 g/cm$^2$), and lets assume the equation for hydrostatic pressure ($P=\rho gh$) can be naively applied under an ice sheet, an assumption that I argue is good enough. Other numbers we'll use are the surface gravity on Europa (1.315 m/s$^2$), 20km depth of surface ice on Europa (estimates range between 10-30km)
$$P=\rho gh,$$ $$P = \left(\frac{0.9167 g}{cm^3}\right)\left( \frac{1 kg}{1000 g}\right)\left(\frac{1000000 cm^3}{1 m^3}\right)\left(\frac{1.315m}{s^2}\right)\left(20000m\right)$$ $$P=24.1 MPa = 237 Atm$$
That is equivalent to about 2500m below the ocean on earth. Building habitats with the strength of a submarine wouldn't be hard, but they are rated to about 500m tops. Building a habitat to handle that pressure would be hard.
Inside the ice is just right
On the other hand, you can find a happy medium in the middle. The 1MeV gamma tenth-thickness of water is about 0.6m. The tenth-thickness is the distance of material needed to attenuate radiation by a factor of 10. I couldn't find the tenth-thickness of ice, so I will assume it is the same (possibly a horrible assumption). Therefore, under 85m of ice, the radiation from Jupiter is about $$540 rem \cdot 10^\left(-\frac{85m}{0.6m}\right) \approx 0. $$
At this depth the pressure is $$P = \left(\frac{0.9167g}{cm^3}\right)\left(1.315 \frac{m}{s^2}\right)\left(85m\right) = 102.5kPa = 1.01 atm.$$
No radiation and atmospheric pressure. Sounds about right to me! Of course, this is not to say that there is atmospheric pressure in the air of a habitat, just because we are 85m below the surface of the ice; since ice is solid it doesn't work like that. But structures built at this depth won't have any pressure related problems that they don't already have on earth...unless the ice is moving.
What building materials are available at 85m below the surface of Europa?
Well....ice. Everything else you are going to have to bring yourself. The rocky surface is below another 20km of ice and maybe 100km of ocean. Pretty technically challenging to get something from there.
However, Jupiter is just surrounded by moonlets and rings and what have you. It you want metal, just mine it out of loose material in the Jovian system and bring it down to the surface.
