I've had dreams about going to Mars, and have spent the last few days thinking about some of the other technical* challenges (aside from system level CELSS development) that might be important for setting up a habitat there, and enroute. Since I am a materials engineer by trade, I'll confine myself to that arena.
*A few years ago at the first 100 Year Starship Symposium, I asked a panel of economists if they thought the challenges for such an interstellar mission were more technical or more fiscal. They all thought economics were the greater challenge. Naturally. Being an engineer, I disagreed :)
Shelter from the traditional elements is not so much the concern on the way to Mars or even on the suface (contrary to The Martian film). Shielding against radiation and shielding against micrometeors is however required.
The Earth itself uses a magnetic field to deflect much of the ionizing radiation, but it is unlikely artificial magnetic shielding will ever be economical without cheap fusion energy to power it.
Readily available regolith (Martian dirt) also would work on the surface where reaction mass is not a concern and transportation costs nil. Regolith thick enough to protect against radiation is generally sufficient to protect against micrometeoroids. However, the technology to make it structurally load bearing would be nice. Sandbag construction, brickmaking, and thermally compatible concretization chemistry are all options. That said, I somewhat favor the burrowing approach, as I think it is more ultimately extendable toward colonizing asteroids. Tunneling equipment on Earth however tends to be quite complex mechanically, and very heavy. A swarm of self replicating robots seems to me the best solution, if prepared ahead of time. Self-replication implies ISRU mining, refining, and manufacturing. These are all worthy of another post at a later date.
For the trip there, fortunately one of the best radiation shielding materials is water. Rocket fuel also works pretty well. So does most biomass, including manure, compost, or radiation tolerant plants. Actually anything containing hydrogen. Which is good, because we need all those things anyway. Unfortunately, these things all generally outgas horrendously, which if not contained will vent to space and cause the system to lose mass over time.
Most everything leaks. The saying goes that nature abhors a vacuum, but in space, the opposite is true. Gases diffuse through solids. Liquids evaporate, solids sublimate. Especially at higher temperatures and under bombardment of solar and cosmic radiation.
The more traditional approach to containing an atmosphere is to use a solid barrier sufficiently thick to mitigate diffusion, provide structure, and shielding. Aluminum has been the material of choice for most of the space era due to the optimal combination of strength, weight, and cost. Composites are starting to be used more however. Glassy ceramics tend to be the best, and also have low thermal expansion, which will certainly be a consideration for Mars' 100 degree per day temperature swings.
In the midst of all the graphene hype, one of the recently discovered properties of this material is that it is exceptionally impermeable to hydrogen gas. H2 is one of the hardest gases to contain (it causes hydrogen embrittlement in most metals, making it also very dangerous to contain), so this is great news for the hydrogen fuel cell folks. And for space enclosures. Unfortunately bulk production is still years away. The application of such a material would likely be in the form of a composite sandwich layer between polymers, or a coating applied to a structural metal shell, or sandwiched between walls of Martian brick.
Lighting and Power Technology
Plants and algae require light. Despite all the difficulties involved, I find myself still attached to the idea of greenhouses on Mars, or at least light pipes collecting and admitting natural sunlight to a subterranean habitat. Bulk glass production would be needed to accomplish this. Relatively thick windows would be needed just to protect from UV and other radiations. I've also considered a layer of water as a potential solution, but red wavelengths have difficulty penetrating more than a couple meters of water, and the reds are required for most photosynthesis. At least two meters of water and glass combined would be needed for radiation protection. Mars is already receiving less PAR insolation compared to Earth due to its added distance from the sun. Rad hardy plants might be bred (or maybe genetically sampled from Chernobyl...) to provide an outer canopy for a less shielded greenhouse. But as Biosphere 2 found, algae really likes to grow on windows where condensation forms. Due to the temperature differential, this would be nearly inevitable.
It's therefore likely that the first generations of Martians will utilize artificial lighting to great degree. This will require power, either from geothermal sources, photovoltaic or nuclear. In transit, PV has a clear advantage, but I somewhat prefer the latter for Mars itself. LFTR reactors seem to be a good safe solution for this in my opinion, because waste heat could be used for keeping warm on a planet where the average temperature is on par with dry ice.
LED grow lights have come a long ways recently and are probably the best current solution. However, humans will require broad spectrum lighting for psychological and health reasons. Not to mention, for seeing stuff.
Ultimately, I like to think of the holy grail of space materials as something very like cell walls/membranes in function, combining the following: hermetic structure, visible light admission, radiation shielding ideally absorbing the energy and converting it to energy. Also,self repairing and easy to fabricate from readily available materials. It would be ideal to have a single wonder material to simultaneously perfrom all these functions, but that is not likely.This is a technology that will likely need to evolve, and will probablly be a composite of many materials, strategically layered.