There's huge optimism about the prospects of settling Mars amongst space cadets, you just have to look at the photos from the various probes to be drawn to the place, it just looks so benign, it even has a day of similar length to the Earth.
What the pictures don't show, but we each know (at least on an intelectual level) is that it's bloody cold, there's almost no air, and what little there, even if concentrated, would kill you by asphyxiation in a couple of minutes, it looks so inviting, but it's more hostile to human life than any place on the surface of the Earth - that's not news - but do we really understand what that means?
If I suggested to you that you could take a few tonnes of equipment and build a self-sustaining colony on the middle of the Antarctic icecap, or in death valley, you would probably find lots of reasons why it couldn't be done, easy to visit, but hard to stay without continual replenishment from outside.
Having said all that, I think there is still reason for optimism, but that optimism has to be based on the reality of what's actually there, and the knowledge that life won't be easy, after all, people do live in cold and arid places on Earth.
What are the basics that our settlers will need? Lets list them:
Well, water is available on Mars, quite a lot of it if you know where to look, and to cut a long story short, growing food makes air (well, oxygen), and on Mars that food will also require shelter, so all we need is water, and a shelter under which we can grow food, which means it needs to be transparent, after all, we all ready have days of the right length.
We also need to recognize that to hold that air at the pressure we need, we have to have some way of containing it at at least 4 tonnes per square meter pressure with a 50:50 O2/N2 mix, or higher pressures if we want an air nitrogen content closer to that which we enjoy on Earth.
We could hold that pressure in one of two ways; either with a strong container with high tensile strength, which would either need to be brought from Earth or manufactured, or simply by trapping the air under a thick layer of non-permeable material and using gravity to hold it down. For a 40 kPa atmospheric pressure in our shelter we'd need 10.5 tonnes/ m^2 of material in Mars 0.38 gravity, if that material were transparent we could kill two birds with one stone, as, if we wanted to grow crops - and we need to grow crops - a transparent dome will save us having to either produce that light artificially, or else, if we want to use natural light, save us needing a complicated arrangement of mirrors to capture and channel the sunlight.
So what materials are available? Two spring to mind, glass is the obvious one, if we wanted to cover a hectare with glass though, we'd need to pour a sheet of it 4 metres thick with an area of 10,000m^2 thats just over one hundred thousand tonnes of very pure glass (high purity is necessary to allow the passage of light), the other option is water - or ice - we'd still need a hundred thousand tonnes of the stuff, and it would still need to be of high purity, but unlike glass, it doesn't require a great deal of energy and considerable manufacturing to produce, on Earth we manipulate water by the thousands of cubic meters with ease, it costs just a few cents a cubic meter if it's not bottled, I find fixing a broken window reasonably expensive in comparison.
So perhaps we could just inflate lightweight plastic domes which we could build ice domes over by slowly building up ice by simply spraying water on and letting it freeze, then, when we have our dome, putting a thin transparent sealing layer over the top to stop sublimation?
That would save transporting so much from Earth, and it would be the ultimate in ISRU.
It's also worth taking into account that such an ice dome only conducts 0.17 Watts/m^2 if the temperature gradient is 10K per meter through the thickness of the dome, so it's good insulation as well.