> Those 100,000 people have a choice – find a way to make do with what they have, or die. I don't think we should underestimate human ingenuity in such circumstances. They'll lose access to a lot of highly advanced goods – for example, they probably won't have their own leading-edge semiconductor fabs – but they may be able to survive without them, using more basic goods to take their place.
The answer will be die. There is no way to survive on Mars for any period of time without access to high technology, you will be dependent on it for electricity, for water, for farming, for pressurized ventilation, for air recirculation, for robots that can work on the surface (e.g. To keep solar panels clean, or for any kind of construction on the surface), to dig tunnels through rock to expand as resources dry up, and to produce the majority of the more basic goods you need. 100k people is very little for a functioning isolated economy on Earth, on Mars it would be much more complicated.
> The atmosphere of Mars is around 2.8% nitrogen. There are likely to also be nitrate minerals in the Martian soil and rock.
The much denser atmosphere ON earth is 78% nitrogen (see also my next point for a comparison), there is plenty of nitrogen accumulated in the soil, and still it is one of the major factors limiting plant growth. Any nitrite deposits on Mars are likely to be contaminated with other toxic chemicals, like much of the Martian soil and probably much of the water ice.
> Mars' atmosphere is 95% CO2. You can extract oxygen from CO2 by electrolysis. The MOXIE experiment on Perseverance has demonstrated this.
O2 is about 21% of the Earth's atmosphere.
Mars's atmosphere is less than 0.6% of the Earth's atmosphere, even at the ridiculously low Martian surface temperatures.
So even if you can convert CO2 to O2 with 100% efficiency, and even if Mars' atmosphere were 100% CO2, you would still only get 0.02 times the O2 you have on Earth.
This is extremely relevant not only for breathing, but because the only kinds of batteries we can imagine that have anything close to the power efficiency (Wh/kg) of fossil fuels utilize atmospheric oxygen to get most of the power.
> On human timescales (decades to millennia), Martian atmosphere lost to space isn't significant.
Fair enough, but it's still going to be impossible to recover water or helium or other gases we leak from Mars' rarefied atmosphere.
> I think Musk is going to try to establish a settlement on Mars, initially with tens of people, then hundreds, then thousands. And becoming fully self-sustaining is going to be their long-term goal. But I don't think actual full self-sustaining status is going to be achieved for several centuries.
I don't believe Musk will achieve anything more than tragically killing some volunteers on Mars. I also don't think a permanent self sustaining colony on Mars is actually possible or even desirable, for all of the reasons I listed, with any kind of foreseeable future tech (of course, paradigm changes in physics or chemistry could have completely unpredictable effects).
Edit to add: it's also important to remember that we do not know if humans can actually survive long term in 0.3g, or with what impact on their health. This will have to be tested on animals for years before actually attempting a human base.