I appreciate the thought you put into this response. You make good points.

>For a self sustaining colony on Mars you need to span a huge part of the Martian globe to get access to enough raw materials to build things like the microprocessors required for basic life sustaining devices.

I don't see any reason that microprocessors are required for basic life sustaining devices. They are certainly convenient and valuable. Even if there were required, a simple microchip fab is a known technology. Expensive but does not require new technology.

An interesting question here is how big such a colony needs to be to be self-sustaining.We have clear answers because our present complex Terran economy assumes a planet wide network of trading industrial economies. Perhaps other, smaller solutions exist but we haven't bothered exploring them here because there is no market for it on Earth. Then again I suspect that as long as Earth has an industrial space faring civilization, there will be continuous trade between Earth and Mars. We might not be able to determine how self-sufficient Mars ever is because trade between Earth and Mars may never be shut off.

>And one of the biggest potential limiters is water: we don't actually know how much water there is on Mars.

I agree with you here and it is important point. If it turns out that extracting useable water on Mars is exceptionally difficult, a self-sustaining Mars colony is much harder to create. However we don't really know how true this is. This degree of uncertainly does not support the statement given earlier that "it is certainly not possible with known technologies."

>You'll also need massive ecosystems to sustain an industrial chain with 0 access to fossil fuels, since you'll need to be able to produce bioplastics and biofuels in addition to food. Again, you'll consume huge quantities of water to achieve this.

Yep, strong agree this is why most of these architectures assume large quantities of usable water. I wonder when we get the answer to this question?

>But the more you scale to reach the required resources, the more of those resources you will need - especially when you'll more or less have to tunnel your way everywhere if you don't want everyone to die of cancer in their thirties.

You tunnel for living spaces, you tunnel for resources, a self-sustaining Mars colony will likely require a sufficient investment to get going. Part of that investment will be in energy used for tunneling. Light weight nuclear batteries and generators. Bootstrapping a self-sustaining Martian colony would be extremely difficult but taking a dependent colony and growing it to a self-sustaining system seems easier.

Probably the longer Earth sustains it, the cheaper it will be to become fully self-sustaining.

>Finally, energy production will be extremely difficult, as solar panels are extremely inefficient, efficient forms of solar require boiling water compounding your water problems (and so does nuclear), and both wind and solar of any kind will have massive dust problems, probably insurmountable with today's technology.

I don't know the science but I would assume wind is out because of the thin atmosphere. Solar might be inefficient but how much electricity do you need? I suspect you want to build farms and living spaces that can survive at least a few days to a few weeks without electricity. Low electricity usage and dependency is probably a critical component of robust martian colony architectures.

You can run nuclear on things other than water. US and Soviet Submarines use liquid metal cooled reactors [0]. Why not run underground breeder reactors on Mars, if something goes wrong collapse 5km of soil on them?

While I think fusion is going to play a big role on Earth, I suspect it won't be well used on Mars for a while due to its inherent complexity and dependence on rare materials.

[0]: https://en.wikipedia.org/wiki/Liquid_metal_cooled_reactor#Pr...