To a large extent we have good confidence that Mars used to be a very wet world with a thick atmosphere. Now it's clearly very dry and the atmosphere is extremely thin. So if we colonize Mars with the long-term goal of expanding the colony and terraforming Mars so that it becomes more habitable, will the same things that destroyed Mars' environment initially destroy any efforts we make to colonize it now?
closed as primarily opinion-based by David Hammen, Nathan Tuggy, Jan Doggen, GdD, Jack Jun 25 '18 at 12:49
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When Mars lost its water and atmosphere, there were no humans with machinery on its surface to prevent it. Mars can't be terraformed to a stable, fully self-sustaining system - at least according to our current theories of how it could work (maybe our descendants will think of something better.) It will always be a resource hog, requiring constant resupply of water and raw materials to keep it going.
...about the same way as cities. Cities don't produce nearly enough food to sustain themselves. They depend on the out-of-city agriculture for sustenance, they require raw materials for production and construction, and for maintenance of existing infrastructure - and any city without external supply of goods will quickly fall, using up all of its stockpile.
Is it a waste of time and resources to build cities?
The artificial ecosystem of Mars would quickly fall apart without these supplies, without human intervention. But that doesn't mean we shouldn't do it - it just means streamlining and automating maintenance of Mars will be a necessity. We can't create a fully self-sustainable natural system - but we can create an artificial one, with autonomous spacecraft hunting for comets to deliver water, autonomous maintenance and service robots for keeping production of air running, to replace what is lost to vacuum of space, systems monitoring and maintaining ecosystem balance. I
It's possible it would all fall apart if there's no-one to maintain it, keep it running. But if it came to that, who would be left to care?