alt Hacker NewsThey're not viable candidates as backup plans for humanity because they have the same vulnerabilities to comet strikes, global nuclear war and pandemic as the rest of the Earth.
OTOH, if one of those took out human life on Earth, people living on Mars could re-colonize Earth.
Earth after a global nuclear war or comet strike is still more hospitable than Mars.
It's hard to imagine anything that would make Mars a better place to live than simply staying on earth.