I take it the contingency plan is for, say, another Chicxulub sized impactor hitting the Earth. At which point even undersea or desert habitats will be in troub

I take it the contingency plan is for, say, another Chicxulub sized impactor hitting the Earth. At which point even undersea or desert habitats will be in trouble, unless they were totally self-sufficient. My impression is the (current) lack of amenities on Mars demands establishing a self-sufficient colony; hard to imagine anything on Earth rising to that level.