Building a colony in a salt mine would be an easier backup plan than a Martian one. Maybe you need to scrub radiation and smallpox out of your air but at least

Building a colony in a salt mine would be an easier backup plan than a Martian one. Maybe you need to scrub radiation and smallpox out of your air but at least you have air! Admittedly solar power would be off the table, but almost everything else is easier.