> This insinuates that surviving the aftermath of a Chicxulub event is what threatens us - but it isn't the aftermath, it is the event itself.
We have plenty of ways that a small group of humans could be isolated and fed for the time it would take to make humans survive. Humans would not go extinct due to an asteroid impact, rest assured.
There's even a few places that would have natural protection from most of the phenomena. And of course, there'd be a few weeks where, starting 500km from the impact site or so, there would be essentially nothing wrong on other than a stripe in the sky. That time can be used to prepare for what's coming, and as long as you only try to save 0.1% of the population, you could do that just fine.
And there's plenty of calamities that would affect mars just as much as they'd affect earth. A supernova within 50 lightyears or so of earth (depending on size can be a bit more) is not survivable. That would destroy the atmosphere in a matter of seconds and kill the power grid, and there would be no warning at all (it's like an earthquake, we would "know it's coming" give or take 100000 years or so, and yes, there's one "ready to blow" within that range). Within 20 light years or so and we'd see wind speeds on the surface would go absolutely insane within a few minutes, lasting a few hours (there'd be a VERY hot wind with speeds that'll probably exceed the speed of sound for a few minutes). Anything above the surface is gone. Hell, this will erode the himalayas by several hundred meters at least, maybe kilometers. Ocean water levels will drop dozens of meters and there'll be zero water left on land.
But, on earth, some installations would probably survive even this. It won't be pleasant, because life on earth will take decades to centuries to come back to the point that the planet is covered in green (depending on what it does to the soil, might even be hundreds of thousands of years).
For all these calamities, the same thing keeps coming back. What you need is
a) a huge, huge mass surrounding you (the earth counts as "surrounding" us since there's always 50% of the surface that's protected by the mass of the earth), preferably with lots of metal inside of it.
b) an energy source that isn't dependent on anything (which means: a nuclear reactor). Obviously solar panels won't do.
c) a way to grow food and scrub co2 directly using just energy.
d) we'll need to do this using extremely well established technology. For instance, we can probably use computers from, say, a 50nm process, but we can't really go higher than that as we wouldn't be able to replace them. Because these events would wipe industry from the earth that means we can only depend on technology that can be made within whatever base we use.
That will let us have large amounts of survivors on our planet. The biggest problem seems to me to be C. We can do it, sort of, but not very efficiently.
Also, once we leave earth, we shouldn't go to Mars. Once we escape Earth's gravity we'd be wise never to dive into a gravity well again. We should migrate populations into asteroids and use them with ion engines as spaceships. With the people living deep into the interior. They will be far, far safer than Mars or any planet.