Except for Earth's complete physical destruction, no event would make Earth as hostile to life as Mars is.

“um he talks about the need for mars as a lifeboat for humanity in the event that something horrible happens to earth like an asteroid as big as the one that killed off the dinosaurs 66 million years ago uh impacting earth again uh there was nothing that could really happen to earth that would make it as bad as mars is to live on short of the actual physical complete destruction of the earth um because even that day when that asteroid hits”