To be fair, we're desperately trying to get to Mars, and we can't even breathe there.
Yeah, but we're taking our own life-support systems if/when we go; we aren't just gonna pop open the hatch on the spaceship and take a big, deep lungful of death. 'Cause we've taken the time to ascertain that there is a potential hazard to our health on the planet we're visiting before turning up to invade and stuff...
As for the OP, sounds like a classic case of withholding too much info for the BAM! factor on the twist. In which case, as you've discovered, it comes across less as a clever punchline and more as an eye-rolling attempt to shock the reader. The ending has to take place in the same universe that the rest of the world occurred in - meaning the same events have to logically lead up to and include the twist. Otherwise it's a fatal blow to a reader's suspension of disbelief.