Apart from a gamma-ray burst, what do you imagine could happen to the Earth to make it as inhospitable as Mars? Asteroid hits wouldn't come even close. The wor

Apart from a gamma-ray burst, what do you imagine could happen to the Earth to make it as inhospitable as Mars? Asteroid hits wouldn't come even close. The worse disease imaginable would not make the air as permanently dangerous to life as Mars' atmosphere is. So what exactly would motivate this move?