“Chiron, a recently-discovered planetesimal, is on an unstable orbit near Saturn and measures 180 km across. The consequences of it hitting the Earth are too horrible to contemplate. And Chiron is by no means the largest known minor planet. Four billion years ago such objects would have been far more common than they are today. [...]
An impactor 500km in diameter would excavate a hole 1500 km across and at least 50 km deep. A huge volume of rock would be vaporized in a gigantic fireball that would spread rapidly around the planet, displacing the atmosphere and creating a global furnace. The surface temperature would soar to more than 3000 ºC, causing all the world's oceans to boil dry, and melting rock to a depth of almost a kilometre. As the crushingly dense atmosphere of rock vapour and superheated steam slowly cooled over a period of a few months, it would start to rain molten rock droplets. A full millennium would elapse before normal rain could begin, presaging a 2000-year downpour that would eventually replenish the oceans and return the planet to some sort of normality.”
"The Origin of Life" by Paul Davies, p140-141.
I read The Origin of Life while on holiday (I'm back now! Hi! Photos to follow!) and have got quite a few quotes from it. It is not groundbreaking stuff anymore, but it also gave me a good understanding of the hardy little critters that we call extreme, but they'd call cosy. It is us, desperately relying on the sun for heat and production, living on the cold, weather-prone surface of the planet that are the extreme ones, trekking out to the surface where we have to build our own metabolisms because everything is so cold!