What happened…when the object apparently responsible for the extinction of the dinosaurs hit the Earth 65 million years ago?
“First, there was a gigantic fireball brighter than the Sun as the comet plunged to its death, not with a whimper, but a bang. One casualty was the ozone layer, which temporarily vanished. Seconds after the big comet first encountered Earth’s upper atmosphere, it carved out a crater - now buried - 200 kilometers wide and 25 kilometers deep. All that debris shot up into the sky and came back again, all over the Earth. No place would have been spared a hit of at least a tiny particle.
Reacting to this incredible bombardment, the air temperature rose quickly until, for mor ethan two hours, the worldwide temperature reached that of an oven set to broiling. The sky glowed like an electric heater. Ground fires flared everywhere. Then the temperatures started to drop, and drop, and drop. A thick cloud of dust blackened the world, setting off a several-month period without sunlight. Rains poisoned with sulfuric and nitric acid added to the misery.
With blow after blow to the biosphere…most large land-roving dinosaurs probably died within weeks. Other creatures took longer; those who survived one disaster would perish in the next one. Slowly, the great cloud dissipated, and temperatures began to rise again, this time due to a greenhouse effect that lasted for centuries or millennia. Overall, perhaps 70 percent of all the species of life died during the siege, and in North America at least, about half of the species of flowering plants.
But not everybody. Some of the hardier representatives of many species, including the ones equipped to hibernate, made it through the impact winter. Enough small mammals survived that, when the biosphere finally started to recover, they began to proliferate and flourish.
Impacts clear the decks for new forms of life. The fossil record shows that after major impacts, there is a burst of speciation. New life forms fill the niches that the old ones leave behind. If there were no impacts, the thrust of evolution might have slowed down, and today there would be a different set of species inhabiting the Earth.”
David H. Levy, Gene Shoemaker in an exchange about comets and cosmic collisions| Impact Jupiter: The Crash Of Comet Shoemaker-Levy 9