NYTimes Article on Early Earth

An interesting New York Times article on new evidence for a more benign early Earth has some information on early Earth bombardment. I would recommend the entire article but here is a section on early impact events...

Earth, like the other planets, coalesced more than 4.5 billion years ago. It is commonly hypothesized that almost immediately, a Mars-size object about 4,000 miles wide hit it — a true cataclysm that vaporized much of the object and Earth. Some of the debris ejected into orbit became the Moon. The molten Earth cooled quickly, probably within a few million years, and nothing that large ever struck again.

Dr. Sleep [Norman H. Sleep, professor of geophysics at Stanford] said his calculations suggested that during the 700 million years of the Hadean period about 15 objects 100 miles wide or wider hit the Earth. About four of the objects were wider than 200 miles, and those collisions would have been violent enough to boil off most of the oceans. (By contrast, the more recent object that hit the Earth 65 million years ago and helped kill off the dinosaurs was about 6 miles wide.)

But in numerical simulations that will be presented this month at a meeting of the American Geophysical Union in San Francisco, Dr. Mojzsis and Oleg Abramov, a postdoctoral researcher at the University of Colorado, show that the Late Heavy Bombardment impacts were not quite as lethal as had been thought.

“Things are hurt really bad,” Dr. Mojzsis said. But the computer calculations indicated that even rocks up to 300 miles wide would not kill everything, that pockets would exist where organisms that thrive in high-temperature environments like hydrothermal vents could survive.

Genetic studies of current life support that notion, pointing to an organism that lived in a high-temperature environment as the last common ancestor. That does not mean that life started there, but that is almost certainly where survivors of the giant impacts would have huddled.

Link: NYTimes Article: A New Picture of the Early Earth