This landmark paper describes the analysis of the oldest-known earth materials, 4.4 billion year old zircon grains. Studies of these zircons suggest that some continental crust formed as early as 4.4 Ga, 160 m.y. after accretion of the Earth, and that surface temperatures were low enough for liquid water. The hypothesis of a cool early Earth suggests long intervals of relatively temperate surface conditions from 4.4 to 4.0 Ga that were conducive to liquid- water oceans and possibly life. Meteorite impacts during this period may have been less frequent than previously thought.
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