Cool early Earth
The cool early Earth theory posits that for part of the Hadean geological eon, at the beginning of Earth's history, it had a modest influx of bolides and a cool climate, allowing the presence of liquid water. This would have been after the extreme conditions of Earth's earliest history between 4.6 and 4.4 billion years ago, but before the Late Heavy Bombardment of 4.1 to 3.8 Ga ago. In 2002 John Valley et al. argued that detrital zircons found in Western Australia, dating to 4.0–4.4 Ga ago, were formed at relatively low temperatures, that meteorite impacts may have been less frequent than previously thought, and that Earth may have gone through long periods when liquid oceans and life were possible.
In 2016 Gavin Kenny et al. replied to suggestions that zircons were formed by melting during tectonic subduction at plate boundaries, and argued that at least some of them were formed by meteorite impacts.
