Early Earth

The term “Cool Early Earth” refers to the new vision of the Hadean-early Archean Earth characterized by much more temperate surface conditions than believed before. The term was coined by the American geochemist John Valley in 2002. A popular image of the early Earth is indeed of a planet covered by hot magma. This image is at the origin of the term Hadean (from the Greek god of the underworld, Hades) for the first eon. The fractionated oxygen isotopic compositions of 4.4–4.3 Ga-old Jack Hills zircons indicate, however, that they crystallized from magma that had been in contact with water or sediments; thus, surface temperatures may have been low enough to permit liquid water on the Earth’s surface. It is therefore possible that liquid water was present at the surface of the Hadean Earth and that Earth cooled rapidly from its “hot phase” during and soon after core formation.

The chemical composition of the 4.4–4.3 Ga Jack Hills zircons implies...