The Hadean eon

The Hadean eon

Volcanic eruptions would have been common in Earth's early days.

The early Earth, during the very early Hadean eon, was very different from the world known today. There were no oceans and no oxygen in the atmosphere. It was bombarded by planetoids and other material left over from the formation of the solar system. This bombardment, combined with heat from radioactive breakdown, residual heat, and heat from the pressure of contraction, caused the planet at this stage to be fully molten. During the iron catastrophe heavier elements sank to the center while lighter ones rose to the surface producing the layered structure of the Earth and also setting up the formation of Earth's magnetic field. Earth's early atmosphere would have comprised surrounding material from the solar nebula, especially light gases such as hydrogen and helium, but the solar wind and Earth's own heat would have driven off this atmosphere.

This changed when Earth was about 40% its present radius, and gravitational attraction allowed the retention of an atmosphere which included water. Temperatures plummeted and the crust of the planet was accumulated on a solid surface, with areas melted by large impacts on the scale of decades to hundreds of years between impacts. Large impacts would have caused localized melting and partial differentiation, with some lighter elements on the surface or released to the moist atmosphere.

The surface cooled quickly, forming the solid crust within 150 million years; although new research suggests that the actual number is 100 million years based on the level of hafnium found in the geology at Jack Hills in Western Australia. From 4 to 3.8 billion years ago, Earth underwent a period of heavy asteroidal bombardment. Steam escaped from the crust while more gases were released by volcanoes, completing the second atmosphere. Additional water was imported by bolide collisions, probably from asteroids ejected from the outer asteroid belt under the influence of Jupiter's gravity. The planet cooled. Clouds formed. Rain gave rise to the oceans within 750 million years (3.8 billion years ago), but probably earlier. Recent evidence suggests the oceans may have begun forming by 4.2 billion years ago. The new atmosphere probably contained ammonia, methane, water vapor, carbon dioxide, and nitrogen, as well as smaller amounts of other gases. Any free oxygen would have been bound by hydrogen or minerals on the surface. Volcanic activity was intense and, without an ozone layer to hinder its entry, ultraviolet radiation flooded the surface.