Hadean

Hadean rocks are very rare, largely consisting of granular zircons from one locality (Jack Hills) in Western Australia.

[8][9][10] Earth in the early Hadean had a very thick hydride-rich atmosphere whose composition likely resembled the solar nebula and the gas giants, with mostly water vapor, methane and ammonia.

The eon's name "Hadean" comes from Hades, the Greek god of the underworld (whose name is also used to describe the underworld itself), referring to the hellish conditions then prevailing on early Earth: the planet had just been formed from recent accretion, and its surface was still molten with superheated lava due to that, the abundance of short-lived radioactive elements, and frequent impact events with other Solar System bodies.

The term was coined by American geologist Preston Cloud, originally to label the period before the earliest known rocks on Earth.

[14][15] Prior to the 1980s and the discovery of Hadean lithic fragments, scientific narratives of the early Earth explanations were almost entirely in the hands of geodynamic modelers.

One example occurs in the Guiana shield from the Iwokrama Formation of southern Guyana where zircon cores have been dated at 4.22 Ga.[20] A sizable quantity of water would have been in the material that formed Earth.

Part of the ancient planet is theorized to have been disrupted by the impact that created the Moon, which should have caused the melting of one or two large regions of Earth.

[23] The initial magma ocean solidified within 5 million years,[24] leaving behind hot volatiles which probably resulted in a heavy CO2 atmosphere with hydrogen and water vapor.

Studies of zircons have found that liquid water may have existed between 4.0 and 4.4 billion years ago, very soon after the formation of Earth.

[26] Bombardment origin of a substantial amount of water is unlikely, due to the incompatibility of isotope fractions between the Earth and comets.

[7] The lower viscosity was due to the high levels of radiogenic heat and the fact that water in the mantle had not yet fully outgassed.

[7] Abundant Hadean-like geothermal microenvironments were shown by Salditt et al. to have the potential to support the synthesis and replication of RNA and thus possibly the evolution of a primitive life form.

[45] Although the early part of the Late Heavy Bombardment happened during the Hadean, the impacts were frequent only on a cosmic scale, with thousands or even millions of years between each event.