Hadean

The Hadean is the geologic eon before the Archean. It started at Earth's formation about 4.6 billion years ago (4600 Ma), and ended roughly 3.8 billion years ago, though the latter date varies according to different sources. The name "Hadean" derives from Hades, Greek for "unseen" or "Hell" and suggesting the underworld or referring to the conditions on Earth at the time. The geologist Preston Cloud coined the term in 1972, originally to label the period before the earliest-known rocks. W. B. Harland later coined an almost synonymous term: the "Priscoan period". Other older texts simply refer to the eon as the Pre-Archean, while during much of the 19th and 20th centuries, the term Azoic (meaning without life) was commonly used.

Subdivisions
Since few geological traces of this period remain on Earth there are no official subdivisions. However, several major divisions of the lunar geologic timescale occurred during the Hadean, and so these are sometimes used unofficially to refer to the same periods of time on Earth.

Hadean rocks
In the last decades of the 20th century geologists identified a few Hadean rocks from Western Greenland, Northwestern Canada and Western Australia. The oldest known rock formations (the Isua greenstone belt) comprise sediments from Greenland dated around 3.8 billion years ago somewhat altered by a volcanic dike that penetrated the rocks after they were deposited. Individual zircon crystals redeposited in sediments in Western Canada and the Jack Hills region of Western Australia are much older. The oldest dated zircons date from about 4400 Ma - very close to the hypothesized time of the Earth's formation.

The Greenland sediments include banded iron beds. They contain possibly organic carbon and imply some possibility that photosynthetic life had already emerged at that time. The oldest known fossils (from Australia) date from a few hundred million years later.

The late heavy bombardment happened during Hadean times and affected the Earth and the Moon.

Atmosphere and oceans
A sizeable quantity of water would have been in the material which formed the Earth. Water molecules would have escaped Earth's gravity until the planet attained a radius of about 40% of its current size; after that point, water (and other volatile substances) would have been retained. Hydrogen and helium are expected to continually leak from the atmosphere, but the lack of denser noble gases in the modern atmosphere suggests that something disastrous happened to the early atmosphere.

Part of the young planet is theorized to have been disrupted by the impact which created the Moon, which should have caused melting of one or two large areas. Present composition does not match complete melting and it is hard to completely melt and mix huge rock masses. However, a fair fraction of material should have been vaporized by this impact, creating a rock vapor atmosphere around the young planet. The rock vapor would have condensed within two thousand years, leaving behind hot volatiles which probably resulted in a heavy carbon dioxide atmosphere with hydrogen and water vapor. Liquid water oceans existed despite the surface temperature of 230°C because of the atmospheric pressure of the heavy CO2 atmosphere. As cooling continued, subduction and dissolving in ocean water removed most CO2 from the atmosphere but levels oscillated wildly as new surface and mantle cycles appeared.

Study of zircons has found that liquid water must have existed as long ago as 4400 Ma, very soon after the formation of the Earth. This requires the presence of an atmosphere.