Hadean

The Hadean (pronounced /ˈheɪdiən/) is the geologic eon before the Archean. It started at Earth's formation about 4.6 billion years ago (4,600 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 "Underworld", 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. Brian Harland later coined an almost synonymous term: the "Priscoan period". Other older texts simply refer to the eon as the Pre-Archean.

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.

The Lunar divisions are:

* Pre-Nectarian, from the formation of the Moon's crust up to about 3920 Ma
* Nectarian ranging up to about 3850 Ma, in a time when the Late Heavy Bombardment, according to that theory, was in a stage of decline.

Hadean Eon
The Hadean is not formally recognized by the International Commission on Stratigraphy. The following subdivisions represent one proposal that is loosely based on the lunar geologic time scale.
Cryptic Basin Groups Nectarian Lower Imbrian

Hadean rocks
Further information: Oldest dated rocks

In the last decades of the 20th century geologists identified a few Hadean rocks from Western Greenland, Northwestern Canada and Western Australia. Rock formations in Greenland comprise sediments dated around 3.8 billion years ago and are 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 4,400 Ma[1] – 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.

Atmosphere and oceans
An artist's impression of a magma ocean on the early Earth.

A sizeable quantity of water would have been in the material which formed the Earth.[2] Water molecules would have escaped Earth's gravity more easily when it was less massive during its formation. 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.[dubious – discuss]

Part of the ancient 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.[3] 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.[4]

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.[5][6][7] This requires the presence of an atmosphere. The Cool Early Earth theory covers a range from about 4400 Ma to 4000 Ma.

A September 2008 study of zircons found that Australian Hadean rock holds minerals that point to the existence of plate tectonics as early as 4 billion years ago.[8] If this holds true, the previous beliefs about the Hadean period are far from correct. That is, rather than a hot, molten surface and atmosphere full of carbon dioxide, the Earth's surface would be very much like it is today. The action of plate tectonics traps vast amounts of carbon dioxide, thereby eliminating the greenhouse effects and leading to a much cooler surface temperature and the formation of solid rock, and possibly even life.[8]