The Hadean ( ) is the first and oldest of the four known geologic eons of Earth's history, starting with the planet's formation about 4.6 billion years ago[ set by the age of the oldest solid material in the Solar System found in some about 4.567 billion years old]
Hadean rocks are very rare, largely consisting of Hadean zircon from one locality (Jack Hills) in Western Australia.[
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Earth in the early Hadean had a very thick hydride-rich atmosphere whose composition likely resembled the solar nebula and the , with mostly water vapor, methane and ammonia. As the Earth's surface cooled, vaporized atmospheric water condensation into liquid water and eventually a superocean covering nearly all of the planet was formed, turning Earth into an ocean planet. Volcanism outgassing and asteroid impact event further altered the Hadean atmosphere eventually into the nitrogen- and carbon dioxide-rich, weakly reducing Paleoarchean atmosphere.
Etymology
The eon's name "Hadean" comes from Hades, the Greek god of the Greek underworld (whose name is also used to describe the underworld itself), referring to the 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 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.
Rock dating
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 Geodynamics modelers.
In the last decades of the 20th century, geologists identified a few Hadean rocks from western Greenland, northwestern Canada, and Western Australia. In 2015, traces of carbon minerals interpreted as "remains of Biotic material" were found in 4.1-billion-year-old rocks in Western Australia.
The oldest dated zircon crystals, enclosed in a Metamorphism sandstone conglomerate in the Jack Hills of the Narryer Gneiss Terrane of Western Australia, date to 4.404 ± 0.008 Giga-annum.[—around 200 million years after the hypothesized time of Earth's formation.
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In many other areas, xenocryst (or relict) enclosed in older rocks indicate that younger rocks have formed on older terranes and have incorporated some of the older material. One example occurs in the Guiana Shield from the Iwokrama Formation of southern Guyana where zircon cores have been dated at 4.22 Ga.
Atmosphere
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. Earth's present composition suggests that there was not complete remelting as it is difficult to completely melt and mix huge rock masses.[
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Oceans
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.[
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The most likely source of the water in the Hadean ocean was outgassing from the Earth's mantle.[
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Asteroid impacts during the Hadean and into the Archean would have periodically disrupted the ocean. The geological record from 3.2 Gya contains evidence of multiple impacts of objects up to in diameter.[ However, the frequency of meteorite impacts is still under study: the Earth may have gone through long periods when liquid oceans and life were possible.][
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The liquid water would absorb the carbon dioxide in the early atmosphere; this would not be enough by itself to substantially reduce the amount of .[
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Plate tectonics
A 2008 study of zircons found that Australian Hadean rock contains minerals pointing to the existence of plate tectonics as early as 4 billion years ago (approximately 600 million years after Earth's formation).[ Geophysical models are underconstrained, but can paint a general picture of the state of Earth in the Hadean.]
Mantle convection in the Hadean was likely vigorous, due to lower viscosity.[ 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.]
Subduction due to plate tectonics would have removed carbonate from the early oceans, contributing to the removal of the -rich early atmosphere. Removal of this early atmosphere is evidence of Hadean plate tectonics.
If plate tectonics occurred in the Hadean, it would have formed continental crust.
Continents
The amount of exposed land in the Hadean is only loosely dependent on the amount of continental crust: it also depends on the ocean level.[ In models where plate tectonics started in the Archean, Earth has a global ocean in the Hadean.][ Continents may have appeared in the mid-Hadean, and then disappeared under a thick ocean by the end of the Hadean.][
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Possible life
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.[ Such a primitive RNA system also may have been able to undergo template strand switching during replication (genetic recombination) as occurs during the RNA replication of extant .]
See also
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Formation and evolution of the Solar System
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– the first sections describe the formation of Earth
Further reading
External links
