"Hadean" (from Hades, the Greek god of the underworld, and the underworld itself) describes the hellish conditions then prevailing on Earth: the planet had just formed and was still very hot owing to its recent accretion, the abundance of short-lived radioactive elements, and frequent collisions with other Solar System bodies.
Since few geological traces of this eon remain on Earth, there is no official subdivision. However, the Lunar geologic timescale embraces several major divisions relating to the Hadean, so these are sometimes used in an informal sense to refer to the same periods of time on Earth.
In 2010, an alternative scale was proposed that includes the addition of the Chaotian and Prenephelean Eons preceding the Hadean, and divides the Hadean into three eras with two periods each. The Paleohadean era consists of the Hephaestean (4.5–4.4 Ga) and the Jacobian periods (4.4–4.3 Ga). The Mesohadean is divided into the Canadian (4.3–4.2 Ga) and the Procrustean periods (4.2–4.1 Ga). The Neohadean is divided into the Acastan (4.1–4.0 Ga) and the Promethean periods (4.0–3.9 Ga). As of February 2017[update], this has not been adopted by the IUGS.
In many other areas, xenocryst (or relict) Hadean zircons 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 and oceans
A sizable quantity of water would have been in the material that formed the Earth. 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 escape (even to the present day) due to atmospheric escape.
Part of the ancient planet is theorized to have been disrupted by the impact that created the Moon, which should have caused melting of one or two large regions of the Earth. Earth's present composition suggests that there was not complete remelting as it is difficult 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 CO 2 atmosphere with hydrogen and water vapor. Liquid water oceans existed despite the surface temperature of 230 °C (446 °F) because at an atmospheric pressure of above 27 atmospheres, caused by the heavy CO 2 atmosphere, water is still liquid. As cooling continued, subduction and dissolving in ocean water removed most CO 2 from the atmosphere but levels oscillated wildly as new surface and mantle cycles appeared.
Studies of zircons have found that liquid water must have existed as long ago as 4.4 billion years ago, very soon after the formation of the Earth. This requires the presence of an atmosphere. The cool early Earth theory covers a range from about 4.4 to about 4.1 billion years.
A September 2008 study of zircons found that Australian Hadean rock holds minerals pointing to the existence of plate tectonics as early as 4 billion years ago (approximately 600 million years after Earth's formation).
If this is true, the time when Earth finished its transition from having a hot, molten surface and atmosphere full of carbon dioxide, to being very much like it is today, can be roughly dated to about 4.0 billion years ago. The actions of plate tectonics and the oceans trapped vast amounts of carbon dioxide, thereby reducing the greenhouse effect and leading to a much cooler surface temperature and the formation of solid rock, and possibly even life.
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^Shaw, D.M. (1975). "Early History of the Earth". Proceedings of the NATO Advanced Study Institute. Leicester: John Wiley (London): 33–53.
^Jarvis, Gary T.; Campbell, Ian H. (December 1983). "Archean komatiites and geotherms: Solution to an apparent contradiction". Geophysical Research Letters. 10 (12): 1133–1136. doi:10.1029/GL010i012p01133.
^ abAbramov, Oleg; Mojzsis, Stephen J. (December 2008). "Thermal State of the Lithosphere During Late Heavy Bombardment: Implications for Early Life". AGU Fall Meeting Abstracts. Fall Meeting 2008: American Geophysical Union. 1 (2008 Fall Meeting). Bibcode:2008AGUFM.V11E..08A.CS1 maint: location (link)
Valley, John W.; Peck, William H.; King, Elizabeth M. (1999), "Zircons Are Forever", The Outcrop for 1999, University of Wisconsin-Madison, retrieved January 10, 2006 – Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago.
Wyche, S.; Nelson, D. R. & Riganti, A. (2004), "4350–3130 Ma detrital zircons in the Southern Cross Granite–Greenstone Terrane, Western Australia: implications for the early evolution of the Yilgarn Craton", Australian Journal of Earth Sciences, 51 (1): 31–45, Bibcode:2004AuJES..51...31W, doi:10.1046/j.1400-0952.2003.01042.x.