The eon's name "Hadean" comes fromHades, theGreek god of theunderworld (whose name is also used to describe the underworld itself), referring to thehellish conditions then prevailing onearly Earth: the planet had just been formed from recentaccretion, and its surface was still molten with superheatedlava due to that, the abundance of short-lived radioactive elements, and frequentimpact events with other Solar System bodies.
The term was coined by American geologistPreston Cloud, originally to label the period before the earliest knownrocks on Earth.[11][12]W.B. Harland later coined an almost synonymous term, thePriscoan period, frompriscus, a Latin word for 'ancient'.[13] Other, older texts refer to the eon as thePre-Archean.[14][15]
Prior to the 1980s and the discovery ofHadean lithic fragments, scientific narratives of the early Earth explanations were almost entirely in the hands ofgeodynamic modelers.[16]
In the last decades of the 20th century, geologists identified a few Hadean rocks from westernGreenland, northwesternCanada, andWestern Australia. In 2015, traces of carbon minerals interpreted as "remains ofbiotic life" were found in 4.1-billion-year-old rocks in Western Australia.[17][18]
In many other areas,xenocryst (or relict)Hadean zircons enclosed in older rocks indicate that younger rocks have formed on olderterranes and have incorporated some of the older material. One example occurs in theGuiana 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.[21] Water molecules would have escaped Earth's gravity more easily when the planet was less massive during its formation.Photodissociation by short-waveultraviolet insunlight couldsplitsurface water molecules intooxygen andhydrogen, the former of which would readily react to form compounds in the then-reducing atmosphere, while the latter (along with the similarly lighthelium) would be expected to continually leave the atmosphere (as it does to the present day) due toatmospheric escape.
Part of the ancient planet is theorized to have been disrupted by theimpact 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.[22] However, a fair fraction of material should have been vaporized by this impact. The material would have condensed within 2,000 years.[23] The initialmagma ocean solidified within 5 million years,[24] leaving behind hot volatiles which probably resulted in a heavyCO 2 atmosphere withhydrogen andwater vapor. The initial heavy atmosphere had a surface temperature of 230 °C (446 °F) and anatmospheric pressure of above 27standard atmospheres.[23]
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.[19][25][26] Liquid water oceans existed despite the high surface temperature, because at an atmospheric pressure of 27 atmospheres, water remains liquid even at those high temperatures.[23]
The most likely source of the water in the Hadean ocean was outgassing from theEarth's mantle.[27]Bombardment origin of a substantial amount of water is unlikely, due to the incompatibility ofisotope fractions between the Earth and comets.[21]
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 100 kilometres (62 mi) in diameter.[28] Each such impact would have boiled off up to 100 metres (330 ft) of a global ocean, and temporarily raised the atmospheric temperature to 500 °C (932 °F).[28] 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.[25]
The liquid water would absorb the carbon dioxide in the early atmosphere; this would not be enough by itself to substantially reduce the amount ofCO 2.[23]
Evolution of continental crust and ocean depths (from Korenaga, 2021)[7]
A 2008 study of zircons found that Australian Hadean rock contains minerals pointing to the existence ofplate tectonics as early as 4 billion years ago (approximately 600 million years after Earth's formation).[29] However, some geologists suggest that the zircons could have been formed by meteorite impacts.[30] The direct evidence of Hadean geology from zircons is limited, because the zircons are largely gathered in one locality in Australia.[7][31] Geophysical models are underconstrained, but can paint a general picture of the state of Earth in the Hadean.[7][32]
Mantle convection in the Hadean was likely vigorous, due to lowerviscosity.[7] The lower viscosity was due to the high levels ofradiogenic heat and the fact that water in the mantle had not yet fully outgassed.[33] Whether the vigorous convection led to plate tectonics in the Hadean or was confined under a rigid lid is still a matter of debate.[7][10][31][34] The presence of Hadean oceans is thought to have triggered plate tectonics.[35]
Subduction due to plate tectonics would have removed carbonate from the early oceans, contributing to the removal of theCO 2-rich early atmosphere. Removal of this early atmosphere is evidence of Hadean plate tectonics.[36]
If plate tectonics occurred in the Hadean, it would have formedcontinental crust.[37] Different models predict different amounts of continental crust during the Hadean.[9] The work of Dhiumeet al. predicts that by the end of the Hadean, the continental crust had only 25% of today's area.[8] The models of Korenaga,et al. predict that the continental crust grew to present-day volume sometime between 4.2 and 4.0Gya.[37][38]
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.[7] In models where plate tectonics started in the Archean, Earth has a global ocean in the Hadean.[39][40] The high heat of the mantle may have made it difficult to support high elevations in the Hadean.[41][42] If continents did form in the Hadean, their growth competed with outgassing of water from the mantle.[7] Continents may have appeared in the mid-Hadean, and then disappeared under a thick ocean by the end of the Hadean.[43] The limited amount of land has implications for theorigin of life.[7]
Abundant Hadean-likegeothermalmicroenvironments were shown by Saldittet al. to have the potential to support the synthesis and replication ofRNA and thus possibly the evolution of a primitive life form.[44] Porous rock systems comprising heated air-water interfaces were shown to allowribozyme-catalyzed RNA replication of sense and antisense strands followed by subsequent strand dissociation, thus enabling combined synthesis, release and folding of active ribozymes.[44] 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 extantcoronaviruses.[45]A study published in 2024 inferred thelast common ancestor of all current life to have emerged during the Hadean, between 4.09 and 4.33 Gya.[46]
Although the early part of theLate 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. As Earth already had oceans, life would have been possible, but vulnerable toextinction events caused by those impacts. The risk would not be on the frequency, but on the size of the impactor, and remains on the Moon suggest impactors bigger than theChicxulub impactor that caused theextinction of dinosaurs. An impactor big enough may erase all life on the planet, although some models suggest that microscopic life may still survive if underground or in the oceanic depths.[47]
^abCohen, Kim (October 2022)."New edition of the Chart - 2022-10".International Commission on Stratigraphy. Retrieved16 January 2023.2022/10 - Hadean: GSSA instated as ratified by IUGS (5-10-2022). The GSSA is 4,567.30 ± 0.16 Ma.
^Shaw, D.M. (1975).Early history of the Earth. Proceedings of the NATO Advanced Study Institute. Leicester: John Wiley. pp. 33–53.ISBN0-471-01488-5.
^Jarvis, Gary T.; Campbell, Ian H. (December 1983). "Archean komatiites and geotherms: Solution to an apparent contradiction".Geophysical Research Letters.10 (12):1133–1136.Bibcode:1983GeoRL..10.1133J.doi:10.1029/GL010i012p01133.
^Harrison, T. Mark (2020).Hadean earth. Cham: Springer. p. 4.ISBN978-3030466862.
^abcWilde, Simon A.; Valley, John W.; Peck, William H.; Graham, Colin M. (2001). "Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago".Nature.409 (6817):175–178.Bibcode:2001Natur.409..175W.doi:10.1038/35051550.PMID11196637.S2CID4319774.
^Reis, HLS; Sanchez, EAM (2020). "Precambrian". In Alderton, David; Elias, Scott (eds.).Encyclopedia of Geology. Elsevier Science. p. 30.ISBN9780081029091.
^abLowe, DR; Byerly, GR (2015). "Geologic record of partial ocean evaporation triggered by giant asteroid impacts, 3.29–3.23 billion years ago".Geology.43 (6):535–538.Bibcode:2015Geo....43..535L.doi:10.1130/G36665.1.
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.
Carley, Tamara L.; et al. (2014). "Iceland is not a magmatic analog for the Hadean: Evidence from the zircon record".Earth and Planetary Science Letters.405 (1):85–97.Bibcode:2014E&PSL.405...85C.doi:10.1016/j.epsl.2014.08.015.
