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Hadean

For the Romanian chef, seeAdrian Hădean.

TheHadean(/heɪˈdiːən,ˈheɪdiən/hay-DEE-ən,HAY-dee-ən) is the first and oldest of the four knowngeologic eonsofEarth'shistory,starting withthe planet's formationabout 4.6billion years ago[3][4](estimated 4567.30 ± 0.16million years ago[1]set by the age of the oldest solid material in theSolar Systemfound in somemeteoritesabout 4.567 billion years old[5]), and ended 4.031 billion years ago. Theinterplanetary collisionthat created theMoonoccurred early in this eon. The Hadean eon was succeeded by theArcheaneon, with theLate Heavy Bombardmenthypothesized to have occurred at the Hadean-Archean boundary.

Hadean
4567.3 ± 0.16 – 4031 ± 3Ma
Chronology
Etymology
Synonym(s)Priscoan Period
Harlandet al.,1989
Usage information
Celestial bodyEarth
Regional usageGlobal (ICS)
Definition
Chronological unitEon
Stratigraphic unitEonothem
First proposed byPreston Cloud,1972
Time span formalityFormal
Lower boundary definition(4567.30 ± 0.16) Ma[1]
Lower GSSA ratifiedOctober 5th, 2022[1]
Upper boundary definitionTen oldest U-Pb zircon ages
Upper boundary GSSAAlong the Acasta River,Northwest Territories,Canada
65°10′26″N115°33′14″W/ 65.1738°N 115.5538°W/65.1738; -115.5538
Upper GSSA ratified2023[2]

Hadean rocks are very rare, largely consisting ofgranular zirconsfrom one locality (Jack Hills) inWestern Australia.[6]Hadeangeophysicalmodels remain controversial amonggeologists:plate tectonicsand the growth ofcratonsintocontinentsmay have started in the Hadean, but there is still uncertainty.[7][8][9]

Earth in the early Hadean had a very thickhydride-richatmospherewhosecompositionlikely resembled thesolar nebulaand thegas giants,with mostlywater vapor,methaneandammonia.As the Earth's surface cooled, vaporized atmospheric watercondensedintoliquid waterand eventually asuperoceancovering nearly all of the planet was formed, turning Earth into anocean planet.Volcanicoutgassingandasteroidbombardmentsfurther altered the Hadean atmosphere eventually into thenitrogen- andcarbon dioxide-rich,weakly reducingPaleoarchean atmosphere.

Etymology

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The eon's name "Hadean" comes fromHades,theGreek godof theunderworld(whose name is also used to describe the underworld itself), referring to thehellishconditions then prevailing onearly Earth:the planet had just been formed from recentaccretion,and its surface was still molten with superheatedlavadue to that, the abundance of short-lived radioactive elements, and frequentimpact eventswith other Solar System bodies.

The term was coined by American geologistPreston Cloud,originally to label the period before the earliest knownrockson Earth.[10][11]W.B. Harlandlater coined an almost synonymous term, thePriscoan period,frompriscus,a Latin word for 'ancient'.[12]Other, older texts refer to the eon as thePre-Archean.[13][14]

Rock dating

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Further information:Oldest dated rocks

Prior to the 1980s and the discovery ofHadean lithic fragments,scientific narratives of the early Earth explanations were almost entirely in the hands ofgeodynamicmodelers.[15]

Backscatter electron micrograph of detrital zircons from the Hadean (4.404 ± 0.008 Ga)metasedimentsof theJack Hills,Narryer Gneiss Terrane,Western Australia

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.[16][17]

The oldest datedzirconcrystals, enclosed in ametamorphosedsandstoneconglomeratein theJack Hillsof theNarryer Gneiss Terraneof Western Australia, date to 4.404 ± 0.008Ga.[18]This zircon is a slight outlier, with the oldest consistently dated zircon falling closer to 4.35 Ga[18]—around 200 million years after the hypothesized time ofEarth's formation.

In many other areas,xenocryst(or relict)Hadean zirconsenclosed in older rocks indicate that younger rocks have formed on older terranes and have incorporated some of the older material. One example occurs in theGuiana shieldfrom the Iwokrama Formation of southern Guyana where zircon cores have been dated at 4.22 Ga.[19]

Atmosphere

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A sizable quantity of water would have been in the material that formed Earth.[20]Water molecules would have escaped Earth's gravity more easily when the planet was less massive during its formation.Photodissociationby short-waveultravioletinsunlightcouldsplitsurface watermolecules intooxygenandhydrogen,the former of which would be readily removed by 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.[21]However, a fair fraction of material should have been vaporized by this impact. The material would have condensed within 2,000 years.[22]The initialmagma oceansolidified within 5 million years,[23]leaving behind hot volatiles which probably resulted in a heavyCO
2atmosphere withhydrogenandwater vapor.The initial heavy atmosphere had a surface temperature of 230 °C (446 °F) and anatmospheric pressureof above 27standard atmospheres.[22]

Oceans

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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.[18][24]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.[22]

The most likely source of the water in the Hadean ocean was outgassing from theEarth's mantle.[25]Bombardmentorigin of a substantial amount of water is unlikely, due to the incompatibility ofisotopefractions between the Earth and comets.[20]

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.[26]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).[26]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.[24]

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.[22]

Plate tectonics

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Evolution of continental crust and ocean depths (from Korenaga, 2021)[6]

A 2008 study of zircons found that Australian Hadean rock contains minerals pointing to the existence ofplate tectonicsas early as 4 billion years ago (approximately 600 million years after Earth's formation).[27]However, some geologists suggest that the zircons could have been formed by meteorite impacts.[28]The direct evidence of Hadean geology from zircons is limited, because the zircons are largely gathered in one locality in Australia.[6][29]Geophysical models are underconstrained, but can paint a general picture of the state of Earth in the Hadean.[6][30]

Mantle convectionin the Hadean was likely vigorous, due to lowerviscosity.[6]The lower viscosity was due to the high levels ofradiogenic heatand the fact that water in the mantle had not yet fully outgassed.[31]Whether the vigorous convection led to plate tectonics in the Hadean or was confined under a rigid lid is still a matter of debate.[6][9][29][32]The presence of Hadean oceans is thought to have triggered plate tectonics.[33]

Subductiondue 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.[34]

If plate tectonics occurred in the Hadean, it would have formedcontinental crust.[35]Different models predict different amounts of continental crust during the Hadean.[8]The work of Dhiumeet al.predicts that by the end of the Hadean, the continental crust had only 25% of today's area.[7]The models of Korenaga,et al.predict that the continental crust grew to present-day volume sometime between 4.2 and 4.0Gya.[35][36]

Continents

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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.[6]In models where plate tectonics started in the Archean, Earth has a global ocean in the Hadean.[37][38]The high heat of the mantle may have made it difficult to support high elevations in the Hadean.[39][40]If continents did form in the Hadean, their growth competed with outgassing of water from the mantle.[6]Continents may have appeared in the mid-Hadean, and then disappeared under a thick ocean by the end of the Hadean.[41]The limited amount of land has implications for theorigin of life.[6]

Possible life

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Abundant Hadean-likegeothermalmicroenvironmentswere shown by Saldittet al.to have the potential to support the synthesis and replication ofRNAand thus possibly the evolution of a primitive life form.[42]Porous rock systems comprising heated air-water interfaces were shown to allowribozyme-catalyzedRNA replication of sense and antisense strands followed by subsequent strand dissociation, thus enabling combined synthesis, release and folding of active ribozymes.[42]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.[43] A study published in 2024 inferred thelast common ancestor of all lifeto have emerged during the Hadean, between 4.09 and 4.33 Gya[44].

See also

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References

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