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The Hadean or Präarchaikum is the first eon of Earth's history. It begins with the formation of the Proto-Earth about 4.6 billion years ago and ends geochronologically defined 4 billion years ago. It was around this time that the first continental crustal blocks were formed through magmatic differentiation . The Archean followed the Hadean . Classically, the end of the Hadaic era is often given as 3.8 billion years BP , since this is where the great meteorite bombardment ended.
The name Hadaikum, English Hadean , is derived from Hades ( ancient Greek ᾍδης or Ἅιδης, Doric Ἀΐδας), the Greek god of the underworld .
Redefinition of the Precambrian
In the course of the redefinition of the Precambrian , the Hadaikum was shortened by 30 million years in 2012 and ends 4030 million years BP with the beginning of the first period of the Paleoarchean , the Acastum . It is divided into two eras , the chaotic , which lasts from the formation of the earth to 4404 million years BP, and the subsequent zirconium . With the zirconium, the first preserved minerals appear - the zircons .
At the beginning of the Hadaikum, the earth's mantle froze, disturbed several times by impacts from other protoplanets , the last of which probably led to the formation of the moon . After that, for a long time the earth was covered exclusively by oceanic crust , which, like today, was constantly renewed by mantle convection . Volcanoes fed the primordial atmosphere and provided water vapor for the ocean, which was cool under a still weak sun . Possibly originated around black smokers the life .
In the Hadean, the formation of the planet earth took place first. After accretion from a disk of dust and gas surrounding the young sun , the proto-earth was formed . The earth's material gradually differentiated , with the heavier elements such as iron and nickel forming the core of the earth , the light ones, such as silicon and bound oxygen , forming the earth's mantle and a likely basaltic crust , with the material of the earth's interior being hot and partially melted. The heat comes to a large extent from the formation phase of the earth, in which the potential energy of the planetesimals , from which the earth was formed, was mainly converted into heat energy. The liberated gravitational binding energy totaled 2.487 x 10 32 J . This energy corresponds to the total radiant power of the sun on 7.5 days or the explosive power of 10 16 = 10,000,000,000,000,000 Tsar bombs .
After the collision theory of lunar origin of the impact of a transported about Mars-sized proto-planet , Theia called much cladding material into orbit, where the moon was formed, even at a distance of only about a fifth time today. He therefore exerted about 5 3 = 125 times stronger tidal forces ; the fissured and partially liquefied earth's mantle did not come to rest, especially since the collision had greatly accelerated the earth's rotation . By tidal friction was angular momentum of this rotation to orbital angular momentum, mainly of the moon, so that it slowly away.
In the quieter time after these large impacts, the earth's mantle slowly heated up due to the decay heat of radionuclides , mainly due to potassium -40 and uranium -235.
In the Hadean, the earthly hydrosphere could have formed, perhaps only temporarily . The origin of the earthly water has not yet been fully clarified; The main sources were the one hand outgassing from magma from the earth's interior and also impacts water heavenly bodies, and studies rather objects from the asteroid belt rather than comet and trans-Neptunian objects point. Depending on the atmosphere at the time, liquid water may also have existed at high temperatures, e.g. B. in an atmosphere with high pressure and a large amount of carbon dioxide , similar to today's atmosphere of Venus . The existence of oceans in the Hadean is controversial. The brilliance of the young sun was the model calculations considerably weaker than it is today (about 70-75%). As a result, the temperatures could have been well below zero, which in any case was not the case in the Archean. In this context one speaks of the paradox of the weak young sun .
Towards the end of Hadaikums probably started the Great bombardment ( Late Heavy Bombardment ), a phase in which it too many impacts of large meteorites came and planetesimals on Earth and Moon, whose traces can be seen today only on the lunar surface. It is possible that the oceans that existed at that time were completely evaporated by the energy of the impacts.
Exploration of the Hadaikum - The oldest rocks and minerals
Microscopic remains of weathered zircons , which have been dated to 4,404 million years ago, are the oldest minerals from the Hadaic era found on earth to date. They were found in Western Australia in the Narryer Gneiss Terran in the Yilgarn Craton . Their age is interpreted as the age of crystallization of the zircon. Especially since the discovery of the macroscopic, 4030 million year old Acasta gneiss in the Canadian Shield , it has been clear that already in this early age - a few hundred million years after the formation of the earth - there was an ocean and at least one mainland island - not yet crystallized has given. The rocks of the Nuvvuagittuq greenstone belt in the Canadian province of Québec are similarly old, possibly 4280 million years old (dating is controversial, possibly only 3800 million years) even older than the Acasta gneiss . From such early Hadaic cratons, the hypothetical first supercontinent Ur could have formed in the Archean .
Today only four other zircons older than 4050 million years are known which do not originate from Western Australia. The Western Australian zircons may have been exposed to significantly different conditions in the Hadean than the rest of the crust, which is why you may not be able to draw conclusions about the general conditions of the earth at that time. The up to 400 million years younger Chinese zircons had crystallization temperatures of approx. 910 ° C, while the Western Australian zircons mostly crystallized at approx. 690 ° C and a few specimens at approx. 800 ° C. The proven low heat flow in Western Australian zircons also suggests that they experienced a constellation that was special for that time, possibly similar to today's converging plate boundary. The source material of the Western Australian zircons is highly controversial; In particular, whether these originated not only from igneous rocks, but possibly also from existing sediments, is part of today's scientific debate. The evidence of sediments that existed at that time would be a strong indication of an already existing hydrosphere with precipitation and oceans.
It is possible that there are largely unchanged fragments of rocks from the Hadaic era outside of the earth that were released into space by meteorite impacts. There is evidence that part of a stone found during the Apollo 14 moon mission originally crystallized on earth. With a dating of 4.0 to 4.1 billion years, this would probably be the oldest stone on earth and would allow a completely new view of this eon.
In the Hadean, the rocks were degassed probably 4.35 billion years ago and the first atmosphere was created. Research today suggests that the atmosphere at the time was not reducing.
Possible origin of life
If earthly life arose through chemical evolution (and did not come to earth from space according to the panspermia hypothesis), then this step took place in the Hadean.
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- ↑ a b Guang-Fu Xing, Xiao-Lei Wang, Yusheng Wan, Zhi-Hong Chen, Yang Jiang, Kouki Kitajima, Takayuki Ushikubo, Phillip Gopon: Diversity in early crustal evolution: 4100 Ma zircons in the Cathaysia Block of southern China . In: Scientific Reports . 4th volume, June 3, 2014, 5143, doi : 10.1038 / srep05143 (English).
- ^ Frank D. Stacey , Conrad HB Stacey: Gravitational energy of core evolution: implications for thermal history and geodynamo power . In: Physics of the Earth and Planetary Interiors . tape 110 , no. 1-2 , January 1999, ISSN 0031-9201 , pp. 83–93 , Table 2, Model C , doi : 10.1016 / S0031-9201 (98) 00141-1 (English, freely available online through mcgill.ca [PDF; 109 kB ]).
- ↑ a b Elizabeth Bell: Petrology: Ancient magma sources revealed . In: Nature Geoscience . tape 10 , no. 6 , June 2017, p. 397–398 , doi : 10.1038 / ngeo2955 (English, limited preview in readcube.com ).
- ↑ Simone Marchi, WF Bottke, LT Elkins-Tanton, M. Bierhaus, K. Wünnemann, A. Morbidelli, DA Kring: Widespread mixing and burial of Earth's Hadean crust by asteroid impacts . In: Nature . tape 511 , no. 7511 , July 31, 2014, p. 578–582 , doi : 10.1038 / nature13539 (English, freely available online from boulder.swri.edu [PDF; 4.0 MB ]).
- ↑ Jonathan O'Neil et al .: Neodymium-142 Evidence for Hadean Mafic Crust . In: Science . tape 321 , no. 5897 , September 26, 2008, p. 1828–1831 , doi : 10.1126 / science.1161925 (English, freely available online through researchgate.net ).
- ↑ Michelle Hopkins, T. Mark Harrison, Craig E. Manning: Low heat flow inferred from> 4 Gyr zircons suggests Hadean plate boundary interactions . In: Nature . tape 456 , no. 7221 , November 27, 2008, p. 493–496 , doi : 10.1038 / nature07465 (English, freely available online through researchgate.net ).
- ↑ This may be Earth's oldest rock — and it was collected on the moon. January 25, 2019, accessed January 28, 2019 .
- ↑ Dustin Trail, E. Bruce Watson, Nicholas D. Tailby: The oxidation state of Hadean magmas and implications for early Earth's atmosphere . In: Nature . tape 480 , no. 7375 , December 1, 2011, p. 79–82 , doi : 10.1038 / nature10655 (English, freely available online through wustl.edu [PDF; 199 kB ]).