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geologic time scale > geological age > Precambrian > Hadean

Preferred term

Hadean  

Definition

  • The Hadean is the first and oldest of the four known geologic eons of Earth's history, starting with the planet's formation about 4.54 Bya, now defined as (4567.30 ± 0.16) Mya set by the age of the oldest solid material in the Solar System found in some meteorites about 4.567 billion years old. The supposed interplanetary collision that created the Moon occurred early in this eon. The Hadean ended 4.031 billion years ago and was succeeded by the Archean eon, with the Late Heavy Bombardment hypothesized to have occurred at the Hadean-Archean boundary. Hadean rocks are very rare, largely consisting of granular zircons from one locality (Jack Hills) in Western Australia. Hadean geophysical models remain controversial among geologists: it appears that plate tectonics and the growth of continents may have started in the Hadean. Earth in the early Hadean had a very thick carbon dioxide- and methane-rich prebiotic atmosphere, but eventually oceans made of liquid water were formed. The eon's name "Hadean" comes from Hades, the Greek god of the underworld. A sizable quantity of water would have been in the material that formed Earth. Water molecules would have escaped Earth's gravity more easily when the planet was less massive during its formation. Photodissociation by short-wave ultraviolet in sunlight could split surface water molecules into oxygen and hydrogen, the former of which would be readily removed by the then-reducing atmosphere, while the latter (along with the similarly light helium) would be expected to continually leave the atmosphere (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 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. However, a fair fraction of material should have been vaporized by this impact. The material would have condensed within 2,000 years. The initial magma ocean solidified within 5 million years, leaving behind hot volatiles which probably resulted in a heavy CO2 atmosphere with hydrogen and water vapor. The initial heavy atmosphere had a surface temperature of 230 °C (446 °F) and an atmospheric pressure of above 27 standard atmospheres. (Adapted from: https://en.wikipedia.org/wiki/Hadean)

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http://data.loterre.fr/ark:/67375/QX8-TP6FRFLP-S

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