Cosmic timeline 18
Starting with the Earth’s formation by accretion from the solar nebula 4.54 billion years ago, the first eon in the Earth’s history is called the Hadean. It lasted until the Archaean eon, which began 3.8 billion years ago. The oldest rocks found on Earth date to about 4.0 billion years ago, and the oldest detrital zircon crystals in some rocks have been dated to about 4.4 billion years ago, close to the formation of the Earth’s crust and the Earth itself.
Because not much material from this time is preserved, little is known about Hadean times, but scientists think that at an estimated 4.53 billion years ago, shortly after formation of an initial crust, the proto-Earth was impacted by a smaller protoplanet, which ejected part of the mantle and crust into space and created the Moon.
The Hadean era
The name “Hadean” derives from Hades, Greek for “Underworld”, referring to the conditions on Earth at the time. The geologist Preston Cloud coined the term in 1972, originally to label the period before the earliest-known rocks.
During the Hadean, the Earth’s surface was under a continuous bombardment by meteorites, and volcanism must have been severe due to the large heat flow and geothermal gradient. The detrital zircon crystals dated to 4.4 billion years ago show evidence of having undergone contact with liquid water, considered as proof that the planet already had oceans or seas at that time. From crater counts on other celestial bodies it is inferred that a period of intense meteorite impacts, called the “Late Heavy Bombardment”, began about 4.1 billion years ago, and concluded around 3.8 billion years ago, at the end of the Hadean.
Artist’s impression of the moon during the Late Heavy Bombardment (Lunar Cataclysm) and today. The Late Heavy Bombardment, called the lunar cataclysm, is a period of time approximately 4.1 to 3.8 billion years ago during which a large number of impact craters are believed to have formed on the Moon, and by inference on Earth, Mercury, Venus, and Mars as well.
The Archaean era
The name comes from the ancient Greek “Αρχή” (Arkhē), meaning “beginning, origin”.
By the beginning of the Archaean, the Earth had cooled significantly. It would have been impossible for most present day life forms to exist due to the composition of the Archaean atmosphere, which lacked oxygen and an ozone layer. Nevertheless it is believed that primordial life began to evolve by the early Archaean, with some possible fossil finds dated to around 3.5 billion years ago. Some researchers, however, speculate that life could have begun during the early Hadean, as far back as 4.4 billion years ago, surviving the possible Late Heavy Bombardment period in hydrothermal vents below the Earth’s surface.