Hadic Aeon

The Hadic eon, Hadeic, Hadean or Katarchei, is a division of the geological time scale and the first division of the Precambrian. It begins at the time the Earth was formed about 4,567 million years ago and ends 4,000 million years ago, lasting about 567 million years, when the Archean eon begins. The International Commission on Stratigraphy considers it a formal term as of 2022.
Geologist Preston Cloud coined the term in 1972, originally to label the period before the first known rocks on Earth. W. Brian Harland later coined a near-synonymous term, the "Priscoan period", from priscus , the Latin word for "ancient". Other older texts refer to the aeon as the Pre-Archaic. Etymologically, the word "Hadic" comes from the Greek word Hades which denominated the Greek underworld, probably because it is related to a stage of heat and confusion.

During this period, the solar system was probably forming within a large cloud of gas and dust. The Earth was formed when part of this incandescent matter was transformed into a solid body. This is the period during which the earth's crust was formed. This crust underwent many changes, due to the numerous volcanic eruptions.

The oldest known rocks are about 4.4 billion years old and are found in Canada and Australia, while the oldest rock formations are the 3.8 billion-year-old rock formations of Greenland.

During this eon, the late heavy bombardment occurred, affecting the inner planets of the solar system 3.8-4 billion years ago.

According to molecular clocks and fossil evidence, the last universal common ancestor of all living things lived at the end of this aeon, and the origin of the first prokaryotes was estimated in the middle of this aeon. At the beginning of this eon, the self-replicating biomolecules that would form the protobionts were present, abiotic structures that would later give rise to the last universal common ancestor and to life in general. (See Abiogenesis)

Hadic rebuke.

Hadic Rocks

A zircon seen under optical microscope

In the last decades of the 20th century geologists identified some Hadic rocks in western Greenland, northwestern Canada and Australia Western.

The oldest known minerals are single crystals of zircon redeposited in sediments of western Canada and the Jack Hills region of Western Australia. The oldest dated zircons are 4.4 billion years old, very close to the estimated date of Earth formation.

The oldest known rock formation, the Isua supracrustal belt, consists of Greenland sediments dated to about 3.8 billion years old, somewhat disturbed by volcanic dikes that penetrated the rocks after they were deposited.

Greenland's sediments include banded iron formations. They possibly contain organic carbon, which would indicate that the first self-replicating molecules (RNA world hypothesis) date from this time and a small probability that photosynthesis had already emerged. The oldest known fossils (from Greenland) date from a few hundred million years later.

High impact hypothesis

Among the material from which the earth formed must have been a certain amount of water. The water molecules would have been escaping Earth's gravity until the planet reached a radius of approximately 40% of its present size; after that point, water and other volatile substances would have been conserved. Hydrogen and helium would be expected to continuously escape the atmosphere, but the lack of dense noble gases in the modern atmosphere suggests that something catastrophic happened in the early atmosphere..

It is hypothesized that some of the material from the young planet was contributed by the impact that created the Moon. The current composition of the Earth does not match that which it would have in a complete meltdown, and, moreover, it is difficult to completely melt and mix huge masses of rock. However, a significant fraction of material must have been vaporized in this impact, creating an atmosphere of vaporized rocks around the young planet.

The condensation of the vaporized rocks would take two thousand years, leaving a heavy atmosphere of carbon dioxide with hydrogen and water vapor. Oceans of liquid water would form despite a surface temperature of 230 °C, due to the strong atmospheric pressure of CO₂. As cooling continued, subduction and dissolution in ocean water removed most of the CO₂ from the atmosphere, but levels oscillated strongly as the surface and mantle cycles appeared.

The study of zircons has revealed that liquid water must have existed as early as 4.4 billion years ago, very shortly after the formation of the Earth. This requires the presence of an atmosphere.

Subdivisions

Since few geologic traces of this period have survived on Earth, the International Commission on Stratigraphy has not recognized any Hadic subdivisions. However, there are several main divisions of the Hadic eon on the lunar geologic time scale, which are sometimes used unofficially to refer to the same periods on Earth.

Super	Eon Eonotema	It was Eradication	Period System	Home, in millions of years
Precámbrico	Proterozoic	Neo-proterozoic	Ediacárico	~635
			Criogenic	~720
			Tonic	1000
		Meso-proterozoic	Stem	1200
			Ects	1400
			Calymic	1600
		Paleo-proterozoic	Estatérico	1800
			Orosirico	2050
			Risual	2300
			Sidérico	2500
	Arcaico	Neoarchaico		2800
		Mesoarcaico		3200
		Paleoarcaic		3600
		Eoarcaico		4000
	Hádico			4567