Hadean

Artist's depiction of Earth during the Hadean eon

Scale: 100 Million years

The Hadean eon was the first unit of geologic time in Earth's history, as well as the first unit of the Precambrian and the first of four eons. The eon lasted a total of roughly 536 million years (4.567-4.031 Gya), the shortest of the four eons. It is followed by the Archean eon and the Eoarchean era.

Definition and Etymology

The lower boundey of the eon is defined with the formation of the Earth, estimated to be approximately 4.567 Gya, and the upper boundry is defined by the oldest dated rocks which is approximately 4.031 Gya.

The name comes from the name of the Greek God of the underworld, Hades, so named because of the basal position it holds in Earth's geologic history.

Formation of the Solar System and the Earth

Artist's depiction of the early Solar System with a protoplanetary disk

The age of the solar system is estimated to be approximately 4.5682 billion years, which was found from dating the oldest material in the Solar System. The most widely accepted hypothesis on the formation of the solar system is that formed from the gravitational collapse of a large rotating cloud of dust consisting mostly of hydrogen and helium. Most of the material clumped in the center to form what eventually became the sun, while the remaining material formed a protoplanetary disk.

The Planets (including the Earth) is hypothesized to have from the clumping of dust particles in the disk, and increasing in size through collisions with other large celestial bodies.

Impact with Theia and Formation of the Moon

Artist's depiction of the collision between Earth and Theia

Dating of material from the Moon has shown that it formed much later than the Earth. Several theories have been brought forward to explain how the Moon formed but the most widely accepted is the Giant Impact theory.

The theory proposes that early into the Hadean eon (roughly 4.5 Gya) the Earth collided with a Mars-sized object that has been termed as 'Theia'. Most of the debris from the collision was absorbed into the Earth while the remaining formed into the Moon.

Geology

Little is known about the geologic processes of the Hadeon due to slim chance of materials from this time surviving to modern day. The oldest dated materials are detrial zircons found incase in metamorphised sediments in Western Australia, the oldest of which been dated to 4.404 Gya, found in the Jack Hills. Most of the knowledge of the Hadean comes from studying these zircons.

The existence of plate tectonics during the Hadean is still debated. It has been theorized that due to lower viscosity in the Hadean, mantle convection was more vigorous, and hence, would have led to plate tectonics, as a result, carbonate from the oceans would be reduced leading to the depletion of CO₂ from the atmosphere through the process of subduction, all of which points to the existence of plate tectonics in the Hadean.

Geography

Oceans most likely formed from outgassing of water from the Mantle. The oceans would absorb the CO₂ which was abundant in the atmosphere at this time, leading to depletion. The oceans stayed in liquid form due to the high atmospheric pressure of 27 atmospheres.

By the end of the Hadean, the Planet would have been covered by a large global ocean, with limited pieces of land above the sea level.

Climate

The atmosphere in the early years was thick and heavy consisting of hydrides, methanes, ammonia, and water vapours, and possessed an atmospheric pressure of 27 atmospheres and a surface temperature of 230°C or 446°F.

By the end of the eon, the Earth had cooled down and was covered by a global ocean. Nitrogen and CO₂ levels in the atmosphere had increased.

Origins of Life

The earliest lifeforms likely lived around hydrothermal vents like this one

The presence of a large global ocean in the later stages of the Hadean makes it likely that life originated in this eon, however, no concrete proof has been discovered to back up life originating the Hadean. The Last Universal Common Ancestor (LUCA) of everything on Earth alive today most probably originated in the Hadean.

The most widely accepted theory of the origins of Life is abiogenesis in deep sea hydrothermal vents, where RNA can be synthesisedand replicated. The deep sea environment of early microscopic life would have shielded life from being wiped out during the Late Heavy Bombardment during the late Hadean eon, which continued into the Eoarchean era.