Hadean Eon & the formation of Earth

The Hadean Eon Basal part of the former Precambrian Era

Hadean Eon subdivided into several unofficial “eras” based on lunar geology Basin Groups Cryptic 4.56 GA 4.1 GA Early Bombardment Phase Nectarian Lower Imbrian 3.8 GA Late Bombardment Phase

The Cryptic “Era” – 4.56 to 4.1 Ga Cryptic because not much geological evidence on Earth has survived to the present day. Correlative to the Early Bombardment Phase of Earth/Moon history Late Bombardment Phase 4.1 to 3.8 Ga (beginning of Archaean Eon).

image taken by Hubble Space Telescope. Nebular Hypothesis for formation of our solar system It is currently believed that the Earth and the other planets formed at essentially the same time from a cloud of gas – mostly hydrogen and helium (nebula). Nebula image taken by Hubble Space Telescope.

The Nebular Hypothesis: 15-18 Ga Big Bang occurs – instantaneous formation of all matter in our universe. ~ 5 billion years ago, swirling mass of the nebula began to contract. A flat, disk-shaped cloud with thickened center forms. The inner planets begin to accrete from metallic and rocky clumps (protoplanets). ~ 10Ma Material at center coalesces, shrinks, becomes heated to several millions of degrees; hydrogen atoms begin to fuse (sun).

Nebular Hypothesis

Formation of Earth’s layered structure As Earth formed, the decay of radioactive elements and heat from high-velocity impacts (meteors and asteroids) caused the temperature to increase. Iron and nickel began to melt and sink toward the center. Lighter rocky components migrate outward toward the surface. Gaseous material escapes from Earth’s interior, producing primitive atmosphere.

Formation of Earth’s inner layers

Structure of the Moon

c) Impactor hypothesis A Mars-sized object collides with a young Earth . Material ejected from the collision of the impactor and from the Earth forms the Moon. The differences in chemical composition between the Earth and the Moon is explained Earth’s high density from contribution of the impactor’s core. Moon’s lower density because it originated from Earth’s crust. Mechanics of collision explain Earth-Moon orbit.

Meteorites – fragments of shattered planets Fe - Ni Stony Note olivine xls Note Ca- and Al-rich minerals

Mars-size object collides approx Mars-size object collides approx. 60 Ma after formation of Earth – results in a planet with a larger than normal core and the formation of the Moon.

moon Earth (South Pole) Theia

Meanwhile back on Earth 4.5 – 4.4 Ga: Magma seas, komatiite crust, and liquid water

Hadean Factoids First Earth crust destroyed during formation of the Moon Subsequent crust formed from a molten “ocean” of magma (much higher temps than today’s magmas). Composition of first crust was ultramafic komatiite (4.5 Ga). Oldest zircons from Australia indicate granitic rocks were already forming 4.4 Ga komatiite

- vapor condenses & results in CO2 atmosphere with H and H20 vapor - Rock vapor atmosphere forms after moon-forming collision - vapor condenses & results in CO2 atmosphere with H and H20 vapor - liquid water even though surface temps over 250deg C – due to high CO2 pressure Oldest sedimentary rocks in Greenland 3.8 Ga contain banded iron and possible organic carbon – alternating bands of oxygen-rich and oxygen poor iron layers