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Birth of our planet

The earth formed around 4.578 billion years ago by accretion from the solar nebula. In one description, accretion can be explained as the growth of a massive object by gravitationally attracting more matter, typically gaseous matter in an accretion disc. Another description, in the nebular theory, explains accretion as a collision and sticking of cooled microscopic dust and ice particles electrostatically, in protoplanetary discs and Gas giant protoplanet systems, eventually leading to planetesimals which gravitationally accrete more small particles and other planetesimals. Planetismals are those bodies which have reached sizes of approximately one kilometer, after which they can attract each other directly through their mutual gravity, enormously aiding further growth into moon-sized protoplanets

The atmosphere: Volcanic outgassing likely created the primordial atmosphere, but it contained almost no oxygen and would have been toxic to humans and most modern life. Much of the Earth was molten because of extreme volcanism and frequent collisions with other bodies. One very large collision is thought to have been responsible for tilting the Earth at an angle and forming the Moon. Over time, such cosmic bombardments ceased, allowing our planet to cool and form a solid crust. Water that was brought here by comets and asteroids condensed into clouds and the oceans took shape. Earth was finally hospitable to life, and the earliest forms that arose enriched the atmosphere with oxygen. Life on Earth remained small and microscopic for at least a billion years. Then, during the Cambrian period of the Phanerozoic eon, it experienced a rapid diversification into many of its modern forms.

The oldest meteorite fragments found on Earth are about 4,540 million years old; this, coupled primarily with the dating of ancient lead deposits, has put the estimated age of Earth at around that time. The Moon has the same composition as Earth’s crust but does not contain an iron-rich core like the Earth’s. Many scientists think that about 40 million years later a planetoid struck the Earth, throwing into orbit crust material that formed the Moon. Another hypothesis is that the Earth and Moon started to coalesce at the same time but the Earth, having much stronger gravity, attracted almost all the iron particles in the area.

Until recently the oldest rocks found on Earth were about 3,800 million years old, leading scientists to believe for decades that Earth’s surface had been molten until then. Accordingly, they named this part of Earth’s history the Hadean eon, whose name means “hellish”. However analysis of zircons formed 4,400 to 4,000 million years ago indicates that Earth’s crust solidified about 100 million years after the planet’s formation and that the planet quickly acquired oceans and an atmosphere, which may have been capable of supporting life.

Evidence from the Moon indicates that from 4,000 to 3,800 million years ago it suffered a Late Heavy Bombardment by debris that was left over from the formation of the Solar system, and the Earth should have experienced an even heavier bombardment due to its stronger gravity. While there is no direct evidence of conditions on Earth 4,000 to 3,800 million years ago, there is no reason to think that the Earth was not also affected by this late heavy bombardment. This event may well have stripped away any previous atmosphere and oceans; in this case gases and water from comet impacts may have contributed to their replacement, although volcanic outgassing on Earth would have contributed at least half compared to the hydrogen burning phase.

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