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1. Origin and Evolution of Earth

Nebular Hypothesis : Earlier Theory of formation of earth from a cloud containing matter (nebula) eventually condensing to form solar system.
The theory had many drawbacks and Hubble proposed another theory. Big Bang Theory

‘Big Bang Theory’ aka ‘Expanding Universe’ theory by Edwin Hubble.

Early Stages
1. Small point of infinite density, infinite temperature, and unimaginably small volume was existing.
2. This ‘tiny ball’ exploded violently and quickly expanded. Universe is expanding ever since. It lead to distribution of energy in space. This took place around 13.7 billion yrs ago. After 3 minutes from Big Bang, first atom began to form. And energy started converting in matter
3. The distribution of matter and energy was not uniform throughout the universe. Around 300,000 yrs after explosion temperature dropped to 4500K and universe became transparent.

Star Formation
The distribution of matter and energy was not uniform throughout the universe. This gave rise to formation of galaxies.
Galaxies form large clouds of Hydrogen gas called as nebula. These nebula, eventually develop into localized clumps of gas. These clumps continued to grow denser, giving rise to formation of stars. It is believed to have taken place 5-6 billion yrs ago.

Formation of Planets
The stars formed had denser hydrogen gas at center surrounded by huge clouds of gases and dust. In process of rotation of these clouds around sun, the dust and gas particles started accumulating to give rise to small spherical bodies. These were hold together by cohesion and gravitational forces.
These bodies further accreted to form small bodies called planetesimalsThese planetesimals further accumulated in large numbers to give rise to planets.

Our Solar System


It consists of Sun, 8 planets, 63 moons, 5 known dwarf planets and many smaller bodies.
Terrestrial  planets/ inner planets – Planets Inside Asteroid Belt are called as inner planets.
Jovian Planets / Gas Giants Planets – thick atmosphere of Helium and Hydrogen.

Why Inner planets are solid and Outer planets are gaseous?
1) Due to solar winds, the lighter gases like helium and hydrogen were driven away from planets nearer to sun, where influence of winds was maximum.
2) These planets due to their small size had less gravitational force and therefore couldn’t hold gases.
3) Terrestrial planets were very close to sun, thus receiving more heat of sun, which made it impossible for gases to cool down and condense in such temperature.

1) Dumb Bell theory – It separated from Earth like a dumb bell.
2) Pacific Depression OR “Giant Impact” or “the big splat.” – A comet splat on earth, separating big chunk out of earth, creating Moon and Pacific Ocean.

Evolution of Earth.
4.6 billion yrs ago it formed.
It was barren, rocky before.
Why earth is made of layers?
Earth was volatile in initial time. As matter got accumulated its density grew. Increasing the temprature inside it.
It caused lighter material to rise and heavy material like iron to sink, thus giving layered structure to earth.

Evolution of Atmosphere
3 stages
1) Hydrogen and helium was surrounding earth, which was blown off by solar winds.
2) Then due to materials inside earth and continuous outpour of gases from earth’s interior. (Degassing)  the earth was dominated by water vapour (H2O), Co2, N2, Nh3, Ch4 and very little free oxygen (O2). (6 in total) Then it rained and rained for good amt of time and oceans formed.
3) Life developed in sea around 3.8 billion yrs ago, it modified atmosphere. 1st life was in ocean, oceans were saturated with oxygen, then it began surfacing 2000 million yrs ago.