The document discusses various theories regarding the origin of Earth, including the nebular hypothesis, planetesimal hypothesis, and modern theories like the Big Bang theory that explain the formation of planets from a protoplanetary nebula. It describes Earth’s early formation, initial conditions, and the significant geological and atmospheric changes that occurred during its development. The document also highlights the impact of the Moon on Earth's rotation and climate, as well as the emergence of life approximately 3.8 billion years ago.

1. Origin of Earth
By- S M Zaki Ahmad

2. Earth is part of the solar system. Although it is the most
studied planet, it cannot be completely understood in isolation.
The chemistry of meteorites and Sun provide constraints on the
composition of the bulk of the Earth.
Early Theories
Nebular Hypothesis (Kant and Laplace)
A huge mass of swirling cold gas and dust (Nebula) in an area in
the Milky way Galaxy, this cloud of gas and dust began to condense
or pull together under the force of its own gravity.
As a result nebula conserved angular momentum of the material
drawn the centre, it spanning anticlockwise, due to this the material
around the centre of the condensing nebula flattened out into disc
like shape. Cont…

3. The centre of the nebula continued to contract due to gravity.
Eventually , pressure and temperatures in this mass became high
enough that nuclear fusion started and central mass become a star, the
Sun.
Cont…

4. Planetesimal Hypothesis (Chamberlin-Moulton 1905)
This is the theory of biparental origin of solar system.
According to this hypothesis the origin of the planets has been
due to severe tidal eruptions and disruption of the sun’s mass.
According to this
total mass of the
planets is only a small
friction (1/700) of the
mass of the whole
solar system, but they
carry nearly 98% of
its energy of
revolution. Cont…

5. Russell's binary star hypothesis
In the beginning the planets were closer together and the satellites
owe their birth to the mutual gravitational attraction between them.
The third star pass close to the companion star of the sun which
resulted in the ejection of gaseous matter from the latter in the form of
filament which ultimately separated from it.

6. Modern Theories
Big Bang theory
Hubble’s findings about the expansion of the universe have a very
interesting implication.
If the motion of the Galaxies is traced back in the time, it implies
that they were once all in the same place.
They were greatly compressed and therefore very dense and hot.
This scenario – of a universe that “exploded” out of extremely tiny,
dense and hot initial state – became known as “Big Bang theory”
In the 1920s Georges Lemaitre and aleksandr Friedmann
proposed early version of this theory.
In the 1940s George Gamow and other cosmologist modified it.
Cont...

7. Modern Theories
Cluster of Galaxies
Galaxy
Solar System

8. Interestingly the term “Big Bang” was originally intended as a
derisive one; it was coined in the 1940s by Fred Hoyle, who
championed a competing model known as the “Steady state theory”
Steady State theory- According to this theory “ the universe is
expanding, but its general appearance and composition remain the
constant through the time, as new matter is gradually created to fill in the
gaps left by matter that has spread out.”
---1950s significant support.
Discoveries in the 1960s, weighed heavily against the steady state
theory.
In 1965 – evidence support “ Big Bang theory”
Nearly uniform glow of microwaves is indeed coming from every
direction in the sky.
In 1989 Cosmic Background Explorer (COBE)
Cont..

9. Exactly how it all comes about is still up for debate and there
actually be many different planet formation processes.
2047 Extrasolar planets have been discovered till 12th January 2016 in
our Galaxy.
The common thread of all cosmogonic theories is that the planets
formed from dispersed material, that is from a protoplanetary
nebula.
The modern and widely accepted theory is the “Solar Nebular Disk
Model” (SNDM) – proposed by Vicktor Safronov in his book
“Evolution of the protoplanetary cloud and formation of the Earth
and the planets” – which was translated in 1972.
According to this theory sun initially possessed a uniform gas dust
nebula. The nebula evolves in to a torus and then in to a disk.
Cont…

10. A terrestrial world is born
Cont…

11. Metal dust
Like other planets, Earth was initially formed as Protoplanet by and
accumulation of dust and large sized material. Due to further collisions
with debris Earth mass increased and the released gravitational energy
increasingly heated the Earth – initially Earth was liquid.
Planetesimal Protoplanet Planet and Satellite
The denser siderophiles (iron-loving elements) were enriched in the
Earth core, whereas the lighter lithophilic ( rock- loving) elements
were pushed to the planet outer areas.
As a result , the Earth comprises an iron – rich core and a silicate-
rich mantle and crust.
The Hadean was the first stage of Earth’s development, covering a
period from 4.7 to 4 billion years ago. In this phase Earth was
subjected to frequent collisions with meteorites.
Cont…

12. The moon was formed approximately 4.5 billion years ago, probably due
to a collision of the Earth with a very large body. Since then, the moon’s
mass slow down the Earth’s speed of rotation.
A day on Earth in early “Precambrian” lasted only around 8 hours.
The moon create the preconditions for a constant seasonal climate and
the occurrence of stable climate zone on Earth.
The primordial atmosphere consisted mainly water vapour (H2O- 80%),
Co2 - 20% and some amount of H2S, NH3, CH4.
Prior to around 4.2 billion years ago surface temperature of Earth sank
below 100ºC. The earth crust subsequently solidified and liquid water first
appeared.
The Earth’s oceans were formed within 500million years from the
formation of the Earth.
The chemical evolution started in the Archean and life appeared around
3.8 billion years ago.
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