Earth and Our Universe...solar systems. • A galaxy is held together by gravity. 3 Earth and Our Universe OBJECTIVE IAS Nebula • It is a Latin word which means mist or cloud. Nebulae

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Earth and Our Universe

General Geography

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Earth and Our Universe OBJECTIVE IAS

The Universe

• The Universe is all of space and time and their contents, including planets,

stars, galaxies and all other forms of matter and Energy.

• Evolution of Universe: There are three main theories put forward to

explain the origin and evolution of the Universe are: -

• The Big Bang Theory: In 1916, Einstein published his theory of general

relativity with this he also proposed a theoretical model of the Universe

which was not expanding.

• 1922 Russian cosmologist Alexander Friedman developed what is known

as the Friedman equations which were derived from Einstein equations

for general relativity: contrary to Einstein was advocating at the time with

his cosmological constant. Friedman work showed that the Universe was

likely in a state of expansion.

• 1924, Edwin Hubble's measurement of the great distance to the nearest

spiral nebulae showed that these systems were indeed other galaxies.

• 1927, Georges Lemaitre, a Belgian physicist and Roman Catholic Priest

independently derived the same result as Friedman Equations and

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proposed that the inferred recession Galaxies was due to the expansion

of the Universe.

• 1929, Hubble discovered a correlation between distance and decussation

velocity, which is known as the Hubble law.

• 1931, Georges Lemaitre was suggesting that the current expansion of the

Universe meant that the farther back in time, one went, the smaller the

universe would be.

• At some point in the past, he argued, the entire mass of the Universe

would have been concentrated into a single point from which the very

fabric of space and time originated. ● 1949, Fred Hoyle coined the phrase

"Big Bang" for Lemaitre Hypothesis.

• 1965: The discovery and confirmation of the cosmic microwave

background Radiation (M.B.R) practically supported the Big Bang theory.

• 1981, Physicist Alan bath theorized of a period of rapid cosmic expansion

that resolved other theoretical problems.

• 1990, the discovery of the rise of Dark energy resolves many issues of Big

Bang theory.

• Some other experiments also supported the Big Bang theory.

o Cosmic Background Explorer (COBE)

o Hubble Space Telescope

o Wilkinson Microwave Anisotropy Probe (WMAP)

o Planck observatory

o Large Hadron collider experiment (LHC)

• According to Big Bang theory at starting the whole mass of the Universe

concentrated in an extremely dense and hot fireball like structure 13.7

billion years ago, it exploded (Big Bang). The matter was broken into

pieces, which were thrown out with high speed in all directions forming

stars and galaxies which are still moving away from one another.

GALAXIES

• A galaxy is a huge collection of gas, dust and billions of stars and their

solar systems.

• A galaxy is held together by gravity.

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Nebula

• It is a Latin word which means mist or cloud. Nebulae are not only

massive clouds of dust, hydrogen and helium gas and plasma: they are

also often Steller nurseries – i.e., the place where stars are born.

• A Nebula is formed when a portion of the interstellar medium undergo

gravitational collapse. Mutual gravitational attraction causes matter to

clump together, forming region of greater density. Its ultraviolet ionizing

radiation causes the surrounding gas to become visible at optical

wavelength.

• Orion sward is nebulae of our galaxy. Nebula divided into four categories:

(a) Diffuse Nebulae (b) Dark Nebulae (c) supernova Remnant Nebulae (d)

Planetary Nebulae

Quasars

Quasi-stellar radio sources or Quasars was discovered in 1960 by Allan

standage. Quasars are among the brightest and most distant known celestial

objects and crucial to understanding the early Universe. Quasars live only in

galaxies with supermassive Black holes.

Black Holes

• These are infinitely dense points in space with deep gravity sinks so that

not even light can escape the powerful tug of its gravity. Therefore, it is

also known as the cosmic vacuum cleaner.

• Anything that ventures too close will be stretched and compressed like

putty in a theoretical process aptly known as spaghettification.

Constellation

• A constellation is a group of stars that forms definite imaginary outline

or pattern on the celestial sphere.

• 89 constellation known in our galaxy.

• Hydra is the largest constellation according to the area and smallest

constellation according to number of stars (68 stars)

• The crux is the smallest constellation according to the area.

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• Centaurus is the largest constellation, according to the number of stars.

(92 stars)

• Most popular constellation is Ursa major which lies in the northern sky.

In Latin, it is also known as the Great Bear or the larger Bear. In Indian

mythology, it is known as Saptrishi.

Pole star

In the northern hemisphere, it is also known as the north star. It is the brightest

star that appears nearest to either celestial at a particular time.

• At present, the pole star is Polaris (α Ursa minoris) ● 2700 BCE Theban (α

Draconis) is our pole star.

• 1400 CE Vega become our pole star.

• The present southern pole star is Polaris Australis (α octant is).

Stars

Stars are big exploding balls of gas, mainly Hydrogen and helium; which

secretes a huge amount of energy.

• Sun is the nearest Star of Earth.

• Proxima Centauri is the nearest Star of Sun

• Star colour is linked to temperature

Life cycles of stars

A star’s life cycle is determined by its mass. The larger it mass, the shorter its

life cycle.

• Molecular Star: Stars start out as vast clouds of cold molecular gas. The

gas cloud could be floating near a galaxy for millions of years, but then

some event causes it to begin collapsing under its own gravity. For

example, when galaxies collide, regions of cold gas are given the kick

they need to start collapsing. As it collapses, the interstellar clouds break

up into smaller and smaller pieces, and each one of these collapses

inward on itself. Each of these pieces will become a star.

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• Proto star: As the Steller material pulls tighter and tighter together, it

heats up pushing again further gravitational collapse. At this point, the

object is known as Proto star.

• T Tauri star: At Tauri star begins when material stop falling into the

• protester and it is releasing a tremendous amount of energy. The T Tauri

phase lost for about 100 million years.

• Main sequence: It starts when the core temperature of a star will reach

the point that fusion its core can begin (Exothermic reaction).

• Red Giant: When a star exhausts its fuel of Hydrogen at its core, its

internal nuclear reactions stop. Now star begins to contract inward

through gravity. This process heats up a shell of Hydrogen around the

core which then ignites in fusion and causes the star to brighten up again.

This causes the outer layers of the star to expand outwards, increasing

the size of the star many times. The temperature and pressure at the core

of the star will eventually reach the point that helium can be fused into

carbon and known as red Giant/ dwarf.

• White dwarf: The Star will eject its outer layers into space and then

contract down, eventually becoming a white dwarf.

• Supernova: It is a giant dying Star of high mass that has come to the end

of its life by a spectacular explosion.

• Neutron star: It is the incredibly compact core that remains after a

supernova explosion.

Solar system

• The solar system contains the Sun, eight planets with their satellites,

dwarf planets, asteroids, comets and other near-earth objects.

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The Sun

• Our Sun is a normal

main-sequence G-2

yellow dwarf star, a

hot ball of glowing

gases, one of more

than 100 billion

stars in our galaxy.

• Its gravity holds the

solar system together, keeping everything from the biggest planets to

the smallest particles of debris in its orbit.

• Diameter – 13,90,000 km

• Mass – 1.989e30 kg

• Temperature – 5800 k (surface) 15,600,000 k (core)

• The Sun is personified in many mythologies; the Greeks called it Helios,

and the Romans called it sol.

• Sun contains more than 99.8% of the total mass in the solar system.

• The Sun is primarily made up of hydrogen (H) (70%) helium (He) (28%)

together with a small amount of carbon, oxygen, iron, neon and other

elements.

• The outer layers of the Sun exhibit differential rotation → At the equator

the surface rotates once every 25.4 days – and near the pole, it is as much

as 36 days. This odd behaviour is due to its gaseous structure.

• The core of the Sun covers approximately the inner 25% of its radius. At

the centre of the core the sun density is more than 150 times that of the

water.

• The Sun's power is produced by the nuclear fusion reaction. Each second

more than 70 million tons hydrogen undergo this process and produce

energy in the form of Gamma Radiations.

• The surface of the Sun called photosphere (5800 k).

• Sunspots are cool regions (3800 k)

• A small region known as the chromosphere lies above the photosphere

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• The highly rarefied region above the chromosphere called the corona,

visible during a total solar eclipse.

• The outermost part of the Sun is known as the coronal sphere.

• The Sun's magnetosphere/Heliosphere (magnetic field area) extends well

beyond Pluto.

• Sun also emits low density streams of charged particles like electron,

protons etc known as solar wind which propagates throughout the solar

system. During the solar minimum of the solar cycle the solar wind

emanating from the polar regions flows at nearly double the rate. During

the solar maximum the solar wind moves at an intermediate speed. ● The

age of the Sun is 4.6 billion years.

Planets

• According to the International Astronomical Union Summit 2006; a

planet is a celestial body that–

o Is in orbit around any star

o Having area at least 3000 km2

o Got their energy and light from that star.

o Has sufficient mass for itself gravity to overcome rigid body forces

so that it assumes a hydrostatic equilibrium shape (Nearly round)

o Has cleared the neighbourhood around its orbit

• In our solar system, the total number of planets is 8; which is divided into

two categories: –

• Terrestrial or Inner Planet – They found inside asteroids belts. They are

4 in number Mercury, Venus, Earth, Mars.

• Gas Giant or Outer Planet – They are 4 in number Jupiter, Saturn, Uranus,

Neptune.

• Planets which orbit other stars (not the Sun) are called exoplanets.

1. Mercury

• The closest planet to the Sun and a very hot planet.

• Smallest Planet in the solar system with a diameter of 4900 Km.

• Fastest Planet with a speed of 172500 Km per hour to complete

revolution around the Sun in 88 days.

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• The Planet with no water and gases like Nitrogen, Hydrogen, Oxygen and

Carbon Dioxide.

2. Venus

• Hottest Planet in the solar system with the surface temperature of 478

degree Celsius.

• Also known as "Earth's Twin". It is because of similarity in size and mass

between Venus and Earth.

• One of the two planets in the solar system which rotate around the axis

in a clockwise direction.

• The brightest star in the Solar system. It can be seen in the morning and

evening with open eyes. So known as "Evening Star" and "Morning Star".

3. Earth

• The only Planet to give support to life with a pleasant atmosphere.

• Also known as "Blue Planet" because of the presence of water on it.

• It has one natural satellite named "Moon".

4. Mars

• Known as "Red Planet" because of Iron-rich red soil.

• Second smallest Planet in the solar system after Mercury.

• Has two natural moons "Phobos" and "Deimos".

• Has a thin atmosphere and surface with valleys, craters, deserts and ice

caps etc.

• "Olympus Mons" – Largest volcano and the tallest mountain in the solar

system lies on Mars.

5. Jupiter

• Largest Planet of the solar system with the shortest rotation

• Has atmosphere filled with Hydrogen, Helium and other gases

• The third brightest object in the night sky after the Moon and Venus.

• Great Red Spot, a giant storm in the solar system exists on this Planet.

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• Has at least 69 moons, including four large Galilean Moons "Io, Europa,

Ganymede, and Callisto" which were discovered by Galileo. "Ganymede"

is the largest among them.

• It has an unclear ring around it.

6. Saturn

• The second-largest planet in the solar system and a gas giant.

• Has bright and concentric rings around it which are made up of tiny rocks

and pieces of Ice.

• Saturn can float on water because it has less density than water.

• Has at least 62 moons and Titan is the largest among them.

7. Uranus

• Has the third-largest planetary radius and fourth-largest planetary mass

in the Solar system.

• Greenish in colour.

• Discovered by William Herschel in 1781.

• Known as "Ice Giant". The atmosphere of Uranus is composed of

Hydrogen and Helium primarily, but it also contains more water,

ammonia etc.

• Has coldest planetary atmosphere in the solar system.

• Rotates clockwise on its axis like Venus but unlike other planets

• Has at least 25 moons. Famous moons- Miranda, Ariel and Umbriel

8. Neptune

• The farthest planet from the Sun.

• It is also "Ice Giant". Atmosphere primarily composed of Hydrogen and

Helium.

• Bluish in colour because of Methane.

• The fourth-largest Planet and the third most- massive planet in the solar

system

• Discovered by Johann Galle and Urbain Le Verrier in 1846. The only planet

in the solar system found by Mathematical Predictions.

• Has known 14 satellites. Famous moon – Triton

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Other

Pluto

• As per the new definition of Planets determined by the International

Astronomical Union (IAU), Pluto had been omitted from the list of planets

in 2006.

• Pluto is considered as a dwarf planet (size between planets and asteroids)

now, and it is a member of the Kuiper Belt.

Kuiper Belt

• It is a spherical boundary outside the orbit of Neptune containing a

number of asteroids, rocks, and comets.

Goldilocks Zone

• It refers to the habitable zone around a star where the temperature is just

right - not too hot and not too cold - for liquid water to exist on a planet.

Other Space Objects

1. Asteroids

• These are small objects; rocks (mostly debris) revolve around the Sun.

• They are mostly found in the Asteroid Belt which lies between the orbits

of Mars and Jupiter.

• These are also known as Minor planets.

• Ceres, Vesta, Psyche are some famous and largest asteroids in the solar

system.

2. Meteors and Meteorites

• These are also known as Shooting stars.

• Meteors are the small-sized rocky material which is generally formed due

to asteroid collision and approaching the Earth.

• Because of Earth's atmospheric layers, these small rocks burn before

reaching the surface. ● But there are some meteors which do not burn

completely and land on Earth's surface. They are called as Meteorites.

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• Willamette, Mbozi, Cape York, and El Chaco are some meteorites found

on the Earth.

• Lonar lake, Maharashtra in India is supposed to be created by a meteor

impact in Pleistocene Epoch.

3. Comets

These are shiny, luminous "Tailed Stars". These are rocky and metallic materials

surrounded by frozen gases.

• These are generally found in the Kuiper Belt. They travel towards the Sun.

• Their tail faces opposite of the Sun and head faces towards the Sun.

• They become visible when they travel close to the Sun.

• Halley's Comet is famous which appeared last time in 1986 and which

reappears after every 76 years.

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ORIGIN OF THE EARTH

A number of theories have been propounded by various scientists and

philosophers across the world regarding the origin of the Earth.

The earlier theories given by scientists are as follows:

A. Monoist Theories:

Immanuel Kant’s Gaseous Hypothesis:

• According to Kant, there existed

a primordial matter scattered in

the universe. It contained a

slowly rotating cloud of gas

(nebula) in which particles

collided with each other due to

the gravitational force. Due to

collision heat was generated,

and thus, the temperature of the

primordial matter started rising.

• The increase in the temperature

changed the state of primordial

matter from solid to gaseous

particles. With the continuous

increase in temperature and the

rising rate of rotatory motion,

the nebula started expanding in

size.

• The size of this gaseous cloud increased continuously, which in turn

increased the rotation speed of the particles due to which centrifugal

force became greater than the centripetal force. This resulted in the

formation of concentric rings, and a residual central mass was created,

which was considered as the sun.

• The irregularity of the rings caused the development of the cores for the

formation of the corresponding planets.

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Nebular Hypothesis of Laplace:

• Laplace’s theory is the

modification to Kant’s theory.

• Laplace assumed that there was

a huge and hot gaseous Nebula

in the space which was rotating

on its axis from the very

beginning.

• The continuous cooling of the

nebula was due to loss of heat

from its outer surface through

the radiation. It was

continuously reduced in size

due to the contraction of

cooling.

• To sum up, this hypothesis

considered that the planets

were formed out of the rapidly

spinning nebula. The nebula condensed, cooled and solidified into

celestial bodies such as stars, planets etc.

B. Binary Theories:

Planetesimal theory of Chamberlain and Moulton:

• Chamberlain & Moulton

considered that there

was a proto-sun which

was intruded upon by a

wandering star

(companion star).

• The companion star

passed very close to the

proto-sun which was cold, solid and rocky and infinite number of small

particles (planetesimals) were detached from the outer surface of the

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proto-sun due to massive gravitational pull exerted by the giant intruding

star.

• Numerous small planetesimals accreted together and grew into the

planets, and with the passage of time, the remaining proto-sun changed

into the present-day sun.

Tidal Hypothesis of Jean & Jeffery:

• James Jeans and Harold

Jeffrey supported the theory

of Chamberlain and Moulton.

• This hypothesis considers the

proto-sun to be a hot burning

mass of gas, unlike

Chamberlain’s theory which

considered the proto-sun to be cold, solid and rocky.

• Due to massive gravitational force of the intruding star, a huge amount

of matter was ejected from the primitive sun, which later on became the

building material of future planets.

• The tidal force of the intruding star had a great impact on the surface of

the protosun.

Revised Nebular Hypothesis of Otto Schmidt and Carl Weizascar:

• They considered that the sun was surrounded by solar nebula containing

mostly the hydrogen and helium along with what may be termed as dust.

• Due to the friction and collision of particles, accretion took place, and a

disk-shaped cloud and the planets were formed.

C. Trihybrid Theory:

The Binary Star Hypothesis of Russel:

• According to Russell, there were two

stars near the primitive sun in the

universe (binary star system).

• A giant star named approaching star

came near the companion star, and a

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large amount of matter of the companion star was attracted towards the

giant approaching star because of its massive tidal force.

• As the approaching star came nearer to the companion star, tidal force

and the gravitational force continued to increase.

• When the giant approaching star came nearest to the companion star, a

large amount of matter was ejected from the companion star due to

maximum gravitational force exerted by the giant approaching star.

• From the ejected matter, planets were formed.

THE BIG BANG THEORY:

• Initially, all matter constituting the universe existed in the form of a

singular atom. This singular atom (tiny ball) had an unimaginably small

volume, infinite temperature and infinite density.

• At the event of the Big Bang, nearly 13.7 billion years ago, the tiny ball

(singular atom) exploded violently, which resulted in a huge expansion.

The expansion occurred rapidly within fractions of a second after the

bang after which it had slowed down. The expansion continues even to

the present day.

• As a result of expansion, some energy was converted into matter. The

first atom began to form within the first three minutes from the Big Bang

event. After the first three minutes, the protons and neutrons had

assembled into hydrogen and helium nuclei. The abundance of helium is

a key prediction of big bang theory, and it's been confirmed by scientific

observations.

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• Within 300,000 years from the Big Bang, the temperature came down to

4,500 K, and the atomic matter was formed. With the passage of time,

the matter cooled and more diverse kinds of particles began to form,

which eventually condensed into the stars and galaxies of our present

universe. The universe became transparent.

FORMATION OF THE STAR:

• The formation of

stars is believed to

have taken place

nearly 5-6 billion

years ago.

• The matter and

energy were

distributed unevenly

in the early universe. Initially, the differences in density gave rise to

differences in gravitational forces. As a result, the matter got drawn

together. This led to the formation of galaxies.

• A galaxy contains a large number of stars. The formation of a galaxy starts

with the accumulation of hydrogen gas in the form of a very large cloud

called nebula.

• The growing nebula developed

localised clumps of gas. The clumps

continue to grow into further denser

gaseous bodies, eventually giving

rise to the formation of stars.

FORMATION OF THE PLANETS:

• The planets were formed nearly 4.6

billion years before the present.

• The localized lumps of gas within a

nebula started forming a core to the

gas cloud due to the gravitational

force.

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• As a result of gravitational force and formation of core, a huge rotating

disc of gas and dust developed around the gas core.

• The condensation started within the gas cloud, and the matter around

the core developed into small rounded objects. The process of cohesion

developed between these small rounded objects which developed them

into planetesimals. Planetesimals are a large number of smaller bodies.

• As a result of the collision, larger bodies started forming and gravitational

attraction resulted in the materials sticking together.

• Finally, the large number of small planetesimals accreted together due to

cohesion to form fewer large bodies in the form of planets.

FORMATION OF THE MOON:

• The moon is the only

natural satellite of the

Earth which was

formed nearly 4.4

billion years before

the present. The

scientists have made a number of attempts to explain the formation of

the moon.

• Sir George Darwin, in 1838 suggested that initially, the Earth and the

moon formed a single rapidly rotating body. The whole rotating mass

became a dumb-bell shaped body and broke eventually. Sir Darwin also

suggested that the material forming the moon was separated from what

we have at present the depression occupied by the Pacific Ocean.

• However, these explanations were not accepted by the present scientists.

• Presently, it is believed that the formation of the moon, as a satellite of

the Earth, is a result of ‘giant impact’ or the phenomenon described

as ‘the big splat’.

• A body of the size of one to three times that of mars collided into the

Earth shortly after the formation of the Earth. It separated a large part of

the Earth into space which then continued to orbit the Earth and

eventually formed into the present moon about 4.44 billion years ago.

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SOLAR SYSTEM:

• Our solar system consists of the sun, eight planets, a number of moons,

millions of smaller bodies such as comets and asteroids and a huge

quantity of gases and dust particles.

• The nebula from which our Solar system is believed to have been formed

started its core formation nearly 5-5.6 billion years ago, and the planets

were formed about 4.6 billion years ago.

• Terrestrial planets: Mercury, Venus, Earth and Mars, are called the inner

planets or the terrestrial planets. Terrestrial means Earth-like. They lie

between the sun and the belt of asteroids. They are called Terrestrial

planets because they are made up of rock and metals, and have relatively

high densities.

• Jovian planets: Jupiter, Saturn, Uranus and Neptune are called the outer

planets or Jovian or Gas Giant planets. Jovian means Jupiter-like. These

planets are much larger than terrestrial planets and have a thick

atmosphere made mostly of helium and hydrogen.

• The difference in the formation and composition of terrestrial and jovian

planets are attributed to the few reasons. The terrestrial planets were

formed in the close vicinity of the parent star. It was too warm there for

the gases to condense into solid particles. Jovian planets, on the other

hand, were formed at quite a distant location. The intense solar wind blew

off lots of gas and dust from the terrestrial planets. The intensity of solar

wind was not that high to cause similar changes in Jovian planets. Also,

the smaller size and lower gravity of the terrestrial planets did not allow

them to hold the escaping gases.

• All the planets were formed in nearly the same period around 4.6 billion

years ago.

EVOLUTION OF THE EARTH:

• Initially, the Earth was a rocky, barren and hot object having a thin

atmosphere of helium and hydrogen. This was very different from the

present-day Earth.

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• There have been some events and processes, which caused this change

from rocky, barren and hot Earth to the present day earth being a

beautiful planet with ample amount of water and conducive atmosphere

favouring the existence of life.

• The Earth has a layered structure and matter is distributed on the basis

of density. The atmospheric matter has the least density.

• The Earth’s interior has been divided into different zones, and each of

these contains materials with different characteristics.

Development of Lithosphere:

• During its primordial stage, the Earth was mostly in a volatile state. The

gradual increase in density resulted in an increase of the inside

temperature.

• Due to this differentiation, the material inside started getting separated

depending on their densities. The heavier materials (like iron) sank

towards the centre of the Earth and the lighter ones to moved towards

the surface.

• Further, cooling started with the passage of time, and it solidified and

condensed into a smaller size. This led to the development of the outer

surface in the form of a crust.

• During the formation of the moon (the giant impact), the Earth was

further heated up. The process of differentiation led the materials getting

separated into different layers in terms of density.

• Differentiation of different layers resulted in the formation of the crust,

mantle, outer core and inner core with density increasing from the crust

to the core.

Evolution of Atmosphere and Hydrosphere:

• Nitrogen and oxygen dominate the present composition of Earth’s

atmosphere. However, the early atmosphere was nowhere near the

present-day composition of the atmosphere.

• The present-day atmosphere is the result of three stages in the evolution

of the atmosphere. In the first stage, the loss of the primordial

atmosphere took place. The hot interior of the Earth contributed to the

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evolution of the atmosphere in the second stage. In the final stage, the

composition of the atmosphere was modified by the living world through

the process of photosynthesis.

• The early atmosphere contained a large amount of hydrogen and helium

which was stripped off as a result of the solar winds. This happened in

case of all the terrestrial planets, including the Earth. Due to the intense

solar winds, the terrestrial planets were supposed to have lost their

primordial atmosphere.

• When the cooling of the Earth started, the gases and the water vapour

were released from the interior of the Earth. The process of the

outpouring of gases from the interior is known as ‘degassing’. This

started the evolution of the present atmosphere.

• The early atmosphere largely contained nitrogen, carbon dioxide, water

vapour, methane, ammonia and very little amount of free oxygen.

• Water vapour and gases were getting added to the atmosphere due to

the continuous volcanic eruptions. As the Earth cooled, the water vapour

released started getting condensed, and the carbon dioxide in the

atmosphere got dissolved in rainwater. The temperature further

decreased, causing more condensation and more rains.

• The rainwater falling onto the surface got collected in the depressions

and gave rise to oceans. The Earth’s oceans are supposed to have been

formed within 500 million years from the formation of the Earth. Thus,

the oceans are nearly 4,000 million years old, and life was confined to the

oceans for a long time.

• The evolution of life began nearly 3,800 million years ago. However, the

process of photosynthesis evolved nearly around 2,500-3,000 million

years before the present.

• The process of photosynthesis led the oceans to have the contribution of

oxygen. When the oceans were saturated with oxygen, the oxygen began

to flood the atmosphere nearly 2,000 million years ago.

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Origin of Life:

• The last phase in the evolution of the Earth is related to the origin and

evolution of life. The evolution of life began nearly 3,800 million years

ago.

• Since the initial atmosphere of the Earth was not conducive for the

development of life, the origin of life must have been started through the

complex chemical processes.

• According to modern scientists, the origin of life started as a kind of

chemical reaction, which first generated complex organic molecules and

assembled them.

• The assemblage of the organic molecules was such that they could

duplicate themselves converting inanimate matter into living substance.

Fossils found in rocks are the source of information about the record of life that

existed on this planet in different periods. The microscopic structures closely

related to the present form of blue algae have been found in geological

formations that are much older than these were some 3,000 million years ago.

Thus, it can be assumed that life began to evolve nearly 3,800 million years ago.

Interior of the Earth

The interior of Earth can be observed through direct evidence such as rock

samples from mining, deep ocean drilling project, volcanic eruptions and

indirect evidence such as seismic waves, meteorite investigation, gravitation

force, magnetic field etc.

Structure of the Earth

Crust

• The crust is the outermost brittle solid part of Earth ranging from 5 – 70

kms.

• The crust is of two types:

o Continental Crust: Mean thickness is around 30 km, made of SIAL

(Silica and Aluminium) and is thicker than oceanic crust. Its density

is around at 2.7 g/cm3

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o Oceanic Crust: Mean thickness is about 5 km made of SIMA (Silica

and Magnesium).

• Oceanic crust is basaltic in origin and relatively of a younger age than the

continental crust. The basaltic crust is denser at 3.0 g/cm3

Mantle

• The mantle extends up to 2890

km.

• Asthenosphere: The upper portion

of the mantle which extends up to

around 400 km. It is the primary

source of magma.

• The density of mantle is 3.4 g/cm3

• The lower mantle is in the solid

state which extends up to the

Core-Mantle boundary. This layer

is called as the D″ (pronounced

deedouble-prime) layer.

Note: The Crust and Upper part of mantle combined are called as Lithosphere.

Core

• The core extends to 2870 – 6370 km.

• It is divided into

o Liquid Outer Core

o Solid Inner Core:

Made of NIFE –

Nickel and

Ferrous.

• Note: Inner core rotates

slightly faster than the

rest of the planet.

• The density at the outer

core is at 5.5 g/cm3, which increases to 13 g/cm3 in the inner core.

23


Dynamo theory: It suggests that convection in the outer core, combined with

the Coriolis effect, gives rise to Earth's magnetic field.

Schematic sections through the Earth:

• Continental crust

• Oceanic crust

• Upper mantle

• Lower mantle

• Outer core

• Inner core

Boundaries in the Earth's interior

• Conrad Discontinuity: Between Upper and Lower Continental Crust.

• Mohorovičić discontinuity, "Moho": Crust-Mantle boundary

• Gutenberg discontinuity: Core-Mantle boundary

• Lehmann discontinuity: Boundary between Outer and Inner Core

Important Facts

• Earth's radius: 6370 km

• Earth's diameter: About 12756 km at equator & about 12715 km at the

poles

• Crust: 0.5 % of the volume of the Earth Mantle: 83 % of the volume of the

Earth

• Core: 16 % of the volume of the Earth

• Temperature, Pressure and Density increases with the increasing distance

from the surface to the interior in deeper depths

• Gravitation force is higher near the poles and lesser near the equator

• Gravity anomaly is the difference in gravity value according to the mass

of the material

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