The Origin And Evolution Of The Earth

The origin of the Earth is connected with the origin of the universe, the formation of galaxies, the birth of the Solar System and the gradual evolution of Earth’s lithosphere, atmosphere, hydrosphere and life.

A large number of hypotheses were given by philosophers and scientists to explain how the Earth and other planets came into existence. Some of these were early hypotheses, while the modern explanation of the universe is mainly linked with the Big Bang Theory.

The Universe And Early Hypotheses

Before the Big Bang Theory became widely accepted, different scientists proposed different explanations for the formation of the Solar System.

Gaseous Hypothesis Of Immanuel Kant, 1755

Immanuel Kant assumed that hard matter was scattered in the universe.

According to this hypothesis:

• Particles of matter collided with each other.
• These collisions formed a cloud of gases.
• The cloud started rotating.
• It took the shape of a disc.
• Nine rings separated from this disc and later became planets.
• The remaining central mass became the Sun.

This theory tried to explain the formation of planets from rotating gaseous matter.

Nebular Hypothesis Of Laplace, 1796

Mathematician Laplace revised Kant’s gaseous hypothesis and called it the Nebular Hypothesis.

According to Laplace:

• Planets were formed from a cloud of material.
• This cloud was associated with a youthful Sun.
• The youthful Sun was slowly rotating.
• Matter separated from this rotating cloud and formed planets.

This hypothesis is important because it introduced the idea of planet formation from a rotating nebular cloud.

Planetesimal Hypothesis Of Chamberlin And Moulton, 1905

The Planetesimal Hypothesis was proposed by Chamberlin and Moulton.

According to this theory, two heavenly bodies were present:

• Proto-Sun
• Companion star

The companion star passed close to the Proto-Sun. Due to this close passage, particles detached from the outer surface of the Proto-Sun.

These particles were called planetesimals.

With time, these planetesimals combined and formed planets.

Tidal Hypothesis Of James Jeans And Harold Jeffreys

The Tidal Hypothesis was proposed by James Jeans in 1919 and later modified by Harold Jeffreys in 1929.

According to James Jeans:

• The Solar System was formed from the Sun and an intruding star.
• The intruding star was much bigger than the Proto-Sun.
• When it came near the Proto-Sun, a huge amount of material was pulled out.
• This material later became the building material of planets.

A cigar-shaped tidal filament thousands of kilometres long was created on the outer surface of the primitive Sun.

When the intruding star was far away, only a small amount of matter was ejected.

When it came nearest to the Proto-Sun, the maximum amount of matter was ejected.

This was used to explain why bigger planets formed in the middle part of the Solar System.

Harold Jeffreys modified this theory by adding a collision idea. According to him, three bodies were involved:

• Proto-Sun
• Companion star
• Intruding star

The companion star was shattered because of the intruding star, and the shattered material started revolving around the Proto-Sun.

Binary Star Hypothesis Of Russell, 1937

Russell assumed that instead of one star, two stars were present:

• The Sun
• A companion star

The companion star was orbiting around the Sun.

A third star, called the approaching star, came near the companion star. Due to the tidal force of the approaching star, a bulge of material was ejected from the companion star.

This ejected material later became planets.

Supernova Hypothesis Of Hoyle, 1946

Hoyle proposed the Supernova Hypothesis in his essay Nature of the Universe.

According to this hypothesis:

• The Proto-Sun and a companion star were present close to each other.
• The companion star was giant.
• It became a supernova due to internal nuclear processes.
• The violent explosion spread an enormous mass of dust.
• This dust started revolving around the primitive Sun.
• The gaseous matter changed into a circular moving disc.
• Planets were formed from this disc.

Interstellar Dust Hypothesis Of Otto Schmidt

According to Otto Schmidt, the Sun captured interstellar matter during its galactic revolution around the centre of the Milky Way.

This matter was called interstellar dust.

According to this hypothesis:

• Interstellar dust started revolving around the Sun.
• Dust particles combined.
• They condensed into a flat disc.
• This flat disc continued to revolve around the Sun.
• Planets were formed from this material.

Big Bang Theory

The Big Bang Theory explains the origin and expansion of the universe.

According to this theory, about 13.8 billion years ago, all matter and energy of the universe existed in an extremely hot and dense state.

From this state, the universe began expanding and has continued to expand ever since.

Main Features Of Big Bang Theory

• The universe began from an extremely hot and dense state.
• The universe has been expanding since then.
• As the universe expanded, it cooled.
• Particles combined to form atoms.
• Later, atoms formed stars, galaxies and other celestial bodies.

Scientists Related To Big Bang Theory

The Big Bang Theory was proposed by Georges Lemaître in 1927.

Edwin Hubble supported this idea by observing that galaxies are moving away from us.

This observation showed that the universe is expanding.

Redshift

Redshift is the phenomenon in which light from distant galaxies shifts toward longer wavelengths, or the red end of the electromagnetic spectrum.

It indicates that galaxies are moving away from us.

Redshift supports the idea that the universe is expanding.

Evidence For Big Bang Theory

The main evidence for the Big Bang Theory includes:

• Redshift
• Cosmic Microwave Background Radiation
• Abundance of light elements

Cosmic Microwave Background Radiation

Cosmic Microwave Background Radiation, or CMBR, is one of the strongest proofs of the Big Bang Theory.

When electrons and protons combined to form neutral hydrogen atoms, radiation was released and the universe became transparent.

This radiation is observed today as relic microwave radiation.

CMBR was discovered in 1964.

Arno Penzias and Robert Wilson received the Nobel Prize in 1978 for its discovery.

The CMB is also called relic radiation.

Big Crunch

The Big Crunch is a hypothetical end of the universe.

According to this idea:

• The universe may stop expanding.
• Gravity may overcome expansion.
• The universe may begin to contract.
• It may become denser and hotter.
• It may collapse into a very hot and dense state.

It is discussed as an alternative to the Heat Death scenario.

However, present evidence suggests that the expansion of the universe is accelerating, making the Big Crunch much less likely.

Key Aspects Of Big Crunch

Reversal Of Expansion

The Big Bang describes an expanding universe, while the Big Crunch suggests that this expansion could stop and reverse due to gravity.

Density And Temperature Increase

As the universe contracts, it would become denser and hotter.

Hypothetical End State

The universe could collapse into an extremely dense state, possibly similar to a singularity.

Alternative To Heat Death

The Big Crunch is an alternative to the idea that the universe will keep expanding and cooling forever.

Factors Influencing Big Crunch

Dark Energy

Dark energy is believed to accelerate the expansion of the universe.

Its nature is important in deciding whether the universe will expand forever or not.

Density Of Matter

If the total matter density of the universe were high enough, gravity could potentially cause collapse.

Blue Shift

Blue shift is the phenomenon in which light from an object shifts toward shorter wavelengths, or the blue end of the electromagnetic spectrum.

It shows that the object is moving toward the observer.

It is the opposite of redshift and is related to the Doppler effect.

Examples Of Blue Shift

• Andromeda Galaxy
• Barnard’s Star
• Binary star systems
• Blazars

The Andromeda Galaxy is moving toward the Milky Way and its light exhibits blue shift.

Gravitational Waves

Gravitational waves are ripples in spacetime.

Albert Einstein predicted gravitational waves in 1916 in his General Theory of Relativity.

They are caused by violent and energetic processes in the universe.

Examples include:

• Neutron stars orbiting each other
• Black holes orbiting each other
• Colliding black holes

These waves travel outward at the speed of light.

In 2015, the Laser Interferometer Gravitational-Wave Observatory, or LIGO, directly detected gravitational waves from two colliding black holes about 1.3 billion light-years away.

Black Holes

A black hole is a region in space where gravity is so strong that even light cannot escape.

The term Black Hole was popularized by John Archibald Wheeler in 1967.

Einstein’s General Theory of Relativity predicted the possibility of black holes.

The first image of a black hole was released in April 2019 by the Event Horizon Telescope Project.

Types Of Black Holes

There are three main types of black holes:

• Miniature or primordial black holes
• Stellar black holes
• Supermassive black holes

Miniature Or Primordial Black Holes

These are hypothetical very small black holes.

Stellar Black Holes

Stellar black holes form from the collapse of massive stars, often after a supernova.

Supermassive Black Holes

Supermassive black holes are found at the centres of large galaxies.

They have masses of millions to billions of Suns.

Sagittarius A* is the supermassive black hole at the centre of the Milky Way.

India’s AstroSat has observed high-energy X-ray variability from black hole systems.

Our Galaxy

A galaxy is a vast system made up of:

• Stars
• Gas
• Dust
• Dark matter

The Milky Way is our galaxy.

It contains roughly 100 billion stars.

Many stars visible to the naked eye are within a few thousand light-years of us.

Important Facts About The Milky Way

• The neighbouring major galaxy of the Milky Way is Andromeda.
• M33, or Triangulum Galaxy, is another nearby galaxy.
• The average distance between nearby major galaxies in our Local Group is of the order of millions of light-years.
• The Milky Way rotates.
• The Solar System lies about 25,000-28,000 light-years from its centre.
• The diameter of the Milky Way is about 100,000 light-years.

Basis Of Development Of Galaxies

In the early universe, matter and energy were unevenly distributed.

These density differences created differences in gravitational force.

Due to gravity:

• Matter came together.
• Hydrogen gas accumulated.
• A large cloud called a nebula formed.
• The nebula developed localized clumps of gas.
• These clumps became denser.
• Stars were formed.

Formation Of Planets

Planets form from a rotating disc of gas and dust around a newly formed star.

This rotating disc is called a protoplanetary disc.

Inside this disc:

• Particles collide.
• Particles stick together through accretion.
• Planetesimals form.
• Planetesimals grow into protoplanets.
• Protoplanets finally become planets.

The Solar System formed about 4.6 billion years ago.

The inner planets are generally rocky, while the outer planets are mainly gaseous or icy.

Our Solar System

The Solar System consists of:

• The Sun
• Eight planets
• Their satellites
• Asteroids
• Comets
• Meteoroids
• Interplanetary dust

The eight planets are:

• Mercury
• Venus
• Earth
• Mars
• Jupiter
• Saturn
• Uranus
• Neptune

Pluto was once considered the ninth planet, but it is now classified as a dwarf planet.

The Sun makes up about 99.8% of the Solar System’s mass.

It is mainly composed of hydrogen and helium.

The Sun

The Sun is the star of our Solar System.

Its radius is about 696,000 Km.

The Sun is described through its internal zones and outer atmosphere.

Structure Of The Sun

Core

The core is the source of the Sun’s energy.

In the core, hydrogen is converted into helium through nuclear fusion.

The temperature of the core is about 15 million°C.

Radiative Zone

In the radiative zone, energy moves outward in the form of radiation.

Convective Zone

In the convective zone, energy is transported by convection currents.

Photosphere

The photosphere is the visible surface of the Sun.

Its temperature is about 5,500°C.

Chromosphere

The chromosphere lies above the photosphere.

Corona

The corona lies beyond the chromosphere.

It is visible during a total solar eclipse.

Important Distance Units

Light Year

A light year is used to measure interstellar and intergalactic distance.

1 Light Year = about 9.46 trillion Km

Astronomical Unit

An Astronomical Unit, or AU, is used to measure the average distance between the Earth and the Sun.

1 AU = about 149.6 million Km

It is also about 8.3 light minutes.

Planets In Order

After Pluto’s exclusion, the Solar System has eight planets.

Inner Planets

The inner planets are rocky.

They are:

• Mercury
• Venus
• Earth
• Mars

Outer Planets

The outer planets are gas giants or ice giants.

• Jupiter and Saturn are gas giants.
• Uranus and Neptune are ice giants.

Beyond Neptune, dwarf planets such as Pluto exist.

Mercury

Mercury is the closest planet to the Sun.

It is also the smallest planet in the Solar System.

Important Facts About Mercury

• Closest planet to the Sun
• Smallest planet
• Difficult to observe because of the Sun’s glare
• Completes one revolution in 88 days
• Has no satellite
• Rotation period is about 59 Earth days
• Mass is about 1/18 of Earth’s mass

Venus

Venus is often called Earth’s twin planet because of its similar size.

It is one of the brightest planets in the sky.

It is known as:

• Morning Star
• Evening Star

Important Facts About Venus

• Completes one revolution in about 225 days
• Rotates in about 243 days
• Rotation period is longer than revolution period
• Has no moon
• Rotates from east to west, opposite to Earth’s direction
• Hottest planet in the Solar System

Earth

Earth is the only known planet that supports life.

Life is possible on Earth due to:

• Suitable distance from the Sun
• Moderate temperature
• Presence of water
• Atmosphere
• Ozone layer

Important Facts About Earth

• About 71% of Earth’s surface is covered with water.
• The Moon is Earth’s only natural satellite.
• Earth’s 23.5° axial tilt causes seasonal changes.
• Earth is the fifth-largest planet in the Solar System.

Mars

Mars is known as the Red Planet because of iron oxide on its surface.

Important Facts About Mars

• Takes 687 days to complete one revolution.
• Has two small natural satellites: Phobos and Deimos.
• Olympus Mons is the tallest volcano in the Solar System.
• India’s Mangalyaan, or Mars Orbiter Mission, was launched on 5 November 2013.
• Mangalyaan entered Mars orbit on 24 September 2014.

Jupiter

Jupiter is the largest planet in the Solar System.

About 1300 Earths could fit inside Jupiter by volume.

Important Facts About Jupiter

• Largest planet in the Solar System
• Rotates rapidly
• Has faint rings
• Has 95 officially recognized moons
• Ganymede is the largest moon in the Solar System
• The Great Red Spot is a giant long-lasting storm in Jupiter’s atmosphere

Saturn

Saturn is the second-largest planet.

It is famous for its ring system.

Important Facts About Saturn

• Takes about 29.5 years to complete one revolution around the Sun.
• Has 274 confirmed moons.
• Titan is its largest moon.
• Saturn has very low average density, less than that of water.

Uranus

Uranus was discovered in 1781 by William Herschel.

It is the seventh planet from the Sun.

Important Facts About Uranus

• Takes 84 years to complete one revolution around the Sun.
• Rotates from east to west.
• Bluish-green colour is due to methane in its atmosphere.
• Has 28 known moons.

Neptune

Neptune is the eighth and farthest planet from the Sun.

It was discovered by German astronomer Johann Galle.

It is named after the Roman God of the Sea.

Important Facts About Neptune

• Takes 165 years to complete one revolution around the Sun.
• Has the longest year in the Solar System.
• It is the fourth-largest planet in the Solar System.
• Has 14 known natural satellites.
• Triton is its largest satellite.

Why Pluto Is Not A Planet

Pluto was discovered in 1930.

In 2006, the International Astronomical Union, or IAU, gave Pluto the status of a dwarf planet.

IAU Conditions For A Planet

According to the IAU, a planet must satisfy three conditions:

• It must orbit the Sun.
• It must be massive enough to become spherical under its own gravitational force.
• It must have cleared its neighbourhood.

Pluto could not meet the third condition.

It remains in the neighbourhood class of Neptune for around 20-22 years.

Because of this, Pluto was removed from the list of planets and given the dwarf planet status as 134340 Pluto.

It is the second-largest dwarf planet after Eris.

It is the largest member of the Kuiper Belt, which is a ring of objects located about 30 AU-50 AU from the Sun.

Terrestrial And Jovian Planets

Basis	Terrestrial Planets	Jovian Planets
Position	Close to the Sun	Far from the Sun
Orbits	Closely spaced	Widely spaced
Mass	Small masses	Large masses
Radius	Small radii	Large radii
Composition	Predominantly rocky	Predominantly gaseous
Surface	Solid surface	No solid surface
Density	High density	Low density
Rotation	Slower rotation	Faster rotation
Magnetic Field	Weak magnetic fields	Strong magnetic fields
Moons	Few moons	Many moons
Rings	No rings	Many rings

Interesting Facts About Solar System

Fact	Answer
Biggest planet	Jupiter
Biggest satellite	Ganymede
Blue Planet	Earth
Green Planet	Uranus
Brightest planet	Venus
Brightest star outside Solar System	Sirius
Closest star to Solar System	Proxima Centauri
Coldest planet	Neptune
Evening Star	Venus
Farthest planet from Sun	Neptune
Planet with maximum satellites	Saturn
Fastest revolution	Mercury
Hottest planet	Venus
Densest planet	Earth
Fastest rotation	Jupiter
Morning Star	Venus
Nearest planet to Earth	Venus
Nearest planet to Sun	Mercury
Red Planet	Mars
Slowest revolution	Neptune
Slowest rotation	Venus
Smallest planet	Mercury
Smallest satellite	Deimos
Earth’s twin	Venus
Only moon with atmosphere similar to Earth	Titan

Planet Data Table

Planet	Diameter	Rotation Period	Revolution Period	Average Temperature	Surface Composition	Rings And Moons
Mercury	4,897 Km	58.66 Earth days	88 Earth days	179°C	Rocky	0
Venus	12,104 Km	243 Earth days	224 Earth days	482°C	Rocky	0
Earth	12,756 Km	23 hours 56 minutes 4 seconds	365.25 Earth days	15°C	Rocky	1 moon
Mars	6,756 Km	1.03 Earth days	687 Earth days	-63°C	Rocky	2 moons
Jupiter	142,984 Km	0.42 Earth days	4,332 Earth days	-121°C	Gaseous	95 moons + rings
Saturn	120,536 Km	0.45 Earth days	10,775 Earth days	-125°C	Gaseous	147 moons
Uranus	51,118 Km	0.71 Earth days	30,681 Earth days	-193°C	Gaseous	27 moons + rings
Neptune	49,528 Km	0.67 Earth days	60,193 Earth days	-173°C	Gaseous	13 moons + rings

The Earth

Pythagoras, who lived between 582-507 B.C., believed that the Earth was a sphere.

He was the first to suggest that Earth was shaped like a globe.

Earth has two basic movements:

• Rotation
• Revolution

Rotation Of Earth

Rotation is the movement of the Earth on its own axis.

The Earth completes one rotation in 23 hours, 56 minutes and 4.09 seconds.

It rotates in an eastward direction, opposite to the apparent movement of the Sun.

Earth’s axis is tilted at an angle of 23.5° from a perpendicular to the ecliptic plane.

Velocity Of Rotation

The velocity of Earth’s rotation varies according to distance from the Equator.

• At the poles, rotational velocity is nearly zero.
• At the Equator, rotational velocity is greatest.

Effects Of Earth’s Rotation

Earth’s rotation causes:

• Apparent rising and setting of the Sun
• Alternate occurrence of day and night
• Difference in time between different places
• Coriolis force
• Deflection of winds and ocean currents
• Tides, along with the gravitational pull of the Sun and Moon

Revolution Of Earth

Revolution is the movement of Earth around the Sun in its orbit.

Earth revolves around the Sun in an anti-clockwise direction, from west to east.

Earth takes 365 days and 6 hours to complete one revolution.

Because Earth’s orbit is elliptical, its distance from the Sun changes from time to time.

Perihelion

On January 3, Earth is closest to the Sun.

This position is called perihelion.

At perihelion, Earth is about 147 million Km from the Sun.

Aphelion

On July 4, Earth is farthest from the Sun.

This position is called aphelion.

At aphelion, Earth is about 152 million Km from the Sun.

Effects Of Revolution

Earth’s revolution causes:

• Cycle of seasons
• Variation in length of days and nights
• Variation in distribution of solar energy
• Formation of temperature zones

Seasons

Seasons are caused by the combined effect of Earth’s revolution and the tilt of its axis.

The four seasons are:

• Spring
• Summer
• Autumn
• Winter

On 21 March and 23 September, the Sun rises precisely in the east and sets exactly in the west.

Equinoxes

During equinoxes, the periods of daylight and darkness are equal all over the world.

Equinoxes occur on:

• 21 March
• 23 September

Spring Equinox

On 21 March, the Sun is directly overhead at the Equator.

This is called the spring equinox.

Autumn Equinox

On 23 September, the Sun is directly overhead at the Equator.

This is called the autumn equinox.

Solstice

A solstice is one of the two dates in the year when the Sun reaches its greatest altitude north or south of the Equator.

It is directly overhead along one of the tropics.

Summer Solstice

On 21 June, the Sun is overhead on the Tropic of Cancer, or 23.5° N.

This day is called the summer solstice.

On this date:

• The North Pole is tilted toward the Sun.
• At the Arctic Circle, the Sun never sets for 24 hours.
• At the North Pole, there are 6 months of daylight.
• The South Pole is tilted away from the Sun.
• At the Antarctic Circle, the Sun never rises for 24 hours.
• At the South Pole, there are 6 months of night.

Winter Solstice

On 22 December, the Sun is overhead on the Tropic of Capricorn, or 23.5° S.

This day is called the winter solstice.

During this time:

• The Southern Hemisphere receives direct rays of the Sun.
• Southern Hemisphere has summer.
• Northern Hemisphere has winter.
• Northern Hemisphere has longer nights and shorter days.

Earth’s Position With Respect To Moon

Apogee

Apogee is the point where the Moon is farthest from Earth.

Perigee

Perigee is the point where the Moon is nearest to Earth.

Eclipses

An eclipse is the complete or partial obscuration of light from a celestial body when it passes through the shadow of another celestial body.

There are two types of eclipses:

• Lunar eclipse
• Solar eclipse

Lunar Eclipse

A lunar eclipse occurs when Earth passes directly between the Sun and the Moon.

Earth casts its shadow on the Moon.

A lunar eclipse occurs only during a full moon.

It does not occur on every full moon because the Moon’s orbital plane is inclined about 5° to Earth’s orbital plane.

Solar Eclipse

A solar eclipse occurs when the Moon passes directly between the Sun and Earth.

The Moon casts its shadow on Earth.

A solar eclipse occurs only during a new moon.

It does not occur on every new moon because the Moon’s orbital plane is inclined about 5° to Earth’s orbital plane.

Latitude And Longitude

Latitude and longitude are used to locate places on Earth.

Latitude

Latitude is the angular distance of a point on Earth’s surface from the centre of Earth.

It is measured in degrees.

Latitude specifies a location’s distance north or south of the Equator.

Important Parallels Of Latitude

• Equator – 0°
• Tropic of Cancer – 23.5° N
• Tropic of Capricorn – 23.5° S
• Arctic Circle – 66.5° N
• Antarctic Circle – 66.5° S
• North Pole – 90° N
• South Pole – 90° S

Equator

The Equator is an imaginary line running on the globe.

It divides Earth into two equal parts:

• Northern Hemisphere
• Southern Hemisphere

Heat Zones Of The Earth

The parallels of latitude divide Earth into three heat zones based on the angle of the Sun’s rays.

Torrid Zone

Location:
Between the Tropic of Cancer and the Tropic of Capricorn.

Feature:
It receives maximum heat because the midday Sun is exactly overhead at least once a year.

Temperate Zones

Location:
Between the Tropic of Cancer and Arctic Circle in the Northern Hemisphere.

Between the Tropic of Capricorn and Antarctic Circle in the Southern Hemisphere.

Feature:
These zones experience moderate temperature because the midday Sun is never directly overhead.

Frigid Zones

Location:
Between the Arctic Circle and North Pole.

Between the Antarctic Circle and South Pole.

Feature:
These zones are extremely cold because the Sun’s rays are always slanting.

Longitude

Longitude is the angular distance of a point on Earth’s surface along the Equator, east or west from the Prime Meridian.

The semi-circles running from pole to pole are called meridians of longitude.

Prime Meridian

The Prime Meridian is the 0° longitude line.

It passes through Greenwich in London.

Greenwich Mean Time

The time at 0° longitude is called Greenwich Mean Time, or GMT.

International Date Line

The 180° meridian is exactly opposite to the Prime Meridian.

It is known as the International Date Line, or IDL.

The International Date Line runs over the Pacific Ocean.

Indian Standard Time

The Indian government has accepted 82.5° E longitude as the standard meridian.

Indian Standard Time is 5 hours and 30 minutes ahead of GMT.

Relationship Between Longitude And Time

Earth rotates 360° in 24 hours.

Therefore:

• Earth rotates 15° in 1 hour
• Earth rotates 1° in 4 minutes

Places east of the Prime Meridian are ahead of GMT.

Places west of the Prime Meridian are behind GMT.

International Date Line

The International Date Line roughly follows the 180° meridian.

It is the line where a change of date officially occurs.

When crossing the IDL from east to west, a person gains a day.

When crossing from west to east, a person loses a day.

The IDL is not a straight line. It zigzags over the Pacific Ocean to avoid dividing landmasses and island groups.

Latitude Vs Longitude

Basis	Latitude	Longitude
Also called	Parallels	Meridians
Shape	Full circles, except poles	Semi-circles
Length	Not equal	Equal
Direction	East-west	North-south
Distance	Parallel to each other	Farthest at Equator, meet at poles
Use	Heat zones	Time calculation

Additional Members Of Our Solar System

Apart from the Sun and planets, the Solar System also includes asteroids, meteors, meteorites and comets.

Asteroids

Asteroids are minor planets, especially of the inner Solar System.

They orbit the Sun mainly between Mars and Jupiter in the Asteroid Belt.

Features Of Asteroids

• Solid, rocky and irregular bodies
• No atmosphere
• No rings
• Cannot support life
• Some have small companion moons

More than 150 asteroids are known to have small companion moons.

The first asteroid-moon system discovered was Ida and Dactyl in 1993.

NASA’s Dawn mission became the first mission to orbit a main-belt asteroid, Vesta, in 2011.

Ceres is the largest object in the asteroid belt and is now classified as a dwarf planet.

Meteors And Meteorites

Meteoroids are small particles and fragments that orbit the Sun.

When a meteoroid enters Earth’s atmosphere and burns, it forms a meteor.

If a meteoroid survives its passage through the atmosphere and reaches the ground, it is called a meteorite.

Fireballs are bigger meteoroids.

Comets

Comets are often called dirty snowballs.

They are mostly made of:

• Dust
• Rocks
• Ice

When comets come close to the Sun, they heat up and release dust and gases.

The released gases form a glowing head, and the debris forms a long tail.

Each time a comet passes near the Sun, it loses some of its material and may eventually disappear.

Short-Period Comets

Short-period comets take less than 200 years to orbit the Sun.

They are found in the Kuiper Belt.

Halley’s Comet

Halley’s Comet is one of the most famous short-period comets.

It reappears every 76 years.

It will next be seen in 2062.

Oort Cloud Comets

Less predictable comets are found in the Oort Cloud.

The Oort Cloud is about 100,000 AU from the Sun.

Some comets in this cloud can take as long as 30 million years to complete one revolution around the Sun.

Moon

The Moon is Earth’s only natural satellite.

It is about one-quarter the diameter of Earth.

It is the fifth-largest satellite in the Solar System.

It is the largest satellite in the Solar System relative to its major planet.

The Moon is larger than any known dwarf planet.

Important Facts About The Moon

• The Moon formed about 4.6 billion years ago.
• It formed around 30-50 million years after the formation of the Solar System.
• It is in synchronous rotation with Earth.
• The same side of the Moon always faces Earth.
• The Moon revolves around Earth in 27 days, 7 hours, 43 minutes and 11.47 seconds.
• It rotates on its own axis in the same time.
• The total forces of the Moon and the Sun are in the ratio 9:4.
• The study of the Moon is called Selenology.

Evolution Of The Earth

Earth formed from a disc of dust and gas left over from the Sun’s creation.

This material accumulated into a single hot, rocky body.

Early Earth And Differentiation

Accretion

Accretion refers to the process by which Earth formed through the accumulation of dust and gas into a single body.

Differentiation

Over millions of years, Earth’s increasing density caused internal materials to separate by weight.

• Heavy materials like iron sank to the core.
• Lighter materials moved outward.
• The crust formed from cooling and hardening of molten matter.

Evolution Of Lithosphere

The lithosphere evolved as Earth cooled and its crust solidified.

Important processes were:

Solidification

Earth’s crust solidified as the planet cooled and shrank.

Tectonic Activity

Volcanic activity on the early unstable Earth released gases and molten material, shaping the crust.

Continent Formation

Plate tectonics formed and continues to reshape continents.

Evolution Of Atmosphere And Hydrosphere

Primordial Atmosphere

The first atmosphere was made of hydrogen and helium.

It was lost due to solar winds.

Degassing

Gases were released from Earth’s interior during cooling.

These gases included:

• Water vapour
• Nitrogen
• Carbon dioxide
• Methane
• Ammonia

These gases formed the second atmosphere.

Continuous volcanic eruptions also contributed water vapour and gases to the atmosphere.

Ocean Formation

As Earth cooled, water vapour condensed.

Carbon dioxide dissolved in rainwater.

Temperature decreased further, causing more condensation and more rain.

This led to the formation of oceans.

Earth’s oceans formed within 500 million years of Earth’s formation.

Thus, oceans formed around 4000 million years ago.

Photosynthesis And Oxygenation

Early life forms such as cyanobacteria, or blue-green algae, performed photosynthesis.

They converted carbon dioxide into oxygen.

This gradually increased oxygen levels in the atmosphere and helped form a breathable atmosphere.

Origin Of Life

Life first appeared as simple, single-celled organisms in the oceans around 3.5 to 4 billion years ago.

Over billions of years, this simple life evolved into complex multicellular species.

This process led to the enormous biodiversity seen today.

Geological Time Scale

The Geological Time Scale is a hierarchical framework that divides Earth’s 4.6-billion-year history into different time units.

These divisions are based on major changes in:

• Earth’s geology
• Life forms
• Mass extinctions
• Continental drift
• Climate changes

Four Major Divisions Of Geological Time Scale

• Eons
• Eras
• Periods
• Epochs

Eons

Eons are the largest divisions of geological time.

Earth’s history is divided into four major eons:

• Hadean
• Archean
• Proterozoic
• Phanerozoic

Hadean Eon

Time: 4.6 to 4 billion years ago

The Hadean marks the time from Earth’s formation to the formation of the first solid crust.

It was marked by:

• High temperature
• Volcanic activity
• Absence of life

Archean Eon

Time: 4 to 2.5 billion years ago

During the Archean, Earth’s crust cooled enough to allow continent formation.

The earliest life forms, mainly single-celled organisms or prokaryotes, appeared during this time.

Proterozoic Eon

Time: 2.5 billion to 541 million years ago

This eon saw the rise of more complex life forms, including multicellular organisms.

Oxygen began to accumulate in the atmosphere.

Phanerozoic Eon

Time: 541 million years ago to present

This is the most recent eon.

It is marked by abundant plant and animal life.

Most fossil records belong to this eon.

Eras

Eons are divided into eras.

The Phanerozoic Eon is divided into three major eras:

• Paleozoic Era
• Mesozoic Era
• Cenozoic Era

Paleozoic Era

Time: 541-252 million years ago

This era is known for the Cambrian Explosion, a period of rapid diversification of life forms.

It ended with the largest mass extinction in Earth’s history, the Permian-Triassic extinction.

Mesozoic Era

Time: 252-66 million years ago

The Mesozoic Era is called the Age of Reptiles.

It saw:

• Dominance of dinosaurs
• Rise of mammals
• Rise of birds
• Breakup of the supercontinent Pangaea

It ended with the Cretaceous-Paleogene extinction, which wiped out dinosaurs.

The Mesozoic includes:

• Triassic
• Jurassic
• Cretaceous

Cenozoic Era

Time: 66 million years ago to present

The Cenozoic Era is called the Age of Mammals.

It saw the rise of:

• Mammals
• Birds
• Flowering plants
• Humans

The Cenozoic includes:

• Paleogene
• Neogene
• Quaternary

Periods

Periods are subdivisions of eras.

Cambrian Period

Time: 505-570 million years ago

The Cambrian Explosion led to rapid diversification of marine life and development of most modern animal phyla.

Ordovician Period

Time: 438-505 million years ago

This period saw the rise of marine life and ended with a major ice age.

Silurian Period

Time: 408-438 million years ago

The first vascular plants appeared, and coral reefs began to form.

Devonian Period

Time: 360-408 million years ago

This period is known as the Age of Fishes.

It also saw the development of the first land-dwelling vertebrates.

Carboniferous Period

Time: 286-360 million years ago

Extensive coal-forming forests developed, and the first reptiles appeared.

Permian Period

Time: 245-286 million years ago

The Permian ended with the most extensive mass extinction in Earth’s history.

This paved the way for the rise of dinosaurs.

Epochs

Periods are further divided into epochs.

Paleocene Epoch

Time: 57-65 million years ago

It marked the beginning of the Cenozoic Era after the extinction of dinosaurs.

Eocene Epoch

Time: 37-58 million years ago

It was marked by the rise of many modern mammal groups.

Oligocene Epoch

Time: 24-37 million years ago

Major climatic changes led to the evolution of more open habitats.

Miocene Epoch

Time: 5-24 million years ago

Grasslands expanded, and mammals diversified.

Pliocene Epoch

Time: 2-5 million years ago

The first hominins, or early human ancestors, appeared.

Pleistocene Epoch

Time: 10,000 years to 2 million years ago

It is known for repeated ice ages and the evolution of modern humans.

Holocene Epoch

Time: 0-10,000 years ago to present

It is the current epoch.

It is marked by:

• Human civilization
• Agriculture
• Industrialization

Geological Time Scale Table

Eon / Era	Period	Epoch	Age / Years Before Present	Life / Major Events
Cenozoic	Quaternary	Holocene	0-10,000 years	Modern Man
Cenozoic	Quaternary	Pleistocene	10,000-2 million years	Homo sapiens
Cenozoic	Tertiary	Pliocene	2-5 million years	Early human ancestor
Cenozoic	Tertiary	Miocene	5-24 million years	Ape, flowering plants and trees
Cenozoic	Tertiary	Oligocene	24-37 million years	Anthropoid ape
Cenozoic	Tertiary	Eocene	37-58 million years	Rabbits and hare
Cenozoic	Tertiary	Paleocene	57-65 million years	Small mammals like rats and mice
Mesozoic	Cretaceous	–	65-144 million years	Extinction of dinosaurs
Mesozoic	Jurassic	–	144-208 million years	Age of Dinosaurs
Mesozoic	Triassic	–	208-245 million years	Frogs and turtles
Paleozoic	Permian	–	245-286 million years	Reptiles dominated and replaced amphibians
Paleozoic	Carboniferous	–	286-360 million years	First reptiles, vertebrates, coal beds
Paleozoic	Devonian	–	360-408 million years	Amphibians
Paleozoic	Silurian	–	408-438 million years	First trace of life on land, plants
Paleozoic	Ordovician	–	438-505 million years	First fish
Paleozoic	Cambrian	–	505-570 million years	No terrestrial life, marine invertebrates
Proterozoic	–	–	570-2,500 million years	Soft-bodied arthropods
Archean	Pre-Cambrian	–	2,500-3,800 million years	Blue-green algae, unicellular bacteria
Hadean	Pre-Cambrian	–	3,800-4,800 million years	Oceans and continents formed, atmosphere rich in carbon dioxide
Origin of Stars	–	–	5,000 million years	Origin of the Sun
Supernova	–	–	5,000-13,700 million years	Origin of the universe
Big Bang	–	–	13,700 million years	Beginning of universe

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FAQs On The Origin And Evolution Of The Earth