A. ORIGIN OF THE EARTH
of the earliest to made popular
arguments regarding the origin of
the earth. Laplace revised
this hypothesis in 1796.
cloud of a material
associated with a youthful sun, which was
slowly rotating.
approached the sun. As a
result, a cigar-shaped extension of
material was separated from
the solar surface. As the passing star
moved away, the material
separated from the solar surface
continued to revolve around
the sun and it slowly condensed into
planets.
somewhat revised the
‘nebular hypothesis’. They considered that
the sun was surrounded by
solar nebula containing mostly the
hydrogen and helium along
with what may be termed as dust.
The friction and collision
of particles led to formation of a diskshaped cloud and the planets were formed
through the process of accretion.
The Big Bang Theory
considers the following stages in the
development of the
universe.
place in the form of a “tiny ball” (singular atom) with an
unimaginably small volume, infinite temperature and infinite
density.
(ii) At the Big Bang the “tiny ball” exploded violently. This led to a
huge expansion.
(It is now generally accepted that the event of big bang took
place 13.7 billion years before the present. The expansion
continues even to the present day. As it grew, some energy was
converted into matter. There was particularly rapid expansion
within fractions of a second after the bang. Thereafter, the
expansion has slowed down. Within first three minutes from the Big
Bang event, the first atom began to form).
(iii) Within 300,000 years from the Big Bang, temperature dropped
to 4,500 K and gave rise to atomic matter. The universe became
transparent.
The expansion of universe means increase in space between the
galaxies. An alternative to this was Hoyle’s concept of steady
state. It considered the universe to be roughly the same at any
point of time.
A light year is a measure of distance and not of time.
Light travels at a speed of 300,000 km/second.
Considering this, the distances the light will travel in one year is
taken to be one light year. This equals to 9.461×1012 km.
The mean distance between the sun and the earth is 149,598,000
km. In terms of light years, it is 8.311 minutes of a year.
A galaxy contains a large number of stars.
thousands of light-years.
The diameters of individual galaxies range from 80,000-150,000
light years.
A galaxy starts to form by accumulation of hydrogen gas in the
form of a very large cloud called nebula.
Eventually, growing nebula develops localised clumps of gas.
These clumps continue to grow into even denser gaseous bodies,
giving rise to formation of stars.
All the planets were formed in the same period sometime about
4.6 billion years ago.
The following are considered to be the stages in the development
of planets :
(i) The stars are localised lumps of gas within a nebula. The
gravitational force within the lumps leads to the formation of a
core to the gas cloud and a huge rotating disc of gas and dust
develops around the gas core.
(ii) In the next stage, the gas cloud starts getting condensed and
the matter around the core develops into small rounded objects.
These small-rounded objects by the process of cohesion develop
into what is called planetesimals.
Larger bodies start forming by collision, and gravitational
attraction causes the material to stick together. Planetesimals
are a large number of smaller bodies.
(iii) In the final stage, these large number of small planetesimals
accrete to form a fewer large bodies in the form of planets.
E. SOLAR SYSTEM
The nebula from which our Solar system is supposed to have been
formed, started its collapse and core formation some time 5-5.6
billion years ago and the planets were formed about 4.6 billion
years ago.
Our solar system consists of the sun (the star), 8 planets, 63 moons,
millions of smaller bodies like asteroids and comets and huge
quantity of dust-grains and gases.
Out of the EIGHT planets, mercury, venus, earth and mars are
called as the inner planets as they lie between the sun and the
belt of asteroids the other four planets are called the outer
planets.
Alternatively, the first four are called Terrestrial, meaning earth-like
as they are made up of rock and metals, and have relatively high
densities.
The rest four are called Jovian or Gas Giant planets. Jovian means
jupiter-like. Most of them are much larger than the terrestrial
planets and have thick atmosphere, mostly of helium and
hydrogen.
The difference between terrestrial and jovian planets can be
attributed to the following conditions:
(i) The terrestrial planets were formed in the close vicinity of the
parent star where it was too warm for gases to condense to solid
particles.
Jovian planets were formed at quite a distant location.
(ii) The solar wind was most intense nearer the sun; so, it blew off
lots of gas and dust from the terrestrial planets.
The solar winds were not all that intense to cause similar removal
of gases from the Jovian planets.
(iii) The terrestrial planets are smaller and their lower gravity could
not hold the escaping gases.
F. THE MOON
The moon is the only natural satellite of the earth.
In 1838, Sir George Darwin
suggested that initially, the earth and
the moon formed a single
rapidly rotating body. The whole mass
became a dumb-bell-shaped
body and eventually it broke. It was
also suggested that the
material forming the moon was separated
from what we have at
present the depression occupied by the
Pacific Ocean.
It is now generally
believed that the formation of moon, as a
satellite of the earth, is
an outcome of ‘giant impact’ or what is
described as “the big
splat”.
A body of the size of one
to three times that of mars collided into
the earth sometime shortly
after the earth was formed. It blasted a
large part of the earth
into space. This portion of blasted material
then continued to orbit the
earth and eventually formed into the
present moon about 4.44
billion years ago.
The earth has a layered structure. From the outermost end
of the
atmosphere to the centre of
the earth, the material that exists is
not uniform.
The atmospheric matter has
the least density. From the surface to
deeper depths, the earth’s
interior has different zones and each of
these contains materials
with different characteristics.
The earth was mostly in a
volatile state during its primordial stage.
Due to gradual increase in
density the temperature inside has
increased. As a result the
material inside started getting separated
depending on their
densities. This allowed heavier materials (like iron) to sink
towards the centre of the earth and the lighter ones to
move towards the surface. With passage of time it cooled further
and solidified and condensed into a smaller size. This later led to
the development of the outer surface in the form of a crust.
During the formation of the moon, due to the giant impact, the
earth was further heated up. It is through the process of
differentiation that the earth forming material got separated into
different layers.
Starting from the surface to the central parts, we have layers like
the crust, mantle, outer core and inner core. From the crust to the
core, the density of the material increases.
Evolution of Atmosphere and Hydrosphere
The present composition of earth’s atmosphere is chiefly
contributed by nitrogen and oxygen.
There are three stages in the evolution of the present atmosphere.
The first stage is marked by the loss of primordial atmosphere.
In the second stage, the hot interior of the earth contributed to
the evolution of the atmosphere.
Finally, the composition of the atmosphere was modified by the
living world through the process of photosynthesis.
The early atmosphere, with hydrogen and helium, is supposed to
have been stripped off as a result of the solar winds. This
happened not only in case of the earth, but also in all the
terrestrial planets, which were supposed to have lost their
primordial atmosphere through the impact of solar winds.
During the cooling of the earth, gases and water vapour were
released from the interior solid earth. This started the evolution of
the present atmosphere.
carbon dioxide, methane, ammonia and very little of free oxygen.
The process through which the gases were outpoured from the
interior is called degassing. Continuous volcanic eruptions
contributed water vapour and gases to the atmosphere. As the
earth cooled, the water vapour released started getting
condensed.
The carbon dioxide in the atmosphere got dissolved in rainwater
and the temperature further decreased causing more
condensation and more rains.
The rainwater falling onto the surface got collected in the
depressions to give rise to oceans. The earth’s oceans were
formed within 500 million years from the formation of the earth.
of the earliest to made popular arguments regarding the origin of
the earth. Laplace revised this hypothesis in 1796.
cloud of a material associated with a youthful sun, which was
slowly rotating.
approached the sun. As a result, a cigar-shaped extension of
material was separated from the solar surface. As the passing star
moved away, the material separated from the solar surface
continued to revolve around the sun and it slowly condensed into
planets.
somewhat revised the ‘nebular hypothesis’. They considered that
the sun was surrounded by solar nebula containing mostly the
hydrogen and helium along with what may be termed as dust.
The friction and collision of particles led to formation of a diskshaped cloud and the planets were formed through the process of accretion.
The Big Bang Theory considers the following stages in the
development of the universe.
place in the form of a “tiny ball” (singular atom) with an
unimaginably small volume, infinite temperature and infinite
density.
(ii) At the Big Bang the “tiny ball” exploded violently. This led to a
huge expansion.
(It is now generally accepted that the event of big bang took
place 13.7 billion years before the present. The expansion
continues even to the present day. As it grew, some energy was
converted into matter. There was particularly rapid expansion
within fractions of a second after the bang. Thereafter, the
expansion has slowed down. Within first three minutes from the Big
Bang event, the first atom began to form).
(iii) Within 300,000 years from the Big Bang, temperature dropped
to 4,500 K and gave rise to atomic matter. The universe became
transparent.
The expansion of universe means increase in space between the
galaxies. An alternative to this was Hoyle’s concept of steady
state. It considered the universe to be roughly the same at any
point of time.
A light year is a measure of distance and not of time.
Light travels at a speed of 300,000 km/second.
Considering this, the distances the light will travel in one year is
taken to be one light year. This equals to 9.461×1012 km.
The mean distance between the sun and the earth is 149,598,000
km. In terms of light years, it is 8.311 minutes of a year.
A galaxy contains a large number of stars.
thousands of light-years.
The diameters of individual galaxies range from 80,000-150,000
light years.
A galaxy starts to form by accumulation of hydrogen gas in the
form of a very large cloud called nebula.
Eventually, growing nebula develops localised clumps of gas.
These clumps continue to grow into even denser gaseous bodies,
giving rise to formation of stars.
All the planets were formed in the same period sometime about
4.6 billion years ago.
The following are considered to be the stages in the development
of planets :
(i) The stars are localised lumps of gas within a nebula. The
gravitational force within the lumps leads to the formation of a
core to the gas cloud and a huge rotating disc of gas and dust
develops around the gas core.
(ii) In the next stage, the gas cloud starts getting condensed and
the matter around the core develops into small rounded objects.
These small-rounded objects by the process of cohesion develop
into what is called planetesimals.
Larger bodies start forming by collision, and gravitational
attraction causes the material to stick together. Planetesimals
are a large number of smaller bodies.
(iii) In the final stage, these large number of small planetesimals
accrete to form a fewer large bodies in the form of planets.
The nebula from which our Solar system is supposed to have been
formed, started its collapse and core formation some time 5-5.6
billion years ago and the planets were formed about 4.6 billion
years ago.
Our solar system consists of the sun (the star), 8 planets, 63 moons,
millions of smaller bodies like asteroids and comets and huge
quantity of dust-grains and gases.
Out of the EIGHT planets, mercury, venus, earth and mars are
called as the inner planets as they lie between the sun and the
belt of asteroids the other four planets are called the outer
planets.
Alternatively, the first four are called Terrestrial, meaning earth-like
as they are made up of rock and metals, and have relatively high
densities.
The rest four are called Jovian or Gas Giant planets. Jovian means
jupiter-like. Most of them are much larger than the terrestrial
planets and have thick atmosphere, mostly of helium and
hydrogen.
The difference between terrestrial and jovian planets can be
attributed to the following conditions:
(i) The terrestrial planets were formed in the close vicinity of the
parent star where it was too warm for gases to condense to solid
particles.
Jovian planets were formed at quite a distant location.
(ii) The solar wind was most intense nearer the sun; so, it blew off
lots of gas and dust from the terrestrial planets.
The solar winds were not all that intense to cause similar removal
of gases from the Jovian planets.
(iii) The terrestrial planets are smaller and their lower gravity could
not hold the escaping gases.
The moon is the only natural satellite of the earth.
In 1838, Sir George Darwin suggested that initially, the earth and
the moon formed a single rapidly rotating body. The whole mass
became a dumb-bell-shaped body and eventually it broke. It was
also suggested that the material forming the moon was separated
from what we have at present the depression occupied by the
Pacific Ocean.
It is now generally believed that the formation of moon, as a
satellite of the earth, is an outcome of ‘giant impact’ or what is
described as “the big splat”.
A body of the size of one to three times that of mars collided into
the earth sometime shortly after the earth was formed. It blasted a
large part of the earth into space. This portion of blasted material
then continued to orbit the earth and eventually formed into the
present moon about 4.44 billion years ago.
The earth has a layered structure. From the outermost end of the
atmosphere to the centre of the earth, the material that exists is
not uniform.
The atmospheric matter has the least density. From the surface to
deeper depths, the earth’s interior has different zones and each of
these contains materials with different characteristics.
The earth was mostly in a volatile state during its primordial stage.
Due to gradual increase in density the temperature inside has
increased. As a result the material inside started getting separated
depending on their densities. This allowed heavier materials (like iron) to sink towards the centre of the earth and the lighter ones to
move towards the surface. With passage of time it cooled further
and solidified and condensed into a smaller size. This later led to
the development of the outer surface in the form of a crust.
During the formation of the moon, due to the giant impact, the
earth was further heated up. It is through the process of
differentiation that the earth forming material got separated into
different layers.
Starting from the surface to the central parts, we have layers like
the crust, mantle, outer core and inner core. From the crust to the
core, the density of the material increases.
Evolution of Atmosphere and Hydrosphere
The present composition of earth’s atmosphere is chiefly
contributed by nitrogen and oxygen.
There are three stages in the evolution of the present atmosphere.
The first stage is marked by the loss of primordial atmosphere.
In the second stage, the hot interior of the earth contributed to
the evolution of the atmosphere.
Finally, the composition of the atmosphere was modified by the
living world through the process of photosynthesis.
The early atmosphere, with hydrogen and helium, is supposed to
have been stripped off as a result of the solar winds. This
happened not only in case of the earth, but also in all the
terrestrial planets, which were supposed to have lost their
primordial atmosphere through the impact of solar winds.
During the cooling of the earth, gases and water vapour were
released from the interior solid earth. This started the evolution of
the present atmosphere.
carbon dioxide, methane, ammonia and very little of free oxygen.
The process through which the gases were outpoured from the
interior is called degassing. Continuous volcanic eruptions
contributed water vapour and gases to the atmosphere. As the
earth cooled, the water vapour released started getting
condensed.
The carbon dioxide in the atmosphere got dissolved in rainwater
and the temperature further decreased causing more
condensation and more rains.
The rainwater falling onto the surface got collected in the
depressions to give rise to oceans. The earth’s oceans were
formed within 500 million years from the formation of the earth.