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A mixture of gases surrounds a celestial object that has a strong
gravitational field. The gravitational force of the celestial body
prevents the gases from escaping. So, the celestial body is
enveloped in a layer of gaseous mixture. Since the earth fulfils
these requirements, we have a gaseous envelope around us we call the
atmosphere.
The gaseous mixture that make up the atmosphere contains 78 per cent
nitrogen, 21 per cent of oxygen and the remaining one percent is
made up of argon, carbon dioxide, water vapor and traces of
hydrogen, methane, ozone, carbon monoxide, helium, neon, krypton and
xenon. Obviously, nitrogen is the main constituent of the
atmosphere. This composition of gases has evolved over 4.5 billion
years, as scientists have estimated.
Gradual changes in
the atmospheric composition:
The earth has seen numerous volcanic
eruptions over the years. Gases that emanate from volcanic
eruptions, however, do not contain oxygen. Volcanic gases are mainly
composed of water vapor, carbon dioxide, sulphur dioxide and
nitrogen.
Various other processes must have been responsible for the
composition of the atmosphere as it is today, considering the
percentage of oxygen. There are many scientific theories that have
tried to account for the occurrence of oxygen in the atmosphere.
A plausible explanation is that condensation of water vapor from
the atmosphere resulted in the large quantity of water present on
the earth. This must have also led to many chemical reactions. With
the evolution of primitive life, the process of photosynthesis would
have started. As a result, oxygen was the probable release, slowly
becoming a part of the earth.
Scientists believe that carbon dioxide in the air must have reacted
with rocks on the earth’s crust and formed carbonates. Some of these
might have also got dissolved in water.
The first primitive life on earth is believed to have been marine
life. Dissolved oxygen in the water made such an event possible, as
scientists believe. And the surviving marine life forms survived,
and slowly evolved as well.
It is believed that the amount of oxygen present in the atmosphere
increased slowly over the years. And so did the number and variety
of life forms on earth. And with the increase of the level of
oxygen, the earth was capable of supporting all its species of
animals and plants.
Oh, the Ozone's
going, gone !
An allotropic form of oxygen is ozone and it contains
three atoms in each molecule. About nineteen to forty eight
kilometers above the earth’s surface lies the ozone layer. This
layer is formed as a result of sunlight acting on the oxygen in the
atmosphere. The ozone layer is very important for sustenance of life
on earth, as it protects life on earth from the ultraviolet
radiation of the sun.
It is, however, a frightening fact that the ozone layer is slowly depleting.
Certain chemicals that contain chlorine when released, react with
sunlight and destroy ozone molecules. Such chemicals are generally
used in refrigerants and as propellants in aerosol spray cans. So,
such chemicals must be avoided to protect the stratospheric ozone
layer.
Emissions from vehicles
also damage the ozone layer. That’s why, strict vehicle emission
norms have been brought into force. These norms need to be followed
diligently for the sake of our environment and, therefore, life.
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