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WHY
DOES LIFE EXIST ON EARTH?
 Earth is unique in the
Solar
System as
being the only planet which is able to support life in all its
forms: from basic living micro-organisms to highly sophisticated
and intelligent human beings. There are many reasons why this
happens.
REASON ONE:
ATMOSPHERE
Earth has a breathable atmosphere. Oxygen is the gas that is
required for the life of most creatures. This is present in
Earth's atmosphere, along with nitrogen, and in water, along with
hydrogen. Oxygen is constantly put into the atmosphere by plants
and trees. Fortunately, Earth's atmosphere does not contain much
carbon dioxide, a poisonous gas which makes up most of the
atmosphere of planets like Venus and Mars. In densely-populated
areas, carbon dioxide is more present. This is because cars and
factories produce it. This causes the air to be polluted. The
Earth's atmosphere is kept on the planet by its pull of gravity. Mars and Mercury are too small to keep
atmosphere. As a result, Mercury has no atmosphere, and Mars' atmosphere is very thin,
containing gases which have not managed to escape into space yet.
Earth's atmosphere is thick enough to prevent poisonous rays of
radiation from getting through it (this is what has happened on Mars).
REASON
TWO: CLIMATE
Earth has a suitable climate. This is caused by the moderate
amount of carbon dioxide in the planet's atmosphere, which is
constantly refreshed whenever there is a volcanic eruption. The
temperature on Earth does not go from one extreme to the other
either. Mercury can be anything from 200°c below freezing
to 375°c above. At 375°c, water would only exist as a gas, and
the planet would be completely dry. Venus has a surface temperature
of 480°c, which would be much too hot for anybody to live in. Mars, although it can reach
25°c, can be as cold as -140°c, a temperature which would
freeze blood and water. The other planets are colder still.
REASON
THREE: WATER
Earth has water! Water is believed to be the most important
chemical necessary for life. It contains the oxygen needed for
life. Other liquids contain poisonous elements. Water doesn't
burn skin (like liquids containing acids do), it is drinkable,
and it allows life-providing molecules to move around easily.
Other moons in the Solar System, such as Europa, a moon of Jupiter, are believed to have
oceans of water under its icy surface. Scientists believe that,
if they could find traces of water on Mars, the possibility of
life existing on that planet
would be greatly increased. Water on Earth can be found anywhere,
in its three states. It can be frozen, taking the form of ice. It
can be liquid, seen in seas and oceans and lakes. It can also be
a gas, seen as clouds. In the picture below, we can see water in
its three states; a solid, a liquid and a gas. The blue glow at
the top of the Earth is the planet's thin atmosphere.
REASON
FOUR: LIGHT
All planets receive light from the Sun, but no planet uses it as
usefully as Earth. Trees and plants on the planet produce oxygen
through a process called photosynthesis. Plants need the Sun to grow. Look at plants in
windows and notice how they usually seem to grow towards the Sun. Try growing a plant in a
dark room and in a light room. Notice which one grows quicker.
The one which has grown quickest is the one which also produces
more oxygen. It is believed that if we were able to get plants
to grow on another planet, such as Mars, they would begin putting
oxygen into the planet's atmosphere and increase the possibility
of life. Saturn's moon, Titan, has an atmosphere
containing mainly nitrogen. If this moon was to possess oxygen
too in its atmosphere, by a plant photosynthesising on it, it
could have a similar atmosphere to Earth. Something else which helps the plants
to photosynthesise on Earth is the length of time the planet takes to spin
once on its axis. Taking just under 24 hours means that each side of the
planet receives sunlight regularly. If we look at a planet like Venus, which
takes 243 days to spin on its axis, it means that for a large period of time
certain parts of the planet are in complete darkness. So even if the planet
could support life, it would struggle to do so.
REASON
FIVE: THE SUN
All of the reasons
given above for life existing on Earth are only possible because of one main
reason. The Sun!
Put simply, if there was no Sun,
there would be no life on Earth. Technically, Earth probably wouldn't exist
either! Because of Earth's ideal distance from the Sun,
it receives the perfect amount of heat and light to allow life to be created
and to support it. Imagine what would happen if the Sun
suddenly vanished. How would you keep warm? How would you see? How would you
get food and drink? How would plants and trees grow? How would they
photosynthesise? Where would Earth go? The Sun's
gravity keeps Earth in its orbit, but if the Sun
vanished, Earth would simply float away.
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