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:
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:
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:
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.