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Why is there Life 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: AtmosphereEarth has a breathable atmosphere. Oxygen is the gas that is required for the life of most creatures. This is present in Earth's atmosphere and also in water. Oxygen is constantly put into the atmosphere by plants and trees. Earth's atmosphere also contains a small amount of carbon dioxide. This is a poisonous gas which makes up most of the atmosphere of planets like Venus and Mars and makes them unable to support human life. However, its smaller presence on Earth is useful as it helps to moderate the planet's temperature and is absorbed by plants during photosynthesis to produce oxygen.
Reason Two: ClimateEarth 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, is usually freezing and can be as cold as -140°c, a temperature which would freeze blood and water. Other planets are colder still.
Reason Three: WaterEarth has water! Water is considered to be the most important chemical necessary for life. It contains the oxygen needed for life. Other liquids can 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 the presence of water on other objects in the solar system greatly increases the chances of life existing on them. Water on Earth is abundant and can be found in its three states of matter. 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. This process is sometimes referred to as terraforming and is a requirement for the possible existence of human beings on other planets. 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. But Mars, with a day length similar to that of Earth, and quite a bit of light, it could be a possibility.