Solar System information #5: Earth

Earth - our home in space

We live on a unique planet named Earth - the only place we know of that has air and water and is able to support life. Earth supports many forms of life - on land, in the waters, and in the air. 70% of the Earth's surface is covered by water. Our air and oceans were formed from gases that seeped from the Earth as it cooled after formation.

We are fortunate to have this type of atmosphere - it is quite dense in the lowest region (up to 10km high) and acts as a filter to shield the surface from harmful rays such as ultra-violet and X-rays. Higher up, it also acts as a buffer zone to protect us from bombardment by meteors: most meteors are vaporized by frictional heating, 85 to 100km above the Earth's surface. The air we breathe is a complex mixture of gases of which only two - oxygen and water vapour - are essential for supporting life.

The Earth has a solid core made up mainly of iron and nickel. The outer part of the core is probably a very dense liquid, surrounded by the mantle and then the crust. When the Earth had just formed, over 4 billion years ago, radioactive material in its interior was generating a great deal of heat. The heat couldn't dissipate quickly enough and the interior grew hotter and hotter, keeping the interior molten and allowing the heavy iron to sink to the centre, forming the core. As the Earth cooled, a thin solid crust formed on the surface, but most of the interior is still either liquid, or under so much pressure that it flows slowly, like plasticine, even though it’s more than hot enough to flow like water when the pressure is released, as in a volcano.

The Earth's extensive oceans, ice caps and cloud cover make it very reflective: it reflects approximately half the energy it receives from the Sun. Seen from space, as the Apollo astronauts saw it in 1969-72, our planet looks very bright: a living, vibrant jewel in space.