our Earth

Earth is the third planet from the Sun. It is the first planet from the Sun to have a moon, a common feature of all of the other planets beyond our orbit. The Earths atmosphere reaches to approximately 80km above the surface, but this is only about one hundredth of the planets radius, and so occupies a relatively thin band around the planet. It is composed mainly of nitrogen (78%) and oxygen (21%), with the remainder built up from small traces of other gases. These relative proportions remain fairly constant up to around 80km, but the heavy gases become rare at greater heights and only hydrogen and helium are found in the outermost regions. At the height of the stratosphere, there is a concentrated layer of ozone, at about 25km. This ozone layer is vital to life on the planet, as it stops nearly all of the Suns harmful ultraviolet radiation from reaching the surface.

Atmosphere & Surface The Earth's atmosphere also contains a large amount of water vapour, although its concentration varies in different places, and is subject to seasonal variations. Most of it is visible as clouds, and most of these extend only to the top of the troposphere - up to 13km. It is in this region that the dominant weather systems are found. Above this lies a layer of warm air which holds cooler air below containing most of the water vapour, and the clouds that we see.

The surface of the planet is subject to much geological activity, and is shaped by continental drifting (plate tectonics). This makes the planet's surface relatively young, and taken in conjunction with other factors, such as the existence of an atmosphere and weathering, explains why few visible impact craters are to be found. The surface of the moon, which is heavily cratered, gives a more accurate a more accurate guide as to the quantity of collisions with debris making its way through the solar system. Here, the surface features date back to the formation of the solar system, and have remained undisturbed due to the moon's geological inactivity.