Earth

Earth is the only planet in our Solar System capable of sustaining life. Located 149,600,000 kilometers (1AU) from the Sun, the Earth is the third of the four terrestrial planets. It is also the largest, with a diameter of 12,756 kilometers. Earth's atmosphere is much less dense than Venus', but much more dense than Mars'. It consists of mainly Nitrogen and Oxygen. Earth's clouds, which are composed mainly of water vapour, are spread roughly over about half the planet at any given time, in constantly changing patterns. These clouds exist in the seven mile deep troposphere, which is the lowest and densest layer of the Earth's atmosphere. Above the troposphere is the stratosphere, which is 40 kilometers thick. The ozone (O3) in the ozone layer in the stratosphere prevents solar ultraviolet radiation from reaching the Earth. Above this layer is the mesosphere, which is 32 kilometers think, and above that is the ionosphere, which is 160 kilometers thick. A total of 250 kilometers above the surface. Manned spacecraft (e.g: Russian (formerly Soviet) space stations and the ISS) operate at about 400-480 kilometers above ground level, and are well above the Earth's atmosphere. At this altitude, the troposphere and stratosphere appear as a narrow blue band on the Earth's horizon, while the other layers are too faint to be seen at all.

Temperatures on Earth vary from 60 Degrees C, to -100 degrees C, although the mean temperature is 15 degrees C, suitable for sustaining life. The Earth rotates on its axis every 24 hours, and an Earth year is approximately 365 days. Water covers 3/4 of the Earth's surface. The Earth's crust is 40 km thick and is composed mainly of silicate rock. beneath this surface is a mantle, which consists of semi-molten silicate rock 2900 km thick. The Earth's core is a sphere of molten iron (with some nickel), roughly 6720 miles in diameter. Earth is the second most geologically active body in the solar system (Io is the first).