The third planet from the Sun. From the astronomical perspective, Earth belongs to the group of terrestrial planets, which also includes Mercury, Venus and Mars. It is with this group, and also the Moon, that its origin, structure and evolution are often compared.
Earth has an atmosphere intermediate in density between those of Venus and Mars. It is unique in possessing vast oceans of liquid water. The complex interaction between ocean, atmosphere and planetary surface determines the energy balance and the temperature regime. Cloud cover is typically 50 per cent, and heat trapped within the atmosphere (the greenhouse effect) raises the average temperature by more than 30 degrees.
The present composition of the atmosphere is 77 per cent molecular nitrogen, 21 per cent molecular oxygen, 1 per cent water vapour and 0.9 per cent argon. Carbon dioxide is the most important trace constituent. The high concentration of oxygen, which dates from 2,000 million years ago, is a direct result of the existence of plants. The presence of oxygen allowed the formation of the high-level ozone layer, which shields the surface from solar ultraviolet radiation damaging to life.
Earth is the only major planet known for certain to be geologically active. Its large-scale features have all been determined by the creation, destruction, relative movement and interaction of a dozen or so crustal plates - comprising the lithosphere - which slide over the the less rigid asthenosphere below. Collisions between plates produce folded mountains and zones of seismic activity are concentrated along the plate boundaries.
Seismic waves generated during earthquakes reveal the internal structure of the Earth by the way they propagate. At the centre, there is a molten metallic core of iron and nickel, possibly with a solid core at the very centre. The central temperature is around 4,000íC. A silicate mantle overlies the core. The outermost crust is about 10 kilometres (6 miles) thick under the oceans and 30 kilometres (20 miles) thick where there are continents.
In planetary terms, the surface of the Earth is very young. The basaltic rocks forming the ocean floors are among the youngest. The Precambrian shields - which occupy about 10 per cent of the surface - are the oldest, and the nearest approximation to the cratered terrain that forms a large part of other planetary surfaces. Weathering has removed all but a few traces of whatever impact craters there were.
The molten metallic core gives rise to the Earth's magnetic field and magnetosphere. A layer of electrically charged particles between heights of about 50 and 600 kilometres (30 and 400 miles) form the ionosphere. The funnelling of charged particles by the magnetic field to regions between latitudes of 60í and 75í create the phenomenon of the aurorae. Satellite measurements have shown that the Earth is also an intense source of radio waves at kilometre wavelengths, though these are generated high up and are not detectable at ground level.
