The Earth is the third closest planet to the Sun and it just so happens to be the planet we live on. This is no coincidence. The Earth is the only planet in our Solar System capable of supporting the human race. Its temperature, proximity to the Sun, mass, density, chemical composition and even its size are all important factors in allowing life to develop, and subsequently in shaping the human race. However, it has not been just huumans living on this amazing planet. In fact, we've hardly been here at all when you consider that the oldest rocks so far discovered in the Earth's crust are 4,000 million or so years old, and the first fossils of living organisms are about 3,900 million years old.

    Scientists can generally tell quite a lot about a planet's development by looking at its surface and studying its impact craters, mountains, ridges etc, but in the case of the Earth this is not possible because of erosion by wind and rain, as well as the the natural rock cycle which 'recycles' rocks. Consequently most of the Earth's surface is less than 100 million years old - relatively young compared with other known planets and celestial rocks - and there are no really large impact craters on the Earth's surface. However, scientists have instead been able to study the interior of the Earth, to the point of constructing detailed 3 dimensional models of its internal structure, using the technique of 'Seismic Tomography', which we will now discuss.

Seismic Waves

    One of the key ways in which scientists can study the Earth's structure is by analysing the after-effects of earthquakes, and looking at how seismic waves spread through the Earth. They have been able to discover that there are 3 main types of seismic waves - Primary Waves, Secondary Waves and Surface waves. As the name suggests, surface waves just travel around the surface of the Earth, causing the very obvious and devastating effects which we associate with earthquakes. Primary waves travel rather like sound waves, compressing and then stretching the rock or liquid they are travelling through, whilst Secondary waves travel through solids in the typical 'wave' pattern, by creating a momentary sideways displacement (think of a rope flicked at one end).

    So how does this information help scientists? Well, firstly the fact that Secondary Waves do not travel through liquids whilst Primary waves do has helped ascertain that the solid core of the Earth is surrounded by a liquid core, as Secondary waves are not be detected on the far side of the Earth after earthquakes, whilst Primary ones are. Also, by studying the speed of these waves as they travel through the Earth, geophysicists are able to work out the divisions in the Earth's interior, and their respective temperatures.

Birth of the Earth

    Initially, scientists thought that the Earth was formed by a gentle 'rain' of dust and small particles slowly accumulating on the then small Earth's surface, causing a gradual increase in its mass. After a while the Earth would have began to heat up due to the slow decay of radioactive elements, and melt, forming a bouyant crust and dense core. However modern views lean more towards the theory that the Earth grew relatively quickly (100,000 years to 1 million years rather than 100 million to 1,000 million years) through larger collisions. The Earth was probably molten, or at least partially molten, through its period of growth, allowing heavy materials to sink towards the centre, forming a core and lighter ones to float to the surface. This process is called differentiation.

    Many astronomers currently believe that slightly less than 4,500 million years ago, whilst the Earth was still hot and young, a massive, violent off-centre collision with another large object led to large amounts of material being ejected into the Earth's orbit. This material cooled down and formed what we know as the Moon a few years later. According to this hypothesis, the core of this unknown object would have been incorporated into the Earth's mantle, thus changing the Earth's composition.

Other Facts

    Of all of the Earth's mass, most is to be found in the liquid mantle layer, just below the Earth's crust. Of the remaining mass, most is to be found in the outer and then inner cores. In fact, only a tiny percentage of the Earth's mass forms the crust, and an even smaller percentage forms our oceans! The inner core itself is thought to be at a temperature of 7,500°K.

    Closer to home, the atmosphere that envelopes us is predominantly formed of Nitrogen (around 78%), followed by Oxygen(21%) and 1% of other gases. The atmosphere plays an absolutely crucial role in determining the state the Earth is in today. As well as regulating long- and short-term weather, disintegrating small meteors (and waste from the international space station!) and protecting us from harmful Ultra-Violet rays, it provides humans, animals and plants with the necessary ingredients for sustainable life, powered by the Sun.

    Water at least 4 km deep covers 70% of the Earth's surface. The largest land mass is Eurasia (most of Europe and most of Asia) which measures 53,698,000 km². The Earth is only slightly larger than Venus yet Venus has only 80% the mass of the Earth. This is mostly due to the fact that we have a dense iron-nickel core at the centre of our planet.

Magnetic Field

    The Earth's magnetic field is much larger than that of any of the other rocky planets and this is also due to our iron-nickel core spinning so fast. Our magnetic field is actually pear shaped, pointing away from the Sun (it's blown in that direction by the Solar Wind) and it deflects most harmful radiation away from the planet. The process of charged particles from the Sun's solar wind becoming trapped by the Earth's magnetic field and colliding with air molecules above the the planet's poles gives rise to the phenomenon of aurorae, or the spectacular Northern and Southern lights.

    In 1958, James Van Allen discovered that a large number of energetic charged particles surround the Earth, forming two distinctive radiation belts around the Earth, at altitudes of 19,000 km to 41,000 km and 13,000 km and 7,600 km. The source of these radiation belt particles is mainly neutrons produced when solar wind particles and cosmic rays from outer space bombard the Earth's atmosphere. Though most of these fly off into space, some decay into protons and electrons, which are quickly trapped by the magnetic field.

    The Russian Sputnik was the first artificial satellite of the Earth in 1957, and this was followed by the Luna 1, also Russian, the first spacecraft to leave Earth's gravity. The first weather satellite (the TIROS 1) was launched in 1960 by NASA. Then, in 1961, Russian Cosmonaut Yuri Gagarin became the first man to leave the Earth's atmosphere, in his Vostok 1. The first humans orbited the moon in 1968 and in 1969 Neil Armstrong was the first man to walk on the moon. The first space station, the American Skylab was launched in 1973. In 1998 the first satellite of the Earth Observing System was launched. In 2000, the first permanent crew inhabited the International Space Station, now orbiting the Earth many miles above your head.