Leicester University Physics Department

Earth

picture : The Earth taken from Apollo 9, courtesy of NASA

Solar System Index

The Sun

Mercury

Venus

Earth

The Moon

Mars

Jupiter

Saturn

Uranus

Neptune

Pluto

The Kuiper Belt

Introduction

The third planet from the Sun, Earth is, for obvious reasons, the planet we know the most about. It has one moon and the only breathable atmosphere known to man. Liquid water on the surface of a planet is essential for life as we know it and, as 71% of the Earth's surface is covered by water, it is an ideal life-supporting planet. It is so far the only place in the solar system known to have water existent as a liquid, a solid and a gas. In fact, to our knowledge there are no other worlds capable of developing, or even supporting life in the solar system, and we have not been able to reach other stars to find out whether life could exist elsewhere, although in all probability another Earth-like planet exists somewhere.

Climate

Earth has a great variety of different climates, varying by geography and season. The Sun is the main cause of these variations. It causes the oceans and atmosphere to heat, leading to vast movements of gases and water. Partial evaporation of the oceans leads to the formation of clouds, which can cause rain, snow, thunderstorms, or any one of many other types of weather depending on local conditions. Climatologists and Meteorologists seek to understand and model the entire weather system. This is no simple task, and despite state-of-the-art technology and some of the keenest scientific minds in the country, accurate weather forecasting is still extremely difficult, and many mistakes have been made. The quest for greater and greater accuracy will go on for a long, long time. One of the many climatic effects man has been studying recently is Global Warming. This seems to have two causes : the breakdown of the Ozone Layer and the increase of the Greenhouse Effect.

The Ozone Layer is a layer of clear gas, similar to the oxygen that we breathe, several miles above the Earth's surface. This blocks out many of the more harmful rays from the Sun which cause sunburning, as well as possible skin cancer. Unfortunately, due to chemicals that Man has released through industry, manufacturing and waste disposal of many household items, this protective layer is breaking down. The main chemicals which cause damage are known as CFCs - Chloro-Fluoro-Carbons and their effect has been observed to be most severe around the north and south poles (as shown in the image, courtesy of GSFC, NASA).

This may lead to the melting of much of the ice around these poles, causing the sea levels to rise. In future years, floods are inevitable, and unless mankind stops the production of these chemicals, the environmental effects may be devastating. Now that the problem is better understood global measures have been agreed on to slow the rate of distruction of the ozone layer. Some damage has already been done and effects on the environment are inevitable. However, it is not too late to prevent it from being disastrous.

The Greenhouse Effect is caused by too much of several gases, notably Carbon Dioxide, in the atmosphere. These gases cause the Earth to heat up like a greenhouse in the sunlight, and can greatly change the Earth's climate. Carbon dioxide is released by car exhausts, man breathing, and when almost anything is burned. The destruction of the rainforests, especially in South America, is one of the major causes of this carbon dioxide. In fact, the effect is two-fold, since trees, and indeed any other kind of plantlife, absorb carbon dioxide to produce oxygen, preventing the build up of greenhouse gases. The results of the effect are similar to those caused by the depletion of the Ozone Layer: global heating, rising of sea-levels, etc. If the Earth doesn't sort itself out, mankind must eventually take responsiblity for his own actions and clean up the atmosphere, or else the Earth will become like Venus where the Greenhouse Effect went out of control long ago, and where life simply could not exist due to the harsh conditions.

The Interior

Geologists have discovered that the Earth's interior is separated into several, discrete layers through the use of seismology. This involves the study of vibrations caused by earthquakes, meteorite impacts, or man-made explosions. Seismographs can be used to detect the shocks reflected off each of the layer boundaries, and if several are used, a profile of the internal structure can be built up. Over the years, a standard model of the solid Earth has been constructed, and the diagram (courtesy of LANL), shows a simplified representation of this.

The top layer of the Earth is known as the crust. There are two types of crust, continental and oceanic (which is the thinnest of the two), which have been formed largely through volcanic activity. Being the surface of the Earth, the crust is also the coldest part, and as such is extremely rigid. Together with the upper mantle which is solid near the crust and gradually softens with depth, the crust forms the lithosphere, which is divided into several plates. There are at present eight plates, and the study of them is known as Plate Tectonics. It is also the study of earthquakes, since these are mostly caused by movements of plates relative to each other.

The transition region between the mantles is sometimes also known as the fertile layer. It is the source of much of the magma, known as lava when it erupts, that we see spewing forth from volcanoes, and upon it the crust floats.

The lower mantle is similar in nature to the upper mantle, except for the increased density and slightly different chemical structure.

The core, both inner and outer, consists mostly of iron or nickel, and at the centre temperatures may be as high as almost 8000 celsius, hotter than the surface of the Sun. Due to the incredible pressures however, the inner core is solid.