The History of Earth's Atmosphere

Soon after its formation (about 4,500 million years ago) the Earth may have resembled present-day Jupiter or one of the other giant planets, with a thick layer of gases surrounding a very dense core. In the Earth's past much of this original atmosphere was eventually lost, possibly having been burned off during a period of intense solar activity. This theory is supported by the fact that, compared to their cosmic abundance, the rare gases (neon and xenon, for example) are found in only infinitesimal amounts in the modern atmosphere.

The secondary atmosphere

In place of the cosmic gases a secondary atmos­phere developed, deriving from the Earth itself. For many millions of years after its formation, the Earth's surface was probably molten and, even after a thin outer crust had solidified, there was incessant volcanic action. During this period of great activity, the rocks gave off large quantities of gases, including nitrogen, ammonia, carbon mon­oxide, methane, and probably carbon dioxide and water vapour as well - a mixture similar to that given off by volcanoes and fumaroles today. Like modern volcanic gases, the primeval atmosphere is thought to have contained only the slightest trace of oxygen, and so it would have been poisonous to almost all modern life forms.

Photosynthesis and oxygen

The Earth gradually cooled, and after its surface temperature had fallen to below 100°C, water vapour in the atmosphere condensed and fell as rain, which filled hollows and created shallow lakes and seas. In these waters, at depths of more than 10m (the limit of penetration by lethal ultraviolet radiation), the first primitive plants evolved. They were probably algae and, according to the fossil record, appeared approximately 3,000 million years ago. They produced their own food by photosynthesis (a form of metabolism in which light is used to convert inorganic molecules such as carbon dioxide and water into more complex organic molecules, such as sugar), and released oxygen - a by-product of photosynthesis - into the atmosphere.

In the air molecules of oxygen (O2) were broken down by ultraviolet radiation into single atoms (0), and some of these atoms combined with oxygen molecules to form ozone (O3). Ozone is an unstable gas, the molecules of which absorb ultraviolet radiation. In doing so, they are often broken down and turned back into oxygen mol­ecules and single atoms. Hence ozone forms and reforms continuously in the atmosphere at a rate that varies according to the amount of ultraviolet light, which itself depends on other factors such as sunspot activity, the season, and even whether it is day or night.

Because of the photosynthetic activity of the early plants, the concentration of oxygen and ozone in the primeval atmosphere steadily increased, thereby providing more and more protection from harmful ultraviolet radiation. Eventually the amount of this radiation reaching the Earth's surface had diminished to such an extent that it penetrated only the top few centimetres of the oceans, as a result of which marine organism could begin to develop more fully.

The modern atmosphere

The chief gases in the atmosphere today are nitrogen (78.09 per cent), oxygen (20.95 per cent) and argon (0.93 per cent). The remaining 0.03 per cent is made up of carbon dioxide (which is photosynthesized by plants), very small quantities of neon, helium, ozone and hydrogen, and minute traces of krypton, methane, xenon and other gases. Another important constituent of the atmosphere is water vapour, which makes up four per cent of the atmosphere by volume and three per cent by weight. The atmosphere also contains tiny partic­les of salt, smoke, dust, and man-made pollution. Volcanic dust erupted into the lower stratosphere - well above the various weather processes that would return it to the surface - may remain there for several years.

The composition of the atmosphere is relatively constant up to 50km above the Earth's surface, with the exceptions of ozone (which is concentrated in the stratosphere, between 15 and 35km above the Earth) and water vapour. Water vapour originates on the Earth's surface and so its concentration decreases with height; it is almost completely absent beyond 10 to 12km above the surface.