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Nature Gallery (Earth)

Atmosphere

Wrapped around the Earth is a thin blanket of gases referred to as the atmosphere. Without this blanket, our planet would be as lifeless as the Moon. The atmosphere gives us the air we need to breathe, provides us with clean water to drink, traps the Sun's warmth, and protects us from harmful ultraviolet rays.

Structure of the Atmosphere

The atmosphere has a distinct vertical structure comprising four broad layers, each with its own characteristics. Each layer is warmed by different portions of the Sun's radiation, so the temperature of the atmosphere varies between layers.

The lowest layer, the troposphere, is the layer in which we live. It gets its warmth from the ground, which is heated by the Sun. Temperatures in the troposphere decrease steadily with distance from the ground. The rate of cooling, known as the environmental lapse rate, is remarkably even at around 6°C (42.8F) per 1,000 metres (3,280 feet). The troposphere contains 75 per cent of the atmosphere's gas. It also holds huge amounts of dust and water vapour, and is often dense with clouds and mist. Air pressure is greatest in the troposphere, because gravity pulls the atmosphere towards the Earth, squeezing most of its weight into this lowest layer. 

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As the Sun heats the ground, it keeps this thick mixture churning around, bringing us everything we call weather.

The boundary that separates the troposphere from the stratosphere is called the tropopause. The height of this boundary varies between about 15 kilometres (9 miles) at the Equator and 8 kilometres (5 miles) at the North and South Poles. In the stratosphere, temperatures begin to rise. This is due to the presence of the ozone layer, which absorbs the Sun's ultraviolet (UV) light and, at the same time, protects the Earth from the dangerous effects of UV rays. The ozone actually soaks up so much UV light that the stratosphere gets quite warm towards the top. Since the air gets warmer beyond the tropopause, moisture evaporated from the sea can never rise into the stratosphere, because it is carried by colder, denser air in the troposphere. Because the air in the stratosphere contains little moisture, and is heated from above as ozone absorbs UV light, the stratosphere is still and calm, which is why jet airliners climb up to this level for long distance flights. The only clouds are faint noctilucent (nightshining) clouds and mother-of-pearl clouds. The stratosphere contains 19 per cent of the atmosphere's gas.

Higher still, the mesosphere is heated as oxygen and nitrogen are warmed by extreme ultraviolet light, but temperatures begin to drop with height as the gases get thinner and thinner. The air in the mesosphere is very thin, but thick enough to slow down meteorites, which burn up as they hurtle into it, leaving fiery trails in the night sky.

The mesopause is the boundary that separates the mesosphere from the fourth layer of the atmosphere, the thermosphere. Gases in the thermosphere are even thinner than those in the mesosphere, but because they are exposed to the full glare of the Sun, temperatures soar to 2,000°C (3,632C). However, because there is so little gas, there is very little real heat.

The upper part of the mesosphere and the lower part of the thermosphere are together referred to as the ionosphere, since this layer contains many electrically charged particles called ions. Ions are atoms or molecules that have lost or gained one or more negatively charged electron. Ions in the atmosphere are formed when gas molecules, such as nitrogen and oxygen, are energized by ultraviolet rays from the Sun to such an extent that they lose one or more of their electrons. Because ions are electrically charged, they are capable of reflecting radio signals. During the day, the Sun's ultraviolet rays turn more and more atoms into ions, and so the ionosphere is most highly charged just after sunset. By dawn, the ionosphere is much weaker because the electrons slowly recombine with ions during the night.

The electrical nature of the Earth's atmosphere is responsible for the coloured lights known as aurorae. Aurorae are permanent features of the Earth's upper atmosphere, although they vary in vividness. There are two aurorae—the aurora borealis at the North Pole, and the aurora australis at the South Pole. Aurorae are gigantic, stretching far up through the atmosphere. The lowest fringes hang about 64 kilometres (40 miles) above the ground, while the upper rays extend more than 965 kilometres (600 miles) into space—three times as high as the space shuttle's orbit. The coloured lights we see in the sky are the atoms and molecules of the atmosphere glowing as they are bombarded by charged particles streaming from the Sun. These atoms and molecules are then deflected towards Earth's magnetic poles.

The outer layer of the atmosphere, the exosphere, lies more than 483 kilometres (300 miles) above the ground. At this height, gases become so rarefied that they drift off into space. Even further out are indistinct regions called the heliosphere and protonosphere. In the heliosphere, the atmosphere has thinned out to a near vacuum, but slight frictional drag on spacecraft indicates that gas is present - mostly helium, which is why it is called the heliosphere. The protonosphere, which stretches out more than 60,000 kilometres (37,200 miles) above the Earth, is even more rarefied, and probably consists of a sparse scattering of charged hydrogen particles, known as protons, hence the name.

Composition of the Atmosphere

The Earth's atmosphere consists mainly of the harmless inert gas nitrogen (78 per cent), and the vital oxygen we need to breathe (21 per cent). It also contains tiny traces of argon, ozone, carbon dioxide, neon, krypton, xenon, helium, methane, and hydrogen. Water vapour and solid particles, such as dust, pollen, and salt spray from the oceans are also present in the lowest level of the atmosphere, the troposphere.

No other atmosphere in the solar system is remotely like that of the Earth. The atmosphere of Venus, for instance, is 96 per cent carbon dioxide, while that of Mars is 95 per cent carbon dioxide. Earth's atmosphere contains practically no carbon dioxide, because most of this gas was absorbed by the oceans early in the planet's history, where it combined with calcium to form the mineral calcium carbonate, or limestone. Mars and Venus have no oceans, so their carbon dioxide is still present in their atmospheres.

The air is almost always moist, even when it is not raining, because it contains water vapour. This water vapour is normally invisible, but if the air is cooled enough, it condenses into drops of liquid water or solid ice and forms clouds, fog, mist, dew, rain, or snow. Water is continually being recycled between the atmosphere and the oceans in a process known as the water cycle.

The air is filled with a wide range of minute airborn particles, known as aerosols. Most of these aerosols are natural, such as volcanic ash, ash from forest fires, pollen, and fungal spores. The biggest sources of aerosols are salt from the sea and dust from soil. More than a billion tons of sea salt joins the air from sea spray every year, and almost a quarter of a billion tons of soil dust is whipped up by the wind. Without these aerosols, there would be nothing for water in the atmosphere to condense on, and there would be no mists, clouds, or rain.

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Recycling the Atmosphere's Components
Green plants take in carbon dioxide from the air during photosynthesis, a process by which they use energy from sunlight to make food, and oxygen is a by-product of this process. Burning and rusting processes throughout the world use up oxygen at the same rate that plants produce it. As a result, oxygen levels in the Earth's atmosphere remain fairly constant.

In fact, all the major gases of the air are continually recycled between the atmosphere and the living world. Plants take in carbon dioxide and split it into carbon and oxygen. The oxygen returns straight to the air, most of the carbon is stored as energy, and some of it is returned to the air when the plants decay. Alternatively, animals may eat the plants and breathe out some carbon as carbon dioxide. Some carbon is retained as plants turn into fossil fuels, and burned later as coal and oil, but either way it eventually returns to the air.

Plants need nitrogen as well as carbon dioxide, but they cannot draw it directly from the air. Instead, soil bacteria absorb nitrogen from air in the soil during a process called nitrogen fixation. Some plants can then take it up directly via symbiotic nitrogen fixers, but most take it from the soil as solid nitrate compounds, made by other bacteria during a process called nitrification. The plants turn the nitrates into complex compounds, which are returned to the soil when the plants die. In the soil, the compounds are taken up by bacteria, turned into nitrogen, and released into the air in a process called denitrification.

Atmospheric Pressure

The atmosphere becomes more dense as it gets closer to the ground. This is because air is being squeezed into an increasingly small space by the weight of air above. At the same time, gravity is pulling the air towards the Earth's surface, which adds to the squeezing effect. The denser the air, the more air molecules there are being squeezed together. The combined force of the air molecules is known as atmospheric pressure, which is essentially the force the air exerts on its surroundings. Typically, this force is about 1 kilogram per square centimetre (14 pounds per square inch) at sea level, but drops steadily with height as the air thins out. Air pressure is usually given in terms of millibars and measured with a barometer. Lines on weather maps joining places with equal air pressure are called isobars.

Air pressure is not even throughout the world, but constantly fluctuates from time to time and from place to place, as areas are heated to greater or lesser degrees by the Sun. Much of the time, for instance, an area of high pressure, called an anticyclone, sits over the North and South Poles, where the air is cold and dense. Close to the Equator, where the overhead Sun heats the Earth most strongly, warm, moist air rises in strong currents, creating a low pressure zone called the doldrums. In between, at the subtropics, there is a high pressure belt where warm air rising over the equator cools and sinks back to the ground. Besides these large-scale, persistent pressure zones, there are much smaller, short-lived highs and lows all over the globe. Their fluctuations play a major part in our weather.

Evolution of the Atmosphere

When the Earth was formed some 4.6 billion years ago, its very first atmosphere of hydrogen and helium probably drifted away into space very quickly because the Earth was too small and too near the Sun to hold it. Before long, a new atmosphere formed as volcanoes burst through nearly every part of the Earth's surface and poured forth fumes of water vapour, carbon dioxide, nitrogen, and other gases from the hot interior in a process called 'outgassing'.

Within a few hundred million years, the Earth was cool enough for some of the water vapour to condense and form the oceans, while the rest, together with some carbon dioxide, created a greenhouse effect that has kept the world warm ever since. Most of the carbon dioxide in the atmosphere dissolved in the newly formed oceans, where it combined with other substances, sank to the sea floor, and eventually formed carboniferous sedimentary rocks. As a result, nitrogen was soon left as the main atmospheric constituent.

Oxygen came from living organisms. The very first organisms were bacteria that fed on substances such as sulphur. About 3.5 billion years ago, an organism called cyanobacteria emerged. Cyanobacteria fed by using the action of sunlight upon a green pigment, chlorophyll, to split carbon dioxide into carbon and oxygen. They used the carbon for food, but released the oxygen into the atmosphere as waste. Over a billion years, all the tiny puffs of oxygen from cyanobacteria gradually began to change the atmosphere. 

At first, the oxygen rusted the iron in the sea, forming ancient red rocks called Banded Iron Formations, which date back 3.5 billion years. Once all the iron in the sea was rusted, the oxygen started to rust iron on land, forming red rocks called Red Beds, which are about 2 billion years old. 

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Finally, about 700 million years ago, with most of the iron on the Earth's surface rusted, oxygen levels began to build up rapidly in the atmosphere.

The evolution of bigger plants accelerated this process. For the first time, there was enough oxygen in the air for animals that breathed to develop. A few hundred million years later, enough oxygen had been converted into ozone to provide the protective layer in the stratosphere for life to develop on land.