The Earth's early atmosphere

The Earth formed about 4.5 billion years ago. Scientists believe that its early atmosphere was produced by volcanic activity.

Composition

The early atmosphere probably contained:

 little or no oxygen  a large amount of carbon dioxide  water vapour  small amounts of other gases, such as ammonia and methane

Scientists cannot be sure about the early atmosphere, since it formed so long ago. They can only analyse evidence from other sources.

Oxygen

Some ancient rocks contain iron compounds that break down in the presence of oxygen. They could only have formed if there was little or no oxygen at the time.

Carbon dioxide and water vapour

A volcano releases large volumes of carbon dioxide and water vapour. Since the early atmosphere was produced by volcanic activity, it was likely to have contained a large amount of carbon dioxide and water vapour.

Forming the modern atmosphere

Compared to the Earth's early atmosphere, the modern atmosphere contains:

 less water vapour  less carbon dioxide  more oxygen

The composition of the modern atmosphere

Water vapour

The Earth cooled after it formed. When it became cool enough, water vapour in the atmosphere condensed to liquid water. It fell as rain, creating the oceans.

Carbon dioxide

Carbon dioxide is a very soluble gas. It dissolves readily in water. As the oceans formed, carbon dioxide dissolved and its amount in the atmosphere decreased.

Oxygen

Plants make their own food by photosynthesis. In this process, carbon dioxide from the atmosphere reacts with water. Glucose is produced, with oxygen as a by-product:

carbon dioxide + water → glucose + oxygen

6CO2(g) + 6H2O(l) → C6H12O6(aq) + 6O2(g)

Photosynthesis by primitive plants and algae caused the removal of carbon dioxide from the air, and the release of oxygen. This had two effects:

 the amount of carbon dioxide decreased further  the amount of oxygen in the atmosphere gradually increased The Atmosphere’s Layers

Introduction

The Earth’s atmosphere layers are similar to the layers in the interior of the Earth. There are five major layers of atmosphere above the Earth that are separated by temperature.

Atmosphere gases

The concentration of atmosphere gases varies depending on the atmosphere layers. The troposphere contains primarily nitrogen (78%) and oxygen (21%). The remaining 1% of the atmosphere is trace gases. The stratosphere contains a layer of relatively high concentrations of ozone.

The lower part of the thermosphere is the ionosphere. This layer has free ions and electrons that are the result of ionization of gas molecules. As the distance increases from Earth the atmosphere becomes thinner as the molecules move further and further apart.

Troposphere

Lower boundary is the surface of the Earth. The upper boundary varies between 7 km (23,000 ft.) above the poles and 14 km (56,000 ft.) above the equator. This is the layer of the atmosphere where we live. The troposphere contains 75% of the total mass of the atmosphere. The air cools 6.5°C for each kilometer above sea level. Most of the energy from the sun travels through the atmosphere and is absorbed by the ground. The ground heats up warming the air above creating air currents. The rising of warm air and falling of cooler air creates convection currents that cause air circulation in this layer of the atmosphere.

Stratosphere 7-17 km to 50 km

The upper boundary of the stratosphere is 50 km (32 miles; 170,000 feet). The bottom layer of the stratosphere remains constantly at approximately 60°C. Jet streams form here as cold air from the poles meets the warmer air rising from the equator.

The stratosphere contains the ozone layer. The layer acts as a shield from ultraviolet radiation from the sun. The upper part of the stratosphere warms up to approximately 18°C as the ozone reacts with ultraviolet radiation giving off enough heat to warm this layer.

Mesosphere

The mesosphere is the third layer of the atmosphere. It extends to an altitude between 80 and 85 km. The temperature begins to drop in this layer until it reaches - 90°C. The upper region of the mesosphere is the coldest region of the Earth’s atmosphere layers.

Water vapor is sometimes present in this layer and it can be seen from Earth as thin feathery clouds of ice crystals. Many meteors entering the atmosphere burn up in this layer. The heat causes enough friction between a meteoroid and gas particles in the mesosphere to burn them up. Shooting stars are the trail of hot glowing gases as a meteoroid burns.

Thermosphere

Location of layer above Earth: 640 kilometers (400 miles; 2,100,000 feet)

The atmosphere is very thin in the thermosphere. The space shuttles orbit in the thermosphere. Radio waves bounce off the thermosphere allowing communication with countries overseas. The temperature in this region can reach 1500°C.

Exosphere

The exosphere is the highest layer of the atmosphere. It extends up to 10,000 km (6,200 miles; 33,000,000 ft.) above the Earth. Satellites orbit the Earth in the exosphere.

Life in the Troposphere

Weather

The troposphere is the lowest part of Earth's atmosphere. It contains about 75% of the atmosphere's mass and 99% of its water vapor.

The troposphere is where Earth's weather like rain, snow, thunder, or storms occur. Clouds can form up to a height of 10–15 km. The troposphere is where people live, because it reaches down to ground level.

In the troposphere, the temperature goes down as the altitude increases. This also means that the troposphere is quite unstable: gases can easily rise up or fall down. Hence the troposphere is well mixed. This vigorous atmospheric convection also causes the general atmospheric circulation.

Greenhouse effect Generating energy from fossil fuels produces a build-up of gases which are thought to be a cause of global warming.

Key points

 Burning fossil fuels increases the concentration of greenhouse gases, such as carbon dioxide and methane, in the atmosphere.

 The gases allow more of the sun's rays to enter the atmosphere and absorb solar radiation when it is reflected back which traps heat.

 Global warming could melt the world's ice caps and glaciers, causing an increase in sea levels and making many coastal areas uninhabitable.

 It could also affect weather patterns, causing droughts, flooding and hurricanes.

Photosynthesis

Animals need to eat food to get their energy. But green plants and algae do not. Instead they make their own food in a process called photosynthesis. Almost all life on Earth depends upon this process. Photosynthesis is also important in maintaining the levels of oxygen and carbon dioxide in the atmosphere.

Photosynthesis takes place inside plant cells in small objects called chloroplasts which contain chlorophyll. This absorbs the light energy needed to make photosynthesis happen.

For photosynthesis plants need:

 carbon dioxide  water  light

Plants get carbon dioxide from the air through their leaves, and water from the ground through their roots. Light energy comes from the Sun.

During photosynthesis plants create:

 oxygen  glucose (for food)

The oxygen produced is released into the air from the leaves and is used by animals to breathe.