Atmosphere: Composition, Troposphere, Exosphere, Properties

Atmosphere is a sphere of vapor or gases, retained above the surface of a celestial body by gravitational force. Air is invisible to us and we know about its existence due to the constant pressure of a mixture of gasses. Thus, air and atmosphere are interrelated. Based on the planetary object’s gravitational force and temperature conditions, the extent of the atmosphere outward into space varies. Planets like mercury and the moon, a natural satellite, have extremely thin atmospheres in comparison to planets like Earth, Mars, Jupiter, etc. This happens because of the high temperature and lower gravitational force. 

Earth’s atmosphere, also known as the protective blanket, consists of five layers and extends to approximately 10,000 km (6,214 miles) above the earth’s surface. The transition point between earth and space is based on the Karman line, which scientists believe to be somewhere around 100 km (62 miles) above sea level. About 99% of Earth’s atmosphere falls well below this point thereby allowing water to be contained in all of its three forms (solid, liquid, and gas). 

Key Terms: Atmosphere, Gas-Aerosol Envelope, Protective Blanket, Greenhouse gases, Ozone Layer

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What is Atmosphere? 

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Atmosphere is a composition of layers of gases surrounding a planet or other celestial body. Earth's atmosphere is composed of about 78% nitrogen, 21% oxygen, and one percent other gases. All living beings on earth depend on the atmosphere for their survival.

Oxygen in the Atmosphere is essential for all living beings on earth for survival. It also protects from harmful ultraviolet solar radiation. It creates pressure without which liquid water couldn't exist on planet's surface. Atmosphere is a warm blanket for our planet that keeps temperatures habitable for Earth.

What is Atmosphere?

Theory Behind Formation of Atmosphere

Numerous theories have been explained about the formation and existence of the earth’s atmosphere. About 5 billion years ago, the earth was believed to be devoid of this protective layer. However, following a collision with a planet the size of Mars, Earth on cooling developed gases like carbon dioxide from the volcanic eruptions. Later on, as the earth’s surface further cooled, the atmosphere got enriched with oxygen, allowed water to be collected on the surface, and led to the evolution of life forms. Hence the current composition of the earth’s atmosphere is a result of several biochemical changes that have taken place from the metabolic activities of the various life forms.

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Composition of Atmosphere 

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Earth’s atmosphere is a heterogeneous mixture of gases, the majority of which is nitrogen. Oxygen and inert gases like argon and neon and trace amounts of carbon dioxide, methane water vapor, and some other gases make up this composition. The gravitational pull of the earth retains these gases, and other suspended particles like dust, soot, smoke, etc. inwards, preventing them from escaping into space. The following chart shows the distribution:

Distribution of gases in Earth’s atmosphere

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Characteristic Properties 

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On moving outwards from the earth’s surface, the atmosphere becomes less dense, gradually merging into outer space. The proportion of gases in the earth’s atmosphere makes the thin layer unique to earth and plays a crucial role in sustaining life on it.

1. Maintaining the temperature on Earth

The solar heat before entering the surface of the earth passes through the atmospheric layers. The solar energy thus gets radiated back into the higher atmospheric layers by the greenhouse effect. During the day, the sun’s rays keep the temperature of the earth’s surface high and at night, the surface cools down releasing heat back into the air. Some of this heat gets absorbed by the greenhouse gases like carbon dioxide, and the rest is pushed back into the atmosphere. This movement helps in regulating the temperature on earth and keeps it warm at night. 

2. Protecting life forms

Ultraviolet radiation from the sun is extremely harmful to all living organisms, especially humans. The atmosphere acts like a blanket shielding its surface from harmful radiation. The required warmth for survival gets trapped and the rest is waved back to space.

3. Regulating Earth’s water cycle

Water from the surface of the earth evaporates and condenses in the clouds as water vapor. This further comes down as it precipitates and replenishes the underground water. The earth's atmosphere thus helps in regulating this movement of water.

4. The atmosphere also helps regulate climates, provides living forms with needed gases like oxygen and carbon dioxide, and protects the earth from foreign objects entering into the earth from micro collisions in space.

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Atmospheric Layers 

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The earth’s atmosphere is broadly classified into 5 layers based on characteristic traits such as temperature and pressure. Following is the classification when moving outwards from the earth's surface:

1. Troposphere- the closest layer to the earth’s surface where we live
2. Stratosphere- ozone layer is present here
3. Mesosphere- there are no air molecules present here making it appear darker due to the absence or minimal refraction of light
4. Thermosphere- the widest layer that absorbs the majority of the harmful radiation
5. Exosphere- the thin layer that separates the earth from outer space

The five layers of the earth’s atmosphere

Let us look at the Layers of the Atmosphere

Layers of Atmosphere are as follows:

Troposphere

This is the lowest layer with 99 percent of water vapor and 75 percent of air thereby constituting the major mass of the earth’s atmosphere. The earth’s clouds, fog, hailstorms, cyclones, and all other weather occur here. Helicopters and light aircraft take flight in this layer owing to the high-pressure systems formed here due to the rapid air movement.

The troposphere extends to an altitude of 8-14 km above ground level and consists mainly of water vapor, ash particles, and dust. The lowest layer of the troposphere is called the boundary and the uppermost layer is called the tropopause. The temperature drops rapidly along with the pressure on moving outwards towards space. Through the tropopause, the temperature reverses and increases again. The temperature here can go as low as -56^oC.

Stratosphere

This is one of the most prominent layers of the atmosphere as it contains a high concentration of ozone. The ozone layer is responsible for absorbing the harmful ultraviolet radiation from the sun. The low wavelength radiations are absorbed that cause agitation inside the layer due to an increase in kinetic energy of the gas molecules. This increases the temperature on moving up the layer.

Temperature ranges from -56^oC to -15^oC. The stratospheric layer extends from the tropopause to an altitude of 50 km above up to the stratopause (highest layer). The pressure continues to decrease across this layer and there is no turbulence due to the absence of air resulting in almost negligible weather activities. Commercial airplanes hence use the lower layers of the stratosphere for their flight to skip the turbulence from the troposphere. 

Mesosphere

This layer extends to approximately 85 km above the stratopause and is one of the coldest regions of the atmosphere with temperature reducing to about -90^oC. The topmost layer of this region is called mesopause. One can see the mesosphere about 1-2 hours after sunrise and 1-2 hours after sunset. The mesosphere is also a destructive region for most of the meteors as they burn upon reaching here. The layer is so thin that one can hardly breathe and air pressure is less than 1 percent above the sea level and continues to reduce on moving outwards into space. The high clouds or noctilucent clouds can be observed in this layer.

Thermosphere

This layer extends to about 600 km above the mesopause. The temperature in this layer is found to increase with altitude and can go up to 2000^oC. The higher temperatures are caused by the absorption of ultraviolet and X rays. The air is very thin in this region making it a freezing environment. Famous visual effects like Northern and Southern Lights and Aurora are observed in this region. These effects are an outcome of the reaction between the electrons and protons from the sun and the isolated ions formed by the breakdown of the gas molecules. The highest layer in this region is called the thermopause. Helium and hydrogen ions are found in the outer layers of the thermosphere. Space shuttles and international space stations are located in this region.

Exosphere

The outermost layer of the atmosphere is the exosphere which extends to 1600 km from sea level. There is no definite boundary that differentiates this layer from outer space. The air in this region is extremely thin and the effect of gravity is almost nil. This makes the gases escape into space. This layer is the closest and first layer to get exposed to the incoming solar radiation. The atoms and molecules move very fast in this region offering very little resistance to the orbiting spacecraft. 

Ionosphere

This is a variable active layer ranging from the mesosphere to the exosphere. There is no distinctive boundary and the layer grows and shrinks based on the energy it absorbs from the sun. The ions from the sun collide with the atmospheric oxygen and hydrogen atoms causing them to get excited to a higher energy state. Once the atoms move back to their parent positions, they emit enormous photons of light. This is what we commonly witness as a visual effect called the Aurora Borealis and Aurora Australis. Due to the presence of ions in this layer, this region is called the ionosphere. 

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Things to Remember

• A layer of gases that are held by gravitational force and envelops the planets of the solar system is called the atmosphere. 
• The earth’s atmosphere is a result of several biochemical changes that occurred millions of years ago as part of the earth’s formation and metabolic activities of the various life forms.
• It is a heterogeneous mixture of gases such as nitrogen, oxygen, carbon dioxide, water vapor, and other inert gases.
• The earth’s atmosphere acts as a shield in maintaining the temperature of the earth, protecting the life forms from harmful radiations of the sun, and regulating the water cycle, and climatic changes.
• The earth’s atmosphere is divided into 5 layers- Troposphere, Stratosphere, Mesosphere, Thermosphere, and Exosphere.
• The ionosphere is an active layer of the atmosphere without a distinctive boundary extending from the mesosphere to the exosphere.
• Temperature and pressure vary across the different layers of the atmosphere, giving them their characteristic properties.

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Previous Year Questions

1. Due to increasing air-borne allergens and pollutants… [NEET 2019]
2. Steps taken by the Government of India to control air pollution include… [NEET 2009]
3. The CO2 content by volume, in the atmospheric air is about… [NEET 1997]
4. Which of the following is NOT a green house gas? [KCET 2020]
5. When concentration of carbon dioxide increases in blood, respiration becomes… 
6. Smoke is an example of… 
7. Ozone depletion over Antarctica is due to the… 
8. Formation of ozone in the upper atmosphere from oxygen takes place by… 
9. The chemical entities present in thermosphere of the atmosphere are… 
10. The basic component of the smog is…