Earth’s Atmosphere: Composition & Comparison

Atmosphere is a gaseous surrounding layers present on some planets and satellites because of gravitational forces. 

Atmosphere is an important part of the earth. It is not as dense as either land or water but it has weight and exerts pressure. It is a mixture of gases held to the earth due to gravitational attraction. It is composed of gases, water vapour and many solids and liquid particles, collectively called aerosols. Its composition is not absolutely constant, as it varies with height, e.g., oxygen is negligible at the height of 120 km. In a similar way, carbon dioxide and water vapour are found only up to 90 km from the surface of earth. 

PERMANENT GASES OF THE ATMOSPHERE

• Concentrations of minor gases such as CO₂, H₂ and Ozone (O3) in Earth’s atmosphere are controlled primarily by reactions in the stratosphere caused by UV solar radiation.
• Solar photons fragment gaseous molecules (such as O2, H2 and CO2) in upper atmosphere producing free radicals (C, H, O), a process called photolysis. Ex.
• Atmosphere of Earth is broadly divided into two main layers: Homosphere and Heterosphere. 
• Homosphere (made up of Troposphere, Stratosphere, and Mesosphere) is the lower atmosphere where the chemical composition is uniform. 
• Heterosphere is the upper atmosphere, comprising the Thermosphere and Exosphere. 

Due to constant mixing and turbulence, the lighter gases are unable to separate to form individual layers in the lower atmosphere. The atmosphere plays a crucial role in supporting life on Earth by blocking harmful UV radiation, providing oxygen and carbon dioxide for living organisms, and maintaining a livable environment. Additionally, the active gases in the atmosphere also help regulate the surface temperature of the Earth at around 15°C.

EARTH’S ATMOSPHERE COMPARED TO OTHER PLANETS

• Earth’s atmosphere is unique among planetary atmospheres of solar system. 
• Venus and Mars have atmospheres composed largely of CO2. 
• Surface pressure on Venus is about 90 times that on Earth while surface pressure on Mars in 1/100th of that on Earth.
• Surface temperatures of Earth (-20 to 40oC), Venus (400-500oC) and Mars (-140 to 25oC) are also very different.
• Outer planets are composed largely of hydrogen and helium and their atmospheres consist chiefly of hydrogen and in some cases, helium and methane.

GASES

• Two gases nitrogen and oxygen make up about 99 percent of the clean dry air.
• Remaining gases are mostly inert (Neon, Helium, Xenon etc.) and constitute about 1 percent of the atmosphere. 
• Carbon dioxide is an important gas as it is transparent to the incoming solar radiation but opaque to the outgoing terrestrial radiation. It absorbs a part of terrestrial radiation and reflects back some part of it towards the earth’s surface. It is largely responsible for the greenhouse effect. Carbon dioxide is the heaviest of the gases in the atmosphere, which is why the lower layers have a higher concentration of it compared to the upper layers.
• Ozone, a vital gaseous component present in atmosphere, is found between 10 and 50 km above the Earth’s surface. Its function is to act as a filter and absorb the ultra-violet rays emitted by the sun, preventing them from reaching the Earth’s surface.
• Water vapor, a variable gas, varies in percentage in the Earth’s atmosphere. In tropical regions, it can make up 4% of the air, but in dry, cold areas like d
• eserts and polar regions, it may only make up 1% or less. Additionally, water vapor decreases as one moves from the equator to the poles. It acts as a blanket by absorbing solar radiation and helping to retain heat on Earth. It also plays a role in both stabilizing and destabilizing the air.

DUST PARTICLES

• There are small solid particles present in atmosphere that come from various sources, such as sea salts, fine soil, smoke-soot, ash, pollen, dust, and debris from meteors. They are collectively called as Dust Particles.
• Dust particles, which are more abundant in subtropical and temperate regions with dry winds, tend to be located in the lower layers of the atmosphere but can be lifted to higher altitudes by convectional air currents. They are less prevalent in equatorial and polar regions.
• These particles act as nuclei for water vapor condensation, resulting in cloud formation.