Topic 1.2: Structure of Atmosphere and Earth

Structure of Atmosphere:

It is divided into five layers. Each layer is separated by an increase or decrease in the temperature of the gases in the layer.

Troposphere (0-15 km):

It is the closest layer to the earth’s surface. It contains the largest percentage of the mass of the total atmosphere. It acts as a blanket to the earth because it keeps the average temperature of the earth nearly constant.

It consists of clouds, rain, and snow. All weather conditions are formed in this layer. We live in this layer.
In this layer, weather conditions are constantly changing due to the mixing up of gases. That’s why it is named as troposphere layer.
Clouds are formed in this layer because 99 % of the water vapor in the atmosphere is present in this layer.
This layer contains more than 75 % of the atmospherics’ mass. So, the air is densest in this layer. Air consists of 78 % Nitrogen, 21 % oxygen, and 1 % other gases (argon, water vapor, and carbon dioxide).

The temperature of the troposphere decreases with the increase in altitude.
As we move upward in this layer, temperature decrease by about 6.5 ^oC per kilometer. The decrease in temperature with height is a result of the decreasing pressure.
If a parcel of air moves upwards it expands (because of the lower pressure). When air expands it cools. So, air higher up is cooler than air lower down.
The lowest part of the troposphere is called the boundary layer.
The top of the troposphere is called tropopause.

Stratosphere (15-50 km):

It is the 2^nd lowest layer of the atmosphere.

This layer has its own set of layers. That’s why it is called the stratosphere layer.
The ozone layer is found in the lower portion of the stratosphere, which helps us by absorbing harmful UV radiations from the Sun.
As the ozone layer absorbs high-energy UV radiation from Sun, the temperature of the stratosphere rises with the rise in altitude.

Temperatures in the stratosphere are highest over the summer pole and lowest over the winter pole.
The top part of the stratosphere is hot because the ozone layer generates heat.
The stratosphere is free of all weather conditions. So, airplanes fly in the lower layer of the stratosphere, to avoid any turbulence, although thunderstorms do penetrate the stratosphere.
The top of the stratosphere is called the stratopause.

Mesosphere (50-85 km):

It is the coldest layer of the atmosphere. In this layer, the air is thin to breathe.

It protects our earth from meteoroids. In this layer, meteors burn upon entry.
As compared to the mesosphere, the exosphere & thermosphere don’t have much air due to which meteors make through these upper layers. In the mesosphere, there is enough air to cause friction to produce heat. Moreover, the speed of meteors accelerates due to the gravitational field of the earth as it comes closer to the earth.

The meteors make it through the exosphere and thermosphere without much trouble because those layers don’t have much air. But when they hit the mesosphere, there are enough gases to cause friction and create heat.
The temperature of this layer decreases with the rise in altitude.
The top of the mesosphere is called mesopause.

Thermosphere (85-600 km):

It is above the mesosphere and below the exosphere.

There aren’t enough molecules in this layer for sound waves to travel through.

In this layer, the temperature is extremely hot. Most of the X-ray and UV radiations are absorbed in this layer. However, if you were to hang out in the thermosphere, though, you would be very cold because there aren’t enough gas molecules to transfer the heat to you.
It is home to the International Space Station as it orbits the earth.
The lower part of this layer contains the Ionosphere, where atoms are ionized. As a result, the ionosphere is electrically charged.
Ions in the ionosphere help to transmit radio waves. It allows us to receive radio broadcasts from other parts of the world.
The top of the thermosphere is called the thermopause.

Exosphere (>600 km):

It is the outermost layer of the earth’s atmosphere as it gradually fades into the vacuum of space.

It is composed of low-density gases such as hydrogen and helium.
In this layer, molecules are far apart. They can travel lots of km without colliding with one another. So, there is no air in this layer to breathe.
It no longer behaves like a gas, and the particles constantly escape into space.

Temperature and Pressure Profile of Atmosphere:

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Structure of Earth:

Eduard Suess was the first to study earth.

Earth is divided into three parts based on composition.
As we go down, density, temperature & pressure increase.

We never managed to go beyond the earth’s crust. Till now, scientists have drilled over 12 km. We know indirectly about the earth using seismologic studies.

Crust (5 to 30 km):

It is brittle in nature. It is made up of a large amount of silicon and aluminum.

It is covered with land, air, and water.

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It can be split into two parts i.e., continents and the bottom of the ocean.

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A continent is a large continuous mass of land. There are seven continents i.e., Asia, Africa, North America, South America, Antarctica, Europe, and Australia.
The Mariana Trench is located in the Pacific Ocean. It is the deepest known part of the Pacific Ocean.

As we move from oceanic crust to continental crust, the thickness of the crust varies from 5 km to 30 km.

The Continental crust is less dense than the oceanic crust. They ride over the molten mantle. That’s why during their collision or movement, oceanic crust goes below continental crust.
If the continental crust were light then it would have gone below the oceanic crust. We would have left with no land.
Continents are embedded in tectonic plates.

There are 15 tectonic plates that fit together around the earth. These plates are not fixed and are slowly moving. Their movement causes earthquakes.
Himalaya was formed due to the collision of plates when two continental plates collide it folds the rock at the boundary, lifting it up and leading to the formation of mountains.

Mohorovicic or Moho Discontinuity (54 km):

It is the boundary between the earth’s crust and mantle. It was first identified in 1909. It is named after Seismologist Andrija Mohorovicic.

In 1909, he examined the earthquake data collected from various seismographic stations in Croatia.
He observed a drastic change in the velocity of seismological waves recorded at different seismographic stations which were at different distances from the epicenter of earthquakes.

It could be possible due to the sharp transition in density in the earth’s crust. The depth of Moho was approximated to be 54 km based on velocity data.

Basically, this continuity was identified by observing the change in velocity of seismological waves as they pass through changing densities of rock.
The Moho separates the crust from the underlying mantle. It marks the transition in composition between crust and mantle.

Mantle (up to 2900 km):

It is composed of molten rocks. It covers 84 % of Earth’s volume.
It lies below the crust. It is in a semi-liquid state.
It is composed of iron, magnesium silicate, and magnesium oxide.
It has a density of 4.5 g/cm³.

Layers of Mantle:

Upper Mantle layer (80-100 km):

This layer is more rigid. This layer is a part of the lithosphere. The lithosphere consists of the crust and upper mantle part.

The Moho layer is part of the upper mantle.

Middle Mantle layer (Asthenosphere):

This layer is just below the lithosphere. This layer is also called the asthenosphere. It came from the Greek word “Asthenes” which means weak.

In this layer, the mantle turns into more fluid and flows. So, this layer is ductile.
It is highly viscous and denser than the crust.
Convection currents are generated within this layer. The hot magma rises then cools and sinks.
It is the main source of magma that finds its way to the surface during a volcanic eruption.
The lithosphere floats on this layer. It allows the tectonic plates to drift across the earth.

Lower Mantle layer:

In this layer, the mantle turns more rigid due to high pressure and temperature and doesn’t flow easily.

Guterberg Discontinuity:

It is the boundary mantle and outer core of the earth.

Core (3417 km):

It is composed of heavy materials such as Nickel & Iron.
It is also called NiFe layer. Temperature varies from 4400 to 6000 ^oC.
Further, this layer can be divided into two parts.

Layers of Core:

Outer Core:

It is in a liquid state. It lies beneath the mantle.

Wide ranges of T & P in the outer core cause the molten metal to move this results in the formation of electric currents that produce magnetic field.
This magnetic field helps to protect us from the Sun’s solar radiation.
The boundary between the outer and inner core is called Lehmann discontinuity.

Inner Core:

It is in a solid state.
It is solid due to high pressure.