It is known that the atmosphere is divided into some layers. In which one among the following layers, is the percentage composition of Helium gas maximum?

1. Composition of the Atmosphere: Permanent vs. Variable Gases (basic)

The Earth's atmosphere is not a simple, uniform cloud; it is a complex mixture of gases, water vapour, and dust particles that are held close to the planet by gravity Fundamentals of Physical Geography, Composition and Structure of Atmosphere, p.64. When we talk about the "air," we are mostly talking about a collection of gases that we categorize into two main groups based on how stable their concentration is: Permanent Gases and Variable Gases.

Permanent gases are those whose relative proportions remain virtually constant throughout the lower atmosphere (up to about 80 km). This means whether you are at sea level or on top of a mountain, the ratio of Nitrogen to Oxygen remains the same, even though the air becomes thinner. Nitrogen (78.08%) and Oxygen (20.95%) make up roughly 99% of the atmosphere's volume Physical Geography by PMF IAS, Earths Atmosphere, p.271. Other permanent gases include Argon (0.93%) and trace amounts of Neon, Helium, and Hydrogen.

In contrast, variable gases change in quantity depending on the time and place. The most significant are Water Vapour, Carbon Dioxide (CO₂), and Ozone. For example, CO₂ and water vapour are essential for trapping heat, but they are only found up to about 90 km from the Earth's surface Fundamentals of Physical Geography, Composition and Structure of Atmosphere, p.64. Because gravity pulls harder on heavier molecules, the heaviest gases like Nitrogen and Oxygen dominate the lower layers, while the lighter gases like Hydrogen and Helium eventually become the primary components at extreme altitudes Physical Geography by PMF IAS, Earths Atmosphere, p.280.

Feature	Permanent Gases	Variable Gases
Examples	Nitrogen (N₂), Oxygen (O₂), Argon (Ar)	Water Vapour (H₂O), Carbon Dioxide (CO₂), Ozone (O₃)
Proportion	Fixed ratio in the lower atmosphere (Homosphere).	Changes based on geography, altitude, and human activity.
Role	Provides bulk and supports life/combustion.	Regulates temperature and drives weather patterns.

Sources: Fundamentals of Physical Geography, Composition and Structure of Atmosphere, p.64; Physical Geography by PMF IAS, Earths Atmosphere, p.271; Physical Geography by PMF IAS, Earths Atmosphere, p.280

2. Thermal Stratification: From Troposphere to Thermosphere (basic)

The Earth's atmosphere is not a uniform mass of gas; it is a structured, layered system where density and temperature change significantly with height. Primarily, scientists divide the atmosphere into five distinct layers based on their temperature gradients (how temperature changes with altitude). These layers are the Troposphere, Stratosphere, Mesosphere, Thermosphere, and Exosphere FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Composition and Structure of Atmosphere, p.65. Density is always highest near the surface due to gravity and rapidly decreases as we move upward. The Troposphere is the most critical layer for life, containing nearly 90% of the atmosphere's total mass and almost all its water vapor Environment and Ecology, Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.7. In this layer, temperature decreases with height at an average rate of 6.4°C per kilometer, known as the Normal Lapse Rate. This happens because the atmosphere is largely transparent to incoming solar radiation but is heated from below by the Earth’s surface through outgoing terrestrial (longwave) radiation Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.295. Interestingly, the troposphere is much thicker at the Equator (about 18 km) than at the Poles (about 8 km) because intense heat at the Equator triggers strong convectional currents that push the air upward. Beyond the troposphere, the temperature trends shift. In the Stratosphere (12–50 km), temperatures actually rise with altitude because the Ozone layer absorbs ultraviolet radiation. The Mesosphere (50–80 km) sees temperatures drop again to the lowest levels in the atmosphere. In the Thermosphere (80–700 km), temperatures soar as gas molecules absorb high-energy X-rays and UV radiation from the sun, though the air is so rarefied (thin) that it wouldn't feel "hot" to a human Physical Geography by PMF IAS, Earths Atmosphere, p.274. Finally, the Exosphere represents the outermost fringe where individual atoms of light gases like Hydrogen and Helium gradually leak into space.

Layer	Approx. Altitude	Temperature Trend	Key Feature
Troposphere	0 - 13 km (avg)	Decreases	Weather, 90% mass, Normal Lapse Rate.
Stratosphere	13 - 50 km	Increases	Ozone layer, aircraft flight.
Mesosphere	50 - 80 km	Decreases	Coldest layer, meteor burning.
Thermosphere	80 - 700 km	Increases	Ionosphere, high energy absorption.
Exosphere	Above 700 km	High (Atomic)	Atmospheric escape, light gases.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Composition and Structure of Atmosphere, p.65; Environment and Ecology, Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.7; Physical Geography by PMF IAS, Earths Atmosphere, p.274; Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.295

3. Atmospheric Pressure and Gravity-Induced Density (intermediate)

To understand the atmosphere, we must first view it as a fluid held in place by Gravity. Atmospheric pressure is essentially the weight of the column of air resting on a unit area of the Earth's surface. Because air is a gas, it is highly compressible; gravity pulls the bulk of the atmospheric mass toward the surface, making the air densest at sea level. In fact, nearly 99% of the atmosphere's total mass is concentrated within just 32 km of the Earth's surface. As you ascend, there is less air above you to exert weight, and the gravitational pull weakens slightly, leading to a rapid decrease in pressure Physical Geography by PMF IAS, Pressure Systems and Wind System, p.305.

The relationship between Density, Pressure, and Temperature is dynamic. When air is heated, it expands and becomes less dense, leading to lower pressure; conversely, cooling causes air to compress and become denser NCERT Class XI: Fundamentals of Physical Geography, Atmospheric Circulation and Weather Systems, p.76. This density isn't just about how "packed" the molecules are, but also which molecules are present. In the lower 80 km of the atmosphere (the Homosphere), the air is well-mixed by turbulence. However, above this, in the Heterosphere, gravity begins to sort gases based on their atomic weight. Heavier gases like Nitrogen and Oxygen settle at the bottom, while lighter gases like Helium and Hydrogen float to the outermost margins of the Exosphere Physical Geography by PMF IAS, Earths Atmosphere, p.271.

It is important to note that the decrease in pressure with altitude is not perfectly linear. On average, pressure drops by about 34 millibars for every 300 metres of height, but because air density is so sensitive to changes in temperature and water vapor, this rate varies Physical Geography by PMF IAS, Pressure Systems and Wind System, p.305. This vertical variation is what creates the "buoyancy" that allows warm, less-dense air parcels to rise until they cool and eventually sink again when they become denser than their surroundings Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.298.

Feature	Lower Atmosphere (Homosphere)	Upper Atmosphere (Heterosphere)
Gas Distribution	Uniform mixture of gases due to turbulence.	Layered by atomic weight (Gravity-induced sorting).
Density	High; contains the bulk of atmospheric mass.	Extremely rarefied; individual atoms move freely.

Sources: Physical Geography by PMF IAS, Pressure Systems and Wind System, p.305; NCERT Class XI: Fundamentals of Physical Geography, Atmospheric Circulation and Weather Systems, p.76; Physical Geography by PMF IAS, Earths Atmosphere, p.271; Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.298

4. The Ionosphere and Solar Interaction (intermediate)

The Ionosphere is not a distinct thermal layer like the troposphere; rather, it is a functional layer defined by its electrical properties. It primarily overlaps with the thermosphere, extending from about 80 km to 400 km above the Earth's surface Physical Geography by PMF IAS, Earths Atmosphere, p.278. The story of the ionosphere is one of a violent but invisible interaction: high-energy solar radiation—specifically Extreme UltraViolet (EUV), X-rays, and cosmic rays—bombards the thin gases of the upper atmosphere. This radiation is so powerful that it knocks electrons off neutral atoms and molecules, a process known as ionization Environment and Ecology by Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.8. This creates a region filled with free electrons and positively charged ions, essentially turning the upper atmosphere into a giant conductive plasma shell.

For us on the ground, the ionosphere acts as a "mirror in the sky." When radio waves are transmitted from Earth at certain angles, they interact with this soup of charged particles. Instead of passing through into space, they are refracted (bent) back toward the Earth. This phenomenon is called Skywave Propagation Physical Geography by PMF IAS, Earths Atmosphere, p.278. It allows radio signals to travel thousands of miles beyond the horizon, bypassing the curvature of the Earth. However, this "mirror" has its limits. If a radio signal has a frequency higher than the critical frequency, the ionosphere can no longer bend it back, and the signal escapes into outer space—which is why satellite communication uses much higher frequencies to "punch through" the ionosphere.

Wave Type	Behavior in Ionosphere	Primary Use
Skywaves	Reflected/Refracted back to Earth	Medium and Shortwave radio (long-distance)
Microwaves/Satellite	Passes through the layer into space	GPS, Satellite TV, and space research
Ground Waves	Travels along the Earth's surface; ignored by Ionosphere	Local AM radio (short-distance)

It is important to note that the ionosphere is highly dynamic. Because it is created by solar radiation, its thickness and reflectivity change between day and night. During the day, the radiation is intense, creating multiple sub-layers (D, E, and F layers). At night, the lower layers (like the D-layer) largely disappear as ions and electrons recombine into neutral atoms, often making long-distance radio reception clearer at night because the signals can reach higher layers before being reflected Physical Geography by PMF IAS, Earths Atmosphere, p.278.

Sources: Physical Geography by PMF IAS, Earths Atmosphere, p.278-279; Environment and Ecology by Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.8

5. Chemical Classification: Homosphere and Heterosphere (intermediate)

While we often classify the atmosphere by temperature (Troposphere, Stratosphere, etc.), geographers also use a chemical classification to understand how gases behave at different altitudes. This system divides our air into two primary regions: the Homosphere and the Heterosphere. The fundamental difference between them is whether the gases are mixed together or sorted into layers.

The Homosphere extends from the Earth's surface up to an altitude of approximately 80 km. Even though the air becomes significantly thinner as you climb, the ratio of major gases—Nitrogen (78%) and Oxygen (21%)—remains virtually identical throughout this zone. This uniformity is maintained by constant turbulent mixing and convection currents. There are only a few exceptions to this rule: water vapor varies near the surface, and a concentrated belt of Ozone (O₃) exists between 19–50 km Majid Hussain, Environment and Ecology, p.7. This stable chemical mix is a relatively recent development in Earth's long history, having been reached about 600 million years ago.

Above 80 km, we enter the Heterosphere. Here, the air is so rarefied that molecules rarely collide, meaning there is no turbulent mixing to keep the gases blended. Instead, gravity takes over and sorts the gases into distinct layers based on their atomic weight Majid Hussain, Environment and Ecology, p.6. Heavier gases like molecular nitrogen (N₂) and atomic oxygen (O) stay in the lower parts of the heterosphere, while the lighter elements, Helium (He) and Hydrogen (H), migrate to the outermost fringes.

Feature	Homosphere	Heterosphere
Altitude	Sea level to ~80 km	80 km to ~10,000 km
Gas Mix	Uniform (78% N₂, 21% O₂)	Layered by molecular weight
Driving Force	Convection & Turbulent Mixing	Gravitational Differentiation

The very top of the heterosphere, starting around 480 km, is known as the Exosphere. In this "outer sphere," the atmosphere is so thin it is essentially a vacuum. Individual atoms of hydrogen and helium are only weakly held by Earth's gravity, and many eventually escape into deep space PMF IAS, Physical Geography, p.280.

Sources: Environment and Ecology, Majid Hussain, Basic Concepts of Environment and Ecology, p.6-7; Physical Geography by PMF IAS, Earth's Atmosphere, p.271, 280

6. Gas Stratification by Atomic Weight in the Heterosphere (exam-level)

To understand the upper reaches of our atmosphere, we must distinguish between the Homosphere (below 80 km), where gases are mixed uniformly, and the Heterosphere (above 80 km), where they are not. In the Heterosphere, the air is so thin (rarified) that there is insufficient turbulence to keep gases blended Environment and Ecology, Majid Hussain, Chapter 1, p.7. Instead, gravity acts as a sorting mechanism, arranging gases into distinct layers according to their atomic weight. This process is known as gaseous stratification.

In this stratified environment, heavier molecules are pulled closer to the Earth, while lighter atoms drift toward the edge of space. The Heterosphere is generally organized into four distinct shells:

• Molecular Nitrogen (N₂): Found at the lowest level of the heterosphere. Nitrogen atoms are bonded by strong triple bonds, making them relatively heavy Science, NCERT Class X, Carbon and its Compounds, p.60.
• Atomic Oxygen (O): Forms the next layer. By an altitude of 120 km, oxygen gas becomes negligible in its standard molecular form (O₂) as it dissociates into single atoms Fundamentals of Physical Geography, NCERT Class XI, Composition and Structure of Atmosphere, p.66.
• Helium (He): A light noble gas that dominates the layer above oxygen.
• Hydrogen (H): The lightest element, forming the outermost fringe of the atmosphere.

As we move into the Exosphere (the uppermost part of the heterosphere starting above 480 km), the atmosphere becomes nearly a vacuum. Here, individual atoms of hydrogen and helium are only weakly bonded by gravity, eventually escaping into outer space Environment and Ecology, Majid Hussain, Chapter 1, p.6. While helium is merely a trace gas at sea level (about 5 ppm), it becomes a dominant constituent in these extreme altitudes because the heavier nitrogen and oxygen are virtually absent Physical Geography by PMF IAS, Earths Atmosphere, p.271.

Feature	Homosphere (0–80 km)	Heterosphere (80 km+)
Gas Distribution	Uniform mixture (Uniformity)	Layered by atomic weight (Stratification)
Primary Driver	Convection and Turbulence	Gravity and Molecular Diffusion
Top Layer	Same ratio (78% N₂, 21% O₂)	Hydrogen (lightest gas)

Sources: Environment and Ecology, Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.6-7; Fundamentals of Physical Geography, NCERT Class XI, Composition and Structure of Atmosphere, p.66; Physical Geography by PMF IAS, Earths Atmosphere, p.270-280; Science, NCERT Class X, Carbon and its Compounds, p.60

7. The Exosphere and Atmospheric Escape (exam-level)

The Exosphere is the outermost fringe of our atmosphere, beginning approximately 400 to 480 km above the Earth's surface and extending thousands of kilometers until it merges with the vacuum of space Physical Geography by PMF IAS, Chapter 20, p.279. In this layer, the air is so rarefied (thin) that individual atoms can travel hundreds of kilometers without colliding with one another. Unlike the lower layers where gases are mixed uniformly (the Homosphere), the exosphere is part of the Heterosphere. Here, gravity sorts gases by their atomic weight: heavier nitrogen and oxygen stay lower, while the lightest elements—Hydrogen (H) and Helium (He)—migrate to the very edge of space Environment and Ecology, Majid Hussain, Chapter 1, p.6.

A fascinating phenomenon occurs here known as Atmospheric Escape (or atmospheric stripping). This is the process where gas molecules permanently leave the Earth's atmosphere. This happens when a molecule achieves escape velocity, fueled either by intense thermal energy from direct sunlight or by being 'kicked' by the high-energy particles of the Solar Wind. While Earth is constantly losing these light gases, our Magnetosphere acts as a vital shield, deflecting the majority of the solar wind and preventing it from stripping away our entire atmosphere Physical Geography by PMF IAS, Chapter 20, p.280.

The boundary where the Earth's magnetic pressure balances the solar wind's pressure is called the Magnetopause. Interestingly, the magnetosphere isn't a perfect sphere; it is compressed into a blunt shape on the side facing the Sun and stretched out into a long magnetotail on the opposite side, extending beyond 200 Earth radii Physical Geography by PMF IAS, Chapter 20, p.66.

Feature	Characteristics in the Exosphere
Primary Gases	Hydrogen and Helium (weakly bonded by gravity).
Temperature	Increases with height due to direct exposure to solar radiation.
Molecular Motion	High kinetic energy; particles follow ballistic trajectories into space.

Sources: Physical Geography by PMF IAS, Earths Atmosphere, p.279-280; Environment and Ecology, Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.6; Physical Geography by PMF IAS, Earths Magnetic Field (Geomagnetic Field), p.65-66

8. Solving the Original PYQ (exam-level)

To solve this, you must apply the concept of Atmospheric Stratification. While you learned that the Homosphere (up to 80 km) maintains a uniform mix of gases like Nitrogen and Oxygen, the Heterosphere behaves differently. In this upper region, gravity acts as a filter: heavier gases sink while lighter gases migrate upward. This process, known as gravitational separation, is the fundamental building block that explains why the composition changes as we move toward space. According to Environment and Ecology by Majid Hussain, gases in the heterosphere are distributed in distinct shells based on their atomic weight.

The reasoning follows a simple logic of elimination by density. As you move through the Ionosphere and into the outermost reaches, the density of heavy molecules like Oxygen (O2) and Nitrogen (N2) drops to nearly zero. In the Exosphere—the final frontier starting above 480 km—the air is so rarefied that only the lightest elements, Hydrogen and Helium, remain. Therefore, while Helium is just a trace gas at sea level (about 5 ppm), its relative percentage composition becomes maximum in the (C) Exosphere because there are virtually no other gases left to compete with it. As noted in Physical Geography by PMF IAS, these atoms are only weakly bonded by gravity at such extreme altitudes.

UPSC often uses the Troposphere or Stratosphere as traps because they contain the bulk mass of the atmosphere. However, the question asks for percentage composition, not total volume. In the lower layers, the overwhelming presence of Nitrogen (78%) and Oxygen (21%) keeps the Helium percentage negligible. Similarly, the Ionosphere is a region of ionized particles overlapping the Thermosphere; it is not the terminal layer where the lightest gases finally concentrate. Always distinguish between total quantity and relative proportion when tackling atmospheric chemistry questions.