Module 9 — Components and Layers of the Atmosphere

• The unit opens with questions: Which planets in the solar system have an atmosphere?
• How thick is it?
• What are its components?
• What would happen if Earth had no atmosphere?

• Which planets have atmospheres? All the planets have some gas around them except for Mercury, which has only a vanishingly thin envelope. Venus has a very thick atmosphere of carbon dioxide (surface temperature about 465 °C because of a runaway greenhouse effect); Mars has a very thin atmosphere (mostly CO₂, pressure less than 1 % of Earth's); Jupiter, Saturn, Uranus and Neptune are surrounded by enormous atmospheres of hydrogen and helium.
• Earth's atmosphere is a thin shell compared with the size of the planet: about 99 % of its mass lies within the lowest 30 km, which is less than 0.5 % of Earth's radius (6371 km). Its outer edge fades gradually into space at about 1000 km.
• Without an atmosphere Earth would resemble the Moon or Mercury: no air to breathe; no liquid water (it would boil away or freeze because of the low pressure); extreme temperature differences (+120 °C in sunlight, −150 °C at night); no protection from harmful ultraviolet rays and meteors; no weather, wind or rain; no sound; and no life as we know it.

• In the previous unit you studied the hydrosphere, one of the four spheres of the environment.
• Now we move to another sphere — the atmosphere.

The atmosphere

• is a layer of gases that surrounds planet Earth.
• It is mainly composed of nitrogen, oxygen and carbon dioxide, along with argon and trace amounts of other gases.

The proportions of the gases are not constant: they change continuously through natural processes in which living organisms play a pivotal role.

• Respiration — living organisms consume oxygen from the atmosphere and release carbon dioxide into it.
• Photosynthesis — plants (and algae) absorb carbon dioxide from the air and release oxygen.
• Geological phenomena, such as volcanic eruptions, add large amounts of gases (water vapour, CO₂, SO₂, N₂, H₂S, and dust particles) to the atmosphere.
• Decomposition of dead plants and animals releases nitrogen compounds that bacteria convert to N₂ (denitrification), and the ocean exchanges CO₂ and O₂ with the air (Module 7).
• Human activities — burning fuels, industry, agriculture, deforestation — change the proportions of CO₂, CH₄, NOx, SO₂ and others (Modules 10 and 12).

The cycles of these processes are normally balanced over long times: the atmosphere is a dynamic system.

1. A protective insulating cover. It shields Earth from harmful solar radiation (especially UV, absorbed by ozone) and from objects arriving from space (meteoroids burn up by friction with the air), which could cause significant damage to living things.
2. Temperature regulation. It retains part of the solar radiation (the natural greenhouse effect), keeping the average surface temperature at about 15 °C instead of about −18 °C. This keeps the temperature at moderate levels suitable for life.
3. The water cycle. The atmosphere takes part in evaporation, condensation and precipitation, distributing fresh water over the planet's surface (Module 1).
4. Source of gases for life: O₂ for respiration and combustion; CO₂ for photosynthesis; N₂ (via bacteria and lightning) for protein formation.
5. Pressure that allows liquid water to exist and allows us to breathe; a medium for sound, wind, and flight of birds and aircraft.
6. A medium for radio communication, because the ionosphere reflects radio waves.

(78 + 21 + 0.93 + 0.04 ≈ 99.97 %; the rest is the trace gases. In real air, water vapour varies between almost 0 and 4 % depending on place and temperature.)

• Enters the atmosphere from the decomposition of dead plants and animals and from the gases of volcanic eruptions.
• It is largely inert: it does not easily react with other gases or elements, because the two atoms in the N₂ molecule are joined by a very strong triple covalent bond (N≡N, bond energy ≈ 945 kJ/mol). Reactions of nitrogen need special conditions: lightning or extremely high temperatures (such as in engines).
• As a result, the proportion of nitrogen oxides in the atmosphere is very low. (This answers the question: "although nitrogen is highly abundant its oxides are found in very small quantities because there is a strong triple bond between the two nitrogen atoms, making it largely inert".) Lightning makes NO (N₂ + O₂ → 2NO), which is oxidised to NO₂ and carried down by rain as nitrate, a natural fertiliser.
• Biologically, nitrogen is needed for proteins and nucleic acids, but most organisms cannot use N₂ directly; nitrogen-fixing bacteria in soil and root nodules do it.

• One of the most abundant gases in the atmosphere; it is mainly produced by photosynthesis of food-producing organisms (plants, algae, cyanobacteria).
• It is chemically active; essential for the respiration of organisms; plays a key role in combustion and in natural and industrial oxidation reactions (rusting, burning, making steel, etc.).
• About 21 % of the air by volume (about 23 % by mass).

• A noble (inert) gas, like helium and neon. It does not react with anything. It is used in lamps and welding to provide an inert atmosphere.

• Its concentration varies considerably from one region to another (higher near cities, lower over forests during the day) and from season to season.
• It is a greenhouse gas that traps heat in the atmosphere and prevents it from escaping into space — the greenhouse effect (Module 12). It is also the carbon source for plants (photosynthesis).

• Water vapour (H₂O), sulfur dioxide (SO₂), ozone (O₃), carbon monoxide (CO), nitrogen dioxide (NO₂). Several of these are pollutants or reactive trace gases that you study in Modules 10 and 12.

• Problem. A classroom measures 8 m × 6 m × 3 m.
• Assume it is filled with dry air at the composition given above.
• Calculate the volume of each of nitrogen, oxygen, argon and carbon dioxide.
• What does the answer say about our 'safe' limit of CO₂?

Solution. Volume = 8 × 6 × 3 = 144 m³. N₂: 0.78 × 144 = 112.3 m³; O₂: 0.21 × 144 = 30.2 m³; Ar: 0.0093 × 144 = 1.34 m³; CO₂: 0.0004 × 144 = 0.058 m³ = 58 L. A person exhales about 40 000 ppm (4 %) of CO₂; so in a crowded, unventilated room the CO₂ level rises and above about 1000 ppm (0.1 %) people feel drowsy, which is why classrooms need ventilation.

Problem. Compare the amounts of O₂ and CO₂ in the air.

Solution. 21 %/0.04 % = 525; so air has about 500 times as much oxygen as carbon dioxide (the number used in Module 7).

• The atmosphere is divided into layers according to the way temperature changes with height (see the temperature graph in the book: falls, rises, falls, rises).
• From the Earth's surface upward:

• The layer closest to the surface, extending on average up to about 10 km (higher above the equator, ~16–17 km, and lower at the poles, ~8 km). It contains about 75–80 % of the mass of the atmosphere and nearly all the water vapour; it is the densest layer.
• Weather phenomena occur here: cloud formation, rainfall, wind and other climatic events.
• Temperature: the average temperature at the Earth's surface under this layer is about 15 °C and decreases with altitude at about 6.5 °C per kilometre (a rate of 1 °C per 154 m).

Why does temperature decrease with altitude in the troposphere?

• The air is heated mainly from below (the ground absorbs sunlight and warms the air in contact with it), and air pressure decreases with altitude.
• A rising parcel of air expands because the surrounding pressure is lower; the expansion needs work, which is taken from the kinetic energy of the air molecules, so the air cools.
• (The lower part of the troposphere is also nearer to the warm ground, so it is warmer than the upper part — the "Critical Thinking" question in the book.)

• The temperature fall continues until the tropopause, the upper boundary of the troposphere at an altitude of about 11 km, where temperature is about −56 °C.

Tropopause.

• A thin layer of air at the top of the troposphere which acts as a barrier or transition zone between the troposphere and the stratosphere.
• It prevents the cold air of the troposphere from mixing with the warm air of the stratosphere above.

Biodiversity in the troposphere.

• The troposphere is home to the richest and most diverse forms of life on Earth.
• Biodiversity means all the living organisms in a specific place, from soil microbes to the largest plants and animals.
• Life is possible in the troposphere because it has oxygen and carbon dioxide, water vapour and other gases (the essential components of life), the right temperature for biological processes, and a suitable pressure for most organisms.
• The interaction of these conditions with the enormous variety of organisms makes the troposphere the most biologically active layer and the most important for sustaining life.

• The layer above the troposphere, from about 10 km to 50 km.
• Temperature: relatively constant in the lower stratosphere (about −56 °C) up to about 20 km, where the ozone layer begins. Within the ozone layer, which extends up to about 40 km, the temperature increases with altitude (reaching about 0 °C at 50 km) because the ozone absorbs ultraviolet (UV) radiation from the Sun and converts the energy to heat.
• Because warm air lies above cold air, there is no convection: the air movement is mostly horizontal, which makes the stratosphere stable with little turbulence — a preferred layer for aircraft flight (long-distance airliners cruise at 10–12 km, the lower stratosphere).

The ozone layer.

• It is an indispensable shield for protecting life and sustaining the biosphere.
• Ozone molecules (O₃) absorb most of the harmful UV radiation coming from the Sun.
• If large quantities of UV reached Earth's surface they could cause severe damage to the DNA in living cells and lead to serious diseases such as skin cancer, and could damage land and aquatic ecosystems (for example plankton).
• The formation and depletion of ozone are studied in Module 10.

• From 50 km to about 80 km.
• The coldest layer of the atmosphere, with temperatures down to about −93 °C (some measurements reach −100 °C), because the layer contains few gas molecules able to absorb the Sun's radiation and ozone is scarce.
• Protective role: most meteors entering the atmosphere from space burn up and disintegrate in this layer (the air here is dense enough to heat them by friction/compression). We see them as "shooting stars".

• From 80 km up to about 700 km above the surface.
• Nitrogen and oxygen absorb high-energy solar radiation, so the temperature rises dramatically, reaching over 2000 °C. (This temperature measures the very high kinetic energy of the very few particles present. Because the air is so thin, an astronaut or satellite there would not feel "hot": there are too few collisions to transfer much heat.)

The ionosphere

• is the lower part of the thermosphere, from 80 km to about 550 km.
• There, oxygen and nitrogen atoms absorb very high-frequency solar radiation (X-rays and gamma rays).
• This radiation ionises the gas atoms — they lose or gain electrons and become electrically charged — so this region has a high concentration of charged ions (hence "ionosphere").

• Radio communication: the ionosphere reflects electromagnetic (radio) waves, so signals can travel over very long distances around the curved Earth without satellites or cables (it works particularly for short-wave radio).
• The aurora: near the north and south poles, high-energy charged particles of the solar wind are guided by Earth's magnetic field toward the polar regions. There they collide with ions and atoms in the ionosphere and excite them (raise electrons to higher energy levels). When the atoms return to their ground state they release photons of light of various wavelengths — green and red from oxygen, blue and purple from nitrogen — producing the bright, colourful lights of the aurora (aurora borealis in the north, aurora australis in the south).

• The highest layer. (Some scientists consider it the uppermost part of the thermosphere.) It extends from about 700 km to 1000 km (and fades into space beyond).
• Gas atoms and molecules are very far apart, so they move over vast distances without colliding with one another. They can reach very high speeds, so some particles (especially hydrogen and helium) escape Earth's gravity and drift into space.
• Because of its extremely low density, an almost perfect vacuum, it is an ideal place for satellites and spacecraft, which face practically no air resistance and can stay in orbit for a long time. Satellites support international communication, wireless internet, the Global Positioning System (GPS), and provide accurate data on weather and climate, helping to predict natural disasters and monitor Earth and space.

• Problem. Using the boundaries (troposphere 0–10 km, stratosphere 10–50, mesosphere 50–80, thermosphere 80–700, exosphere 700–1000 km), find the thickness of each layer and the percentage of the total 1000 km.
• What does the answer suggest about the model in the "Activity"?

• Solution. Troposphere 10 km (1 %); stratosphere 40 km (4 %); mesosphere 30 km (3 %); thermosphere 620 km (62 %); exosphere 300 km (30 %).
• The thermosphere is by far the thickest layer, but it contains almost no gas; the troposphere is thin but holds most of the air.
• In a foam model drawn with these relative thicknesses (for instance 1 cm per 50 km: troposphere 0.2 cm, stratosphere 0.8 cm, mesosphere 0.6 cm, thermosphere 12.4 cm, exosphere 6 cm), students see why the layers' thickness and their mass/density are very different things.

Mercury

• is the smallest planet in the solar system and has no gaseous atmosphere (only a trace exosphere).
• The solar radiation reaches its surface in enormous amounts, so its temperature rises greatly (about 430 °C on the sunlit side).
• When the planet rotates and a place is no longer in sunlight, the surface radiates its heat back to space, and the temperature drops quickly (to about −180 °C) because no atmosphere retains the heat.

• Gas molecules move at extremely high speeds.
• The effective (root-mean-square) speed v_rms of the molecules depends on the temperature T and the molar mass M of the gas:

v_rms = √(3RT/M) (R = 8.314 J/mol·K, M in kg/mol, T in kelvin)

It is greater when the molecular mass is smaller and the temperature is higher. Hydrogen (M = 0.002 kg/mol) molecules move about four times as fast as oxygen molecules (M = 0.032) at the same temperature (√16 = 4).

To escape from a planet's gravity, an object — whatever its mass — must reach a particular speed, the escape velocity (v_e), which is a fixed value for each planet:

v_e = √(2GM/R) (M = planet mass, R = radius, G = 6.67 × 10⁻¹¹ N·m²/kg²)

For Earth, v_e ≈ 11.2 km/s. Values for other bodies (approximate): Moon 2.4 km/s; Mercury 4.3 km/s; Mars 5.0 km/s; Jupiter 59.5 km/s.

• If v_rms < v_e, the molecules cannot escape and the planet retains that gas — it keeps its atmosphere.
• If v_rms ≥ v_e, the molecules can escape and drift into space; the gas becomes rare or absent on the surface.

Notes.

• In reality the molecules in a gas have a range of speeds (a few move much faster than the average), so a planet loses a gas gradually when v_rms is more than about one-sixth of v_e over billions of years.
• That is why Earth has lost most of its hydrogen and helium (very light) but keeps nitrogen, oxygen and carbon dioxide (heavy), and why the Moon and Mercury have little atmosphere at all, whereas giant planets keep hydrogen.

Problem. Calculate v_rms for (a) hydrogen (M = 0.002 kg/mol) and (b) nitrogen (M = 0.028 kg/mol) at 300 K. Compare with Earth's escape velocity 11 200 m/s. Then calculate the v_rms of oxygen (M = 0.032) at 700 K on Mercury's sunlit side and compare with Mercury's v_e = 4250 m/s.

Solution. (a) H₂: √(3 × 8.314 × 300 / 0.002) = √(3.74 × 10⁶) = 1934 m/s (using M = 0.002; with 0.002016, 1924 m/s). (b) N₂: √(3 × 8.314 × 300 / 0.028) = √(2.67 × 10⁵) = 517 m/s. Both are far less than 11 200 m/s; Earth keeps nitrogen easily and (in the lower atmosphere) hydrogen; hydrogen at the hot top of the atmosphere (T ≈ 1000 K) has v_rms ≈ 3500 m/s ≈ 31 % of v_e, high enough for the fastest molecules to escape, so hydrogen slowly leaks away. (c) O₂ at 700 K: √(3 × 8.314 × 700/0.032) = √(5.46 × 10⁵) = 739 m/s — much less than 4250 m/s. So the simplified rule does not entirely explain why Mercury lacks an atmosphere: the answer is the combination of the weak gravity, the intense heating and solar wind that continuously strip gases, and the fact that fast molecules in the high-speed tail escape. The book's simplified rule is correct as a first approximation and is the one to use in answers.

Problem. Calculate the escape velocity from the Moon (M = 7.35 × 10²² kg, R = 1.737 × 10⁶ m).

Solution. 2GM/R = 2 × 6.67 × 10⁻¹¹ × 7.35 × 10²² / 1.737 × 10⁶ = 9.80 × 10¹² / 1.737 × 10⁶ = 5.64 × 10⁶ m²/s²; v_e = 2375 m/s ≈ 2.4 km/s.

Activity: Model of the atmospheric layers.

Aim. To understand the structure of the atmosphere through a visual model.

Materials. Foam sheets of different colours, scissors, glue, a ruler, a marker, cardboard backing.

Steps.

1. Decide a scale (for example 1 cm = 50 km) and cut a foam strip for each layer whose thickness is proportional to the layer's real thickness (see worked example 4).
2. Stack the strips in order from the troposphere (bottom) to the exosphere (top), colour-coding and labelling each layer.
3. Write on each layer its altitude range, temperature trend, and main features (e.g. weather, ozone, meteors burn, ionosphere/aurora, satellites).
4. Add small drawings (cloud, aircraft, meteor, satellite, aurora) at the correct heights.
5. Prepare a short description of how each layer affects life (the troposphere supports life and weather; the stratosphere's ozone absorbs UV; the mesosphere burns meteors; the thermosphere/ionosphere reflects radio waves and produces aurora; the exosphere holds satellites).

Expected result. A model in which the thermosphere and exosphere appear very thick while the troposphere is a thin layer at the bottom, supporting the idea that most of the atmosphere's mass is concentrated in a thin region near the surface.

• Writing that the temperature decreases with height throughout the atmosphere. It decreases in the troposphere and mesosphere and increases in the stratosphere and thermosphere.
• Confusing high temperature in the thermosphere with high heat: the density there is extremely low.
• Saying that nitrogen is not found in oxides because it is unreactive "at all". Say: N₂ has a strong triple bond; it reacts only under lightning or very high temperatures.
• Confusing the ionosphere (a region of the thermosphere with ions) with the ozone layer (in the stratosphere).
• Using v_rms and v_e in different units. Use m/s for both.