🌍 Earth as a System: Energy, Matter, and Life – Class 9 Science Short Notes

Chapter 13 | Class 9 Science | NCERT Exploration

🌱 Earth is an interconnected system in which energy and matter continuously move between different spheres. The Sun is the main source of energy driving most processes on Earth.

🌎 1. The Five Spheres of Earth

The Earth is made up of five interacting spheres:

• 🪨 Geosphere: Rocks, soil, landforms and Earth's interior.
• 💧 Hydrosphere: Liquid water such as oceans, rivers, lakes and groundwater.
• ❄️ Cryosphere: Frozen water such as glaciers, snow and polar ice caps.
• 🌬️ Atmosphere: The layer of gases surrounding Earth.
• 🌱 Biosphere: All living organisms and their surroundings.

🔗 These spheres constantly interact. A change in one sphere can affect the others.

Example: Less snowfall → less melting water → less lake water → less vegetation → effect on animals.

☀️ 2. Solar Radiation and Uneven Heating

The Sun's radiation reaches Earth in the form of electromagnetic (EM) waves.

• ⚡ High frequency: Short wavelength and higher energy.
• 🌊 Low frequency: Long wavelength and lower energy.

The complete range of electromagnetic radiation is called the electromagnetic spectrum.

☀️ Most solar energy reaching Earth is in the ultraviolet (UV), visible and infrared (IR) regions.

Important Values 📌

• Speed of light ≈ 3 × 10⁸ m/s
• UV wavelength ≈ 100–400 nm
• Solar constant ≈ 1.4 kW/m²
• Maximum surface insolation under clear skies ≈ 1 kW/m²

🌞 3. Insolation and Solar Constant

Insolation: The amount of solar radiation that reaches Earth's surface.

Solar Constant: The average solar energy received per unit time per unit area at the top of the atmosphere before atmospheric absorption, scattering or reflection.

💡 Remember: Solar constant ≈ 1.4 kW/m².

⚪ 4. Albedo

Albedo is the fraction of solar radiation reflected by a surface.

• ❄️ High albedo: Reflects more sunlight and remains relatively cool.
• 🖤 Low albedo: Absorbs more sunlight and heats up quickly.

Example: Snow has high albedo, while dark surfaces generally have lower albedo.

🌍 5. Latitude and Uneven Heating

Because Earth is spherical, sunlight strikes different latitudes at different angles.

• 🌞 Equator: Solar energy is concentrated over a smaller area → more heating.
• ❄️ Poles: Solar energy spreads over a larger area → less heating.

This unequal heating produces temperature differences between the equator and poles and helps drive winds and ocean currents.

🌬️ 6. Atmosphere

The atmosphere is the layer of gases surrounding Earth. It mainly contains approximately 78% nitrogen and 21% oxygen, along with smaller amounts of other gases.

Layers of the Atmosphere

💨 7. Winds

Wind is the movement of air from areas of high pressure to low pressure.

Uneven heating of Earth's surface creates differences in air pressure, which cause winds.

Local Winds

• 🏔️ Valley Breeze: During the day, mountain slopes heat faster. Warm air rises up the slopes.
• 🌙 Mountain Breeze: After sunset, mountain slopes cool faster. Cooler, denser air moves down into the valley.

Planetary Winds 🌎

Uneven heating between the equator and poles produces global pressure belts and large-scale planetary wind patterns.

🌍 Earth's rotation also causes winds to curve:

• ➡️ Northern Hemisphere: Winds curve to the right.
• ⬅️ Southern Hemisphere: Winds curve to the left.

🌊 8. Ocean Currents

Ocean currents are the continuous movement of large masses of ocean water.

Major factors affecting ocean currents include:

• 🌡️ Differences in temperature
• 🧂 Differences in salinity
• 🌍 Earth's rotation
• 🗺️ Arrangement of land masses

Warm equatorial water generally moves towards the poles, while colder and denser water moves back towards the equator at deeper levels.

Gyres 🌀

Large circular ocean-current systems are called gyres.

🌡️ Ocean currents help regulate Earth's climate by transporting heat from warmer regions towards cooler regions.

🔄 9. Biogeochemical Cycles

Living organisms continuously exchange matter and energy with air, water, soil and rocks. The movement of essential elements between living and non-living components is called a biogeochemical cycle.

Important cycles include:

• 💧 Water Cycle
• 🌳 Carbon Cycle
• 🌱 Nitrogen Cycle
• 💨 Oxygen Cycle

💧 10. Water Cycle

The major processes are:

• ☀️ Evaporation: Water changes into water vapour.
• 🌿 Transpiration: Plants release water vapour.
• ☁️ Condensation: Water vapour forms clouds.
• 🌧️ Precipitation: Water falls as rain, snow or hail.
• ⬇️ Infiltration: Water enters the soil.
• 💧 Groundwater: Some water moves and is stored below the surface.

Climate Change and Water Cycle 🌡️

A warmer atmosphere can hold more moisture, leading to heavier rainfall in some regions and droughts in others. Melting glaciers can also contribute to rising sea levels.

🌳 11. Carbon Cycle

Carbon is an essential element of life and is present in carbohydrates, proteins, fats and DNA.

Carbon moves between:

• 🌬️ Atmosphere – mainly as CO₂
• 🌱 Biosphere – plants and animals
• 🪨 Geosphere – rocks and fossil fuels
• 🌊 Hydrosphere – dissolved carbon dioxide and marine shells

Fast Carbon Cycle

Plants absorb CO₂ through photosynthesis. Carbon then moves through food chains and returns to the atmosphere through respiration and decomposition.

Slow Carbon Cycle

Carbon can remain stored in fossil fuels and rocks for millions of years. Burning fossil fuels releases stored carbon back into the atmosphere as CO₂.

🔥 Deforestation and burning fossil fuels increase atmospheric CO₂ and contribute to global warming.

🌱 12. Nitrogen Cycle

Nitrogen is essential for making proteins and nucleic acids. Although nitrogen gas is abundant in the atmosphere, plants cannot directly use atmospheric N₂.

Main Steps

1. Nitrogen Fixation: Nitrogen gas is converted into usable nitrogen compounds.
2. Nitrification: Ammonia is converted into nitrites and nitrates.
3. Assimilation: Plants absorb nitrogen compounds from the soil.
4. Ammonification: Decomposers convert nitrogen-containing matter into ammonia.
5. Denitrification: Nitrates are converted back into nitrogen gas.

🦠 Rhizobium bacteria in the root nodules of legumes help in nitrogen fixation.

💨 13. Oxygen Cycle

Oxygen moves between the atmosphere, oceans, land and living organisms.

• 🌿 Photosynthesis adds oxygen to the atmosphere.
• 🫁 Respiration consumes oxygen and releases CO₂.
• 🔥 Combustion consumes oxygen and releases CO₂.

⚖️ The balance between oxygen production and consumption helps sustain life on Earth.

🏭 14. Human Impact on Earth's Processes

• 🔥 Burning fossil fuels: Increases atmospheric CO₂.
• 🌳 Deforestation: Reduces photosynthesis and carbon absorption.
• 🌾 Excess fertilisers: Can cause eutrophication in water bodies.
• 🏭 Air pollution: Can contribute to harmful atmospheric changes.
• 🌊 Climate change: Affects water, carbon and other Earth-system processes.

🌍 Steps Towards Restoration

• ♻️ Conserve energy and resources.
• ☀️ Promote renewable energy sources.
• 🌳 Protect and increase forests.
• 🌱 Promote sustainable farming.
• 🌏 Follow environmentally responsible lifestyles.

⭐ Important Exam Questions

Note: Since this is a newly introduced chapter, the questions below are important NCERT/exam-pattern questions, not claimed as genuine previous-year CBSE questions.

📝 1 Mark Questions

1. What is the primary source of energy for Earth?
2. Name the sphere that contains glaciers and polar ice.
3. What is insolation?
4. What is albedo?
5. Which layer of the atmosphere contains the ozone layer?
6. Where does most weather occur?
7. What are large circular ocean currents called?
8. Name the bacterium involved in nitrogen fixation in legume root nodules.

🧠 2–3 Mark Questions

1. Explain why Earth receives uneven heating from the Sun.
2. Differentiate between insolation and solar constant.
3. Explain the difference between valley breeze and mountain breeze.
4. Why do high-albedo surfaces remain relatively cool?
5. Explain how ocean currents influence Earth's climate.
6. What are biogeochemical cycles? Why are they important for life?
7. Explain the role of photosynthesis and respiration in the oxygen cycle.
8. Explain the three steps: nitrogen fixation, nitrification and denitrification.

🔥 Important Higher-Order Questions

1. A region experiences rapid melting of glaciers. Explain how this change can affect the hydrosphere and biosphere.
2. How can excessive use of fertilisers affect aquatic ecosystems?
3. Why can deforestation disturb both the carbon cycle and water cycle?
4. Explain how uneven heating of Earth's surface leads to the formation of winds.
5. Explain how burning fossil fuels affects the carbon cycle and climate.

🚀 Quick Revision – Remember These!

🎯 Exam Tip: Pay special attention to Earth's five spheres, electromagnetic spectrum, insolation, solar constant, albedo, atmospheric layers, local winds, ocean currents, water cycle, carbon cycle, nitrogen cycle and human impacts.