Earth as a System: Energy, Matter and Life Class 9 Notes

These notes on Earth as a System: Energy, Matter and Life for Class 9 follow the chapter in order: the five spheres, energy from the Sun, how surfaces and the atmosphere handle that energy, winds and currents, the cycles of water, carbon, nitrogen and oxygen, and how humans affect them. You get a worked energy calculation, a revision list and important questions with answers.

Earth's five interacting spheres

Earth is one connected system in which energy and matter keep moving between five spheres:

A change in one sphere sets off changes in others. Less snowfall (cryosphere) means lower lake and river levels in summer (hydrosphere), so less grass grows (biosphere), and the animals that eat it suffer. Similarly, warmer oceans increase evaporation, which can make monsoons swing between floods and droughts.

Energy from the Sun

The Sun's energy reaches Earth as electromagnetic (EM) waves. They travel at the speed of light, 3 × 10⁸ m/s, and unlike sound they need no medium. The full range, from high-frequency gamma rays to low-frequency radio waves, is the EM spectrum.

About 99% of the Sun's energy reaching Earth is in three regions:

X-rays and gamma rays are mostly filtered out by the upper atmosphere, so they do not heat the surface much.

Solar constant, insolation and albedo

The solar constant is the average solar energy received per unit time per unit area, perpendicular to the Sun's rays, at the top of the atmosphere: about 1.4 kW/m² (1400 J/s/m²). Clouds, dust and gases scatter and absorb some of it, so the maximum insolation (incoming solar radiation) at the surface on a clear day is about 1 kW/m². Anna Mani mapped solar insolation across India in the 1950s and showed the country's large solar energy potential.

Energy received: E = intensity × area × time

Worked example (from the chapter): 1 m² for one hour (3600 s) at 1 kW/m².

• E = 1000 J/s/m² × 1 m² × 3600 s = 3,600,000 J = 3.6 × 10⁶ J
• This equals one unit of household electricity.

Albedo is the fraction of sunlight a surface reflects. Snow has a high albedo (0.80–0.90) and stays cool. Black soil and ocean water have low albedo, absorb more and warm up. Concrete and asphalt soak up heat, so cities become urban heat islands, warmer than nearby villages. And because Earth is round, the Sun's rays strike the equator directly but hit the poles at a slant, spreading the energy out.

The atmosphere

The atmosphere is about 78% nitrogen and 21% oxygen.

Outer space is considered to start at about 100 km.

Greenhouse effect: the surface absorbs sunlight and re-radiates it as infrared heat. Greenhouse gases such as CO₂, methane (CH₄) and water vapour trap this heat and keep Earth warm. K.R. Ramanathan found in 1934 that ozone levels over the Himalayas were lower than expected, which helped explain how UV absorption changes with altitude.

Winds and ocean currents

Uneven heating creates pressure differences. Wind blows from high pressure to low pressure.

Valley breeze (day): sunlight heats the slopes faster than the valley floor; warm air over the slopes rises, making low pressure; cooler valley air moves up the slope. At night a mountain breeze forms as cold air sinks into the valley.

Globally, air rises at the equator (low pressure) and sinks around 30° North and South (sub-tropical high pressure). Earth's rotation deflects winds to the right in the Northern Hemisphere and to the left in the Southern (the Coriolis effect).

Ocean currents are driven by wind, temperature and salinity. Warm equatorial water flows toward the poles at the surface; cold, dense water returns deeper down. Deflection forms circular gyres. Currents such as the North Atlantic Drift carry heat and regulate climate.

Cycles of matter

Water cycle: evaporation, transpiration, condensation and precipitation link the cryosphere, hydrosphere and geosphere. Climate change is intensifying it: heavier monsoons in some places, droughts in others.

Carbon cycle:

• Fast cycle: plants take in CO₂ for photosynthesis and release it in respiration.
• Slow cycle: buried organic matter turns into fossil fuels over millions of years.
• Burning fossil fuels has raised atmospheric CO₂ from 315 ppm to 420 ppm since 1960.

Nitrogen cycle: N₂ is 78% of air but unreactive. It is fixed by bacteria (Rhizobium, Azotobacter) or lightning; humans fix it with the Haber-Bosch process to make ammonia for fertilisers.

• Nitrification: Nitrosomonas turns ammonia into nitrite (NO₂⁻); Nitrobacter turns nitrite into nitrate (NO₃⁻), which plants absorb.
• Ammonification: decomposers return nitrogen to soil as ammonia.
• Denitrification: bacteria such as Pseudomonas turn nitrates back into N₂.

Oxygen cycle: photosynthesis releases O₂; respiration and burning use it and release CO₂.

Human impact and solutions

• Too much fertiliser causes eutrophication: extra nitrates trigger algal blooms that use up oxygen and kill aquatic life.
• Vehicle exhaust reacts with sunlight to form smog and ground-level ozone, which harm health.
• The Montreal Protocol cut CFCs, helping the ozone layer slowly recover.
• India's Mission LiFE encourages eco-friendly habits such as saving water and switching to solar power.

Remember this

• Five spheres: geo, hydro, cryo, atmo, bio.
• Solar constant ≈ 1.4 kW/m²; surface insolation ≈ 1 kW/m².
• High albedo = more reflection, less heating.
• Troposphere: weather, cooling with height. Stratosphere: ozone, warming with height.
• Nitrosomonas: ammonia → nitrite. Nitrobacter: nitrite → nitrate.

Important questions with answers

1. Which sphere includes the Thar Desert and Earth's interior? The geosphere.

2. How is the solar constant different from surface insolation? The solar constant (about 1.4 kW/m²) is measured at the top of the atmosphere. Surface insolation (about 1 kW/m²) is lower because gases, clouds and dust absorb and scatter some energy.

3. A 2 m² panel receives 1 kW/m² for 30 minutes. How much energy falls on it? 30 minutes = 1800 s. E = 1000 × 2 × 1800 = 3,600,000 J = 3.6 × 10⁶ J.

4. Why are polar regions colder than the equator? Snow and ice have high albedo and reflect sunlight, and the Sun's rays hit the poles at a slant, spreading energy over a larger area.

5. The ground is at 30 °C. Roughly what is the temperature 2 km up in the troposphere? It falls about 6.5 °C per km: 6.5 × 2 = 13 °C. So about 30 − 13 = 17 °C.

6. In which layer does temperature rise with height, and why? The stratosphere, because its ozone absorbs UV radiation and heats the air.

7. How does Earth's rotation affect winds? It deflects them to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

8. What do Nitrosomonas and Nitrobacter do? Nitrosomonas turns ammonia into nitrite; Nitrobacter turns nitrite into nitrate for plants.

9. What is eutrophication? Excess nitrates from fertilisers cause algal blooms that use up oxygen in water and kill aquatic life.

Common mistakes to avoid

• Mixing up hydrosphere (liquid water) and cryosphere (ice and snow).
• Saying snow has low albedo. It reflects most sunlight, so its albedo is high.
• Thinking temperature always falls with height. In the stratosphere it rises.
• Swapping nitrification and denitrification.

To revise the spheres and cycles with quick quizzes, study this chapter with Joy.