Satellites show stunning images of Earth from space. Among billions of planets, what actually makes this one capable of supporting life?
Key idea
Earth's Thin Life-Supporting Layer
All known life exists only in Earth's crust
a very thin layer compared to Earth's total size.
If Earth were an apple...
The crust would be as thin as the apple's skin. This delicate layer is what makes Earth unique among planets.
Key idea
The Greenhouse Effect
1
Venus: too much greenhouse
Venus isn't the hottest planet because it's closest to the Sun — its thick CO2 atmosphere traps heat, making it hotter than Mercury.
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2
Earth: just enough
Earth's greenhouse effect — CO2 and other gases trapping heat — keeps the temperature suitable for life, not too much, not too little.
Key idea
What Makes Earth Suitable for Life
1
The habitable zone
Just the right distance from the Sun for liquid water.
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2
Just enough gravity
Strong enough to hold an atmosphere, not so strong it crushes life.
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3
The ozone layer
Shields us from harmful ultraviolet rays.
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4
A magnetic field
Deflects harmful cosmic rays and solar wind.
Key idea
Earth's Four Life-Sustaining Spheres
1
Atmosphere
Gases for breathing, and the greenhouse blanket.
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2
Hydrosphere
All of Earth's water — oceans, rivers, groundwater.
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3
Geosphere
The solid Earth — rocks, soil, minerals.
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4
Biosphere
All living things, and the places they live.
Key idea
Asexual and Sexual Reproduction
| Parents | Offspring | Example
Asexual
One — Exact copies of the parent — Vegetative propagation, binary fission
Sexual
Two — A mix of traits from both parents — Pollination and fertilisation, animal mating
Key idea
How Plants Reproduce
Pollination → Fertilisation → Zygote forms → Seed & fruit
Key idea
How Animals Reproduce
1
External fertilisation
Gametes meet outside the body, usually in water — fish and frogs.
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2
Internal fertilisation
Gametes meet inside the female's body. Birds then lay fertilised eggs; most mammals develop the embryo inside the mother.
Key idea
The Triple Planetary Crisis
1
Climate change
Burning fossil fuels releases greenhouse gases, warming the planet.
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2
Biodiversity loss
Habitat destruction disrupts food chains and ecosystems.
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3
Pollution
Air, water, and soil pollution harm health, crops, and wildlife.
Key idea
International Agreements
Agreement | Year | Focus
Montreal Protocol
1987 — Ozone-depleting substances
Earth Summit
1992 — Environment and development
Kyoto Protocol
2005 — Greenhouse gas reduction
Paris Agreement
2015 — Limiting warming below 1.5°C
Chapter · Key terms to remember
Key Terms
Habitable zone
Distance from a star where water stays liquid.
Greenhouse effect
Heat trapped by atmospheric gases.
Atmosphere / Hydrosphere
Gases around Earth / all its water.
Chapter · Key terms to remember
More Key Terms
Geosphere / Biosphere
The solid Earth / all living things.
Asexual / Sexual reproduction
One parent, exact copies / two parents, mixed traits.
Pollination / Fertilisation
Pollen transfer / gametes fusing.
Chapter · Key terms to remember
More Key Terms
Triple planetary crisis
Climate change, biodiversity loss, pollution.
Questions for your notebook
Write these down, then discuss
1
What's one major reason Mars can't currently support life like Earth? (i) too many volcanoes (ii) too close to the Sun (iii) lacks a thick atmosphere and liquid water (iv) too strong a magnetic field.
2
Which is an example of geodiversity? (i) Variety of bird chirping (ii) Different landforms like mountains and valleys (iii) Changing monsoon weather (iv) Number of fish species in a pond.
3
If Earth were smaller with the same density, what might happen to its atmosphere? (i) thicker and hotter (ii) escape into space from weaker gravity (iii) freeze (iv) stronger winds.
Project as-is — students copy the questions, then the class discusses answers together.