13.1 A Unique Planet

Probe and Ponder

What would Earth look like with no life at all? Life here has survived billions of years through asteroid impacts, ice ages and upheavals — what lets it keep going? Could plants grow on Mars if we carried soil and water there? These questions all circle one wonder: among perhaps billions of planets, Earth is the only place we know where life exists and thrives.

This is the last chapter of our journey — the place to gather everything we’ve learned and ask why our home is like nowhere else in the known universe.

Life on a sliver

As Thin as an Apple’s Skin Here is a startling fact: all of life — every mountain forest, ocean, animal and person — exists only on a very thin outer layer of the planet, the crust. From the tallest peak to the deepest trench, this life-zone is tiny compared with the whole Earth. If Earth were shrunk to the size of an apple, the crust would be no thicker than the apple’s skin. That delicate, life-supporting skin is what makes Earth so precious.

Activity 13.1 — Things we take for granted List features of Earth we rarely notice but couldn’t live without. The air doesn’t fly off into space (even though gas particles move freely — Chapter 7). Gravity holds us down, yet our hearts can still pump blood up to our heads (Chapter 5). Earth gives us air to breathe, water to drink, soil to grow food, and rock and timber to build with. The more you list, the more remarkable our ordinary planet looks.

Seeing Earth from Space Today, ISRO’s Earth Observation satellites photograph our planet — one famous image was built like a mosaic from nearly 3000 smaller pictures. These are often false-colour images, where scientists assign colours to show different information: they help us study land plants, ocean plankton, sea-surface temperature, oil spills and wind. Our newest tools (Chapter 11) let us study our oldest home.

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