Earth's atmosphere
The layer of gases around Earth. Today it is about 78 percent nitrogen and 21 percent oxygen, with small amounts of argon and carbon dioxide.
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Analogy
The atmosphere is like a protective coat, with air to breathe and a weather system.
Example
Nitrogen: about 78%Oxygen: about 21%Argon and others: about 1%Carbon dioxide: about 0.04%
In practice
Early Earth had little oxygen. Plants and algae made oxygen by photosynthesis over billions of years.
Questions people ask
How did oxygen get into the atmosphere?
Early Earth had mostly carbon dioxide from volcanoes. Around two to three billion years ago, algae and plants began photosynthesis, taking in carbon dioxide and releasing oxygen. Oxygen slowly built up to the 21 percent we have today, making animal life possible.
What causes acid rain?
Burning fossil fuels containing sulfur makes sulfur dioxide, and hot engines make nitrogen oxides. These dissolve in rainwater to form acidic solutions. Acid rain damages trees, harms fish in lakes and eats away stone buildings. Removing sulfur from fuel and fitting catalytic converters reduces it.