Where the air came from
Scientists think the earth’s atmosphere has changed during the billions
of years since the earth was formed. At first, the atmosphere was
probably very different from what it is now. It began to change as some
of the gases went off into space.
For many millions of years the earth was fiercely hot. Clouds of hot
gases from inside the earth came out of cracks and volcanoes. As these
clouds spread out, they became part of the atmosphere. These gases
changed the atmosphere, but, you couldn’t have breathed the air. It had
no oxygen to keep you alive. It was made up mostly of steam, and gases
called nitrogen and carbon dioxide.
But as the earth grew cooler, some of the steam broke up into the two
gases steam is made of—and one of those gases is oxygen. So, some
oxygen became part of the air. Much of the steam became water, and
formed the first ocean.
After millions of years there were billions of tiny green plants in the
ocean. These plants, called diatoms, made their own food out of water,
sunlight, and carbon dioxide gas, just as green plants do today. When
plants make food, they give off oxygen, so for millions of years the
little diatoms put oxygen into the air, helping to make the kind of
atmosphere we have now.
