The atmosphere is the skin of air around the earth. It’s gone through several different stages since the earth formed.
The quick version is that the first atmosphere came from volcanoes and was mostly water and carbon dioxide. As it cooled down it rained and made the oceans and a lot of the carbon dioxide dissolved. Later some types of algae started making oxygen, until eventually the atmosphere was like it is today.
When the earth formed it was so hot the rock was molten, like lava all over. There were lots of chemical reactions that made gases, but they were stripped away by the strong wind that comes out of the sun (solar wind) very quickly. When earth cooled a bit it formed a crust but there were still lots of volcanoes, gases came from them to form the first atmosphere. It was mostly water – the earth was so hot there were no oceans and the water was all steam and clouds.
As earth cooled more there was the first storm – all the steam in the atmosphere came down in a massive rainfall and formed the oceans, and the gases that were left made the second atmosphere, along with more gases from volcanoes. There was a lot more carbon dioxide than today, but once the oceans formed it started dissolving in them. This all took a very long time – hundreds of thousands of years.
Somewhere in the oceans life began. Early life didn’t use oxygen because there wasn’t any, it breathed other gases like methane. But some algae made oxygen, the same way plants make it today. At first it only built up very, very slowly, but oxygen was poisonous to most of the things that were alive. When there was enough of it the oxygen started to kill most things, except a few that could tolerate it and even use it. When these were just about the only living things left they spread very quickly and started making even more oxygen.
Once there was oxygen the ozone layer could form, because ozone is a type of oxygen. It is up high above the earth and protects us from ultra-violet (UV) radiation from the sun – the one that burns you. The reason being sunburnt is dangerous is that UV radiation doesn’t just burn your skin, it can damage the DNA in your skin cells and turn them into cancers. Until the ozone layer formed nothing could live on land because the UV would kill them. But once it was there life could move out of the oceans.
The atmosphere is constantly changing. Today we are putting artificial chemicals into it as air pollution, many of these have had unexpected effects. We are also increasing the carbon dioxide which is making the earth warm up. But even without us, volcanoes, fires, the way the continents move, the sun and of course other living things are all changing the atmosphere all the time.
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