Earth's security blanket
Scientists believe our atmosphere is the third security blanket the planet has had. When the Earth was formed some 4.5 billion years ago, gases released by a slowly cooling crust created a first atmosphere of helium and hydrogen.
After 500 million years, the crust cooled and volcanoes poured carbon dioxide, water vapour and ammonia into the air. As this second atmosphere contained lots of greenhouse gases, the planet was much hotter than it is now.
Over time, water vapour from the atmosphere condensed, fell to the ground (rain) and formed oceans. Microbes that turned carbon dioxide into oxygen appeared and flourished. This early life transformed the air again, creating the third atmosphere we live in today.