Explanation, using text and graphics, of the evolution of Earths atmosphere, from the planets just-formed era (very warm hydrogen and helium); through release, as Earths crust formed, of volcanic gase
The gases which formed the Earth's atmosphere -- and probably its oceans -- did not come from inside the Earth but from outer space, according to a new study.
When our atmosphere formed, it wasn't formed with oxygen. In fact, it was deadly and poisonous. These things called Cyanobacteria, which made oxygen and started the oxygen trend in Earth's atmosphere.
Earth's atmosphere formed as early as 4.6 billion years ago. How it formed is actually pretty interesting. When Earth didn't have an atmosphere, everything on the surface of the Earth was molten and really hot. But as Earth got cooler, gases and solids formed the atmosphere.