Question

what would have happened if there was no atmosphere

Answer 1

We will not be able to live because all airs and gases are present due to atmosphere and if we were not here so there will no option to think more about your question and about UV RAYS heats earth so I am sure about this statement that earth was actually a fire ball billions of years ago

Answer 2

if there is no atmosphere we cannot get air and the uv rais of the sun will hits the earth the life will wanish from the earth