Birth of the Sky

S1, E1: Today, Earth is enveloped by a thin veil of gas, a narrow band of atmosphere that protects a world covered in lush green vegetation, deep blue oceans and abundant life. But 4.5 billion years ago, Earth was a very different place; a hellscape of molten lava and barren rock, under relentless bombardment from meteors, and with no atmosphere whatsoever. So how did our familiar blue sky come to be?