The Thin Blue Line

Professor Brian Cox reveals how something as flimsy as an envelope of gas - an atmosphere - can create some of the most wondrous sights in the solar system. He takes a ride in an English Electric Lightning and flies 18 km up to the top of earth's atmosphere, where he sees the darkness of space above and the thin blue line of our atmosphere below. In the Namib desert in south-west Africa, he tells the story of Mercury. This tiny planet was stripped naked of its early atmosphere and is fully exposed to the ferocity of space.

Against the stunning backdrop of the glaciers of Alaska, Brian reveals his fourth wonder: Saturn's moon Titan, shrouded by a murky, thick atmosphere. He reveals that below the clouds lies a magical world. Titan is the only place beyond earth where we've found liquid pooling on the surface in vast lakes, as big as the Caspian Sea, but the lakes of Titan are filled with a mysterious liquid, and are quite unlike anything on earth.