By Danielle Peck and Chris Holt
Producers, Wonders of the Solar System
Professor Brian Cox explains how Saturn's moon, Titan, maintains its atmosphere
We may take ours for granted, but atmospheres are responsible for creating some of the wonders of the Solar System.
The flimsy, thin blue line seen from space that surrounds Earth makes it a wonderfully diverse place.
There were few people who disputed the interpretation of a river channel. There really wasn't very much doubt
Dr Ralph Lorenz, Huygens scientist
Our atmosphere's unique chemical formula supplies us with the air we breathe, the water that bathes us and provides a soothing blanket that keeps us protected from the violence of space.
And where there are other atmospheres, there are other wonders.
Travel away from Earth at the speed of light for half an hour and you would enter the frozen realm of the outer Solar System.
Out here are planets that are nothing but atmosphere - unimaginably vast balls of churning gas and ice. These are alien, chaotic worlds with storms large enough to swallow Earth three times over and winds screaming across their surface at nearly the speed of sound.
Yet it is also out here amongst these alien planets that we have discovered a world that holds a striking resemblance to Earth. A moon orbiting Saturn called Titan.
When Titan was first discovered, it was thought to be the largest moon in the Solar System, but then astronomers realised their mistake.
At first they had not understood that this moon was unique. It had an atmosphere - a thick, dense orange haze, 10 times as deep as the Earth's atmosphere.
This made Titan look far larger than it really was.
Just like home
When they analysed this mysterious atmosphere, astronomers found that the chemical make-up was the most Earth-like in the Solar System, with plenty of nitrogen and a smattering of methane.
But, if this atmosphere is so similar to ours, what about the world it obscures? Our view of Titan sharpened with the arrival of the Cassini spacecraft, which has been performing a measured dance around Saturn and its moons since June 2004.
Ligeia Mare (sea) covers about 100,000 sq km in Titan's high north
On Christmas Day of that year, the European Space Agency probe Huygens detached itself from Cassini and began a slow descent to the surface of Titan.
"It was amazing because we just had no idea what to expect," says Huygens scientist Dr Ralph Lorenz. "We didn't know if it would be cratered like the moon, or a flat expanse of sand. And then the first pictures came back and it was just astonishingly familiar."
As it descended through the smog, Huygens captured images of a craggy, rocky-looking landscape. The images of its landing site was even more startling - it appeared to have settled on a gravelly plain, dotted with smooth, rounded stones.
According to Dr Lorenz, it was clear what these pictures meant: "There were few people who disputed the interpretation of a river channel. There really wasn't very much doubt."
Finding a river bed on a moon was an extraordinary discovery. By peeking through Titan's hydrocarbon shroud, we had revealed a world familiar to our eyes; a landscape sculpted by flowing liquid, just as the rain, rivers, and seas sculpt the surface of our planet.
The Matanuska glacier region of Alaska shows how water sculpts a landscape
Yet Titan's surface temperature is -180C, far too cold for liquid water to exist. Some other liquid was carving out the features on this moon.
To bring this story to life, the team from BBC Two's Wonders of the Solar System series spent long days poring over images, staring at endless vistas. Finally, they landed on the stunning Matanuska Glacier region of Alaska.
The series presenter Professor Brian Cox explains: "It's an immensely beautiful landscape but the important thing about this region is you can see how water, whether liquid or solid has sculpted the landscape - we were surrounded by landscape completely dominated by water.
"And on Titan you would see a landscape that is exactly the same. Yet it's not carved by water. It's too cold."
The reason sublime landscapes like the Matanuska Glacier exist is because of a delicate balance of temperature and pressure. Our planet is at just the right temperature and atmospheric pressure to allow water to exist as ice, liquid water, and water vapour.
Titan seen as sunlight glistens in one of its methane pools
Just as the conditions here allow water to exist in all three states, the conditions on Titan are perfectly balanced to allow methane to exist as a liquid, a solid and a gas.
On Titan, methane plays exactly the same role water plays here on Earth. Where we have clouds of water vapour, Titan has clouds of methane with methane rain.
Where we have lakes and oceans of water, Titan has lakes of methane.
Titan is the only place beyond Earth where we've found liquid pooling on the surface in enormous quantities, as big as the Caspian Sea. This moon is bathed in fuel.
Alaska was a perfect place to envision this bizarre moon. Craggy mountains covered in ice and snow, the brittle rugged surface of the glacier, rivers grinding away at pebbles and tranquil lakes. All have their twin on Titan.
"The best locations in the series really transport you to the wonders we are talking about. And Alaska was one of the best of them all, and one of the most beautiful places I have been," says Professor Cox.
Titan is a true wonder of the Solar System. It's like staring back in time four billion years and seeing the Earth as it was before life began.
Wonders of the Solar System, presented by Professor Brian Cox, continues on BBC Two on Sunday (2100 GMT).
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.