NARRATOR Bigger than the planet Mercury, Titan is hidden by a thick orange haze. No one has ever seen its surface.

But a small probe named Huygens, released by Cassini, is about to change everything. This mission will challenge long held notions of where life could exist beyond Earth.

These are the actual images Huygens takes, as it breaks through the clouds and haze. Titan is a land of mountains and valleys, a place that looks surprisingly like Earth. Then, images reveal something no one expects: the surface is littered with smooth rocks, the type normally found in riverbeds on Earth.

CHRIS MCKAY (ASTROBIOLOGIST, NASA AMES RESEARCH CENTER) My response was shock. We look out on the surface, and we see what looks like a desert, and, at the same time, the data from the probe told us that the ground around the site was wet.

NARRATOR Hundreds of miles overhead, Cassini's radar sweeps the surface. The images show a landscape covered with what appear to be hundreds of lakes. This one covers an area of 6,000 square miles, about the size of Lake Ontario, one of the Great Lakes. It's a surprising discovery.

CHRIS MCKAY It's the only world other than the earth that has a liquid on its surface.

NARRATOR But what exactly is this liquid? Titan is minus-290 degrees Fahrenheit. If it's water, it should be frozen solid.

Then, one of Cassini's instruments analyzes the infrared light reflected off the lakes. The readings are consistent, not with water, but with liquid methane and ethane, substances that, on Earth, are volatile, flammable gases.

NARRATOR Data now reveals that methane on Titan carves river valleys, forms clouds, and even falls as rain. Liquid methane acts a lot like water on Earth, but could it act the way water does, as an essential foundation for life, allowing organic molecules to dissolve, mix and interact?

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