Last year, the Cassini spacecraft found solid (haha) evidence for the existence of lakes of liquid methane and ethane on the giant moon Titan. Of course, Titan is barely a moon at all -- bigger than Mercury, it would be a planet in its own right if it weren't orbiting Saturn. It has an atmosphere with almost twice the surface pressure as Earth's, which is mostly nitrogen and a trace of hydrocarbons. But that trace is important: because Titan is so cold, methane and ethane can rain from the Titanian sky, forming river systems and lakes. But there's a problem: the north pole of the moon has far more lakes than the south pole. Seven times as many! Why?
First, methane on Titan goes through cycles something like water does on Earth. During Titan's summer, the northern lakes lose methane to evaporation, and the gas gets transported to the colder ...
