A little closer to home than the Diamond planet is Titan, the largest moon of Saturn, which is between 1.2 billion and 1.7 billion miles from Earth, depending on the time of year. Titan is unique in the solar system for two reasons: It is the only moon to have a dense atmosphere and the only one to have stable bodies of liquid on its surface, with definitive lakes numbering in the hundreds found on the surface by the Cassini space probe.
Titan’s lakes are very different from those found on Earth, however. They are believed to hold liquid methane, otherwise known as natural gas. The Cassini observations revealed that some of Titan’s lakes are filled completely, while others are only partially filled, suggesting the planet experiences a “condensable-liquid hydrological cycle,” or rainfall composed of natural gas. Another hypothesis is that there is a subsurface liquid methane table, much like the water tables here on Earth. Some estimatethat the larger lakes on Titan independently contain more than 130 billion tons (6.3 quadrillion cubic feet) of natural gas, which is more than 31 times the total proven reserves found on Earth.