Researchers using data from NASA’s Cassini mission have discovered a striking similarity between Earth and Saturn’s moon Titan. Just as the surface of Earth’s oceans lies at an average elevation referred to as “sea level”, Titan’s seas also lie at an average elevation. Titan is the only world in our solar system other than Earth known to have stable liquid on its surface. Instead of water, Titan’s lakes and seas are filled with hydrocarbons, mostly methane and ethane. Water ice, covered by a layer of solid organic material, forms the bedrock surrounding these lakes and seas.