Titan was discovered by Christiaan Huygens (1629–1695) around 1655. Over the centuries, astronomers discovered that this largest of Saturn’s moons is the only moon in the solar system with a substantial atmosphere—it is even denser than the atmosphere of planet Earth. Titan’s atmosphere appears to be composed mainly of nitrogen and methane, with many other ingredients as well. Observations with the space probe Voyager 1, and with other telescopes, suggested that Titan might harbor liquid nitrogen or methane at its surface, perhaps in lakes and seas, and that its clouds may produce chemical rains and other weather patterns. Any detailed view is blocked by Titan’s thick, opaque atmosphere, however.

The Cassini spacecraft launched the Huygens probe into the atmosphere of Titan in January 2005. That spacecraft slowly descended by parachute to that moon’s surface, taking pictures all the way down. Huygens saw that, despite the numbing cold (–300 degrees Fahrenheit), there are topological features on Titan that look like tall mountains, rocky beaches, rivers, lakes, and even seas and shorelines. Liquid appears in abundance on the surface of Titan, but it is not liquid water. At those temperatures, water is frozen solid and as hard as granite. Rather, the liquid is probably methane—liquid natural gas.