1655

Titan

Christiaan Huygens (1629–1695)

Galileo’s Starry Messenger of 1610, announcing the discovery of four moons of Jupiter, the phases of Venus, and the presence of mountains and valleys on the Moon, generated an enormous amount of excitement and instigated a “space race” of sorts among seventeenth-century astronomers. If such discoveries could be made with Galileo’s relatively simple spyglass, what new wonders would await the even larger telescopes? Soon, bigger telescopes began reaching further into the skies.

Saturn orbits the Sun at nearly twice the distance of Jupiter, and so sunlight there is more than three times less intense than it is at Jupiter. Maybe it’s no surprise, therefore, that it took until 1655 for telescopes to become sensitive enough to detect the faint amount of sunlight reflected off a moon in orbit around Saturn. That discovery was made by the Dutch astronomer Christiaan Huygens, using a telescope of his own design. Huygens called the new moon simply Saturni Luna, Latin for “Saturn’s moon.” It wasn’t until 1847 that it was named Titan, as part of a Greek mythology theme for naming the seven then-known satellites of Saturn.

Modern observations of Titan by the Voyager and Cassini space missions have revealed it to be a strange and unique world. Titan is the second-largest moon in the solar system and, at 3,200 miles (5,152 kilometers) across, it is larger than the planet Mercury. Titan’s density of 1.9 grams per cubic centimeter implies an icy and rocky interior. It is the only moon with a thick atmosphere—a dense mixture of nitrogen and methane smog that shrouds the surface from view. Because Titan has a temperature of 90 kelvins and a surface pressure about 50 percent higher than Earth’s, hydrocarbons created by sunlight in its nitrogen-methane atmosphere are expected to exist as liquids. Indeed, the Cassini orbiter’s radar mapper found rivers and lakes of liquid ethane or propane on Titan.

Titan’s environment is one of sluggish organic chemistry in the absence of oxygen. It is an astrobiology hot spot—the best place in the solar system to study what the early Earth may have been like before life made our atmosphere oxygen-rich. In 2005, the first probe ever to land on a moon of another world was sent to Titan. The successful mission was fittingly named Huygens.

Natural-color view of the haze-shrouded disk of Titan, photographed in 2009 from NASA’s Cassini Saturn orbiter. The icy moon Tethys is in the distance beyond Titan.