An infrared color mosaic showing the sun reflecting off Titan's north polar seas [image: Cassini spacecraft]

Titan was discovered in 1655 by Dutch astronomer Christiaan Huygens.
Huygens, the European Space Agency (ESA) probe, landed on Titan 350 years after the moon's discovery. NASA's Cassini spacecraft took it to Saturn. The spacecraft itself was named after Jean Dominique Cassini. He was an Italian-French astronomer who observed Saturn in the 17th century, and discovered four of its moons.

Second biggest moon in the Solar System
If you add up the mass of Saturn's 145 known moons, Titan's mass is over 96% of the total. Astronomers had thought that it was the biggest moon in the Solar System, but space probes told a different story. Titan has a thick atmosphere that made it look bigger than it is. In fact, it's 5,150 km (3200 mi) in diameter and Jupiter's moon Ganymede is 5,270 km (3275 mi) in diameter. They're bigger than the planet Mercury.

Nearly 10 AU from the Sun
An AU (the astronomical unit) is the distance from the Sun to the Earth. At 9.5 AU, Saturn and its moons are nearly ten times farther from the Sun than we are. Antarctic temperatures are mild compared to the cold of Titan. Its surface temperature is around -178° C (-289°F). When it's that cold, water freezes into ice that's as hard as rock. In a photo taken by Huygens on Titan, the rocks and pebbles are all made of ice.

A lot like Earth?
Some scientists say that Titan is like Earth. Although the two are similar in some ways, they are far from being twins. However, Titan may be like a cold version of the ancient Earth long before life began.

Layered like a planet
Asteroids and small moons don't have layers. They're pretty much the same all the way through. But Titan is layered. There is even a liquid water layer under the surface.

Titan and Earth are the only Solar System bodies with an atmosphere rich in nitrogen.
Earth's atmosphere is about 78% nitrogen and 21% oxygen. Titan's is over 95% nitrogen, and the rest is methane and other hydrocarbons. (Hydrocarbons are substances made of carbon and hydrogen.) Titan is the only moon with a thick atmosphere. It's even thicker than Earth's atmosphere.

Smog
Los Angeles smog is famous, but it's nothing compared to Titan's smog. That is so thick that we can't see Titan's surface. Cassini needed instruments that can “see” through the smog, and used radar to map Titan's surface. With a cleaner atmosphere and nearness to the Sun, daytime on Earth is a thousand times brighter than on Titan.

Mountains, hills, sand dunes
Overall Titan is rather flat. The hills are gentle, and unlike some of the Martian mountains, those on Titan aren't very high. There are dune fields that look very much like those on Earth, but we don't know what the sand is made of.

Earth and Saturn are the only two objects in the Solar System with bodies of liquid on the surface.
On Earth the liquid is water. On Titan it's liquid methane mixed with other hydrocarbons. Titan has lakes and seas, but the seas aren't as big as those on Earth. [Click for a radar image taken by Cassini of the sea named Ligeia Mare.] Titan even has methane rain. Methane is what natural gas is made of, so you might wonder why Titan hasn't caught fire. Just remember that fire needs oxygen and Titan doesn't have any.

Volcanoes
Even though we can't see Titan's surface, many people think that Titan has volcanoes. There are structures that could be volcanoes. If so, they would be cryovolcanoes that erupt water instead of lava. Eruptions like that have been seen on Jupiter's moon Europa, Neptune's Triton and Saturn's Enceladus.