It is the biggest of the planet Saturn's moons. Titan's diameter of 3,200 miles makes it larger than the planet Mercury with a diameter of 3,031 miles and the planet Pluto with a diameter of 1,442 miles. Of all the moons in our Solar System, only the planet Jupiter's moon Ganymede is larger. Its diameter is 3,270 miles. Titan contains nearly twenty times as much matter as all the rest of Saturn's moons and rings combined.
Titan is the only moon known to have a large atmosphere.
Its atmosphere is mostly nitrogen, just like the atmosphere surrounding Earth, but thicker. Its atmosphere contains methane, which is composed of carbon and hydrogen and is found in small amounts in Earth's atmosphere. Only tiny amounts of oxygen, which we need to live, are found in Titan's atmosphere. However, in Titan's atmosphere, the nitrogen and methane are bombarded by high-energy particles and ultraviolet light. That creates a brownish-orange haze that sort of clumps together and rains down on Titan's surface.