Titan is the largest moon of the planet Saturn.
How big is Titan? 
The moon Titan is 5150 km wide. It is the second largest moon in the Solar System. Only Ganymede, a moon of Jupiter, is larger. Titan is bigger than either the planet Mercury or dwarf planet Pluto. It is nearly one and a half times the size of the Earth's Moon, and has nearly twice the mass of the Moon. Titan is the only body besides Pluto, that we know of, that has a colder surface temperature because it has an atmosphere. The surface temperature of Titan is -180 degrees centigrade.
What is its surface like? 
Titan is the only moon in the Solar System to have a thick atmosphere. Until the Cassini spacecraft arrived in orbit around Saturn, we did not know what the surface looked like. The ground is very cold on Titan, and all of the water is frozen. The surface looks like it is smooth, with areas of light and dark material. Most of the craters have been filled in.
It was thought that Titan may have seas of liquid methane, but it now looks like there may only be a few lakes. The Huygens probe was dropped into the atmosphere of Titan and slowly fell to the ground. This gave us our first close-up pictures of the surface. These showed what appear to be channels among hills that lead down to flat regions below. These may have been made by flows of liquid that left dark materials behind.
How long is a day on Titan? 
A day on Titan is the same as the time it takes for the moon to orbit around Saturn, about 15 days, 22 hours, and 41 minutes. The same side of Titan always faces toward Saturn, in much the same way as the same face of our Moon always points toward the Earth. This was caused by the gravity of Saturn creating a tidal force on Titan, which changed its rotation.
What is it made of? 
The atmosphere of Titan is mostly nitrogen, with small amounts of other gases. Many of these gases are called hydrocarbons because they include hydrogen and carbon. These hydrocarbons are created in the upper atmosphere from Ultraviolet sunlight. Ultraviolet is the same light that causes sunburn on your skin. The hydrocarbons create a thick orange smog that hides the surface.
The moon is made of about half ice and half rocks. Most of this rock may be in a core at the center of the moon. The core may still be hot, as is the core of our own world, and there could be some volcanic activity. The surface is mostly ice.
How much would Titan's gravity pull on me? 
The gravitational pull on Titan is about one seventh the Earth's gravitational pull. So if you weighed 100 pounds here you would weigh 14 pounds on Titan.
Who is it named after? 
Titan was named after the race of children borne by Gaea and Ouranos in Greek mythology.
When was it discovered? 
It was discovered in 1655 by the Dutch astronomer Christiaan Huygens.