Titan is Saturn’s largest moon and the second largest moon in the Solar system after Jupiter’s Ganymede. Not only is Titan’s size remarkable, but there are many aspects that make it more like a planet than a moon.
First, Titan has a permanent thick atmosphere that is slightly denser than that of Earth, and is mainly composed of methane gas (a mixture of carbon and hydrogen atoms), which is broken down by the weak sunlight to produce a thick smog that obscures the planet’s surface. You can see surface features in the image (see right) because they were taken in the infra-red part of the spectrum, which can see through the smog a little better.
As with the Earth, winds blow across the moon's surface shifting clouds of methane, and instead of raining water on Titan, it rains liquid methane. This means that the surface is covered with vast permanent lakes of liquid methane and ethane, which then can evaporate off into the clouds and fall as rain again.
The freezing temperatures on Titan, where it averages around -179°C, mean that methane can exist in all three states: solid, liquid and gas, just like water can on Earth. As a result, the surface of the moon has been sculpted by this methane-cycle, resulting in rivers, gorges and coastlines. When the Cassini probe landed on Titan, back in 2005, it came down on a bed of water ice pebbles that had been frozen harder than rock and then worn smooth by the flow of liquid methane.
There are more similarities to Earth; vast dunes are formed around the equator by high winds, but instead of being made of sand, they are composed largely of hydrocarbons. There are also thought to be volcanoes that are spewing liquid water, which then set like rock (ice) in the freezing temperatures. The water is thought to have come from a 100km thick ocean buried beneath Titan’s surface, which some astronomers think may be capable of supporting life.