Voyager 2, as you probably know, went past Jupiter, Saturn, Uranus, and Neptune. All four - because they were lined up, and it was possible to fly past all of them - and this happens only once every couple hundred years; won't happen again in any of our lifetimes. We had this one shot. And we sent the two Voyager probes, both capable of making this grand tour.
But only one did. Why? Because we altered the trajectory of the other one - Voyager 1 - to fly close to Titan. That meant it had to give up Uranus and Neptune - both gas giant systems, all their moons and rings and mysteries, neither of which we'll visit again. Voyager 1 gave up all that, leaving them to its sister alone, just so its path through the Saturn system would take it close to Titan.
And twenty years later, when we launched Cassini towards the Saturn system, Cassini - unlike Galileo, which went to orbit Jupiter - carried a lander with it: the Huygens probe. The first thing Cassini did when it reached Saturn was drop the probe... so the probe could land on Titan, take pictures as it descended, and survive for a couple of minutes on the surface. We didn't bother to send a lander to any of Jupiter's moons. Titan's only the fifth thing we've landed anything on at all, after the Moon, Mars, Venus, and one near-Earth asteroid.
What makes Titan so special?
If you list "things smaller than a gas giant" in the Solar System in order of size, the list starts with Earth, Venus, Mars, Jupiter's moon Ganymede, and Titan. (Mercury's next: Titan is larger than Mercury.) Titan's a lot larger than any other moon in the Saturn system: Rhea, the second-largest moon, is 900 miles in diameter; Titan is 3000.
If you list "worlds with noticeable atmospheres", that list is short: Venus, Earth, Mars, and Titan.
If you list "worlds where you can't see the surface from space", the list is even shorter: Venus and Titan.
Titan is the only moon in the Solar System with more than a trace of atmosphere. Titan's atmosphere is about sixty percent thicker than Earth's. Since Venus' atmosphere is about a hundred times as dense as Earth's, and Mars's about one-hundredth as dense, Titan is the only other place with atmospheric pressure remotely like ours -- if nothing else is within a factor of a hundred, being within a factor of two counts as "pretty much the same".
With atmosphere comes clouds, and haze, and that's why we can't see its surface. When you look at Titan, all you see is a hazy featureless ball, like you took an out-of-focus picture of an orange.
So that's where its mystery came from; why we wanted so much to know more. Here was a world we couldn't see, that starts out with tantalizing similarities to our own world.
And the similarities don't end with the atmosphere. Huygens was a lander probe - when it reached Titan's atmosphere it opened a parachute, pointed a camera down as it descended, and sent us the first pictures we'd ever seen of Titan's surface. Look at what it saw.
Those are exactly what they look like. Rivers. A river delta leading to a coastline and a shallow sea. (Titan's really cold. Ice is a rock - a really common rock. The liquid forming those rivers is methane, not water.)
Huygens took that picture while parachuting down, and then it landed, and for a few minutes survived to take pictures of local rocks. And the rocks look exactly like the sort of smoothed, eroded rocks, cobbles, and pebbles we see in every riverbed on Earth.
Cassini is equipped with radar. In the last eight years it's passed by Titan a lot, and bounced radar off of it every time - so by now we know the large surface features, and atmospheric basics, fairly well. There are seas and rivers, hills and valleys. Titan has thunderstorms and methane rain. It has a "methane cycle" much like the water cycle on Earth - evaporation, condensation, precipitation, collection. It has seasons - lasting thirty times as long as Earth seasons.
It's, in short, far more Earthlike than anyplace else in the Solar System. (Just a lot colder.)
Could there be life? It would have to have a chemistry based on methane instead of water, and that's not simply a matter of swapping out oxygen for carbon. A whole lot of "basic" chemistry just can't happen there because there's not much that isn't simply inert rock at that temperature. But - we don't really know. We have a few pictures, and radar from a long distance. Titan is a fascinating place, a billion miles away, with secrets we've only just begun to unlock.
It's just barely possible we'll get there again in my lifetime.