NASA's Voyager 1 provided the first detailed images of Titan in 1980. They showed only an opaque, orange atmosphere, apparently homogeneous.
It was so thick that you could not see the surface. However, other data revealed exciting things. Similarly to Earth, Titan's atmosphere is mostly nitrogen but there is also methane and many other organic compounds.
Before the arrival of the ESA Huygens probe, planned for January 2005, astronomers will observe Titan using the most powerful ground-based telescopes.
Images from the WM Keck Observatory reveal methane-containing clouds near Titan's south pole. This could mean that Titan has the equivalent of a weather cycle similar to ours on Earth.
This is a major discovery which means that the atmosphere is much more dynamic than previously thought.
The NASA Cassini orbiter will clearly see these clouds, carrying out precise observations before, during and after releasing the Huygens probe.