Ralph Lorenz (JHU-APL)
Titan is a target of outstanding interest from three broad perspectives. First, it is a large icy satellite, with a geophysical structure and evolution that may be compared with other satellites such as Ganymede.
Second, it shows geomorphological and meteorological processes characteristic of the terrestrial planets - most obviously our own Earth. Mountains, lakes, dunefields and fluvial channels are evidently evolving via familiar processes under unfamiliar conditions to yield a varied landscape much like our own. Similarly, methane rainstorms resemble tropospheric moist convection on Earth, and the Titan stratosphere exhibits seasonal change in gas and aerosol composition, with a polar structure that has parallels with the Earth's ozone hole.
Finally, Titan has a rich atmospheric photochemistry which produces an array of organic molecules and macromolecules. Titan's surface is the ultimate sink for this material, forming lakes at the poles and giant seas of sand at low latitude. It has even been shown that terrestrial microbes can metabolize laboratory analogs of this photochemical material ('tholin').
Titan is one of the richest chemical environments in the solar system, and Titan-like moons doubtless circle many extrasolar planets. As our sun evolves into a red giant, Titan may become a habitable or even inhabited world, and many extrasolar Titans may already have done so.
Titan's organic-rich surface environment lends itself to in-situ exploration : there is no radiation hazard and the atmosphere makes it easy to deliver large payloads softly to the surface, and permits global-scale mobility by vehicles such as hot-air balloons.