The European Space Agency's Huygens plunged a probe through Titan's thick nitrogen-and-methane atmosphere and learned that the probe had landed on Titan shortly after a methane rainstorm. The probe released a burst of gaseous methane from liquid just below the surface as it landed. (Methane on Titan is the equivalent to water on Earth.) Once the probe was on the ground, it recorded a 30% jump in methane levels, which indicates that the liquid methane is near the surface. The large quantity of liquid methane surprised scientists who expected to see perhaps higher concentrations of liquid ethane. Photochemical processes were thought to convert atmospheric methane to other molecules like ethene and acetylene, which would then collect on the surface in large pools. Evidence shows photochemical smog rain in the form of dark material that has collected into channels and drained into lake beds. Despite the extensive information the probe has provided scientists with, many questions and mysteries still remain about the chemistry of methane and other organics on Titan.

http://pubs.acs.org/cen/news/83/i04/8304titan.html