This material is extracted from the review article by Chyba &
McDonald (1995, Annual Review of Earth and Planetary Science).

 Titan's atmosphere is a rich mix of nitrogen and methane, from which
organic molecules (i.e., those containing carbon, not necessarily
molecules in living organisms) can be formed.  Indeed, there has been
speculation that Titan's atmosphere resembles that of Earth some 4
billion years ago.  Complex organic chemistry can result from the
ultraviolet light from the Sun or from charged particle impacts on the
upper atmosphere.  Unfortunately, Titan's great distance from the Sun
means that the surface temperature is so low that liquid water is
probably not present globally.  Since we believe that liquid water is
probably necessary for the emergence of life, Titan is unlikely to
harbor any life.  The impact of comets or asteroids on Titan may,
however, warm the surface enough that any water ice could melt.  Such
"impact pools" could persist for as long as 1 thousand years,
potentially allowing life-like chemical reactions to occur.