Titan, seen here behind Saturn's rings, is the largest moon of Saturn and the only non-planetary satellite known to have a defensive atmosphere. It is also the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found — whole lakes, in fact. Although those liquid lakes are not made of water (they are made of hydrocarbons), they make an ideal mixing pot for complex organic chemistry. Furthermore, the moon's thick atmosphere is made of important building blocks for life, including nitrogen.