Europa is a Moon-sized body whose surface is a smooth, relatively young layer of water ice. Liquid water oceans may exist under this ice crust. Liquid water appears to be the critical ingredient for the development and sustenance of life. Internal heating of Europa may produce hydrothermal vents, similar to those on Earth known to harbor non-photosynthetic life forms. Thus, the detection and characterization of any Europa oceans is an integral part of our search for evidence of life in the solar system.
Titan has a thick, reducing atmosphere and a prodigious organic chemistry powered by sunlight.