Why might europa be a likely place to find extraterrestrial life?

Jupiter's moon, Europa has been the topic of investigation about the survival of life due to the strong likelihood of a liquid water ocean underneath its ice surface. Hydrothermal vents on the end of the ocean, if they be existent, may warm the ice and could be adept of supporting multicellular microorganisms. It is also conceivable that Europa could care aerobic macrofauna using oxygen shaped by cosmic rays smashing up its surface ice.