One of four discovered (in 1610) by Galileo, Europa is the smallest of the Galilean moons, but still larger than Pluto.  It is very bright because it is entirely covered in ice.

When Zeus (Jupiter) raped Europa, he “abducted” her in the form of a white bull.  In the above collage, you can see the representations of Jove (Jupiter) and Europa in both astronomical and mythological forms.  The gravity of Jupiter deforms and cracks the icy sheathe covering its moon, allowing water plumes (last observed in 2013) to escape.  It is believed that Europa has as much as or more water than Earth trapped beneath this covering. 

Jupiter’s gravitational kneading of Europa provides enough heat to keep the ocean below the ice liquid, within a sort of global igloo.  The constant cracking of the mantle of ice also allows surface hydrocarbons to mix and dissolve in the ocean below.

more (recent) info: http://disq.us/t/3ocaprp