Jupiter's icy moon Europa is slightly smaller than the Earth's Moon. Like the Earth, Europa is thought to have an iron core, a rocky mantle and a surface ocean of salty water. Unlike on Earth, however, this ocean is deep enough to cover the whole surface of Europa, and being far from the sun, the ocean surface is globally frozen over.
Europa orbits Jupiter every 3.5 days and is phase locked -- just like Earth's Moon -- so that the same side of Europa faces Jupiter at all times. However, because Europa's orbit is eccentric (i.e. an oval or ellipse not a circle) when it is close to Jupiter the tide is much higher than when it is far from Jupiter. Thus tidal forces raise and lower the sea beneath the ice, causing constant motion and likely causing the cracks we see in images of Europa's surface from visiting robotic probes.
This "tidal heating" causes Europa to be warmer than it would otherwise be at its average distance of about 780,000,000 km (485,000,000 miles) from the sun, more than five times as far as the distance from the Earth to the sun. The warmth of Europa's liquid ocean could prove critical to the survival of simple organisms within the ocean, if they exist.
More on Europa
Europa was discovered on 8 January 1610 by Galileo Galilei. The discovery, along with three other Jovian moons, was the first time a moon was discovered orbiting a planet other than Earth. The discovery of the four Galilean satellites eventually led to the understanding that planets in our solar system orbit the sun, instead of our solar system revolving around Earth. Galileo apparently had observed Europa on 7 January 1610, but had been unable to differentiate it from Io until the next night.
How Europa Got its Name:
Galileo originally called Jupiter's moons the Medicean planets, after the Medici family and referred to the individual moons numerically as I, II, III, and IV. Galileo's naming system would be used for a couple of centuries.
It wouldn't be until the mid-1800s that the names of the Galilean moons, Io, Europa, Ganymede, and Callisto, would be officially adopted, and only after it became apparent that naming moons by number would be very confusing as new additional moons were being discovered.
Europa was originally designated Jupiter II by Galileo because it was the second satellite of Jupiter. Europa is named for the daughter of Agenor. Europa was abducted by Zeus (the Greek equivalent of the Roman god Jupiter), who had taken the shape of a spotless white bull. Europa was so delighted by the gentle beast that she decked it with flowers and rode upon its back. Zeus seizing his opportunity rode away with her into the ocean to the island of Crete, where he transformed back into his true shape. Europa bore Zeus many children, including Minos.