As Europa orbits Jupiter, the gas giant's immense gravitational pull stretches and squeezes the moon, creating internal friction that melts its inner ice.

Beneath its thick icy crust, Europa harbors a global saltwater ocean that may contain twice as much water as all of Earth's oceans combined.

Liquid water and warm slush periodically erupt from beneath Europa's crust, freezing over existing craters to create a relatively young, smooth surface.

In Arthur C. Clarke's novel 2010: Odyssey Two, mysterious monoliths turn Jupiter into a mini-sun to thaw Europa and encourage primitive aquatic life to evolve.