NARRATOR Slightly smaller than our own moon, Europa is covered with ice. Data collected by Galileo shows that the surface is minus-260 degrees Fahrenheit, surely hostile to life.

But as the probe gets closer, it takes these images. A mysterious network of dark cracks is etched into Europa's icy surface.

JOHN SPENCER (ASTRONOMER, SOUTHWEST RESEARCH INSTITUTE) We see places where, very clearly, the ice has cracked and two sides have spread apart, material has come up and frozen in the middle to fill the gap.

NARRATOR In addition to the dark cracks, the probe also reveals vast jagged areas of ice that appear to have melted, broken apart and frozen back together again.

JOHN SPENCER There's something very dramatic happening to destroy the existing surface there.

NARRATOR To an expert eye, it's a familiar pattern: sea ice found on Earth looks very similar.

Then Galileo takes readings of Europa's magnetic field. These indicate an electric current flowing inside, consistent with an ocean of salty liquid water.

JOHN SPENCER It's very hard to get that pattern without having an ocean underneath the ice.

NARRATOR The magnetic field data suggests that miles down, beneath Europa's icy surface there is an ocean that could be 60 miles deep. This small moon could have twice as much liquid water as in all the oceans on Earth.

Something must be melting the moon from deep inside. And again, the key is Jupiter. The same gravitational forces that flex Io's rocky interior, turning it into an ocean of magma, are melting Europa's ice to produce its hidden ocean of liquid water and creating the cracks on the moon's icy surface.

JOHN SPENCER The ice is creaking and groaning around. That generates a huge amount of friction and a huge amount of heat.

NARRATOR But the question is, could anything live in this cold, liquid ocean, concealed beneath miles of ice, where there is no energy from the sun?

Print Transcript