This article is adapted from NASA Press Releases.

In the late 1970's, NASA Voyager spacecraft imaged Europa.  Its
surface was marked by complicated linear features, appearing like
cracks or huge fractures in the surface.  No large craters (more than
five kilometers in diameter) were easily identifiable.  One
explanation for this appearance is that the surface is a thin ice
crust overlying water or softer ice and that the linear features are
fractures in that crust.  Galileo images have reinforced the idea that
Europa's surface is an ice-crust, showing places on Europa that
resemble ice floes in Earth's polar regions, along with suggestions of
geyser-like eruptions.

Europa's appearance could result from the stresses of the contorting
tidal effects of Jupiter's strong gravity (possibly combined with some
internal heat from decay of radioactive elements).  If the warmth
generated by tidal heating is (or has been) enough to liquefy some
portion of Europa, then the moon may have environmental niches warm
and wet enough to host life.  These niches might be similar to those
found near ocean-floor vents on the Earth.