EUROPA

Europa’s orbit lies between volcano-racked Io and the larger, more sedate Ganymede and Callisto. Like all the Galilean satellites, it was little more than a pinkish-white disc until the Voyager probes flew past Jupiter in 1979. 

The Voyagers revealed a curious ice ball – a world that seemed white and featureless at first glance, but which enhanced photographs revealed to be stained with pink and blue patches and criss-crossed with countless brownish streaks named lineae (from the Latin for “line”). Craters were few and far between, while in some places the terrain was eerily reminiscent of jumbled arctic pack ice.

SMOOTH SURFACE

Strangest of all was Europa’s smoothness – its surface is the flattest of any world in the solar system (relative to its size, it is smoother than a cueball).

Astronomers were startled to find a world the size of our Moon, with no substantial mountains or cliffs, deep trenches or craters, just rolling hills and valleys with the occasional sunken pit marking the ghost of a meteorite impact. 

It seemed clear from the outset that there was only one explanation – Europa’s surface was nothing more than a dense layer of ice, able to slip, slide, creep and rearrange itself much like the ice in glaciers on Earth.