Of the four large "Galilean" moons of Jupiter (so named for their discoverer), Io is the nearest to the central planet and therefore the most seriously inconvenienced by tidal stress. Next out is Europa, with an appearance completely different than that of Io, but hardly less bizarre. From a distance it looks like a cracked and badly stained cue ball. But up close it's another and far more complex story. With the arrival of the Galileo spacecraft at Jupiter in December 1995 the first probe to go into orbit around the planet scientists had the opportunity to study Europa in great detail over a period of several years. And everything they have seen in that time tends to confirm what was already suspected following the Voyager encounters: beneath Europa's icy outer crust may lie a deep, watery ocean. (Jim Erickson, 2003)
A 20 km wide region of ridged plains on Europa taken by the Galileo spacecraft on December 16, 1997 at a range of 1,300 km. The many parallel and cross-cutting ridges commonly appear in pairs, with dark material in between. Credit: NASA/JPL. The lack of cratering is the first clue. An old surface is heavily pockmarked with large and small impact craters, like battle scars picked up over an aging warrior's long lifetime. Europa has very few craters, and hardly any large ones, suggesting that its present landscape is at most only a few tens of millions of years old. And what a landscape! Nothing like it exists anywhere else in the solar system: an elaborate tapestry of fractures, ridges, bands and spots. The fractures, running everywhere, are cracks in the icy coating. Close study of these cracks their shape and length reveals that they could only form if the surface rose and fell by many meters every day............
