In most moons in the Solar System, this tendency is overpowered by the fact that each moon has acquired a slight permanent tidal bulge in its rocky body, and its home planet's tidal pull urges the moon to keep that bulge pointed straight toward the planet throughout each orbit.

This is probably also the case with Europa's main rocky body -- but it may not be the case with its surface layer of ice, which may be very slowly sliding over Europa's main rocky body.

Greenberg and his colleagues have noted that Europa's surface cracks follow an odd pattern: cracks further to the west tend to be "rotated" as compared to their fellow cracks further to the east -- in a clockwise direction in the northern hemisphere, counter-clockwise in the south.

Their calculations show that this is exactly what you would expect if Europa's surface was rotating slightly faster than its revolution period around Jupiter, so that the point on its surface where the tidal bulging (producing the cracks) is greatest is slowly but constantly migrating further west.

And since Europa's solid main body is probably not migrating, its surface ice crust must be sliding across the underlying rock in response to Jupiter's tidal tuggings -- which can only be the case if there is an underlying soft layer on which the ice can slide.

However, if such surface movement is occurring, it is extremely slow. Comparisons of Galileo's photos with those taken 20 years earlier by the Voyagers show no visible motion, which means that Europa's surface must slide around its main body every 10,000 years at most -- and this is hardly surprising, since Greenberg's own calculations indicate that such a slew should take several million years.

But Greenberg has other pieces of evidence. First, there are Europa's "strike-slip faults" -- regions in which one surface ice plate has apparently slid sideways along the edge of another, judging from the way in which they have broken up older surface features.

Greenberg's theories indicate that when such a fault crack opens up in the ice crust, the rhythmic cycle of vertical and sideways tidal tuggings produced by Jupiter's pull during each of Europa's four-day orbits around the planet should indeed cause such sideways slipping to occur -- but again, the ice plates must be gliding on something.

Then there are Europa's strange "cycloidal cracks" -- chains of repeated, connected curving arcs that can stretch for hundreds of kilometers across the surface.

These were a complete mystery, until Greenberg's team conducted an analysis last year showing how that same rhythmic pattern of tidal tuggings during each of Europa's orbits could not only suddenly split open a new crack in the surface ice, but then cause it to follow that strange "hopping" path -- with each new arc being formed during one of the Europa's 85-hour-long orbits, while following a curving path as the direction of the tidal pull changes during that orbit, so that the entire chain was formed over a period of only a few weeks.

However, for Jupiter's tides to form cycloidal cracks in this way, the ice layer must flex up and down as much as 30 meters as the force of the tides changes -- and since Greenberg is convinced that such great flexing can only occur if a layer of liquid is sloshing around underneath the ice, he regards the cycloidal cracks as the single strongest piece of evidence yet for a subsurface liquid ocean.