Credit: Paul Schenk
Europa is a complex and dynamic moon, but now, global mapping of unusual large circular features on the ice-covered ocean world of Europa has revealed that Jupiter's curious icy moon is even more unstable than previously thought.
Because of the strong pull of Jupiter's gravity, Europa's icy shell bulges slightly at the equator and is flattened at the poles. The shell is also thought to be separated from Europa's core by an ocean, which would permit the shell to move en masse -- a phenomenon called true polar wander.
Europa is not the only planet or moon to have undergone polar wander. Mars has probably tilted over at least once, due to the formation of the Tharsis volcanos. Earth's outer layers have done so, as apparently have Enceladus, and possibly Miranda. Polar wander may be a common occurrence across the solar system, suggesting that planets in general are less stable than we have thought.
I think we might have to install a GPS network on Europa as well, to sort this out! :-)
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