Europa is locked in a tidal resonance with its neighboring satellites, volcanic Io on the inside and large Ganymede on the outside. Like each of the Galilean moons, Europa is spin-locked to Jupiter, rotating about its pole with a period identical to its orbital period of 3.6 Earth days. Europa is in a 1:2:4 resonance with Ganymede and Io for every two times that Europa orbits Jupiter, Ganymede orbits once, and Io orbits four times. This resonance keeps the satellites from perfect circular orbits, causing a forced eccentricity that varies their distances from Jupiter. Jupiters strong gravity raises a bulge on the portion of the surface facing towards the planet, but since the distance between the moons and Jupiter varies, the height of the tidal bulge also varies throughout the satellites orbit. The variation in tidal amplitude results in tidal flexing as the bulge goes up and down, producing an internal dissipation of energy called tidal heating.

The tidal heating is most intense at Io, due to its proximity to the giant planet Jupiter, and results in constant volcanic activity on the small moon. Io is, in fact, the most volcanically active body in the solar system, surpassing even Earth. If the current rate of volcanic activity on Io is typical, then there has been sufficient volcanic processing to recycle the entire crust of Io through volcanoes multiple times in its history. The volcanic activity on Io could have also have driven off most volatiles over time. If any water were present on Io from its formation early in its history, it is now long gone.

Ganymede is also subject to tidal heating, although to a lesser extent than Io and Europa given its larger distance from Jupiter (and thus Jupiters reduced gravitational pull). Still, Ganymede seems to have had sufficient tidal heating to result in some geologic activity in this past, which allowed Ganymede to become internally differentiated into an icy mantle and a rocky/metal core. Ganymedes surface consists of old, cratered terrains, but also of younger grooved and ridged areas that are the result of more recent tectonic activity. An interesting comparison can be made between Ganymede and its neighboring satellite Callisto. Callisto does not participate in the resonance with the other three Galilean satellites, and its surface resembles Earths Moon - it is an old, cratered body with few, if any, signs of geologic activity. Callisto also is thought not to be fully differentiated, although recent measurements suggest that it could have a small core.

A substantial amount of heat is dissipated within Europas core, mantle, and ice shell as well. Debate is still taking place as to exactly how much heat is tidally dissipated within Europa, and where exactly this heating takes place is it near the surface, at the ice/water interface (if there is one), or at the water / rock or ice/rock interface at the bottom of the ice layer?

Thus, the theoretical possibility exists for the presence of liquid water on Europa. Still, though, do we have any evidence? In fact we do, and the next article in this occasional series about life on Europa will consider the lines of evidence for liquid water.