The axis of rotation of many planetary bodies is offset from the orbital axis, a difference known as obliquity. Numerical modelling suggests that, for the moons of the outer Solar System, obliquity may be indicative of the internal structures of the satellites.
Rose-Marie Baland at the Université de Nantes, France, and colleagues developed a model of rotation of Jupiter's satellites assuming entirely solid moons, and compared these results to models employing combinations of rigid and liquid layers of variable thicknesses. For Io, they used a fluid core within a rock mantle, whereas models of Europa, Ganymede and Callisto included a global liquid layer sandwiched between an ice shell and rigid interior. They found that the presence of a liquid ocean influences the obliquity of Europa and — even more significantly — Ganymede and Callisto, but internal structure had little effect on the obliquity of Io.
Future high-precision measurements of the obliquities of Jupiter's moons may help to constrain the presence and depths of subsurface oceans, which represent potential abodes for extraterrestrial life.