Rotational Dynamics of Europa

The rotational state of Europa is only rather poorly constrained at present. It is known to rotate about an axis that is nearly perpendicular to the orbit plane, at a rate that is nearly constant and approximates the mean orbital rate. Small departures from a constant rotation rate and oscillations of the rotation axis both lead to stresses that may influence the location and orientation of surface tectonic features. However, at present geological evidence for either of these processes is disputed. We describe a variety of issues that future geodetic observations will likely resolve, including variations in the rate and direction of rotation, on a wide range of timescales. Since the external perturbations causing these changes are generally well known, observations of the amplitude and phase of the responses will provide important information about the internal structure of Europa. We focus on three aspects of the rotational dynamics: obliquity, forced librations, and possible small departures from a synchronous rotation rate. Europa’s obliquity should be nonzero, while the rotation rate is likely to be synchronous unless lateral shell thickness variations occur. The tectonic consequences of a nonzero obliquity and true polar wander have yet to be thoroughly investigated.