CRACKING EUROPA'S ROTATION HISTORY WITH TIDALLY-DRIVEN FRACTURES

Europa has a rich and diverse geologic record preserved on its icy surface, which overlies a liquid water ocean over 100 km deep. It is a world where tidal response dominates both the surface and interior evolution. We now know that Europa is non-unique; tiny, ice-covered Enceladus displays tectonic activity, plume eruptions, and high heat flow that are all likely governed by tides. Studying tidal-tectonic processes on Europa has provided a framework for interpreting surfaces and constraining rotation states on all bodies in which tidal processes play a role.

Tectonic activity on Europa has been linked to tidal stress caused by its eccentric orbit and possibly non-synchronous rotation (NSR) of the icy shell. Cycloids and other lineaments are thought to form in response to tidal tensile stress while strike-slip motion along preexisting faults has been attributed to tidal shear stress. Our analysis of these features has revealed that Europa most likely had a small but finite obliquity during the most recent epoch of tectonic activity and that spin pole precession was likely an important factor in the formation of these features. The role of non-synchronous rotation is more ambiguous. Stress from NSR is not indicated in fits to observed cycloids, nor is it supported by lineament azimuth variation. However, all cycloids appear to have undergone translations in longitude since their formation, presumably due to NSR.

We present an overview of these results and attempt to fit them into a unified rotational history. We also present new results that incorporate a linear Maxwell viscoelastic rheology into calculations of tidal stress. Past work has shown that tidal stresses in rheologically-layered bodies differ from those predicted for a thin, elastic shell. Based on the paths of observed cycloids, we evaluate the influence of interior structure and discuss the implications for ice shell thickness and NSR.