Tidal heating in an internal ocean model of Europa

Abstract

Considerable evidence suggests that Europe's internal structure might consist of a liquid water layer that decouples a thin (<30 km) overlying ice lithosphere from an underlying silicate core^1,2. A lack of impact features, extremely subdued topography, and positive spectroscopic identification of H₂O all imply recent resurfacing by water. In addition, curvilinear features resembling cracks are ubiquitous over the surface; their orientations are broadly consistent with tidally controlled tectonic activity³. Compositional models of Europea indicate that the H₂O layer could be over 100 km thick¹. Cassen et al.⁴ first showed how tidal heating, if sufficiently intense, might stabilize a thin (<30 km) ice lithosphere over the internal ocean and prevent the water from freezing. Other models of Europa do not include an internal ocean and instead have an ice layer (perhaps as thick as 100 km or as thin as a few kilometres) resting directly on the silicate interior^5,6. However, based on studies of crater relaxation, Thomas and Schubert⁷ have shown the version of this model with a thin ice layer to be unlikely. The plausibility of the internal ocean model vis-a-vis the thick ice model depends critically on the level of tidal heating in Europa. Here we present new calculations of tidal heating based on a more realistic three-layer model of Europa. The tidal distortion of a decoupled ice lithosphere is only half that previously thought. At the current value of orbital eccentricity, tidal heating is only marginally able to prevent the internal ocean from freezing.