THE ROLE OF DIKE INTRUSIONS IN RIDGE FORMATION ON EUROPA

Many generations of linear and curvilinear ridges and troughs cover the surface of Europa, an icy satellite of Jupiter. Previous workers have differentiated between several ridge morphologies, including single ridges, double ridges, complex ridges and cycloidal ridges. A successful model for the formation of Europan ridges should account for the observed diversity of morphology and for the presence of transitions from one morphology to another along strike.

The lack of ridge segmentation motivates us to explore formation mechanisms related to fluid intrusion instead of faulting. We assume that liquid water fills tension cracks that open in the Europan crust in response to tidal stress and possible overpressure of a subsurface ocean. The crack would be long and essentially continuous, similar to dikes on Earth. It may adopt a cycloidal outline under a time-variable diurnal stress cycle. Volume expansion upon crystallization of the cracks would compress and buckle the adjacent crust. We developed initial models of ice deformation around a crystallizing intrusion to test the range of morphologies that may be produced. The model is set up in 2-D, as a cross-section of the ice shell taken perpendicular to the intrusion.

We report on the kinds of morphologies obtained by varying the depth and geometry of the intrusions, as well as the thickness of the ice layer. The geometry of the intruding dike has a major impact on ridge morphology. Specifically, the aspect ratio of the dike is the principal determining factor in whether a double or single ridge is observed on the surface. The mechanism easily transitions between forming a double ridge to forming a single ridge by changing only the aspect ratio of the intruding dike. A surface trough is generated when the intrusion is at greater depth and the double ridges become subdued with increased intrusion depth. As morphology is sensitive to the detailed geometry and depth of the intrusion, this model may allow for the estimation of water depth within or beneath the ice shell, and the thermal structure of the ice shell.