Europan double ridge morphology as a test for hypothesized models of formation

Double ridges on the Jovian satellite Europa consist of two ridges with a central trough. Several hypotheses exist describing their formation. Explosive cryovolcanism would result in granular ice depositing as two self-symmetric ridges flanking a central fracture, lying at or below the angle of repose (AOR). Cryo-sediments deposited by tidal squeezing and low-viscosity cryolavas emplaced by effusive cryovolcanism would likely have shallow slopes, although ridge symmetry is not expected. A second group of hypotheses involves brittle deformation of the crust, namely by diapirism, shear heating, and buckling of the lithosphere due to compression. Because these models involve uplifting vertical fractures, they are expected to result in interior slopes steeper than the AOR, with shallower exteriors. Thus, these formation hypotheses can be grouped into two classes based on expected interior slopes: cryo-sedimentary mechanisms (cryovolcanism, tidal squeezing), and brittle deformation mechanisms (diapirism, compression, shear heating), with symmetry providing a secondary constraint.

To assess the viability of these formation mechanisms, I measured double ridge slopes at multiple locations distributed across Europa through analysis of data from the Galileo Solid-State Imaging camera. Two types of data were used to derive slopes and slope symmetry for 44 double ridges: elevation data from 5 digital elevation models (DEMs) (4 ridges), and ridge shadow length measurements taken on images (40 ridges). DEM slope values in the Cilix and Banded Plains (15°S, 195°W) regions typically fall below the AOR, although values above the AOR occur along one ridge. Interior-exterior slope symmetry is dominant, which favors cryo-sedimentary mechanisms because double ridges created from brittle deformation would maintain slope asymmetry after post-emplacement modification (e.g. mass wasting). Two different shadow measurement techniques were implemented to calculate interior slopes, with one technique accounting for a gap between ridge set interiors. These slopes are also below the AOR (42° maximum for the youngest ridges). Older ridges tend to have shallower slopes, which could possibly be attributed to mass wasting. Shallow interior slopes derived from both techniques suggest that double ridges form from cryo-sedimentary mechanisms. Symmetry of ridge interiors and exteriors may also suggest that explosive cryovolcanism is the dominant shape-forming mechanism.