Flanking fractures and the formation of double ridges on Europa

Europa, a satellite of Jupiter, is one of the most intriguing worlds in the Solar System. Its dearth of impact craters and plethora of surface morphologies point to a dynamic evolution of its icy shell in geologically recent times. Double ridges are a common landform and appear to have formed over a significant fraction of the satellite’s observed geologic history. Thus, understanding their formation is critical to unraveling Europa’s history, and many models have been proposed to explain their creation. A clue to the formation of ridges may lie in evidence for flexure of the lithosphere in response to a load imposed by the ridge itself (marginal troughs and subparallel flanking fractures). When this flexure has been modeled, a simple elastic lithosphere has typically been assumed; however, the generally thin lithospheres suggested by these models require very high heat flows that are inconsistent with Europa’s expected thermal budget (of order 1 W m−2 vs. of order 10 mW m−2). Each of the proposed formational models, however, predicts a thermal anomaly that may facilitate the flexure of Europa’s lithosphere. Here, we simulate this flexure in the presence of these anomalies, as a means to evaluate the different models of ridge formation. We find that nearly all models of double ridge formation are inconsistent with the observation of flexure (specifically the flanking fractures), except for a cryovolcanic model in which the growing ridge is underlain by a cryomagmatic sill that locally heats and thins the lithosphere.