Comparison of Ridges on Triton and Europa

Introduction: Triton and Europa each display a variety of ridges and associated troughs. The resemblance of double ridges on these two satellites has been previously noted [R. Kirk, pers. comm.], but as yet, the similarities and differences between these feature types have not been examined in any detail. Triton's ridges, and Europa's, exhibit an evolutionary sequence ranging from isolated troughs, through doublet ridges, to complex ridge swaths [1, 2]. Comparison of ridges on Europa to those on Triton may provide insight into their formation on both satellites, and thereby have implications for the satellites' histories. Europa ridges: Galileo visited Europa in the late 1990's, obtaining high-resolution images revealing a geologically young surface of bizarre landforms. In addition to features such as " pull-apart " grey bands, and chaos and lenticulae, the surface is criss-crossed by ubiquitous ridges and troughs (see review in [3]). Ridges range from ~200 m to >4 km in width, can exceed 1000 km in length, and can be over 200 m high. Ridge morphology appears to span a genetic sequence beginning with isolated troughs, through double ridges (two ridges separated by a trough), and triple ridges (three ridges separated by two troughs), ending with complex ridges comprising several individual ridges (Fig. 1). Ridge formation is a process that has apparently continued throughout Europa's visible history, and the predominant morphology is the double ridge. Although several distinct models have been proposed for the origin of Europa's double ridges [see review in 4], the most plausible is the newer shear heating model of [5]. They propose that diurnal tidal stresses within Europa's shell result in strike-slip motion along fractures. This process presumably begins with troughs [cf. 2], which are likely tension fractures that have not yet penetrated to the brittle-ductile layer. If the brittle-ductile transition is reached, full-scale slip can occur along the fault plane, resulting in shear heating and the uplift of relatively warm buoyant subsurface ice on either side of the fracture. This model is the most plausible of all those proposed for several reasons: it explains the strike-slip offset documented along many ridges [6], it provides a mechanism for the build-up of the ridge crests, it explains the considerable length and uniformity of the many ridges, it accounts for the morphological transition from troughs to ridges, it accounts for the fact that ridges (but rarely troughs)