Europa: Morphological characteristics of ridges and triple bands from Galileo data (E4 and E6) and assessment of a linear diapirism model

Galileo solid-state imaging (SSI) images of Europa provide high-resolution data on the morphological characteristics of ridges and permit the development of nomenclatural schemes for their description and classification. Key observations are that ridges (1) are remarkably consistent in their along-strike linearity, width, and height, (2) form long linear features in which preexisting structures can sometimes be traced up the outer slopes of the ridges and in other cases appear to be buried, (3) contain narrow apical zones of small-scale, ridge-parallel faulting, (4) are sometimes flanked by narrow troughs and ridge-parallel fractures, and (5) often display associated color variations. On the basis of the characteristics and associated features of ridges, we find that a process in which initial fracturing (most plausibly related to tidal deformation) of a brittle layer overlying a buoyant ductile substrate leading to linear diapiric upwelling provides a consistent explanation for the observed features. In this process the upwelling linear diapir causes flexure (bending and faulting) of the region marginal to the fracture, the deformation and uplift of adjacent plains material and its preexisting structures to form the apical part of the ridge, the exposure of the inner walls of the crack, and the mass wasting of the inner and outer walls of the ridge to modify, but often not completely destroy, the preexisting structure of the adjacent plains. Specifically, this mechanism can account for many characteristics of the ridges, including their linearity, their consistent and regular morphology over their great lateral extent, their positive topography, the presence of preexisting structure on the outer ridges (caused by upbowing of background ridged plains), the formation of marginal troughs (as diapiric rim synclines), the detailed nature of their outer and inner slopes (caused largely by faulting and mass-wasting processes), and their sequential formation with multiple orientations (related to tidal deformation processes). Linear diapirism also provides a possible explanation for color and albedo characteristics, related to thermal effects of the upwelling warm ice (e.g., inducing volatile migration and grain-size variations). As the vast majority of deformation is vertical in this scenario, this mechanism minimizes the necessity for complementary compressional deformation required by some other models.