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 developm ent of nomenclatural schemes for their description and classification. Key observations are that ridges (1) are remarkably consistent in their along-s trike linearity, width, and height, (2) form long linear features in which preexisting structures can sometimes be traced up the outer slopes of the r idges and in other cases appear to be buried, (3) contain narrow apical zon es of small-scale, ridge-parallel faulting, (4) are sometimes flanked by na rrow troughs and ridge-parallel fractures, and (5) often display associated color variations. On the basis of the characteristics and associated featu res of ridges, we find that a process in which initial fracturing (most pla usibly related to tidal deformation) of a brittle layer overlying a buoyant ductile substrate leading to linear diapiric upwelling provides a consiste nt explanation for the observed features. In this process the upwelling lin ear diapir causes flexure (bending and faulting) of the region marginal to the fracture, the deformation and uplift of adjacent plains material and it s 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 out er walls of the ridge to modify, but often not completely destroy, the pree xisting 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, th eir positive topography, the presence of preexisting structure on the outer ridges (caused by upbowing of background ridged plains), the formation of marginal troughs las diapiric rim synclines), the detailed nature of their outer and inner slopes (caused largely by faulting and mass-wasting process es), and their sequential formation with multiple orientations (related to tidal deformation processes). Linear diapirism also provides a possible exp lanation for color and albedo characteristics, related to thermal effects o f the upwelling warm ice (e.g., inducing volatile migration and grain-size variations). As the vast majority of deformation is vertical in this scenar io, this mechanism minimizes the necessity for complementary compressional deformation required by some other models.