Extensional and compressional instabilities in Icy satellite lithospheres

Abstract

A simple mechanical model of planetary lithospheres and asthenospheres is combined with the most recent experimental results concerning the brittle and ductile deformation of ice I_h to determine the plausibility of forming the grooved terrains on Ganymede, Enceladus, and Miranda by means of lithospheric instabilities. The model assumes a lithosphere of uniform strength atop a substrate whose strength decreases exponentially with depth. The thickness of the lithosphere is defined by the intersection of a near-surface Byerlee's law relationship for brittle failure with a standard exponential ductile flow law at depth. For Ganymede we find that the relatively high surface gravity warm temperatures render virtually unachievable an instability strong enough to account for the observed topographic relief. However, a sufficiently strong instability, capable of producing features with the proper spacing, could develop on a smaller, colder body such as Enceladus or Miranda. The requirements in this case would be a strain rate greater than about 10^(−14) sec^(−1) and a near-surface thermal gradient of a few tens of degrees Kelvin per kilometer. The most reasonable way to accomplish these requirements is by the intrusive emplacement of a thick layer of warm ice beneath a preexisting lithosphere of about a kilometer or so in thickness, accompanied almost contemporaneously by a strain event lasting on the order of 10^5 years for extension or 10^3 years for compression. However, observed properties of grooved terrains on Enceladus and Miranda, such as nonuniform feature spacing and low crater counts, force us to conclude that unstable deformation must be more complicated than this simple model would suggest if it is to explain these features.