TECTONICS ON ICY WORLDS

Twenty-five years ago marked the completion of the initial geological reconnaissance of the icy moons of Jupiter and Saturn. Ahead lay visits to the moons of Uranus and Neptune, but more importantly, detailed investigation of the icy Galilean satellites by Galileo and ongoing exploration of the moons of Saturn by Cassini. The dominant geological material on all these bodies is water ice, and because of low surface temperatures far from the Sun (30–150 K), ice there can behave as an elastic and brittle solid. Higher-temperature, ductile behavior (more familiar to glaciologists) is expected at depth, of course, allowing us to propagate such familiar concepts as lithosphere and asthenosphere, faulting and folding, and solid-state convection into these unfamiliar realms. Extension appears to dominate tectonic expression on icy moons, with rifts forming in thicker lithospheres and extensional instabilities (“grooves”) in thinner. Strike-slip faults are less common, but important on satellites undergoing repetitive tidal flexing (e.g., Europa), and compressional structures (e.g., folds) the least important of all. Regions of high extensional strain have been identified on Ganymede and Europa, but sites of compensating crustal loss remain enigmatic or controversial. Tectonic, non-volcanic, resurfacing has been proposed for some icy bodies, but cannot be the whole story. The rheology of water ice argues against possibility of terrestrial style plate tectonics on icy bodies, in that ice “plates” should be strongly coupled to any interior flow patterns. Viscous relaxation of crater forms is seen on Ganymede, possibly Callisto, Enceladus, and Ariel (Uranus), and represents the ultimate expression of isostasy; converting degree of topographic relaxation to meaningful heat flow history remains challenging, but tractable. The time history of tectonic events is naturally poorly constrained, but recent progress in modeling the formation of the outer Solar System suggests the heavily cratered terrains seen on many icy satellites may be linked in time to the Late Heavy Bombardment in the terrestrial planet region. And while determining anything about Titan's tectonics is difficult owing to surface erosion and alteration, a whole new class of evolved icy bodies is being uncovered beyond Neptune, in the Kuiper belt.