Ridge and Trough Terrains on Outer Planet Satellites

Major satellites of all four outer planets display ridges and troughs, occurring individually and in subparallel sets of ridge and trough terrain. Ganymede's "grooved terrain" is the archetype example, but analogous ridge and trough terrains and related landforms occur on Io, Europa, Callisto, Enceladus, Tethys, Dione, Rhea, Miranda, Ariel, and Triton. A variety of geological processes can produce individual landforms or ridge and trough terrains. In tectonic models, they can be produced by extension (as tilt-block or horst-and-graben normal fault blocks), contraction (as thrust blocks and/or folds), or strike-slip (especially as related to fault duplexing). In magmatic models, ridges might form by extrusion (as linear eruptions) or intrusion (as linear laccolith-like bodies) of volcanic or diapiric source materials. The combination of Voyager, Galileo, and Cassini imaging data show a surprising variety of specific ridge and trough morphologies on the outer planet satellites. Extensional tectonism is inferred to be the principal cause of many ridge and trough terrains, and this process is inferred to have at least partially tectonically resurfaced older terrains (erased preexisting features) and/or brightened surfaces (by revealing brighter material beneath a darker surface layer) on Ganymede, Europa, Enceladus, Tethys, Dione, and Rhea. There is also strong evidence that contraction (on Io, Europa, and Enceladus) and strike-slip faulting (on Ganymede and Europa) have shaped some ridge and trough terrains. Moreover, intrusive and extrusive models remain quite viable models for some individual ridge and ridge sets on Europa, Miranda, Ariel, and Triton. While important commonalities of morphologies and process exist, multiple processes have formed ridges and troughs on the outer planet satellites, pointing to rich geological histories.