RIDGE AND TROUGH TERRAINS ON ICY WORLDS

Major satellites of all four outer planets display ridges and troughs, occurring individually and in subparallel sets of “ridge and trough terrain.” Ganymede’s bright “grooved terrain” is the archetype example, where the icy surface is deformed at multiple scales and locally highly strained. Its characteristics are consistent with domino-style normal faulting and extensional necking of an ice-rich lithosphere above a ductile icy asthenosphere.

Other ridge and trough terrains and related landforms also occur on Io, Europa, Callisto, Enceladus, Tethys, Dione, Rhea, Miranda, Ariel, and Triton. A variety of geological processes can produce individual ridge or trough landforms or sets of ridge and trough terrain. 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.

Detailed analyses of imaging data from the Voyager, Galileo, and Cassini spacecraft reveal a wide variety of specific ridge and trough morphologies on the outer planet satellites. Extensional tectonism is inferred as the principal cause of many ridge and trough terrains, and has tectonically resurfaced older terrains at least partially by erasing preexisting features and brightening surfaces by revealing icier 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. Intrusive and extrusive models are viable for some individual ridges and for ridge sets on Europa, Miranda, Ariel, and Triton.

While important commonalities of morphologies and process exist, multiple processes are inferred to have formed ridges and troughs on the outer planet satellites. Observations of Ceres by the Dawn spacecraft and Pluto by New Horizons—both in 2015—will be important in constraining the extent of tidal vs. radiogenicly driven processes in shaping the tectonics of icy worlds, potentially including ridge and trough terrains.