CRYOVOLCANISM ON THE ICY SATELLITES

Evidence of past cryovolcanism is widespread and extremely varied on t he icy satellites. Some cryovolcanic landscapes, notably on Triton, ar e similar to many silicate volcanic terrains, including what appear to be volcanic rifts, calderas and solidified lava lakes, flow fields, b reached cinder cones or stratovolcanoes, viscous lava domes, and sinuo us rilles. Most other satellites have terrains that are different in t he important respect that no obvious volcanoes are present. The preser ved record of cryovolcanism generally is believed to have formed by er uptions of aqueous solutions and slurries. Even Triton's volcanic crus t, which is covered by nitrogen-rich frost, is probably dominated by w ater ice. Nonpolar and weakly polar molecular liquids (mainly N-2, CH4 , CO, CO2, and Ar), may originate by decomposition of gas-clathrate hy drates and may have been erupted on some icy satellites, but without w ater these substances do not form rigid solids that are stable against sublimation or melting over geologic time. Triton's plumes, active at the time of Voyager 2's flyby, may consist of multicomponent nonpolar gas mixtures. The plumes may be volcanogenic fumaroles or geyserlike emissions powered by deep internal heating, and, thus, the plumes may be indicating an interior that is still cryomagmatically active; or Tr iton's plumes may be powered by solar heating of translucent ices very near the surface. The Uranian and Neptunian satellites Miranda, Ariel , and Triton have flow deposits that are hundreds to thousands of mete rs thick (implying highly viscous lavas); by contrast, the Jovian and Saturnian satellites generally have plains-forming deposits composed o f relatively thin flows whose thicknesses have not been resolved in Vo yager images (thus implying relatively low-viscosity lavas). One possi ble explanation for this inferred theological distinction involves a d ifference in volatile composition of the Uranian and Neptunian satelli tes on one hand and of the Jovian and Saturnian satellites on the othe r hand. Perhaps the Jovian and Saturnian satellites tend to have relat ively ''clean'' compositions with water ice as the main volatile (ammo nia and water-soluble salts may also be present). The Uranian and Nept unian satellites may possess large amounts of a chemically unequilibra ted comet-like volatile assemblage, including methanol, formaldehyde, and a host of other highly water- and ammonia-water-soluble constituen ts and gas clathrate hydrates. These two volatile mixtures would produ ce melts that differ enormously in viscosity The geomorphologic simila rity in the products of volcanism on Earth and Triton may arise partly from a theological similarity of the ammonia-water-methanol series of liquids and the silicate series ranging from basalt to dacite. An abu ndance of gas clathrate hydrates hypothesized to be contained by the s atellites of Uranus and Neptune could contribute to evidence of explos ive volcanism on those objects.