TOPOGRAPHIC CONSTRAINTS ON VOLCANISM IN THE OUTER SOLAR SYSTEM: IS THERE ANY?

The notion of erupting water (and other volatile) lavas on the icy satellites has been scientific fodder since Voyager first observed Ganymede in 1979. Volcanism there was inferred from broad (often tectonized) lanes and expanses of ‘bright terrain.’ (I note that melting of solid crustal material and mobilizing it to a planetary surface is “volcanism”, regardless of the composition of that material.) Other putative volcanic features and terrains have been identified on other icy moons since then, most notably Miranda, Ariel, and Triton, but progress has been limited. Smooth plains occur on Tethys, Dione, and Triton; caldera-like features on Ganymede, Triton (and perhaps Dione); and (volcanic?) ridges on Miranda and Ariel. Large volumes are implied for Ganymede, where a minimum of 1-2 km of burial over 66% of the surface are required). The caldera-like features imply some involvement of gas and collapse. Smooth plains of Dione and Tethys appear to fill low-lying topography and may be ‘volcanic’. The ridges on Miranda and Ariel are the only putative locations where we see measureable relief. No compositional signature has as yet been identified on any of these units, and this is a serious impediment to our understanding. On moons like Ganymede and perhaps Dione and Tethys, the implied volume of material suggests that water is the dominant lava, as other ices are likely to be unavailable in large quantities and water ice dominates the spectra over these large areas. The physiochemistry of icy body interiors is likely to be complex, given that methane, CO2, CO, N and simple organics have been observed on comets, Triton and Pluto. The ammonia-water system has been hypothesized and investigated experimentally, and once partially melted, found to mimic the basaltic system, at least rheologically. Addition of other phases to this system only increases complexity. Whether there is evidence in volcanic morphologies for increasing volatile content with distance form the Sun is debatable in the absence of spectroscopic identifications. The physical mechanism of emplacement is also elusive, as traditional flow fronts have not been identified (partly due to resolution limits). We may need to think ‘outside the box’ and look at alternative mechanisms, such as the pressure release melting proposed for Ganymede.