Fluid volcanism on Miranda and Ariel: Flow morphology and composition

Abstract

Several types of volcanic units have been recognized on the icy Uranian satellites Miranda and Ariel. On Miranda, ridges characterized by crest grooves are up to 10 km wide and 500 m high. A continuous flat‐topped flow band also 10 km wide and 500 m high forms the outer southern margin of Elsinore Corona. Elsinore Coronae at least appears to comprise coalesced flow bands and ridges. On Ariel, in addition to at least one ridge unit similar to these on Miranda, flood plains material has covered the floors of deep chasmata (grabens) and an irregular depression. Flows on both satellites are characterized by linear vent geometries and distinct topographic margins, which indicate extrusion of a relatively viscous material. The topography of the flows can be used to estimate flow viscosity or yield strength using a Bingham plastic model. Extrusion viscosity estimates, incorporating plausible volcanologically based emplacement time scales and a rigid crust correction, range from 107 to 109 P (1013 P in the unlikely absence of a chilled crust). Viscosity estimates are dependent on the assumed emplacement time scale, however, and could be as high as 1016 P, if a solid‐stale‐based time scale is assumed. Bingham models indicate that it is yield strength that controls the final flow width and thickness that we see. Yield strength estimates are probably more reliable because of uncertainties in the viscous flow time scale. Yield strength estimates range from 10−1 to 10−2 bars, consistent with yield strengths of partially crystallized silicate melts on other planets. Nevertheless, both the viscosities and yield strengths are well within the range of (and in some cases lower than) silicate flows. Silicate compositions are not possible for these flows because of the ice‐rich composition of these satellites. Measured properties of ammonia‐water liquid‐crystal mixtures are consistent with the observed and estimated flow rheologic properties, however, and extrusion of such material(s) is a viable alternative to solid‐state extrusion for the origin of these flow features.