INVESTIGATING THE DYNAMICS OF DIKE-FED CRYOVOLCANISM

Cryovolcanism has long been postulated for icy satellites, and direct observations of this process include the plumes on Enceladus, observations of possible plume activity on Triton, and more recently, observations of plumes on Europa. While mechanisms for eruptions on Triton include sublimation of methane, eruptions on Enceladus and Europa can be considered “top-down” style eruptions where fracturing is induced within the ice shell by processes such as tidal flexing, fractures then propagate downwards, tap the ocean, and induce explosive volatilization of the ocean materials. This style of volcanism is starkly different from standard silicate volcanism where accumulating mantle melts impinge on the base of the crust, initiate a fracture, and propagate upwards towards the surface in the form of a dike. While there have been several putative identifications of morphologies that would be associated with this dike-fed, “bottom-up”, style of volcanism on Titan and Pluto, there exist several long-recognized mechanical problems with the process.

The single largest mechanical barrier to dike-fed cryovolcanism is that liquid water is denser than the overlying ice shell, and thus will not propagate unless grossly over-pressurized. However, we suggest that exotic cryomagma compositions might not be necessary to allow for dike-fed propagation. Using a series of analyses recently applied to the Moon (another body with magma denser than the overlying crust), we explore how dike-tip degassing, and in-transit magma exsolution might aid cryovolcanic transport. Because the dike tip is inherently a vacuum environment, there will be a pressure gradient in the top of every dike, and this low pressure environment will encourage the exsolution of volatile species in this portion of the dike. The formation of bubbles leads to a magmatic foam with a lower density than the nominal magma density, allowing for continued dike propagation. We examine the effect of dike-tip degassing in cryovolcanic settings for pure H₂O magmas and H₂O+NH₃ magmas. Our preliminary results suggest that while dike-tip degassing can provide significant reduction in magma density and therefore enhance dike propagation; problems are still encountered generating sufficient stresses at the base of the ice shell to allow initial fracture formation.