Rifting and the Elevation of Bands on Europa

Bands are a type of geological feature on Europa that display clear signs of tectonic extension. Whether that extension is passive (rift driven by plate divergence) or active (ascent of a buoyant diapir), interior ice must ascend to replace the preexisting ice that is moving away. As warm, interior ice rises, it carries heat, thinning the overlying ice compared to the surrounding regions.

We present here semi-analytical models of the thermal structure of passively upwelling ice and we discuss the effect of upwelling velocity on ice thickness and topography. We solve the 1-D heat equation for ice shell thickness, considering conduction, heat generation by tidal dissipation, and upward advection. We also manipulate scaling relations from Sotin and Labrosse [1999] to solve for the thickness and interior temperature of a proposed layer of convecting ice beneath the conductive layer. By matching heat flow and temperature between conductive and convecting ice, we construct a temperature profile through the entire ice shell and solve for ice thickness. Importantly, water that crystallizes at the base of the ice shell to replace the upwelling ice releases latent heat, so that the heat flux at the base of the shell increases when there is upwelling. Also, thermal conductivity, viscosity, and therefore heat generation, depend on temperature.

In the absence of convection, the upwelling ice is always thinner than ice at rest, leading to lower elevation inside the band than outside of it. Because bands actually stand 100 m higher than their surroundings, the ice outside the band must be denser than the freshly crystallized ice inside the band, possibly due to non-ice impurities. The band, if currently active, can open no faster than 10^-10 m/s ( 3 mm/yr) or the impure ice would have to be denser than water.

Models with convection exhibit a more complex behavior. For very low basal heat fluxes, the ice is conductive. For higher basal heat fluxes, convection sets in and leads to a thicker ice shell. However, if the basal heat flux is increased further, as happens if upwelling is rapid, convection stops and ice thickness increase due to the relative inefficiency of. In that case, bands could stand a few hundred meters above their surroundings without requiring compositional variations. While unusual, this situation is predicted in our reference model.