Oxidant delivery to Europa’s ocean by brine drainage from chaotic terrains

Abstract:

The habitability of Europa’s interior ocean likely depends on redox gradients established by the delivery of oxidants formed at the surface by irradiation. This requires transport of surface materials through an ice shell that is kilometers to tens of kilometers thick. Given the limited evidence for subduction of surface materials it is unlikely that convective overturn of the ice can generate an oxidant flux that is sufficient to sustain life. We propose that the drainage of dense brines formed near the surface can deliver significant amounts of oxidants to the ocean. Widespread chaotic terrains are the most likely source of large volumes to near surface brine that could entrain oxidants formed on the irradiated surface. Melting near the cold surface is thought to be facilitated by impurities and salts that depress the melting point. These are thought to be enriched in the near surface within the conductive lid. Recent work has assumed that near surface brines generated during the formation of chaotic terrains pond near the surface and refreeze. Here we argue that the underlying convecting ice shell is likely partially molten due to the entrainment of salts and tidal heating. Further we argue that the ice is permeable because brines wet the grain boundaries of ice to establish a percolating melt network.

We present numerical simulations of oxidant transport coupled to brine migration in ductile ice and eutectic phase behavior. We assume a 3 km deep region of increased melt fraction near the surface due to increased salt concentration overlying a low porosity and permeability ice shell. The near surface brine rapidly ponds on top of the low permeability ice. This removes it from the cold surface and prevents immediate refreezing. The overpressure in the brine slowly dilates the underlying ice and eventually leads to the formation of a porosity wave that migrates through the ice shell on time scale of 10⁴ years. What fraction of the oxidants formed on the surface is entrained into the porosity wave depends on their initial distribution. Here we assume that resurfacing mechanisms have distributed oxidants uniformly throughout a depth of up to 1 km beneath the surface. If these oxidants are efficiently mixed throughout the near surface melt region up to 40% of the oxidants are delivered to the ocean below.