Near Surface Water on Europa?

Europa's chaos terrains are generally agreed upon to have formed through disruption of the ice shell and interaction with water, but the exact details are debated. Thrace Macula, one of the largest chaos features, was initially considered to be an extrusive flow due its dark coloration and raised topography. Upon closer inspection, the volcanic interpretation was dismissed, in favor of suggestions that motion of brines through the ice, akin to brine drainage in sea ice, would explain the dark coloration. However no model has clearly explained how both the color and topography of the feature are produced, nor the large ice rafts at its center. In this presentation, we will show that disruption of the surface ice after emplacement of a subsurface water lense can reproduce all of the observations of Thrace Macula if the ice in the upper few kilometers is highly fractured or porous. We will demonstrate with simple hydrologic models that hydraulic gradients within the surrounding ice are sufficient to produce a shallow brine zone that migrates through the ice and creates a distributed network of brine-soaked and refrozen ice. These results suggest that liquid water was still present at Thrace Macula at the time of Galileo, and that future observations of this region may reveal significant changes. Such observations have important implications for crustal recycling, material transport and the long-term habitability of Europa's subsurface and ocean.