Cryomagma ascent on Europa

Abstract

Europa's surface exhibits morphological features which, associated with a low crater density, might be interpreted to have formed as a result of recent cryovolcanic activity. In particular, the morphology of smooth deposits covering parts of the surface, and their relationship to the surrounding terrains, suggest that they result from liquid extrusions. Furthermore, recent literature suggests that the emplacement of liquid-related features, such as double ridges, lenticulae and chaos could result from the presence of liquid reservoirs beneath the surface. We model the ascent of liquid water through a fracture or a pipe-like conduit from a subsurface reservoir to Europa\textquoteright s surface and calculate the eruption time-scale and the total volume extruded during the eruption, as a function of the reservoir volume and depth. We also estimate the freezing time of a subsurface reservoir necessary to trigger an eruption. Our model is derived for pure liquid water and for a briny mixture outlined by Kargel (1991): 81 wt% H$_{2}$O + 16 wt% MgSO$_{4}$ + 3 wt% Na$_{2}$SO$_{4}$. Considering compositional data for salt impurities for Europa, we discuss the effect of MgSO$_{4}$ and Na$_{2}$SO$_{4}$ on the cryomagma freezing time-scale and ascent. For plausible reservoir volumes and depths in the range of $\mathrm{10^{6}\:m^{3}\leq V\leq10^{10}\:m^{3}}$ and $\mathrm{1\:km\leq H\leq10\:km}$ respectively, the total extruded cryolava volume ranges from $10^{3}\,\mathrm{m^{3}}$ to $10^{8}\,\mathrm{m^{3}}$ and the duration of the eruptions varies from few minutes to few tens of hours. The freezing time-scale of the cryomagma reservoirs varies with cryomagma composition and the temperature gradient in the ice shell: from a few days to a thousand years for pure water cryomagma, and from a few months to a 10$^{4}$ years for briny cryomagma.