Incorporating ocean impurities into Europa's ice shell: Salty tree-rings?

The surface of Europa records a storied history of tectonic deformation, including the exposure of new interior material at extensional bands and removal of surface material to the interior at inferred subsumption zones. These geologic processes are critical for transporting material through the brittle ice shell exterior, and therefore critical for understanding the redox state and astrobiological potential of the interior ocean. Some features are associated with the exposure of non-ice materials, indicating spatial or temporal variations in non-ice abundance within the shell. Because the amount of non-ice material incorporated into the ice shell from the ocean depends to first-order on how quickly the ocean water freezes, the current distribution of non-ice materials may offer a record of the geologic evolution of the ice shell and the ice-ocean interface. We numerically simulate an ice I shell resting on a mock ocean, focusing on melting and freezing at the ice ocean interface, the rate at which freezing occurs, and how convection and tectonics transport that material within the ice shell and to the Europan surface. These models capture viscous flow, elastic bending, plastic deformation, and brittle failure. For particles transitioning from the ocean to the ice shell, we record the maximum freezing rate ever experienced as an indicator of potential impurity abundance. Thus, by using freezing rate as an analog for non-ice incorporation, we use maps of freezing rate at the time of ice incorporation to infer the distribution of impurities within the ice shell. We investigate 3 scenarios: (1) An ice shell freezes in from an ocean exposed to space. (2) An initially 130 km thick shell that thins. (3) A frozen-in ice shell thickens in response to a decrease in heating. We find that non-ice distributions record geologic history and interior heat flux, and will help constrain whether the ice shell interior is convecting.