Compositional Controls on the Distribution of Brine in Europa's Ice Shell

摘要

The composition of impurities in ice controls the stability of liquid water and thus the distribution of potential aqueous habitats. We present a framework for modeling the brine volume fraction in impure water ice as a function of temperature and bulk ice salinity, inspired by models originally developed for sea ice. We applied this framework to examine the distribution of brine within the thermally conductive layer of Europa's ice shell, considering binary (NaCl and MgSO_4) and multi-ion “analog” (Cl-dominated and SO_4-dominated) endmember impurity compositions. We found the vertical extent of brine in a conductive ice layer, expressed as a fraction of the total layer thickness, to be <12% for NaCl, <2% for MgSO_4, and <18% for both the analog endmember impurity compositions, suggesting that the depth where brine is stable in an ice shell is more sensitive to composition when only two ionic species are present. For the same temperature and bulk ice salinity, the brine volume fraction is higher in a Cl-dominated ice shell than a SO_4-dominated ice shell. Pressure, governed by the ice thickness, was found to have only a minor effect on the vertical extent of brine within an ice shell, relative to temperature and bulk ice salinity. The minimum stable bulk ice shell salinity formed through freezing of an ocean was found to be insensitive to composition and ultimately governed by the magnitude of the assumed percolation threshold.