Abstract
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 polynomial 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
MgSO4) and multi-ion “analog” (Cl-dominated and SO4-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 MgSO4, 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 SO4-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 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.