We discuss the behavior of salt waters of varying salinity under simulated conditions that are likely to exist in Europa’s hydrosphere. Samples of KCl 5%, 10%, and 15% aqueous solution by weight have been observed at temperatures between 0 and -16 ℃ and from 0 to 69 MPa. The freezing temperature of a solution at a given pressure is found by undercooling a solution in a pressure-controlled stainless steel vessel, and recording the temperature the sample reaches when it undergoes freezing. Multiple trials are run at each pressure to ensure the accuracy of our data, and the freezing points are averaged and plotted to find the dependence of freezing temperature on pressure and salinity. The equations below show the relationship between the freezing temperature of an aqueous KCl solution and percent composition at varying pressures. T(x) is the freezing temperature in degrees Celsius as a function of x, the percent composition of KCl in a solution.
0 MPa: T(x)=-0.01309 x2 - 0.3892 x - 0.4857
17.24 MPa: T(x)=-0.009871 x2 - 0.4685 x - 1.585
34.47 MPa: T(x)=-0.007655 x2 - 0.5121 x - 2.972
51.71 MPa: T(x)=-0.006845 x2 - 0.5197 x - 4.429
68.95 MPa: T(x)=-0.002681 x - 0.5922 x - 5.65
The freezing temperatures of KCl solutions decrease considerably with increasing pressure and salinity. At 69 MPa, the freezing point of KCl 15% solution is measured at -15.02 ℃, which is significantly lower than the freezing temperature of pure water at 0 MPa. Since current simulated models of Europa use the phase diagram of pure water in their calculations, failing to accurately account for the potential salinity of Europa’s ocean likely results in greater uncertainty in the models. Our results can be applied to these models to improve the precision of the simulated thickness of Europa’s ice shell.