A comparison of ice VII formed in the H2O, NaCl–H2O, and CH3OH–H2O systems: Implications for H2O-rich planets

Abstract

High-pressure H2O polymorphs, namely ice VI, ice VII, and ice X, are hypothesized to make up a considerable portion of the interiors of large icy satellites and select extra-solar planets. The incorporation of foreign ions or molecules into these high-pressure phases is possible through ocean–ice interaction, rock–ice interaction at depth, or processes that occurred during accretion. Recent research concerning the effects charged ions have on ice VII has shown that these ions notably affect the structure of ice VII (Frank et al., 2006; Klotz et al., 2009). This study was designed to determine the effects of a molecular impurity on ice VII and compare those effects to both pure H2O ice and ice with an ionic impurity. Ice samples were formed in this study via compression in a diamond anvil cell from either H2O, a 1.60mol% NaCl aqueous solution, a 1.60mol% CH3OH aqueous solution, or a 5.00mol% CH3OH aqueous solution and were compressed up to 71GPa at room temperature. Ice formed from pure H2O had no impurities whereas ices formed in the NaCl–H2O and CH3OH–H2O systems contained the impurities Na+ and Cl− and CH3OH, respectively. Pressure–volume relations were observed in situ by using synchrotron based X-ray diffraction and were used to determine the equations of state for ices formed in the H2O, NaCl–H2O and CH3OH–H2O systems. The data illustrate that ice VII formed from a NaCl-bearing aqueous solution exhibited a depressed volume when compared to that of H2O-only ice VII at any given pressure, whereas ice VII formed from CH3OH-bearing aqueous solutions showed an opposite trend, with an increase in volume relative to that of pure ice VII. The ices within planetary bodies will most likely have both ionic and molecular impurities and the trends outlined in this study can be used to improve density profiles of H2O-rich planetary bodies.