Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p21b1218g&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P21B-1218
Physics
[3924] Mineral Physics / High-Pressure Behavior, [5422] Planetary Sciences: Solid Surface Planets / Ices, [5430] Planetary Sciences: Solid Surface Planets / Interiors
Scientific paper
The ammonia compound decreases very significantly the melting curve of water ices. This property is of fundamental importance in planetology because it constrains the nature of the icy mantle within icy moons of the giant planets (1,2). That is why many experimental studies have been conducted for understanding the binary system H2O - NH3 in the temperature - pressure space (1-6). The most surprising effect of the ammonia compound is that it decreases the melting temperature of the ice polymorphs down to 180 K at the eutectic temperature (1,2). This effect has been observed throughout the pressure domain relevant for icy moons from 0 to 1 GPa. In this paper, a review of these efforts, in addition to new results, obtained by the authors with a sapphire anvil cell in the domains where experimental data were lacking (8), will be presented. A thermodynamic description that uses the chemical potential approach can be used for describing the pure water system (7,9). However, most studies that have used this approach were clearly focused on describing the melting curves of the water phase diagram. A new formulation has recently been proposed (9), which allows to compute phase equilibria in the whole P-T domain where ice polymorphs are encountered. In this work, the model has been improved by incorporating the ice II polymorph, and by describing more accurately the behaviour of the binary liquid mixture which is close to a regular symmetric solution (10). This model has been validated in the pure water domain by checking that all stability domains of the ice polymorphs (Ih, II, III, V, and VI) were predicted with an accuracy better than 1%. A very good reproduction of the stability domain of ice Ih in the H2O-NH3 phase diagram is also achieved. It will be shown that the thermodynamic approach allows to predict the stability of each ice polymorphs whatever the pressure and temperature are, in very good agreement with the available experimental data. It appears that the stability domain of some polymorphs, such as ice II and ice V, enlarges with the ammonia concentration, whereas that of ice III diminishes with NH3 concentration, and this phase is expected to disappear above 10 %wt of ammonia in the system. For each polymorph, a whole description of stability domains and melting surfaces will be presented. Implications for the occurrence of liquid oceans within the largest moons will also be briefly discussed. Ref. : 1. Hogenboom et al., Icarus, 1997. 2. Grasset and Pargamin, Planet. Space Sci., 2005. 3. Croft et al., Icarus, 1988. 4. Johnson and Nicol, J. Geoph. Res., 1987. 5Kargel, Icarus, 1992. 6. Leliwa-Kopystynski et al., Icarus, 2002. 7. Chizhov and Nagornov, J. Appl. Mech. Techn. Phys. 1991. 8. Choukroun M., PhD thesis, 2007. 9. Choukroun and Grasset, J. Chem. Phys, 2007. 10.Wood and Fraser, Oxford Univ. Press, 1977.
Choukroun Mathieu
Grasset Olivier
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