The Ammonia-Water System and the Chemical Differentiation of Icy Satellites

Details The Ammonia-Water System and the Chemical Differentiation of Icy Satellites The Ammonia-Water System and the Chemical Differentiation of Icy Satellites

Jul 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997icar..128..171h&link_type=abstract

Icarus, Volume 128, Issue 1, pp. 171-180.

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42

Scientific paper

We report the discovery of the first high-pressure polymorphs of ammonia hydrates: ammonia monohydrate II and ammonia dihydrate II. The subsolidus transitions and melting curves of these substances are shown by their volume-temperature functions; uncalibrated calorimetry corroborates these phase changes. From 20 to 300 MPa ammonia dihydrate and ice melt at a eutectic to form water-rich liquids; at lower and higher pressures, ammonia dihydrate melts incongruently to ammonia-rich liquids. The new data are consistent with independently known thermodynamic parameters of the ammonia-water system. These results fill in an important region of pressure-temperature space not previously studied; a body of previous data reported by other investigators covers a complementary region (higher pressures), but in the light of the new data those earlier results now appear to have been misinterpreted. We show that a suitable reinterpretation of the previous data supports the identification of at least one high-pressure polymorph of each compound. The behavior of the system H_2O-NH_3 in many ways follows that of MgO-SiO_2, and the roles of ammonia-water in icy satellite evolution may parallel those of magnesium silicates in Earth's structure, volcanism, and deep mantle tectonism. Pressure-related effects, including a pressure influence on the ammonia content of cryomagmas, might be significant in determining some potentially observable aspects of cryovolcanic morphologies, surface compositions, and interior structures of icy satellites.

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