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W. B. McKinnon, A. M. Hofmeister (Washington Univ, Saint Louis)
The proton-ordered counterpart of ice Ih is ice XI, an orthorhombic phase that is stable at low pressure and T < 72 K. The transformation of ice Ih to XI relies on orientational fixation and a slight repositioning of H2O molecules in the lattice, which can be gauged by the dielectric relaxation time \tau (essentially the average time for an for an H2O molecule to reorient). Extrapolating measurements to 70 K gives a transformation time of ~50 yr, while lower temperatures yield longer transformation times (e.g., ~300 Myr at 50 K). It would appear that even pure water ice Ih on satellite surfaces in the outer solar system should covert to ice XI on geologically reasonable time scales so long as temperatures are in the 50-70 K range. The surfaces of cooler, more distant objects (e.g., KBOs) are generally too cold to convert (unless impurities lower \tau sufficiently); elsewhere, diurnal or seasonal cycling to temperatures much above 72 K should cause any ice XI to revert. Subsurface layers on bodies like Pluto and Charon may be optimal locations for ice XI formation (not too cold, not too hot), which may then be exposed by tectonics or impacts. Planetary ice XI should be detectable through infrared spectroscopy. The primitive unit cells of both ice Ih and XI contain 4 H2O molecules, and thus the total number of vibrational modes are expected to be the same. The number of infrared modes is likely to increase, according to symmetry analysis, but whether they will all be seen depends on the degree of frequency overlap. The most obvious change to the spectra expected is that the widths of the IR bands will decrease upon ordering. Physical properties are likewise affected to varying degrees. Measured density and heat capacity (away from the transition) are quite similar. Because of this, and because the total number of modes are the same in both structures, and because the peaks should narrow upon transition to XI, the thermal conductivity should be enhanced.
The author(s) of this abstract have provided an email address for comments about the abstract: mckinnon@wustl.edu
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.