Rheology of Planetary Ices
Laboratory measurements of the brittle and ductile behavior of several of the major icy planetary building materials have been made: water ice phases I through VI, ices in the ammonia-water system, mixtures of water ice plus particulates, and methane clathrate. All the materials measured thus far are sufficiently ductile that they can be expected to participate in convective motion in planetary interiors, and the quantitative rheological laws presented here provide constraints for models of evolutionary and present-day processes on icy moons of the solar system. Some unusual behavior is worth special note: ice III is several orders of magnitude less viscous than the other water ice phases. Ammonia dihydrate has a strength that is so temperature sensitive that, although it melts near 176 K, it becomes as strong as ice I after a decrease in temperature of only 23 K. Because of the extrapolation over many orders of magnitude in strain rate from the laboratory to planetary interiors, some of the material strength values calculated on the basis of the laboratory data may be only upper bounds on strength in the planetary setting.
KeywordsMantle Convection Solar Nebula Outer Solar System Planetary Interior Viscosity Contrast
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