The Jovian satellite Ganymede is composed of ice and silicate minerals. According to a recent analysis (Nature, 292, 225–227, 1981) by French geochemists J. P. Poirier, C. Satin, and J. Peyronneau of the University of Paris, the ice forms of Ganymede may have undergone a complex pressure-temperature history. The mechanism proposed solid state convection of high-pressure phases of H20 driven by heat from radioactive decay of U, Th, and K contained in Ganymede's ‘hard rocks.’ Poirier and his colleagues describe the geologic history of Ganymede as a passage of the ices, from Ice 1 to Ice 8, through the web of phase boundaries in pressure-temperature space.
Viscosity is the clue, it seems. Poirier et al. made visual observations with a ruby-window, high-pressure apparatus positioned for viewing under a microscope. Tap water contained in the sample chamber was frozen directly to Ice 6 by the application of pressure alone, at room temperature. Poirier et al. observed the Ice 6 crystals growing, and then undergoing a creep-flow process over a period of 17 minutes or so, along a superimposed pressure gradient. The Ice 6 crystals were photographed and their positions noted by precise markers.