Introduction

When subjected to high pressures and varying temperatures, ice can form in 12 known ordered phases. On Earth, only ice Ih is present because polar ice sheets are too thin to reach the critical conditions for the formation of ice II and ice III. The situation is very different for the large icy satellites of the outer planets. Temperature and pressure are such that current models of the internal structure of the principal moons of the outer planets suggest that a thick icy shell containing several high-pressure phases of ice surrounds the silicate core. The occurrence and properties of such ices are the subject of numerous studies to understand the tectonics and dynamics of these ices. A review of the main physical properties of high-pressure ices can be found in Klinger et al. (1985), Schmitt et al. (1998) and Petrenko and Whitworth (1999).

The regions of stability of ice crystalline phases on the pressure–temperature diagram are shown in Figure 8.1. Several phases are not mentioned in this figure because they are not stable or not present in icy satellites. In this chapter, the analysis of the mechanical properties of high-pressure ices is restricted to ices II, III, V and VI, the only ices studied in the laboratory. Crystalline structure, density and shear modulus of several phases of water ice are given in Table 8.1.