  The role of water ice in the solar system is reviewed from a fluid-dynamical
point of view. On Earth and Mars, water ice forms ice sheets, ice caps and
glaciers at the surface, which show glacial flow under their own weight. By
contrast, water ice is a major constituent of the bulk volume of the icy
satellites in the outer solar system, and ice flow can occur as thermal
convection. The rheology of polycrystalline aggregates of ordinary, hexagonal
ice Ih is described by a power law, different forms of which are discussed. The
temperature dependence of the ice viscosity follows an Arrhenius law.
Therefore, the flow of ice in a planetary environment constitutes a
thermo-mechanically coupled problem; its model equations are obtained by
inserting the flow law and the thermodynamic material equations in the balance
laws of mass, momentum and energy. As an example of gravity-driven flow, the
polar caps of Mars are discussed. For the north-polar cap, large-scale flow
velocities of the order of 0.1...1 mm/a are likely, locally enhanced by a
factor ten or more in the vicinity of surface scarps/troughs. By contrast, the
colder south-polar cap is expected to be almost stagnant. Tidally heated
convection is discussed for the example of the icy crust of Europa, where a
two-dimensional model predicts the formation of an upper, conductive lid and a
lower, convective layer with flow velocities of the order of 100 mm/a. Very
little is known about the fluid-dynamical relevance of high-pressure phases of
water ice as well as ices made up of other materials.
