Rheological Properties of Water Ice—Applications to Satellites of the Outer Planets

Abstract

The icy moons of the outer solar system have not been quiescent bodies, in part because many have a substantial water component and have experienced significant internal heating. We can begin to understand the thermal evolution of the moons and the rate of viscous relaxation of surface topography because we now have good constraints on how ice (in several of its polymorphic forms) flows under deviatoric stress at planetary conditions. Details of laboratory-derived flow laws for pure, polycrystalline ice are reviewed in detail. One of the more important questions at hand is the role of ice grain size. Grain size may be a dynamic quantity within the icy moons, and it may (or may not) significantly affect rheology. One recent beneficiary of revelations about grain-size-sensitive flow is the calculation of the rheological structure of Europa's outer ice shell, which may be no thicker than 20 km.

Possibility of Convection for Diffusion (Newtonian) Viscosity in the Ice Shell of Europa?

“…An important source of uncertainty in these studies is the rheology of water ice. The flow of water ice I, as observed in experimental conditions, is in fact non-Newtonian (i.e., stress-dependent, e.g., Durham and Stern, 2001;Goldsby and Kohlstedt, 2001). According to these experimental results, it has been proposed that, in planetary conditions, the dominant water ice flow mechanism should be superplastic flow, in which grain boundary sliding is the main process (Goldsby and Kohlstedt, 2001).…”

“…According to these experimental results, it has been proposed that, in planetary conditions, the dominant water ice flow mechanism should be superplastic flow, in which grain boundary sliding is the main process (Goldsby and Kohlstedt, 2001). Thus, superplastic flow (or grain size sensitive creep, as named by Durham and Stern, 2001) has been used in several studies of convection on Europa (Pappalardo et al, 1998;McKinnon, 1999;Nimmo and Manga, 2002;Barr and Pappalardo, 2003;Ruiz and Tejero, 2003).…”

Seas under ice: Stability of liquid-water oceans within icy worlds

“…But those early works considered the ice viscosity (and so its capacity for convection) to be temperature-dependent only (i.e., Newtonian viscosity), while laboratory experiments (e.g., Durham and Stern, 2001;Goldsby and Kohlstedt, 2001), indicate that it is also stress-dependent (i.e., non-Newtonian viscosity). Recently, it has been argued (e.g., McKinnon, 2001;Rainey and Stevenson, 2003) that under low differential stress conditions inside large ice satellites as Callisto, diffusion (Newtonian) creep would be the main flow mechanism for water ice.…”

“…Recently, it has been argued (e.g., McKinnon, 2001;Rainey and Stevenson, 2003) that under low differential stress conditions inside large ice satellites as Callisto, diffusion (Newtonian) creep would be the main flow mechanism for water ice. But diffusion creep has not been experimentally observed; so we follow the interpretation of Goldsby and Kohlstedt (2001) and Durham and Stern (2001), and consider water ice flow to be mainly non-Newtonian under planetary conditions.…”

“…To illustrate this point, here we apply the methodology for nonNewtonian viscosities described in Ruiz (2001) to Titan, using the characteristics of this satellite as given in Morrison et al (1986). Calculations have been performed for two possible deformation mechanisms for water ice, superplastic flow (in which grain boundary sliding is the dominant process; this flow mechanism is grain size-dependent) and dislocation creep (grain size-independent), whose rheological properties are given in Durham and Stern (2001) and Goldsby and Kohlstedt (2001). For superplastic flow, decreasing grain sizes diminish the b value, thus conversely increasing Ra b , as well as layer instability.…”