Effects of dispersed particulates on the rheology of water ice at planetary conditions

Abstract

We have investigated the effects of initial grain size and hard particulate impurities on the transient and steady state flow of water ice I at laboratory conditions selected to provide more quantitative constraints on the thermomechanical evolution of the giant icy moons of the outer solar system. Our samples were molded with particulate volume fractions, ϕ, of 0.001 to 0.56 and particle sizes of 1 to 150 μm. Deformation experiments were conducted at constant shortening rates of 4.4 × 10−7 to 4.9 × 10−4 s−1 at pressures of 50 and 100 MPa and temperatures 77 to 223 K. For the pure ice samples, initial grain sizes were 0.2–0.6 mm, 0.75–1.75 mm, and 1.25–2.5 mm. Stress‐strain curves of pure ice I under these conditions display a strength maximum σu at plastic strains ε ≤ 0.01 after initial yield, followed by strain softening and achievement of steady state levels of stress, σss, at ε = 0.1 to 0.2. Finer starting grain size in pure ice generally raises the level of σu. Petrography indicates that the initial transient flow behavior is associated with the nucleation and growth of recrystallized ice grains and the approach to σss evidently corresponds to the development of a steady state grain texture. Effects of particulate concentrations ϕ < 0.1 are slight. At these concentrations, a small but significant reduction in σu with respect to that for pure water ice occurs. Mixed‐phase ice with ϕ ≥ 0.1 is significantly stronger than pure ice; the strength of samples with ϕ = 0.56 approaches that of dry confined sand. The magnitude of the strengthening effect is far greater than expected from homogeneous strain‐rate enhancement in the ice fraction or from pinning of dislocations (Orowan hardening). This result suggests viscous drag occurs in the ice as it flows around the hard particulates. Mixed‐phase ice is also tougher than pure ice, extending the range of bulk plastic deformation versus faulting to lower temperatures and higher strain rates. The high‐pressure phase ice II formed in ϕ = 0.56 mixed‐phase ice during deformation at high stresses. Bulk planetary compositions of ice + rock (ϕ = 0.4–0.5) are roughly 2 orders of magnitude more viscous than pure ice, promoting the likelihood of thermal instability inside giant icy moons and possibly explaining the retention of crater topography on icy planetary surfaces.