Rheologies of H₂0 Ices I_h, II, And III at High Pressures: A Progress Report

Abstract

Experimental studies of the rheology of ordinary hexagonal ice have, until recently, been restricted to temperatures greater than -30°C (243 K) and pressures less than 30 MPa. We have greatly extended this pressure-temperature field to pressures as high as 350 MPa and temperatures as low as 77 K and have established the basic features of ice I_h rheology. At the lowest temperatures and highest strain rates, ice I_h fails by a very unusual form of fracture. Above a confining pressure of 50 MPa, fracture strength is independent of pressure and varies from about 160 MPa at 77 K to 120 MPa at 158 K. This fracture process appears to be a shear instability and may be related to localized transformation to a high-pressure phase of ice. At higher temperatures, ice displays a steady-state flow behavior at strains greater than about 10%. No fewer than three separate rheological laws are needed to adequately characterize this steady-state ice rheology. Although our rheological data compare well with glaciological studies at high temperatures, the latter are inadequate to describe the flow of ice at temperatures below about 240 K. Plastic relaxation times t_r have been computed from our flow laws at differential stresses that correspond to a range of topographic reliefs on Ganymede and Callisto. For a relief h = 100 m, t_{r >} 3 Ga at temperatures below 110 I and for h >1 km, t_r < 10 Ma at temperatures greater than 100 K. Marked crater profile relaxation is therefore implied for large craters with h < 1 km if pure ice In rheology applies to these Jovian satellites.

We have also performed preliminary experiments on the high-pressure ice polymorphs II and III at a fixed strain rate of about 4x10^–4s^–1. Ice III is markedly weaker than ice I_h in its pressure-temperature stability field and shows a much greater weakening with increasing temperature than ordinary ice. Ice II, on the other hand, is significantly stronger than ice I_h and has a very similar temperature effect on strength as ice I_h. Lastly, both ice II and III are strengthened by increasing confining pressure, in contrast with softening with increasing pressure for ice I_h. These pressure effects on strength parallel the freezing point depression with pressure for ice I_h and its elevation with increasing pressure for ice III, suggesting that all of the high pressure forms of ice will exhibit pressure strengthening.