Rheology of two-phase aggregates of H2O and CO2 ices

Dry ice (CO2) is rheologically much weaker than H2O ice. It has been observed on the surface of Mars’s residual south polar ice cap and is likely to be present on most icy bodies in outer solar system. Thus the rheological behavior of two-phase aggregates of CO2 and H2O ices can potentially influence tectonics and internal dynamics of icy bodies, as well as the stability of the Mars polar cap. We have conducted creep experiments on two-phase mixtures of CO2 and H2O ices at CO2:H2O volume ratios of 5:95, 10:90, 25:75, 50:50, 75:25, under confining pressures of 20-100 MPa and temperatures of 170-190 K using a gas-medium triaxial deformation apparatus. Two-phase aggregates of CO2 and H2O ices were mixed as powders, hydrostatically compacted, and then deformed at constant strain rates from 3e-7 to 1e-5/s. Hydrostatic compression pressures were between 60 MPa and 140 MPa and were generally lower for samples with higher CO2 content. The creep experiments revealed that the flow strength of the two-phase aggregate decreases drastically with increasing CO2 content. Stress exponents and the activation energies in the aggregate flow law gradually change from those of one end member to those of the other. Our results suggest that the rheology of the two-phase aggregate roughly matches an average of isostress and isostrain models. The flow strength in the 5 vol.% CO2 aggregate is less than half of that in pure H2O ice at the strain rate of 1e-6/s. Put differently, the presence of 5 vol.% CO2 ice in H2O ice decreases viscosity by more than one order of magnitude at a differential stress of 0.1 MPa.